The 7th International symposium on Negative Ions, Beams and Sources (NIBS'20)

1 Sep 2020, 08:00 → 11 Sep 2020, 13:20 Universal

Dan Faircloth (STFC) , Neil Broderick (University of Auckland, NZ) , Yuri Belchenko (Institute of Nuclear Physics)

1. Fundamental processes and modelling
2. H– and D– sources for fusion, accelerators and other applications
3. Other negative ion sources
4. Beam formation and low energy transport
5. Beam acceleration and neutralization
6. Beam lines and facilities
7. Applications

Important Dates

Registration Open                                     1 April 2020

Paper Upload Deadline                  23 September 2020

Group Photo Foto NIBS2020 -2.1.jpg

NIBS AWARD Highlights NIBS2020_Award_D-Pace.pdf

Symposium Programme NIBS2020_Programme.pdf

Tuesday, 1 September

08:00 → 10:20 NIBS'20 Opening and Tutorial Lectures. Zoom session – https://ukri.zoom.us/j/98485745248

Conveners: Prof. Yasuhiko Takeiri (National Institute for Fusion Science) , Dr Dan Faircloth (STFC) , Prof. Yuri Belchenko (Budker Institute of Nuclear Physics)

08:00 NIBS2020 Opening

Speakers: Prof. Yasuhiko Takeiri (National Institute for Fusion Science) , Dr Dan Faircloth (STFC) , Yuri Belchenko (Institute of Nuclear Physics)

NIBS2020_T1_Belchenko.pdf

Video of NIBS Opening and Tutorial Lectures

08:15 Negative ion source fundamentals

Speaker: Prof. Motoi Wada (Doshisha University)

NIBS2020_Tu60_Wada.pptx

08:45 Ion source technologies for accelerators

Speaker: Dr Dan Faircloth (STFC)

NIBS2020_Tu_Faircloth.pdf

09:15 Ion source technologies for fusion

Speaker: Prof. Ursel Fantz (Max-Planck-Institut fuer Plasmaphysik)

Fantz_NIBS2020Tutorial_NegIonsFusion.pdf

09:45 Extraction of negative ions

Speaker: Dr Scott Lawrie (STFC ISIS Neutron Source)

NIBS2020_Mon072_Lawrie.pdf

Wednesday, 2 September

05:00 → 09:00 H– and D– sources for fusion: Oral session O1

Conveners: Prof. Motoi Wada (Doshisha University) , Prof. Yuri Belchenko (Institute of Nuclear Physics)

05:00 Initial results of a plasma grid comparison experiment between NIFS and ITER-like types at BATMAN Upgrade

Large-scale sources for negative hydrogen (or deuterium) ions are used in some present and many future fusion devices in neutral beam injectors (NBI). Several test facilities contribute to the development of the RF source for the ITER-NBI in order to achieve simultaneously the ITER requirements, e.g. beam pulse of 3600 s, beam current density of 286 A/m² and co-extracted electron current lower than the ions in deuterium operation at ITER-relevant sources: among them are BATMAN Upgrade (BUG) and ELISE at IPP. Filament-arc (FA) negative ion sources are routinely operated at the NBIs of the Large Helical Device at NIFS. Beside the discharge type, these sources differ in their required parameter range as well as in certain parts of their design, e.g. the existence of a bias plate, magnetic field configurations, and the design of the extraction system, in which the plasma grid (PG) is the plasma-facing component.
In order to gain a better understanding of the influence of the PG design on the plasma and beam properties, a comparison between the NIFS and ITER-like PGs at IPP was started at BUG by a NIFS-IPP international collaboration. The NIFS-type PG is thinner (4 mm) and has a shallower conus around the extraction apertures compared to the ITER-like PG normally used at BUG (thickness of 9 mm and deeper conus). The distance between the PG and the second grid (extraction grid, EG) is kept constant. Hence, due to the different thickness of the PGs, the field strength of the electron deflection magnets (EDM, mounted in the EG) in the plasma close to the PG is stronger for the NIFS-PG. Thus, an influence on the co-extracted electron current is expected. The different conus geometry may impact the transport of negative ions in the plasma as well as the formation of the plasma meniscus (i.e. the transition between plasma and extracted beam), and thus impact the beam optics. For this comparison, source plasma parameters were measured at 32 mm and 27 mm distance upstream of the PG surface in the NIFS and ITER-like PG cases, respectively (difference results from the different PG thickness). This position is outside of the EDM field loop. Initial results have been successfully obtained in hydrogen operation. Similar source plasma parameters with only minor differences were observed, such as local positive ion density in the range of 1x10¹⁷ m^-3 (measured by Langmuir probes) and line-averaged negative-ion density of 3 to 4x10¹⁶ m^-3 (measured by Cavity Ring-Down Spectroscopy) for both PGs at a filling pressure of 0.3 Pa, discharge power of 64 kW, extraction voltage of 5 kV, acceleration voltage of 32.5 kV, PG filter current of 1.5 kA (approximately 3 mT on the PG upstream surface), and bias current of 5 A. The extracted ion current density is with 125 A/m² similar for both PGs, as well as the divergence, measured by Beam Emission Spectroscopy collecting signal from many beamlets. Consequently, it needs to be considered that the respective value of the divergence is strongly affected by the row-wise zig-zag-deflection caused by the EDM field. In contrast, the co-extracted electron current with the NIFS-PG was approximately three times higher than that with the ITER-like PG. An increase of the PG current by approximately 1 kA is necessary for the NIFS-PG to obtain an equivalent co-extracted electron current as for the ITER-like PG. Further systematic investigations will follow, among them also a comparison in deuterium operation, within the scope of the NIFS-IPP international collaboration.

This study was supported by JSPS KAKENHI Grant number 19H01883 and 18KK0080, and NIFS (KEIN1606 and UFEX105).

Speaker: Haruhisa Nakano (National Institute for Fusion Science, National Institutes of Natural Sciences)

NIBS2020_O1_Nakano.pdf

05:20 Validation of the Distribution of Stripping Loss Neutrals in the Accelerator of the Negative Ion Source

Negative-ion based neutral beam injectors (N-NBI) are utilized for plasma heating and current drive in the Large Helical Device (LHD). In the previous operation, we achieved 2.9 MW deuterium beam injection using the negative ion source optimized hydrogen operation. The deuterium negative ion current was reduced to 55.4 A and the average current density was 223 A/m$^3$ which is 0.65 times lower than the nominal hydrogen negative ion current [1]. On the other hand, it is necessary to consider how the stripping loss due to the neutralization reaction in the accelerator affects the deuterium operation. The difference of the stripping loss between hydrogen and deuterium is examined using two different approaches. The first is the measurement of the optical beam emission. An optical line-of-sight (LOS) is set to the angle of 33$^\circ$ to the negative ion beam emitted from the ion source. The wavelength of beam emission spectrum reflects the energy distribution of beam particles by the Doppler effect. The low-energy peaks are distributed in the energy band corresponding to the extraction voltage, and also the flat energy tail is observed in the lower energy region. Secondly, the pressure distribution inside the accelerator is estimated from the accelerator structure and vacuum pressures in the beam line. Since the neutralization cross section is large at the low energy, the loss of negative ions at the extraction section is larger than that in the acceleration section. Particularly inside of the extraction grid, a lot of negative ions are lost and neutral particles with the same energy are produced. These neutrals are the cause of the low-energy peaks in the energy distribution. The beam emission spectrum is estimated from the energy distribution and the cross section of the Balmer alpha emission. It is confirmed that the peak spectrum is asymmetric with a flat tail on the low energy side. The fraction of particles lost inside the accelerator increases from 0.16 for hydrogen to 0.24 for deuterium. Assuming that the negative ion extraction efficiency is followed by the Child-Langmuir law, the value of the negative ion extraction efficiency is 0.84 for hydrogen and 0.53 for deuterium. Therefore, the current ratio of $I_{D^-}/I_{H^-}$ should be 0.63, which is considered to be the cause of the degradation of the injected beam power in deuterium operation.

[1] K. Ikeda, et. al., Nucl. Fusion 59 (2019) 076009.

Speaker: Katsunori Ikeda (National Institute for Fusion Science)

NIBS2020_ikeda.pdf

05:40 Different characteristics of plasma meniscus formation between positive and negative beam extraction

Plasma meniscus is widely believed to be formed in negative ion beam extraction boundary as well as positive ion beam extraction boundary, because very similar perveance dependence of beam divergence is generally observed. However, several difference properties of plasma meniscus formation were revealed in recent studies of negative ion beam focusing. In this talk, the different responses between negative and positive ion meniscus to externally applied perturbation [1-2] and unique phase space structure of negative ion beam [3] are reported. It was also found that the electron current does not affect the beam divergence/meniscus formation [4]. Based on these results, new physical picture of meniscus formation of negative ion beam extraction will be discussed.

This study was supported by JSPS KAKENHI Grant number 17H03002, 17K14903, 19H01883 and 18KK0080, and NIFS (NIFS18ULRR702, NIFS19ULRR031, NIFS20KLER103).

[1] K. Takahashi, T. Imagi, et al., New Journal of Physics 21, 093043, 2019.
[2] Y. Haba, K. Nagaoka, et al., Japanese Journal of Applied Physics 59, SHHA01, 2020.
[3] Y. Haba, K. Nagaoka, K. Tsumori, et al., New Journal of Physics 22, 023017, 2020.
[4] M. Kisaki, K. Ikeada, Plasma and Fusion Research 13, 1205110, 2018.

Speaker: Prof. Kenichi Nagaoka (National Institute for Fusion Science/Nagoya University)

Nagaoka_NBIS2020_v4.pdf

06:00 Development of the directional Langmuir probe for the charged particle flow measurement

Flow of charged particles near the plasma grid in negative ion sources affects the extraction efficiency of negative hydrogen / deuterium (H- / D-) ion beams [1][2]. In our previous research, the flow of the charged particles was measured with a four-pin directional Langmuir probe by rotating the tips around the center axis of the tips [3]. In the case of the four-pin probe, spatial resolution is determined by the distance between the probe tips, while plasma state changes with the direction and strength of Electron Deflection Magnetic (EDM) field. Furthermore, the field direction becomes opposite within the distance of the probe tips at some position close to the plasma grid. To resolve the problem of spatial resolution, a single-pin directional Langmuir probe equipped rotatable flap co-axially to shield charged-particle flow around the tip was newly developed and installed to the NIFS R&D Negative Ion Source (NIFS-RNIS). The probe is available to apply for a usual photodetachment Langmuir probe as well as usual electrostatic one. The first data obtained with the directional photodetachment Langmuir probe with the rotatable flap are going to be introduced.

[1] M. Kisaki et al., Rev. Sci. Instrum. 85, 02B131 (2014).
[2] H. Nakano et al., AIP Conf. Proc. 1515, 237 (2013).
[3] S. Geng et al., Rev. Sci. Instrum. 87, 02B103 (2016).

Speaker: Shingo Masaki (National Institute for Fusion Science)

NIBS2020_O1_Masaki.pdf

06:20 Profile of the LHD negative ion beam source at the plasma meniscus from numerical beam calculation based on experimental onservation

Good understanding of negative ion beam optics is crucial to the development of the ITER NBI system. The Large Helical Device (LHD) has such a negative ion beam. Recently, an unexplained splitting of the beam in the horizontal, but not the vertical, direction has been observed in the emittance diagram. This indicates a large influence of the plasma meniscus on the negative ion beam optics. As part of a larger set of parametric sweeps, a scan of the beam source arc power has been carried out inside the NIFS Neutral Beam Test Stand (NIFS-NBTS), while a Pepper-pot-type phase space analyser (PPSA) has been used to characterize the beam in six-dimensional position-momentum phase space. In this way, the effect of varying the beam source condition on the optics of the beam downstream is investigated. Numerical simulations of beam particle trajectories are carried out based on the phase space measurements to obtain beam intensity profiles and phase space structures (PSSs) at the location of the plasma meniscus. This provides a first step in understanding meniscus structure and formation and its influence on the extracted beam.

Analysis of the PSS observed in the experiment indicates that different beam sub-components after splitting originate from different physical locations on the meniscus. The results motivate further research into negative ion plasma and plasma meniscus modeling and into negative ion beam focusing.

Speaker: Jelle Slief (Eindhoven University of Technology)

NIBS2020_O1_Slief.pptx

06:40 Damage to N-NBI systems due to positive ion back-streaming

Long term stability of a plasma heating system is indispensable for realizing a DEMO nuclear fusion reactor. One possible cause for deteriorating the accelerator/ion source system of negative ion based neutral beam injection (N-NBI) heating is the damage due to energetic positive ions back-streaming from the beam produced plasma. The positive ions strike the surfaces of downstream sides of the extraction/acceleration electrodes and they can acquire the full acceleration energy to bombard the back end-plate of the ion source. The heat flux should be comparable or even greater than that to divertor as the beam energy and the current density reach ITER design goals. The robustness against the damage due to back-streaming ions should now be evaluated by taking the reactor operation life into account.

Particle retention directly affects the collision cascade of a high energy particle injected into solid materials and the resulting sputtering yields. Local temperature of the solid retaining injected particles determines the diffusion of the retained particles, and thus can largely modify the overall erosion rate of the solid. The effects due to long term exposures to intense particle injections of deuterium are studied with the computer simulation code ACAT (Atomic Collision in Amorphous Target) for molybdenum: the candidate material of the back end-plate for an ion sources of a DEMO reactor.

Speaker: Prof. Motoi Wada (Doshisha University)

NIBS2020_O1_Wada.pptx

07:00 Coffee break

07:20 Research activities of RF based negative ion source in the ASIPP

The Comprehensive Research Facility for Fusion Technology (CRAFT) is a large scientific device that is preferentially deployed for the construction of major national science and technology infrastructures. A negative beam source based neutral beam injector (NNBI) with beam energy of 400 keV, beam power of 2 MW and beam duration of 100 s is one of the device. A radio frequency (RF) based negative beam source was designed for the CRAFT NNBI system. In order to understand the physics and pre-study the engineering problems for RF negative beam source, a prototype source with single driver was developed. Recently, this beam source was tested on the RF source test facility with RF plasma generation, negative ion production and extraction. The long pulse of 105 seconds negative ion beam was extracted successfully from a three electrons accelerator. The extracted ion current is 153 A/m2 with Cs injection and the ratio of electron and negative ion is around 0.3. It lays good foundation for the R&D of negative ion source for CRAFT NNBI system. The details of design and experimental results of beam source was shown in this paper.

Speakers: Yahong Xie (Institute of plasma physics, Chinese academy of sciences) , Prof. Yuanlai Xie (ASIPP)

NIBS2020_Y. Xie(ASIPP).pdf

07:40 Beam Extraction and Optics of 200keV beam accelerator for neutral beam injection in China

A Negative-ion based Neutral Beam Injection (NNBI) prototype is under conceptual design in Southwestern Institute of Physics for China Fusion Engineering Test Reactor (CFETR). The aim in the first stage is to extract an H- beam >3A for 1000s and acquire an energy of 200keV. In this presentation, a single beamlet model for optimization of the CFETR NNBI prototype accelerator’s beam optics is developed. The boundary between plasma and beam, namely the meniscus, is calculated with a self-consistent method accomplished in Comsol environment. The optimum perveance is found, which guarantees beam the minimum divergence angle. Magnetic field produced by permanent magnets in the extraction grid is verified to deflect the majority of co-extracted electrons, while inducing a slight offset of the ion beam. As a conclusion, the system is capable to operate as required.

Speaker: Ms Fei Song (Huazhong University of Science and Technology)

NIBS2020_O1_Song.pdf

08:00 Development of External RF Antenna based Cusp Free High Duty Factor Pulsed Negative Hydrogen Ion Source

An external RF antenna based cusp free negative H ion source has been designed and developed as shown in figure 1. This source is operated at 10% duty factor and the key experimental result is shown in figure 2. The extracted H- ion beam current is 11 mA with 2 ms pulse duration and 50 Hz repetition rate at 50 kV beam energy. Operation of the H- ion source at high duty factor results in temperature rise in components, which leads to failure of electronic components and vacuum joints of plasma chamber, igniter chamber and burn out of extraction electrodes. In order to keep the operating temperature within limits, water cooling arrangement was designed and incorporated for (i) the 2 MHz RF antenna operating at 80 A RMS at 7 kVAC. (ii) the extraction electrodes, which is operating at maximum voltage of 15 kVDC (iii) the plasma chamber (made of Aluminium Nitride, transparent to RF field with high thermal conductivity) (iv) the Faraday cup for H- ion current measurement. Forced air cooling was used for RF based pulsed igniter, operating at 13.56 MHz and various current stabilizing electrode biasing networks and RF impedance matching networks. Simulations were carried out on the main components and results obtained were satisfactory.
More details of the H- ion source and experimental results will be presented in the manuscript. The content of manuscript will include thermal, vacuum and electrical simulation and experimental results.

Figure 1 link :Experimental test setup of high duty factor RF based negative hydrogen ion source

Figure 2 link :Recorded pulsed negative hydrogen ion beam current ~ 11 mA (2 ms pulse width at 50 Hz repetition rate, 10 % duty factor) at 50 kVDC accelerating voltage, Scale :X-axis : 0.4 ms/div. and Y-axis : 2.0 mA/div.

Full doc document link :Abstract of the Paper in doc

Full PDF document link : Abstract of paper in pdf

Speaker: Dr Dharmraj Ghodke (Raja Ramanna Centre for Advanced Technology)

Development of External RF Antenna based Cusp Free High Duty Factor Pulsed Negative Hydrogen Ion Source

NIBS2020_O19_Ghodke_28.pdf

NIBS-2020_Wed09_Ghodke.pptx

08:20 Probe for in situ measurement of work function and cesium dynamics

Negative ion production in the hydrogen plasma of a fusion grade negative ion source relies on the surface production mechanism. Surface production is associated with the resonance electron capture (REC) by neutral hydrogen atoms and ions impinging on a low work function surface. In order to reduce the surface work function, a Cesium (Cs) layer is deposited in situ primarily on the first grid of the negative ion extraction system called, plasma grid (PG). For that Cs vapour is injected into the ion source volume, kept under vacuum to avoid reactions of Cs with atmospheric gases and moisture. Too much presence of Cs inside the source possesses a serious maintenance issue because of its deposition on the actively cooled surfaces, mainly in the extractor system and leads to high voltage breakdown among the grids kept at different voltage levels. Therefore, optimization of Cs injection to maintain low surface work function is the need to achieve an efficient negative ion source performance. There is no vacuum compatible probe suitable for ion source application, available which can measure three Cs relevant parameters: (a) Cs flux on a surface, (b) Cs coverage on the surface and (c) correspondingly the work function of the surface in situ and can establish a correlation among all these three parameters.

The present work deals with the development of a vacuum compatible probe for in situ measurement (PRISM) of work function and Cs dynamics, including Cs flux and Cs coverage on a surface.

Speaker: Pranjal Singh (Institute for Plasma Research, Gandhinagar)

NIBS_2020_Wed10_Singh.pptx

08:40 Correction algorithm for cavity ring down based anion density measurement in a negative ion source having continuously fed cesium vapour

Negative ion or anion density measurement is frequently done non-invasively by employing Cavity Ring-Down Spectroscopy (CRDS) diagnostic system. The optical cavity in the CRDS system is created by installing two highly reflecting concave mirrors on two collinear opposite ports of the ion source chamber; so that the cavity encloses the plasma as an absorbing medium. In a continuously fed cesium seeded ion source the CRDS mirror is exposed to Cs vapour environment. As a result, a finite probability of Cs deposition is possible on the mirror surface. In addition, ion sputtering and thermal distortion may degrade the mirror reflectivity and mirror alignment with time of ion source operation respectively. Distorted cavity alignment may affect the CRDS functionality. All the above issues increase themirror loss which can be misinterpreted as absorption losses and lead to an over-estimation of negative ion density for a long ion source operational window.The CRDS sensitivity and accuracy depend on its mirror reflectivity or rather “effective reflectivity”.Due to continuous change in reflectivity of the CRDS mirrors because of continuous Cs deposition, sputtering and distortion, the CRDS sensitivity and accuracy are also function of time and a correction factor is needed to take care of the overestimation in negative ion density value, if the time difference between the reference instance and measurement instance is significantly large.In this article, an algorithm is presented to find the correction scheme.

Speaker: Mr Debrup Mukhopadhyay (Institute for plasma research)

NIBS_2020_Wed11_Mukhopadhyay.pptx

Thursday, 3 September

08:00 → 12:20 O2

Conveners: Dr Gianluigi Serianni (Consorzio RFX) , Prof. Ursel Fantz (Max-Planck-Institut fuer Plasmaphysik)

08:00 Are resonance phenomena creating instabilities in the magnetic filter region in a low-temperature plasma?

Low pressure, low-temperature plasma (LTP) with a spatially varying transverse magnetic filter field configuration has a wide range of applications. In such devices, the magnetic field value is low enough that it magnetizes only the electrons and not the ions and works as an electron cooler by limiting the flux of hot electrons [1]. Due to the gradient of the transverse magnetic filter field, the electron cyclotron frequencies are also changing with location and in some places it comes close to different collision frequencies. A parallel 2D-3V Particle-in-Cell Monte Carlo Collision (PIC MCC) kinetic model is developed [2, 3] to study plasma transport in a negative ion source where spatially varying transverse magnetic filter field is considered in a source geometry similar to that of ROBIN (RF Operated Beam source in India) [4]. Various collision dependent physical phenomena, having different time scales and length scales are studied. It is observed that the magnetic filter increases electron-ion and electron-electron collisions a couple of times than those in the absence of the magnetic filter. In this article, we summarize with observations based on the collision frequencies and try to understand if there are any resonant phenomena happening in the magnetic filter region which may lead to any instability and influence the cross-field transport.
References:
[1] Boeuf, J. P. et al. (2012), Physics of a magnetic filter for negative ion sources. I. Collisional transport across the filter in an ideal, 1D filter. Physics of Plasmas, 19(11).
[2] Chaudhury, B. et al. (2018). Hybrid Parallelization of Particle in Cell Monte Carlo Collision (PIC-MCC) Algorithm for Simulation of Low Temperature Plasmas. In Communications in Computer and Information Science book series (pp. 32–53). Springer, Singapore.
[3] Shah, M (2020). Computational characteristics of plasma transport across magnetic filter in ROBIN using PIC-MCC simulation. Fusion Engineering and Design, 151, 111402.
[4] Bansal, G. et al. (2013). Negative ion beam extraction in ROBIN. Fusion Engineering and Design, 88(6–8), 778–782.

Speaker: Miral Shah (DA-IICT, Gandhinagar)

NIBS2020_02_Shah.pdf

08:20 Experimental results of 40kW, 1 MHz Solid State High Frequency Power Supply with Inductively Coupled Plasma

A Solid State High Frequency (HF) 1MHz, 40kW source is intended for plasma formation in neutral beam source by inductive coupling of RF power. An important design feature of such a HF source is its ability to sustain large transient swings of load(due to impedance transition in microseconds time scale). A 40kW High Frequency Power Supply( HFPS) has been configured with multiple Class-D H-Bridge inverters modules by using latest generation switching semiconductors; each capable of delivering 3kW Power, magnetic combiners, LC tuning network to provide 1 MHz sinusoidal output at 50 Ω standard Load. The developed prototype power supply has been coupled to a single driver RF ion source test bed ROBIN in IPR to characterize the system with actual load conditions.
In recent experimental campaign, tuning of matching network parameters helped to strike and sustain plasma over the range of 1Pa to 0.42 Pa pressure with forward power of 37kW to 22kW. Additional impedance matching network was implemented to map the power supply impedance(50Ω) with impedance offered from source(>90Ω seen at PS end). Configurable frequency with resolution(~1kHz) helped to achieve Power factor close to unity. Experiments helped to study the behavior of power supply in scenarios of dynamic (plasma) impedance. Auto tunable frequency for matching the varying load is being implemented in the HF power supply.

Speaker: Mr Sandip Gajjar (ITER-India, Inst. for Plasma Research)

NIBS2020_Thur02_Gajjar.pptx

08:40 High-intensity polarized and un-polarized H- sources and injector development at BNL

The AGS-RHIC injector complex includes: high-intensity (magnetron type) H- ion source and Optically Pumped Polarized Ion Source (OPPIS); 750 keV RFQ and 200 MeV Linac. In this paper we will focus on the recent Linac LEBT upgrade with three sources: two magnetron sources and OPPIS. This LEBT configuration can be a good prototype of very reliable H- ion beam injectors for the accelerator complexes with the high down time cost. We also present the recent magnetron development to higher duty factor and reliability. The results of the beam production with the new LEBT configuration will be presented. Both magnetron sources produce 120 mA current (600-1000 us pulse duration , 7 Hz repetition rate). The LEBT improvement resulted in beam intensity increase (after RFQ at 750 keV) to 80 mA (maximum 90 mA). The polarized beam efficiency transport will be also improved due to shorter LEBT line and replacement electrostatic Einzel lenses with the magnetic quadrupole lenses.

Speaker: Dr Anatoli Zelenski (BNL)

NIBS2020_O2__zelenski.pptx

09:00 BTR code for NBI Design and Optimization

BTR code is used for NBI beamlines design and studies since 2005. Initially tailored for beam re-ionized particles tracking in NBI Ducts, BTR finally became a universal tool for 3D geometry optimization and thermal loads evaluation in NB lines. BTR simulation includes all variety of neutral beam formation and transport conditions - after the ion beam extraction from the Negative or Positive Ion Beam Source. From the very beginning BTR was created for public usage, and it goes with interactive user-friendly interface (Windows GUI). BTR code tracing model is straight-forward and deterministic (with no random values used), thus it is replicable and easily cross-checked with other beam tracking codes, including analytical models (PDP code). BTR standard beam is a 2D array of beamlets, their spatial positions, focusing and inner angular distributions can be reproduced with high resolution (100-100000 macro-particles per each beamlet). The model 6D statistics leads to precise evaluations of beam direct losses and power deposition profiles at beamline components (currently represented by 50 - 300 surfaces in total). BTR is parallel and able to trace up to 1e10 macro-particles within few hours on average Windows machine, with the best performance achieved on 4-8-processor systems. Today BTR is a lively and evolving code, and all the Users can have a free full-time/full-life support (3FS). Basic applications of BTR code are presented – with a focus on conventional BTR versions (Single-Run).

Speaker: Mrs Eugenia Dlougach (NRC Kurchatov Institute Russia)

BTR for NBI.pptx

09:20 Investigation of the negative ionization of hydrogen particles on metal surfaces with low work function

This work addresses the negative ionization of hydrogen particles on metal surfaces with low work function, which is an important for the field of the surface plasma negative ion sources. The theoretical model for the calculation of the negative ionization probability which takes into account the component of atom/ion velocity, parallel to the surface is described. The influence of parallel velocity on the negative ionization probability for the case of surfaces with low work function is of interest, because for ordinary metal surfaces the parallel velocity effect can enhance the negative ionization probability by order of magnitude. The calculated ionization probabilities quantitatively fit to experimental data for wide range of ion energies. The theoretical analysis reveals, that in the case of low work function converter surfaces, the parallel velocity effect can enhance the negative ionization probability up to ~33%.

Speaker: Dr Ivan Gainullin (Faculty of Physics, Moscow State University)

NIBS2020_Fp05_Gainullin.pdf

09:40 Numerical modeling of the electrons confinement in the multicusp magnetic trap

A charge-exchange target for neutralizing a negative ion beam with energies up to 10 MeV and higher requires the development of a highly efficient plasma trap which allows to form and confine plasma with a linear density up to 1017 cm-2 and higher.
The magnetic systems in which the condition of magnetohydrodynamic stability of the plasma is satisfied are most interesting to obtain a high-density plasma.
The electron confinement efficiency in a magnetic trap with a quasi-spherically symmetric multicusp magnetic field geometry with a "minimum B" at the center of the system, in which all cusps are point-type cusps is studied using numerical methods.
The results of numerical experiments are compared with a collisionless model of particle motion in a trap.

Speaker: Mr Viktor Klenov (INR)

NIBS2020_O2_Klenov.pdf

10:00 Coffee break

10:20 Impact of operational parameters on single beamlet deflection in a negative ion source for NBI applications

For current and future large scale tokamaks, neutral beams for heating and current drive are generated from the neutralisation of large negative ion beams with powers up to 40 MW and energies up to 1 MeV. These beams are created by electrostatically extracting negative ions from a low temperature/pressure hydrogen or deuterium plasma, and accelerating them through a series of grids, resulting in a beam comprised of many hundreds of beamlets. Due to their negative charge, electrons are also extracted from this plasma. Were they to undergo acceleration to the full energy, these co-extracted electrons would reduce efficiency due to the additional accelerated current, and cause high heat loads on beamline components. To prevent this, permanent magnets are embedded in the second grid, the field of which deflects co-extracted electrons out of the beam at a low energy. This field also affects the negative ions as they are accelerated, causing beamlets to exit the grid system with a residual offset and deflection angle. Due to the alternating orientation of the magnetic field, this alternates row-by-row, and causes an observable zig-zag pattern of the beamlets in many ion sources. This adversely affects the overall divergence of the beam, and compensation is foreseen in future devices.

Measurements of the residual deflection of a single beamlet have been carried out at the BATMAN Upgrade test facility by calculating relative beamlet angles from beam emission spectroscopy (BES) spectra, and through the use of one-dimensional carbon fibre composite (1D-CFC) tile calorimetry to find beamlet positions. In this work, the amount of beamlet deflection is shown to change significantly, by up to 0.7° (12 mrad), depending on the operational parameters used. As is to be expected from a simple theoretical treatment, the beamlet deflection angle is observed to be affected by changes to the voltages of the acceleration system. However, the beamlet deflection angle is also observed to change with RF power, which, to a first approximation, should only affect beamlet divergence, and not the deflection. Other source parameters that should only affect the plasma are also demonstrated to impact the beamlet deflection angle, but it was not yet possible to determine which of the source filling pressure, filter field strength, or plasma grid bias current play a role and to which extent.

Whichever are the underlying reasons for changes to the beamlet deflection angle with operational parameters that, to first approximation, should only affect the plasma, it is clear that the deflection is not constant. This has implications for devices that are planning to use compensation methods for beamlet deflection, as these methods may not be effective outside of a narrow operational space.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the EURATOM research and training programme 2014-2018 and 2019-2020 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

Speaker: Andrew Hurlbatt (Max-Planck-Institut für Plasmaphysik)

NIBS2020_O2_Hurlbatt.pptx

10:40 Transferring knowledge gained for pulsed extraction at the ELISE test facility to ITER-relevant CW extraction

The ELISE test facility with its half ITER-size ion source is an essential part of the European Roadmap towards the ITER NBI system. One aim of ELISE is to demonstrate the ITER target values for the extracted current density, the ratio of co-extracted electrons to extracted ions and the uniformity of the extracted beam during long pulses, i.e. $1000\,\mathrm{s}$ in hydrogen and $3600\,\mathrm{s}$ in deuterium and at a filling pressure of $0.3\,\mathrm{Pa}$.
Due to limitations in the available HV power supply, operation of ELISE is currently limited to pulsed extraction, i.e. short extraction phases, so-called extraction blips, of up to $10\,\mathrm{s}$ each ${\approx}150\,\mathrm{s}$. During the past years, a good insight in the physics of this operational mode was gained in both hydrogen and deuterium operation. $1000\,\mathrm{s}$ hydrogen plasma pulses with repetitive extraction blips are possible with an extracted current density of over $90\,\%$ of the ITER target value, while also fulfilling the requirements for the electron-ion ratio and the beam homogeneity. In deuterium roughly 67 % of the ITER target for the extracted current density has been achieved for long pulses.
During such pulses an overall increase of the co-extracted electron current is typically observed between one blip and the next, even though the electron current is observed to actually decrease during each blip. These opposing effects are explained by different caesium dynamics during the source plasma phase when compared to the beam phase and they were one motivation behind the currently ongoing upgrade of ELISE to a CW extraction system. Being able to achieve beam pulses of up to 1 hour will allow knowledge to be gained on the physics of caesium redistribution and conditioning over the long timescales needed for ITER operation.
This update consists of two main hardware changes: i) installation of a new CW high voltage power supply and ii) installation of a CW beam calorimeter. Delivery and commissioning of the CW high voltage power supply as well as first tests using a dummy load are planned for summer 2020. First operation of ELISE using the CW power supply will take place at the end of the year. A CW calorimeter design developed at IPP is currently being tested at the BATMAN Upgrade test facility.
The presentation discusses the physics of pulsed beam extraction and the differences to CW extraction. Additionally, the status of both systems, the CW power supply and the CW calorimeter and first results of the CW calorimeter prototype used at BATMAN Upgrade will be presented.

Speaker: Dirk Wünderlich (Max Planck-Institut für Plasmaphysik)

NIBS2020_O2_Wünderlich.pdf

11:00 First operation of SPIDER and 1MV integrated power test in MITICA

To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 17MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1MeV D- ions for ≤1h, 46A/870keV H- ions for ≤1000s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100keV particle energy (SPIDER), for early investigation of negative ion production and extraction, source uniformity, negative ion current density and beam optics.
This contribution will describe the main results of the first two years of SPIDER operation, devoted to characterizing plasma and beam parameters. Investigation of the efficiency of RF coupling to the plasma in different configurations of the RF circuits and the temporary introduction of a mask reducing the number of beamlets (for a total number of ~100 out of 1280) allowed to extend the operation parameter range, by maintaining the pressure profile within a controlled range to reduce disruptive occurrences. Magnetic filter field effectiveness in reducing the co-extracted electron current was verified, however this field was found to affect the plasma necessitating a modification to the magnetic configuration. A major shutdown, planned for 2021, to solve the issues identified during the operation and to carry out scheduled modifications, will be outlined.
The installation of each MITICA power supply and auxiliary system was completed; mechanical components are under procurement by F4E. Integration. Commissioning and test of the power supplies (PS), procured by different Domestic Agencies, will be presented. In particular, 1.0MV insulating tests were carried out step-by-step and successfully completed. During 2020 PS integrated tests on accelerator dummy load will be carried out, including the resilience to accelerator grid breakdowns using a short-circuit device in vacuum.
The overall programme of the NBTF, aimed at validating the NBI design and at meeting the ITER schedule (requiring NBIs in operation in 2032), will be outlined.

Speaker: Diego Marcuzzi (Consorzio RFX)

NIBS2020_O2_Marcuzzi.pdf

11:20 Controlling the Shape of the ISIS H- Penning Ion Source Beam Pulse

The Penning ion source serving the ISIS Pulsed SpallationNeutron and Muon facility routinely delivers55 mAbeamof negative hydrogen ions (H−) in250μspulses at50 Hzrep-etition rate. The Front End Test Stand (FETS) specificationsrequire60 mA,2 ms, pulses at50 Hz. Extending the ionsource discharge pulse length to2.2 mswill need overcomethe observed beam current droop caused by thermal tran-sients in long pulse operation. Recent experiments at25 Hzhave demonstrated square60 mAbeam pulses up to1.2 mswith the permanent magnet version of the ISIS source and100 mApulses up to1.65 mswith the same source equippedwith a double-width extraction slit. Droop was compen-sated by ramping up the discharge current during the beampulse. The physical phenomena underlying the droop and itscountermeasures are discussed, and further technical devel-opments that are necessary to reach the FETS specificationsare described. In addition, various experimental shapes ofthe ion source discharge pulses and H−beam pulses wereachieved by controlling the ion source discharge currentand accelerated through the ISIS70 MeVlinac. The tech-nique allows almost arbitrary shaping of the H−beam pulsesfor injection studies into the800 MeVrapid cycling protonsynchrotron

Speaker: Tiago Morais Sarmento (Isis Neutron Source)

NIBS2020_TiagoSarmento.pdf

11:40 Experiments on Photo-Assisted O$^-$ and Al$^-$ Production with a Cesium Sputter Ion Source

It has been recently proposed that the production of negative ions with cesium sputter ion sources could be enhanced by laser-assisted resonant ion pair production [1]. We have tested this hypothesis by measuring the effect of pulsed diode lasers at various wavelengths on the O$^-$ beam current produced from Al$_2$O$_3$ cathode of a cesium sputter ion source [2]. Here we summarize the previously reported experimental results demonstrating enhancement of the extracted beam current owing to the laser exposure and amend them with data on photo-assisted Al$^-$ production. The experimental results provide evidence for the existence of a wavelength-dependent photo-assisted enhancement of negative ion currents but cast doubt on its alleged resonant nature as the effect is observed for both O$^-$ and Al$^-$ ions at laser energies above a certain threshold. The beam current transients observed during the laser pulses suggest that the magnitude and longevity of the beam current enhancement depends on the cesium balance on the cathode surface. It is shown that the ions produced by the laser exposure originate from slightly different potential than the surface produced ions, which allows us to constrain the underlying physical mechanisms. It is concluded that the photo-assisted negative ion production could be of practical importance as it can more than double the extracted beam current under certain operational settings of the cesium sputter ion source. We describe experiments designed to unambiguously confirm or dispute the relevance of the ion pair production for negative ion production. Finally, the possibility of ion pair production explaining the beneficial effect of xenon admixture on the negative ion yield of an RF-driven H$^-$ ion source [3] is discussed.

[1] J. S. Vogel, Nucl. Instrum. Methods Phys. Res. B 438, (2019), pp 89-95.
[2] O. Tarvainen et al., submitted to J. App. Phys. (2020); arXiv:2007.00521
[3] T. Kalvas, S.K. Hahto, J.H. Vainionpää, K.N. Leung, S.B. Wilde and P. Mandrillon, AIP Conf. Proc. 925, 136 (2007).

Speaker: Olli Tarvainen (STFC Rutherford Appleton Laboratory)

NIBS2020_ThuO11_Tarvainen.pptx

Friday, 4 September

15:00 → 17:00 H– sources for accelerators: Oral session O3

Conveners: Dr Jacques lettry (CERN) , Dr Dan Faircloth (STFC)

15:00 H- Ion Source Research and Development at the Oak Ridge National Laboratory

The U.S. Spallation Neutron Source (SNS) is a state-of-the-art neutron scattering facility delivering the world’s most intense pulsed neutron beams to a wide array of instruments which are used to conduct investigations in many fields of science and engineering. Neutrons are produced by spallation of liquid Hg by bombardment of short (~1s), intense (~40A) pulses of protons delivered at 60 Hz by a storage ring which is fed by a high-intensity, 1 GeV H- LINAC. This facility has operated almost continuously since 2006, with ion source performance increasing steadily over those years, and now currently providing 50-60 mA of H- ions for maintenance-free runs of ~100 days with near 100% availability. Ion source research and development at ORNL has played a key role in enabling and supporting these achievements, and this report provides a snapshot of our current efforts including infrastructure upgrades to the SNS front end systems as well as R&D related to improving the external and internal antenna ion sources. In particular, we have recently simplified and improved the reliability of the plasma ignition system for the external antenna ion source which is discussed in detail. Overall, experimental ion source R&D is conducted on a 65 keV test stand, 2.5 MeV beam test facility, the SNS accelerator during beam study periods, and on a plasma gun test bench. Finally, future directions and collaborations with other laboratories are discussed.

Speaker: Dr Robert Welton (Oak Ridge National Laboratory)

NIBs 2020-3.pdf

15:20 Recent performance of the SNS H- ion source with a record long run

The Spallation Neutron Source (SNS) accelerator system includes a 65-keV H- injector, a 2.5-MeV RFQ, a 1-GeV linac series, and an accumulator ring. The H- injector consists of an RF-driven, Cs-enhanced H- ion source and a compact, two-lens electrostatic LEBT. SNS routinely operates at 1.4 MW average beam power for three run cycles per year. In the recent two run cycles (FY20A and FY20B), due to conservative operation of the RFQ with reduced power, the H- injector had to deliver >50 mA to the RFQ to achieve ~35 mA linac current required for 1.4 MW. For FY20A, we ran two sources each serving about 60 days. For FY20B, we ran a single source for the entire cycle spanning 116 days. A single dose of cesiation was conducted in the startup which yielded ~54 mA for the entire run with little adjustments of RF power and Cs collar temperature. After service inspections revealed no significant tear or damage that would have limited further operation of the source.

Speaker: Baoxi Han (Oak Ridge National Laboratory)

NIBS2020_FriO3_Han.pdf

15:40 Interface Boundary Conditions for Global Models of Multi-Chamber Negative Hydrogen Ion Sources*

Global models of plasma discharges are a standard tool in plasma fluid simulations incorporating complicated chemistry. These models use global balances of energy and conservation of species number in order to estimate volume averaged number densities and temperatures of plasma components. Simple semi-analytic estimates of species density profiles valid in a wide range of parameters are used in order to include wall fluxes. Due to the nature of wall flux estimation, the global models are limited to single chamber designs.

In this paper we present the development of interface boundary conditions that allows the use of conventional global models for separate chambers. Additional source terms corresponding to interface boundary conditions provide closure for simulations of multi-chamber ion sources. We use an extension of the Global Enhanced Vibrational Kinetic Model (GEVKM) [1] with updated hydrogen plasma chemistry [2] to model multi-chamber negative hydrogen sources. We compare newly developed interface boundary conditions based on the fluid approximation with the thermal flux approximation [3]. We present preliminary results of simulations using different interface boundary conditions for the negative hydrogen ion source at IPP Garching. We compare our new two-chamber model results to previous fluid simulation results and experimental measurements.

[1] Averkin S.N. et al, “A Global Enhanced Vibrational Kinetic Model for High Pressure Hydrogen RF Discharges”, IEEE Trans. Plasma Sci., Vol. 43, N. 6, pp. 1926-1943, 2015.
[2] Yang W. et al, “Benchmarking and Validation of Global Model Code for Negative Hydrogen Ion Sources”, Phys. Plasmas, 25, 113509, 2018.
[3] Averkin S.N. and S. A. Veitzer, “Global Model of Multi-Chamber Negative Hydrogen Ion Sources with Updated Hydrogen Plasma Chemistry”, 10th Int. Partile Accelerator Conf.(IPAC'19), Melbourne, Australia, 19-24 May 2019.

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*This work was performed under the auspices of the Department of Energy, Office of Basic Energy Sciences Award #DE-SC0009585

Speaker: Sergey Averkin (Tech-X Corp)

NIBS2020_O3_Averkin.pdf

16:00 IMPROVING OF THE CONVERTER SURFACE PLASMA SOURCES.

A Large Volume Surface Plasma Source with a converter for the Los Alamos linear accelerator was developed. A large gas-discharge chamber with a multipole magnetic wall and 2 heated cathodes can support a discharge generating a plasma. A cooled converter with a diameter of 5 cm and a potential of up to -300 V emits negative ions, accelerates them and focuses in an emission aperture with a diameter of 6.4 mm. From this SPS, up to 18 mA of H- ions are extracted at a duty cycle of up to 10%.
For dependence of an extracted H- beam current on the discharge current is typical a strong saturation through H- destruction in thick layer of discharge plasma. The H- beam intensity and H- generation efficiency can be increased by decrease of plasma layer thickness between converter surface and emission aperture. It is possible to improve beam characteristics by small modification of this converted SPS. It is proposed to used a thin Penning discharge in front of the converter. Magnetic field for Penning discharge is created by permanent magnets. Decrease of plasma end gas between converter and emission aperture can decrease H- beam loss and increase an extracted beam intensity up to 2 times.

Speaker: Prof. Vadim Dudnikov (Muons, Inc)

NIBS2020_04_Dudnikov (2).pdf

16:20 Quantifying the Cesium and H- Densities Inside the LANSCE H- Ion Source with Laser Absorption Techniques

The Los Alamos Neutron Science Center (LANSCE) H- ion source (LHIS) has provided stable output for decades of LANL mission needs. While operationally well understood, the internal relationship between the LHIS plasma, cesium distribution (the catalyst for producing H- ions), and produced H- beam remains a mystery, only explored indirectly with models.

We will develop fast, accurate, and non-invasive diagnostics techniques to measure the Cs and H- densities inside LHIS. These diagnostics are based on optical absorption spectroscopy that have been developed in the last decade for fusion based H- ion sources that can readily be applied to the accelerator based LHIS. A refined form of optical absorption spectroscopy, the laser absorption technique (LAT), utilizes lasers tuned to a given atomic species to measure its density. In this case a laser tuned to the D2 line of cesium will be used to determine its density inside LHIS. Similarly, a refined version of LAT called the cavity ring-ring down spectroscopy (CRDS) technique utilizes a laser tuned to H- photo-detachment to measure the H- densities at inside LHIS.

With successful development of these diagnostic techniques, any hidden or dormant capabilities in LHIS will be found and capitalized upon, both in its modeling and operation. Also, its potential benefit to LANSCE and LANL future needs will be realized. More generally, this will be the first use of these plasma diagnostic techniques on an accelerator based H- ion sources. We will present on the preliminary status of the diagnostic setup.

Speaker: David Kleinjan (Los Alamos National Laboratory)

2020_09_NIBS20_talk v1.3.pdf

16:40 Modeling filaments in H- ion source from the first principles

We report on the progress in modeling performance of the H-ion source at LANSCE. We have developed a numerical model describing key physics processes from the first principles. The model accurately describes degradation of tungsten filaments and predicts their life time as well as the change in the performance. The model has been benchmarked against experimental data and shows good agreement in various regimes of operation. The model has been used to provide real time information for the degradation of the filaments in the 2019 production cycles at LANSCE. A table with estimates for the filament lifetime depending on the output ion source current has been generated.

Speaker: Nikolai Yampolsky (Los Alamos Natonal Laboratory)

NIBS2020_Ion_Source_Modeling.pdf

17:00 → 17:30 Coffee break

17:30 → 19:30 P1

Conveners: Dr Dan Faircloth (STFC) , Dr Jacques lettry (CERN)

17:30 Operational experience with the ELENA Ion Source

The Extremely Low ENergy Antiproton (ELENA) is a compact ring that was recently installed to complement the antimatter factory at CERN. A local H-/p source has been installed in ELENA for commissioning purposes of the ring and subsequent electrostatic transfer lines toward experiments, allowing to progress with the commissioning phase also in the absence of antiprotons. The ELENA source can produce pulses of H-, with a length of a few us and an intensity of approximately 50 uA at 100 keV, to mimic the antiproton pulse at extraction energy. In addition, the source can deliver proton pulses, if need be.
After a few years of operation, several observations have been collected that will be presented in this work. Of particular interest is the observation of a fast (about 1 MHz) intra-pulse instability for H-, which resulted in poor charge stability of the H- beam injected into ELENA. Possible ways of stabilising the beam pulse have been found and will also be presented.

Speaker: Davide Gamba (CERN)

NIBS2020_P1_Gamba.pdf

17:30 Reference Sheets

Speaker: Tiago Morais Sarmento (Isis Neutron Source)

ReferenceSheets_Combined.pdf

17:30 Vacuum Pressure Considerations on the Performance and Lifetime of Penning Ion Sources

The ISIS pulsed spallation neutron and muon facility uses a Penning surface-plasma ion source to deliver high current, high repetition rate, long pulse H^- beams for accelerator operations. Several variations of the Penning source have been tested, primarily with a view to higher duty-cycle operation in future facilities. It has been noted that the different source configurations vary in their operational stability and wear patterns, despite using the same power supplies, gas feed-rates, magnetic fields, temperature settings etc. It is proposed that the vacuum chamber setup contributes to the observed performance variations. Therefore temporally-resolved vacuum simulations were made of the different setups. The pressure profiles output were used in conjunction with collision cross sections to estimate stripping losses along the beam’s flight path. It was found that up to 20% difference in H^- stripping can occur depending on the vacuum environment. Therefore caution must be used when comparing perveance scans of transported beam current from different ion sources and different experimental setups. In addition, the stripped positive ions can back-stream into the ion source, increasing anode wear.

Speaker: Dr Scott Lawrie (STFC ISIS Neutron Source)

NIBS2020_Fri30_Lawrie_Poster.pdf

17:30 Work function behavior of C12A7 electride materials in low temperature hydrogen plasmas

Since present-day high-power negative hydrogen ion sources are based on the surface conversion mechanism of atomic and ionic hydrogen species, they rely on a low work function converter surface. Currently, the alkali metal Cs (bulk work function of 2.14 eV) is continuously evaporated onto refractory metals during the ion source operation to generate low work function coatings by chemisorption. However, the high chemical reactivity and volatility of Cs coatings are major drawbacks in terms of a stable and long-term reliable ion source performance. Thus, Cs-free alternatives for the H$^-$ converter surface are highly desirable and an active research topic in ion source development. Since the material requirements for an implementation in ion sources are demanding, i.e., machinability and stability at ambient conditions as well as resilience against the plasma load while providing an efficient H$^-$ conversion yield, no viable alternative to Cs has been found so far [1,2].

A promising candidate possibly fulfilling the requirements for an ion source converter surface is the ceramic [Ca$_{24}$Al$_{28}$O$_{64}$]$^{4+}$(e$^-$)$_4$, known as C12A7:e$^-$ [3]. It is the first reported electride which is chemically and thermally stable in ambient atmosphere and the pure surface provides an intrinsic low work function of 2.4 eV at ultra-high vacuum conditions [4]. However, it is well known that the surface work function of this material is particularly sensitive to impurities, and it is very challenging to prepare chemically pure surfaces [4,5]. Thus, C12A7:e$^-$ samples from different manufacturers (Fraunhofer IKTS and AGC Inc.) were investigated at the laboratory experiment ACCesS [6], which provides vacuum and plasma conditions comparable to those close to the converter surface in H$^-$ sources for fusion. At the setup, the surface work function can be measured absolutely via the photoelectric effect [7]. Campaigns regarding the work function dependence on the surface temperature and plasma exposure time in H$_2$ and D$_2$ with and without bias have been performed. It is shown that the work function can be reduced by vacuum heat treatment. The application of hydrogen plasma leads to a further decrease of the work function, and a quick regeneration after surface degradation due to residual gas adsorption. Long-term plasma exposure exhibits a steady-state work function below 3 eV. Biasing has shown a work function dependence from the polarity and the applied bias potential. In general, the C12A7:e$^-$ materials have demonstrated promising properties in terms of plasma resilience, but the reached work function is still substantially higher than what is achieved with the state-of-the-art technique of in situ caesiation.

References
[1] U. Kurutz, R. Friedl, and U. Fantz, Plasma Phys. Control. Fusion 59 (2017), 075008.
[2] U. Fantz, C. Hopf, R. Friedl, S. Cristofaro, B. Heinemann, S. Lishev, and A. Mimo, Fusion Eng. Des. 136 (2018), 340.
[3] S. Matsuishi, Y. Toda, M. Miyakawa, K. Hayashi, T. Kamiya, M. Hirano, I. Tanaka, and H. Hosono, Science 301 (2003), 626.
[4] Y. Toda, H. Yanagi, E. Ikenaga, J. J. Kim, M. Kobata, S. Ueda, T. Kamiya, M. Hirano, K. Kobayashi, and H. Hosono, Adv. Mater. 19 (2007), 3564.
[5] Y. Toda, Y. Kubota, M. Hirano, H. Hirayama, and H. Hosono, ACS Nano 5 (2011), 1907.
[6] R. Friedl, S. Cristofaro, and U. Fantz, AIP Conf. Proc. 2011 (2018), 050009.
[7] R. Friedl, Rev. Sci. Instrum. 87 (2016), 043901.

Acknowlegdements
The authors would like to thank the Fraunhofer IKTS (Institute for Ceramic Technology and Systems, Germany) and AGC Inc. (Japan) for providing the C12A7:e$^-$ electride samples.
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

Speaker: Mr Adrian Heiler (Max-Planck-Institut fuer Plasmaphysik)

NIBS2020_P1_Heiler.pdf

Tuesday, 8 September

02:00 → 06:00 P2

Conveners: Dr Zhimin Liu (a) Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China; b) University of Science and Technology of China, Hefei 230026, China) , Prof. Mahendrajit Singh (ITER-India, Inst. for Plasma Research)

02:00 Electrostatic Simulation and Analysis of Transmission Line Corner Section for the CFETR NNBI test platform

The Gas Insulated Transmission Line (GITL), which is an important link between radio frequency negative ion source and the power supply system, carries all the Ion Source and the Extraction Power Supplies (ISEPS) conductors at -400kV with respect to ground, from High Voltage Deck (HVD) to the beam source and the intermediate potentials of acceleration grids. The corner section, which is inevitable in the GITL, is easy to discharge if the design is unreasonable, so the insulation design should be considered in the design of the corner section. In this paper three insulation schemes are put forward. Through the finite element analysis, only one scheme meets the design requirements. In this analysis, the relationships between the design parameters (the radius and angle of the corner section) and the design indexes (maximum electric field strength and equivalent distributed capacitance) are analyzed. The preliminary design parameters of the corner section: the radius of the corner section can change from 50 mm to 150 mm and the angle of the corner section is chosen as 90°. In this case, the maximum electric field strength of the GITL decreases from 3.18 kV/mm to 3.06 kV/mm, the maximum electric field strength of the inner surface of the grounding shell decreases from 1.08 kV/mm to 0.73 kV/mm, the electric field non-uniformity coefficient decreases from 2.21 to 2.15 and the equivalent distributed capacitance increases from 82.12 pF to 94.70 pF. The simulation results provide a theoretical basis for the engineering design of the corner section of the GITL.

Speaker: Dr Rixin Wang (Institute of Plasma Physics, Chinese Academy of Sciences；University of Science and Technology of China)

NIBS2020_23_Wang.pptx

02:00 Extraction of Negative Hydrogen Ions through a Plasma Electrode Covered by a Ta or Ti Foil

We have discussed in the previous meetings on the possible improvements for the negative ion to electron current ratio by depositing fresh coating of tantalum (Ta) on the plasma grid surface of a negative hydrogen (H-) ion source [1]. A fresh Ta evaporation by burning a thin Ta filament increased H- ion current by 12 % [2]. However, the enhanced H- production lasted only about one hour after burning off the Ta filament in a discharge maintained by the electron emission from a hot tungsten filament. To find out the mechanism responsible for improving H- ion to electron current ratio, a thin (0.1 mm thick) foil of Ta was attached to the area of the extraction hole opened at the center of the plasma grid. The attached Ta foil did not show enhancement as large as that observed by evaporation, while the dependence of H- ion current upon the plasma grid bias [3] showed a change in local plasma potential by putting the foil on the surface of a stainless-steel plasma electrode. When the plasma grid surface facing the hydrogen plasma was covered by a titanium foil, the H- to electron current ratio increased like the case of Ta, while the absolute amount of H- ion current decreased. Correlations among the H- ion to electron current ratio, plasma parameters and the local H- ion density measured by the photodetachment method have been observed.

[1] M. Bacal and M. Wada, AIP Conf. Proc. 1869, 030025 (2017).
[2] K. Maeshiro, M. Wada, S. Masaki, Proc. 34th Int. Conf. Phys. Ionized Gases, PO16PM-027.
[3] M. Bacal et al., Rev. Sci. Instrum. 87, 02B132 (2016).

Speaker: Mr Kenta Maeshiro (Graduate School of Science and Engineering, Doshisha University)

NIBS2020_P2_Kenta Maeshiro.pdf

02:00 Improvements of stable negative ion production for long pulse beam operation toward the negative ion source for JT-60SA

Toward the coming JT-60SA tokamak operation, negative ion sources producing 500 keV and 22 A (130 A/m$^2$) H-/D- beams for 100 s are being developed for the JT-60SA neutral beam injector. So far, the extraction of 15 A H$^-$ beams for 100 s has been achieved, however, there are two remaining issues related to the unclear Cs behavior in the large size negative ion source (~1.2 m x 0.68 m x 0.55 m) during long pulse operation around 100 s. One of the issues is a limitation of available negative ion current due to arcing on filaments in the arc source for the long pulse operation with Cs. Another issue is the slow degradation of negative ion current during a long pulse due to the unexpected Cs flux to the plasma grid (PG) from the Cs-accumulated chamber wall (~8 g, ~2.8 m$^2$) around 100 $^o$C, although the PG temperature is controlled by around 200 $^o$C.
To achieve higher arc power for the negative ion current during the long pulse with Cs, the protection of the filaments is most important. For this purpose, a prototype of fast arcing detection/cutoff system using Field Programmable Gate Array (FPGA) has been developed for a group with 6 filaments. By optimizing the time resolution of the system and the logic of arcing detection, the cutoff time has been reduced to 100 $\mu$s, which is 1/10 faster than the previous system in the power supply and is enough short to reduce the damage to the filament. By applying this prototype, the stable production of negative ion beam with 500 keV, 154 A/m$^2$, 118 s has been achieved for the first time. During this experiment, even in the operational Cs amount of 5 g, 3 times longer life time of filaments were demonstrated. This result has been applied to the JT-60SA ion source by modifying the prototype to detect the arcing in 8 groups with 48 filaments for JT-60SA.
As for the slow degradation of the negative ion current during long pulse, the balance of the Cs layer on the PG surface is a key for the long pulse operation. This time, increase of PG temperature up to 300$^o$C is considered to accept the larger Cs flux to the PG according to the previous R&D. So far, the PG temperature was controlled by the high temperature fluorinated fluid whose limit is around 200 $^o$C due to the boiling point. In order to realize the PG temperature more than 250$^o$C during long pulse, the coolant was changed to the compressed air whose heat transfer coefficient is about 1/10 of the fluorinated fluid. In the preliminary tests, designed temperature of 300 $^o$C was sustained during 300 s up to a half of nominal arc power with heat flux of 30 kW/m$^2$. Based on the results, the PG temperature control system will be modified toward the JT-60SA ion source.

Speaker: Masahiro Ichikawa (National Institutes for Quantum and Radiological Science and Technology)

NIBS2020_43_ichikawa.pdf

02:00 Ion time-of-flight signals from nanosecond laser ablation plasmas excited in constricted cavities

Negative ions as well as singly and multiply charged positive ions are generated in a laser produced plasma. During laser vaporization of the target, various atomic processes related to negative ion production take place involving radiative capture, dielectronic recombination, and ternary collisions, where the last is found to be most probable in the case of a laser plasma due to its high electron density [1]. During ternary collisions, electrons collide with neutral atoms to form negative ions. When the plasma plume is geometrically confined, plasma density is further increased, which also increases the probability of ions and electrons in colliding with neutrals [2]. In this work, laser vaporized plasma is confined by forming a cavity on the target by multiple pulse ablation, wherein the inner wall of the cavity constricts the plasma plume. The ion species and velocity distribution are characterized by time-of-flight (TOF) spectroscopy.

The setup consists of a laser ion source and a TOF spectrometer. The ion source consists of a Q-switched Nd:YAG laser (pulse width = 5 ns, wavelength = 1064 nm, repetition rate 10 Hz) which is focused on the 10 mm diameter graphite target at 12$^\circ$ incidence, with 0.7 mm spot diameter. The laser is incident on the same spot of the target surface while being rotated along its axis, deepening the hole as the number of subsequent pulses increases. Ions passing through an extraction aperture are accelerated towards the TOF spectrometer which consists of a 50-Ω resistor terminated Faraday cup (FC) array positioned 0.6 m from the target axis. Flight times of the ions are recorded by a 250 MHz oscilloscope triggered by a high-speed photodiode. The system is operated inside a stainless steel vacuum chamber with an ambient pressure of 10$^{-6}$ Pa.

Preliminary experiments for initial target ablation of a fresh target surface at 2 GW/cm$^2$ laser power density and -2 kV extraction voltage resulted in an initial burst of fast electrons followed by positive ions. Collected ion current was observed to be highest along the target axis. Experiments are underway to correlate the laser fluence and hole depth with the resulting ion spectra.

References
[1] S.S. Alimpiev, M. E. Belov, V.V.Mlinsky., S.M. Nikiforov, and V.I. Romanjuk, Appl. Phys. A 58 67 (1994).
[2] Y. Qiu, C. Yao, C. Yao,1,2 J. Gan, W. Zhang, N. Xu, J. Sun., and J. Wu, AIP Advances 9, 095021 (2019).

Speaker: James Edward Hernandez (Doshisha University)

NIBS2020_P2_Hernandez.pdf

02:00 Measurements of work function and negative Ion spectra from C12A7 electride immersed in a hydrogen plasma.

Recently the C12A7 electride attracts much attention as one of the possible plasma electrode material of Cs-free negative hydrogen isotope (H⁻/D⁻/T⁻) ion sources[1-2].
In this work, H-/D⁻ spectra from a C12A7 electride target immersed in an ICP plasma was measured in the apparatus, Phisis[3] . The target was biased negatively against the electrically grounded chamber. The temperature of the target was controllable up to 800 C. Negative ions were energy analyzed by a Hiden EQP300 mass spectrometer. By rotating the target, it can be irradiated by an energy-tunable photon beam, Omni Lamda 300i[4], when the plasma off, and the photo-electric current was measured. The work function of the sample can be obtained from the threshold photon energy. The lowest value of the work function observed was ~2.5 eV. This value was confirmed by UPS measurement with the similar conditioning procedure.
Change of the negative ion yield due to the work function was measured.

[1] Y. Toda, et al., Advanced Materials 19, 3564 (2007).
[2] M. Sasao, et al., Applied Physics Express 11, 066201 (2018).
[3] G. Cartry, et al., A, New Journal of Physics 19 025010 (2017).
[4] L. Tahari, et al., presented at JSI 2020 - Journées Surfaces & Interfaces(Sorbonne Université, 22-24 janvier 2020).

Speaker: Mamiko SASAO (Doshisha University)

NIBS2020_P2_SSO_pdf.pdf

02:00 Plasma Electrode Shape Suitable for Negative Hydrogen Ion Production

Introduction of cesium (Cs) into an ion sources enhances the production of negative hydrogen (H$^-$) ions while suppresses the co-extracted electron current by reducing the work function of the plasma grid surface [1-2]. However, cesium injection into an ion sources should require periodical maintenance and can add serious complexity in operation in a strong neutron /gamma radiation environment. Thus, the development of Cs-free H$^-$ / D$^-$ sources is desirable.

We have been investigating the amount of H$^-$ current and the amount of co-extracted electron current when C12A7 electride or molybdenum (Mo) were used for the Plasma electrode (PE) in a compact ECR ion source with an exchangeable PE. The C12A7 electride has a low work function, and it is expected to increase the amount of H$^-$ ion current and suppress the co-extracted electron current when used as the PE material [3]. In this study, we focus on the H$^-$ ion extraction and electron coextraction due to the shape of the PE of a H$^-$ ion source. Two materials: Mo and C12A7 electride are examined for a search of extraction structure suitable for a Cs-free ion source.

Speaker: Keita Bito (Graduate School of Science and Engineering, Doshisha University)

NIBS2020_P2_Bito.pdf

02:00 Some technical verification on infra-red temperature measurement

Radiation temperature measurement is acquired widely to be based on the famous Planck's law of blackbody radiation, which indicate that any object with a temperature higher than absolute zero is constantly radiating infrared rays outward. According to the law, the radiation emission at a given temperature is constant in object and varies among materials which ruled by its emission rate.
Just as other emission model, the received energy of a given radiation source is dominated by its spatial placement and the emission rule of the give source. So, my work here is to verify the radiation rule of the blackbody and the spatially variation rule of the received emission.
For a typical monochromatic radiation temperature measurement, the measured voltage of the transferred emission follows this equation:
$V_{num} = \rho M \cdot \frac{\Delta A_B}{\pi l^2_B} \times D_{decay} \times S_{sensor} \times T_{trans} \times A_{amp}$
Among them, $\rho$ represent the emission rate of the source, $\Delta A_B$ is light collection area and $l_B$ shows its distance, $D_{decay}$ give a sum to the optical attenuation, $T_{trans}$ gives us the conversion factor from radiation to electric signal, $A_{amp}$ is the amplifier magnification, M is the blackbody spectral radiance, which follows Planck blackbody radiation formula:
$M=\int_{\lambda_{1}}^{\lambda_{2} } \cdot \frac{2 \pi h c^{2}}{\lambda^{5}} \cdot \frac{d \lambda}{e^{h c / k_{B} \lambda T}-1}$
Among them, is Planck's constant equals 6.626e-34 J · s, c is the speed of light equals 2.997925e8 m / s, T is the temperature of the source, is spectral wavelength, and is Boltzmann's constant equals 1.38054e-23 J / K.
To verify the emission rule, we keep the spatial placement fixed and change the temperature of the source to collect the data. Under such situation, the output voltage $V_{num}$ is proportional to Planck blackbody spectral radiance M. But considering the zero drift, the presented $M-V_{num}$ relationship is a linear function theoretically. So we did the verification:

Using the above $M-V_{num}$ data, we did a linear fit and get a COD of 0.99969, So $M-V_{num}$ have a typical linear relationship.
To verify the spatially variation rule, we keep the source temperature and the spatial phase fixed and change the observing distance to record the vary of the output voltage $V_{num}$. The distance should be inversely proportional to the output voltage $V_{num}$. And due to the zero drift $b_0$ and the initial distance of the blackbody source $l_0$, the actual relationship follows equation:
$V_{num}=\frac{k}{(l_B+l_0)^2}+b_0$
We used two different temperature sets, each has a zero drift of 0.5V, since $l_0$ is a constant number we use one data set to fitting the k, $l_0$ data with $b_0=0.5V$, another one using the $l_0$ data and fitting the k,$b_0$ to see whether $b_0$ fitted to 0.5V. The result shows that $b_0=0.53V$, so the fitting result meets the expected data and the spatial relationship is verified.

Those two rules are the bases for a more accurate infra-red measurement way of NBI process monitoring, as for infra-red measurement results changes with different arrangement which should be verified beforehand.

Speaker: Mr Chao Shi (Science Island Branch, Graduate School of University of Science and Technology of China)

poster.pdf

02:00 Study of the multi-driver decoupling model of RF negative ion sources

According to the latest physics design of the China Fusion Engineering Test Reactor (CFETR), radio-frequency(RF) driven negative ion sources are selected as the preferred ion source for CFETR neutral beam injector (NBI).To solve the key problems of the CFETR, the engineering design, development, and construction of the Comprehensive Research Facility for Fusion Technology (CRAFT) has begun. Since the RF driven negative ion sources of CRAFT are Large-area high-current extraction beam sources, according to the NBI beam line extraction surface requirements, the RF negative ion sources will adopt the working mode of multi-driver distributed driving. When multiple RF drivers of the same type work at the same time, there may be mutual coupling and interference between them, which results in an asymmetric distribution of the RF magnetic field in the driver, thereby affects the extraction of ion current of the RF negative ion sources. This study analyzes the multi-driver decoupling model of the RF ion negative sources by simulating the mutual coupling of drivers in different distribution modes and obtains the distribution mode with the least mutual coupling and interference between the driver. In this mode, through simulating decoupling effects of adding metal shielding on the periphery of the driver, adding decoupling circuit in the driver circuit, replacing multiple drivers with larger drivers and so on, the best method of eliminating the coupling interference between the RF negative ion sources drivers is obtained, which provides theoretical support for CRAFT's RF negative ion sources to achieve stable operation of the extracted ion current.

Speakers: Ms Na Wang (a) Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China; b) University of Science and Technology of China, Hefei 230026, China) , Dr Zhimin Liu (a) Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China; b) University of Science and Technology of China, Hefei 230026, China) , Dr Yahong Xie (a) Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China; b) University of Science and Technology of China, Hefei 230026, China)

NIBS2020_14_Na.pdf

02:00 The Cs-free Negative Hydrogen Ion Source Project at KAERI: A New Concept Multi-Pulsed Ion Source

Due to reliability issues in using cesium (Cs) for efficient hydrogen negative ion production in ion sources for neutral beam injection (NBI) systems in fusion research, many researchers have explored Cs-free alternatives to Cs for a future DEMOnstration power station (DEMO) NBI system. The Korea Atomic Energy Research Institute (KAERI) recently launched a new project in collaboration with Seoul National University (SNU) in order to identify an efficient Cs-free negative ion source based on the volume production mechanism, not only for fusion application, but for other applications (e.g., semiconductor, space propulsion, and accelerator). In this project, we attempt to improve efficiency of the volume negative ion source by introducing plasma pulsing. The plasma pulsing, which is also called temporal filter, refers to a method of modulating power that sustains the plasma and consequently the electron energy. Supplying negative ions at high densities by the pulsing is, however, inherently transient and its duration is short. In view of a future DEMO, a significant drawback of the pulsing is being unable to continuously supply the negative ions to an extraction system. To remedy the drawback, and consequently to develop a novel promising Cs-free alternative, we devised a multi-pulsed ion source. The multi-pulsed ion source included more than two plasma sources and magnetic filters operates with an alternating pulsing sequence of the plasma sources. The temporal and magnetic filters named spatiotemporal filters may enable this ion source to continuously supplying the negative ions, leading to the development of the efficient Cs-free negative ion source. In this presentation, the overview of the Cs-free negative hydrogen ion source project at KAERI, the new ion source concept, and its preliminary experimental results will be presented and discussed in detail.

Speaker: Dr Sung-Ryul Huh (Korea Atomic Energy Research Institute)

NIBS2020_P2_Huh.pdf

02:00 Transport of a negative ion beam through a hydrogen plasma

The Los Alamos Neutron Science (LANSCE) linear accelerator provides an 800 MeV beam of
negative hydrogen ions (H-) to several facilities, e.g., pRad, a proton radiography studies facility
and the PSR, a proton storage ring which accumulates ions to be delivered to a target at the
Lujan Center for the production of neutrons by spallation.
The negative ion branch of LANSCE starts at an ion source which produces H- ions by surface
conversion of hydrogen atoms. This process takes place inside a chamber which contains a
hydrogen plasma created by an arc-discharge of molecular hydrogen gas. Located inside the
chamber there is a negatively biased converter where the actual conversion of hydrogen atoms
into negatively charged ions takes place. The production of H- ions is enhanced by the addition
of Cesium gas into the plasma chamber.
The H- ions produced at the negatively biased converter flow towards the grounded plasma
electrode which provides an aperture for the extraction of the H- beam by a positive potential.
As the H- ions flow through the hydrogen plasma there are several kinetic reactions which may
provide a destruction mechanism. The main reactions are (i) electron detachment, (ii) mutual
neutralization with positively charge plasma components and Cs ions, (iii) associative and nonassociative neutralization by neutral hydrogen gas components.
Since the amount of H- current that can be extracted from this source depends on the amount
of H- ions that survive at the extraction aperture, it is therefore of the utmost importance to
calculate the rate of H- extinction as they move through the plasma environment.
We will present Particle-in-Cell calculations of the H- beam transport through the hydrogen
plasma including charged and neutral components, taking into account the kinetic reactions
mentioned above.

Speaker: Enrique Henestroza (Los Alamos National Laboratory)

NIBS2020_P2_HENESTROZA.jpg

02:00 Velocity distribution functions of hydrogen atoms in ion source discharges

Many types of negative hydrogen (H$^-$) ion sources are operated by injecting cesium (Cs) into the discharge. The injected Cs reduces the surface work function of the extraction electrode to enhance negative ion current and reduce coextracted electron current. The high H$^-$ ion density in the vicinity of the beam extraction hole is believed to be realized by negative ionization of incoming flux to the electrode including atomic hydrogen (H$^0$) produced from the ion source discharge. The production efficiency of this surface produced H$^-$ ion component should be highly dependent upon the velocity distribution of H$^0$ atoms striking the extraction electrode. A system to measure the change in the H$^0$ velocity distribution functions depending upon the method to excite the plasma has been designed, built and being improved the performance.

The system equips a rotating blade neutral beam chopper to modulate the intensity of the neutral flux passing through the skimmer that separates the downstream chamber for time of flight analysis from the ion source discharge. After about 45 cm free flying vacuum space an electron impact type ionizer converts neutral particles to ions. A magnetic deflection type mass separator guides the produced protons to a secondary electron multiplier detector. The system exhibits the existence of a high-speed component in the H$^0$ velocity distribution when it was tested with an ECR plasma source. The current problem is the detector life that limits the signal accumulation. The ion to electron conversion plate degrades rapidly for the positive hydrogen ion impact and reduces the signal pulse height in several hours. The correlation between the discharge power and the observed time-of-flight spectrum is discussed.

Speaker: Mr Tatsuhiro Tokai (Graduate School of Science and Engineering, Doshisha University, Kyotanabe, Kyoto Japan.Graduate School of Science and Engineering, Doshisha University, Kyotanabe, Kyoto Japan.)

NIBS2020_P2_Tokai.pdf

Wednesday, 9 September

01:00 → 05:00 O4

Conveners: Prof. Katsuyoshi Tsumori (National Institute for Fusion Science) , Prof. Mamiko Sasao (Doshisha University)

01:00 110 mA Operation of J-PARC Cesiated RF-Driven Hˉ Ion Source

Abstract. The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven Hˉ ion source has been stably operated for about six years. The J-PARC 400 MeV LINAC successfully accelerated the 60 mA beam required for the J-PARC, when a 72 mA beam was injected from the source. The high intensity beam with transverse emittances suitable for the RFQ is produced with several unique measures, such as, slight water molecules addition into hydrogen plasma, low temperature (about 70ºC) operation of 45º-tapered plasma electrode with a 16-mm thickness, impurity (argon and/or nitrogen) elimination in the hydrogen plasma along with filter-field optimization, and so on. A 65 keV 110 mA Hˉ ion beam, whose about 103 mA has transverse emittances used for a common RFQ design, was stably operated with a duty factor of 4.5 % (1 ms x 45 Hz). Since the 52.5 keV beam energy (WT) was required to produce an Hˉ ion beam intensity (IH-) of 72 mA, the interesting relationship of IH-(65)~IH-(52.5)x(65/52.5)^2 was derived. The source is the benchmark one for the next generation high intensity and high energy Hˉ LINACs.

Speaker: Akira UENO (J-PARC)

NIBS2020_O4_Ueno.pdf

01:20 High-speed Emittance Measurements for Beams Extracted from J-PARC RF Ion Source

Oscillation of emittance and Twiss parameters in the negative ion $(\text{H}^-)$ beam from the J-PARC 2MHz RF ion source is measured by applications of a double-slit emittance monitor located at the RFQ (Radio Frequency Quadrupole) entrance. The emittance monitor is equipped with a newly-developed 60 MS/s data acquisition system, so that beam current oscillation in a few MHz can be observed with enough time resolution.

From the measurement, it is shown that the beam phase space consists of (1) a DC component in the beam core, (2) a 2MHz oscillating component which takes place both in the beam core and the halo and (3) a doubled RF frequency (4 MHz) oscillation which slightly exists in the beam halo. The major component is the 2 MHz component, which resultantly decides the beam emittance oscillation frequency. A typical value of the beam emittance in the present experiment is 0.34 $\pi$ mm-mrad, while the amplitude of the 2MHz oscillation is around 0.04 $\pi$ mm-mrad. The results indicate that the high-frequency oscillation component occupying about ten-percent of the beam from the RF source travels a few meters passing through a magnetic lens focusing system.

Speaker: Takanori Shibata (J-PARC)

NIBS2020_O4_Shibata.pptx

01:40 Primary electron analysis to improve the negative ion uniformity toward ITER-class long pulse and high power negative ion sources

The multi-cusp negative ion source with a unique end-plug, so-called Eclair ion source, has been designed for high power negative ion sources required in fusion devices such as ITER and JT-60SA. This Eclair ion source shows one of the ideal magnetic field configurations designed by the detailed fast electron analysis to achieve both uniform negative ion production and reduction of a co-extracted electron in large negative ion source with over 1m long.
The non-uniformity of negative ions was widely observed in the large negative ion sources in the world. This is a critical issue in the long pulse beam acceleration because this causes excess grid heat loads. In this study, the magnetic field configurations were examined by referring the semi-cylindrical ion source with 1.2 m long and 0.6 m in diameter used for JT-60SA (500 keV, 22 A deuterium negative ion beams for 100 s in the requirement). In order to produce uniform plasma as a seed of negative ions and reduce the co-extracted electrons in the whole extraction area, fast electrons should be not only well confined but also distributed throughout the driver region. Originally, a tent-shaped filter field was designed and demonstrated to make the uniform distribution of negative ions and electrons instead of the conventional plasma grid current filter (PG filter). The previous study utilizing the tent-shaped filter system showed that the uniform and intense negative ions in 80 % of the longitudinal extraction region. However, co-extracted electrons detected at the EXG (extraction grid) was two times larger than the case of the PG filter, which is not acceptable for the long pulse operation. In this study, the fast electron trajectory is analyzed to understand a route cause of the large electron current and produce uniform negative ions.
The fast electrons emitted from cathode filaments were moving in the longitudinal direction due to the grad-B drift of the filter field but were leaking around the corners of the ion source. It was expected that the electron leak at the corner was occurred by the weak magnetic field there because the magnets are arranged with the 90 degrees of the angle at the corners, and the magnetic field becomes weaker. Although additional magnets were mounted at the corners to suppress the electron leak and enhance the magnetic field, the numbers of leaked electrons to PG were increased contrary to our expectations. This result indicated that the electron leak was influenced by a discontinuous magnetic field at the corner region. Firstly, the proper magnet positions were examined to make smooth and continuous magnetic field respect to the tent-shaped filter configuration. Finally, hemisphere end-plug and magnet locations were designed to circulate the fast electrons efficiently at the corner region. The electron leak to the PG was completely suppressed with the Eclair shaped source chamber and continuous magnet arrangement. The plasma uniformity with the Eclair shaped ion source can be estimated by the experimental results based upon the fast electron trajectories as 21% and 20% improvements in the lateral and longitudinal directions, respectively.
As a result, the new magnetic field geometry with the Eclair type may contribute to the improvement of the negative beam uniformity and the reduction of co-extracted electrons by comparing with conventional PG and tent-shaped filters.

Speaker: Yuji Shimabukuro (National Institute for Quantum and Radiological Science and Technology)

NIBS2020_O4_Shimabukuro.pdf

02:00 Issues in the measured values of Langmuir and photo-detachment probes measuring Cs-seeded plasmas

Diagnostics at the region between magnetic filter to plasma grid of negative ion sources are important to understand the mechanism of the process through production to extraction of negative ions in source plasmas seeded caesium (Cs). Millimeter microwave interferometry and cavity ringdown (CRD) measurement are reliable diagnostic methods to measure line-averaged densities of electrons and negative ions, respectively.
The plasma close to the plasma grid, however, changes due to the vector field of magnetic field changing three dimensionally, and a “local” measurement at any point is required in such cases. Langmuir probe and photo-detachment probe are suitable for the purpose, while those methods have ambiguities caused by magnetic field and electrostatic field resulted from bias potential at plasma grid to obtain accurate physics values. In addition, influence of photoelectrons to a Langmuir probe was observed at the measurement of Cs-seeded plasma formed in the extraction region of LHD ion source.
In this presentation, we report the problems to apply Langmuir and photo-detachment probes and discuss the conditions to correct the magnetic field and to reduce the influences of photoelectron to the measured values.

Speaker: Prof. Katsuyoshi Tsumori (National Institute for Fusion Science)

NIBS2020_O4_tsumori.pdf

02:20 Steady-state charge-exchange ion source of 10 mA H- beam

For injection into a tandem accelerator of the BNCT device at Budker INP, Novosibirsk, a steady-state charge-exchange negative ion source with a current of ~ 10 mA is being developed. The primary beam of hydrogen ions with a current of 1–2 A and an energy of 30 keV is formed by a an RF ion source using a multi-aperture four-electrode ion-optical system with ballistic focusing. The RF plasma source operated at low hydrogen supply produces a plasma with a high content of molecular ions H2+. The formed beam of hydrogen ions passes through a charge-exchange target. Fast molecular ions H2+ in the charge-exchange target dissociate into two half-energy protons and are partially converted into negative ions. The resulting negative beam with half energy is deflected by 90 degrees by a focusing bending magnet and then accelerated by a single-aperture ion-optical system to an energy of 100 keV. Residual fast atoms and protons formed in the charge-exchange target are dumped. To reduce the stripping of the negative ion beam, high vacuum is to be provided in the transport region using differential pumping by turbopumps. In the paper, we discuss a design of the primary molecular ion source and that of beam line. At present, the source design has been completed and its parts are being manufactured. Power supply and control systems of the ion source are already prepared.

Speaker: Igor Shikhovtsev (Budker Institute of Nuclear Physics of Siberian Branch Russian Academy of Sciences)

NIBS2020_O4_Shikhovtsev.pdf

02:40 Coffee break

03:00 Recent Achievements in Studies of Negative Beam Formation and Acceleration in the Tandem Accelerator at Budker Institute

A continuous-wave surface-plasma negative hydrogen ion source is used to inject a negative ion beam into tandem accelerator with vacuum insulation at Budker Institute of Nuclear Physics since 2006. In the ion source, Penning discharge with plasma injection from hollow cathodes and cesium seeding is applied to generate a negative ion beam with a current up to 10 mA at 25 KV acceleration voltage. Due to several updates to the ion source and the tandem, maximum output proton beam current of up to 8 mA at 2 MeV is obtained. The results obtained recently in studies of ion beam formation and acceleration in the tandem accelerator are discussed

Speaker: Andrey Sanin (Budker Institute of Nuclear Physics)

NIBS2020_O4_Sanin.pdf

03:20 Negative Ion Beam Acceleration and Transport in the HV injector prototype

A high-voltage negative ion based neutral beam injector is under construction at the Budker Institute of Nuclear Physics [1]. It consists of the negative ion source, of the low-energy beam transport section (LEBT) to purify the beam before the acceleration, of the multi-electrode single-aperture beam accelerator and of the plasma neutralizer and beam magnetic separator with beam energy recuperators. The test stand for study of the main injector components, consisting of ion source and LEBT, installed at the HV platform and of accelerator section with HEBT was physically launched in 2019. A negative ion beam production and transport through LEBT to the input of the accelerator tube was studied before [2]. In the first half of 2020 the initial beam acceleration to energy up to 180 keV and the accelerated beam transport through the HEBT section with the quadrupole lenses were tested. The parameters of the transported beam were measured by the secondary emission detectors, installed at different points of the beam line and by the beam calorimeter, installed at the beam line exit in 10 m from the source. The power load to the acceleration tube electrodes was measures. The efficiency of the beam transport vs various source and LEBT parameters will be presented and discussed.
[1]. A.A. Ivanov, G. Abdrashitov, V. Anashin, et.al., Rev. Sci. Instrum., 85, 02B102 (2014);
[2]. A. A. Ivanov, Yu. Belchenko, P. Deichuli, A. Sanin, and O. Sotnikov. AIP Conf. Proc., 1771, 030012 (2016).

Speaker: Dr Oleg Sotnikov (Budker Institute of Nuclear Physics)

NIBS2020_O4_Sotnikov.pdf

03:40 Noise Mitigation Techniques in Thermocouple signals in Negative ion sources with RF and HV transients

Negative Ion based inductively coupled plasma sources operate in a high RF power and HV environment for plasma production and beam transport. Due to plasma power coupling dynamics, RF power mismatch causes large reflected fields which affect all the diagnostic signals by degrading the signal to noise ratio. In addition to RF disturbances, during the beam extraction and acceleration, the diagnostic signals are also prone to suffer during HV breakdowns due to high dV/dt associated with fast turn off and turn on of HV system. The breakdowns cause generation of HV transients which in turn disturb the entire signal reference system.
Such operational environment poses challenges for front end electronics design for low voltage signals like the one from thermocouple sensors, which are the most important diagnostic elements in such sources. The surface mounted thermocouples referenced to floating potential pick up noise from HV transients and RF noise. A signal conditioning system [1,2] is therefore needed to arrest the noise sources and provide clean signals for acquisition and control system . Such signal conditioning needs specific RF filters and PCB design. Special attention is also required for shielding and grounding to help reducing noise interference.
The present work discusses the measurements in light of the improvements made to the front end electronics for the thermocouples used on the ROBIN [3] RF based negative ion source test bed. The source operates at 1 MHz RF and with HV power supplies of rating 11 kV 35 A for extraction and 35 kV 15 A for acceleration for which mismatch fields of the order of 90 V/m have been observed. The overall signal chain from field to presentation layer shall be presented with the measures undertaken to solve the noise interference.
References:

[1] Henry Ott, Noise Reduction Techniques, Wiley
[2] L.K. Bansal, et. al.,” Noise Mitigation in Thermocouple Signal Conditioning System for Neutral Beam Calorimeter for NBI SST-1”, IEEE Trans on Plasma Sciences, Volume: 42, Issue: 6, June 2014
[3] K Pandya et. al., “First results from negative ion beam extraction in ROBIN in surface mode”, AIP Conference proceedings 1869 (2017), p. 030009

Speaker: Himanshu Tyagi (ITER-India, IPR)

NIB2020_O4_Himanshu1.pptx

04:00 Prediction of negative hydrogen ion density in permanent magnet-based helicon ion source (HELEN) using deep learning techniques

In the present work, a deep learning model is developed for a permanent magnet-based helicon plasma source. The non-invasive cavity ring-down spectroscopy (CRDS) characterizes the HELEN ion source as a negative hydrogen ion source. This paper discusses different deep learning techniques for modelling the ion source and subsequently predicts the ion source density. The experiments were conducted for measuring the plasma density for the different range of gas pressure (mTorr), magnetic field (B-Field) and RF-power. Consequently, experimental data trains deep learning models. The performance of various deep learning models has been assessed by the root mean squared error and the coefficient of determination values. The deep learning techniques also develop a correlation between the electron temperature and plasma densities which reasonably mimics the behaviour of HELEN ion source and can classify the helicon plasma generation at high power range (800-850 W). Also, the influence of other input parameters such as gas pressure and the magnetic field is assessed using the correlation matrix.

Speaker: Mr Vipin Shukla (PANDIT DEENDAYAL PETROLEUM UNIVERSITY)

NIBS2020_O4_Vipin.pptx

04:20 Design of FPGA based Triggering and Synchronization System (TSS) for Laser Photo Detachment diagnostic in ROBIN

ROBIN [1] is a single RF driver based negative ion test bed currently in operation at IPR, Gandhinagar, India. To understand and have deeper insights of physical phenomenon, several diagnostics have been interfaced with ROBIN system.
To quantify the negative ion density in ROBIN source, laser photo detachment (LPD) diagnostic is configured. LPD at ROBIN is based on single high power Nd:YAG laser which has a pulse width of 3 ns and repetition rate of 20 Hz. Successful integration of LPD needs control on laser energy, precise triggering and detection of weak signals. Precise triggering has two aspects. One is to control laser energy via delay between Flash lamp and Q switch pulses and second is to trigger it in phase with 1 MHz RF signal. This required a triggering system which can work with accuracy of few nano seconds.
A triggering and synchronization system (TSS) based on a custom FPGA (Field Programmable Gate Array) is designed and integrated in order to assure reliable and synchronized operation of the laser. Precise delay between laser pulses is necessary to obtain appropriate laser energy and to prevent the active medium, the Nd:YAG rod from thermal damage. In order to synchronize laser operation with 1 MHz RF, Rogowskii coil based peak detection is used and interfaced with TSS. The LPD setup is HV referenced; hence the interfaces were designed with optical isolation.
Another challenge is to detect weak signal from LPD probe. For this a custom low noise pre amplifier circuit is designed to measure signal in range of 1-2 mA in presence of large RF noise. The engineering solution designed for LPD interface contains mix of analog and digital systems. The TSS is based on low cost FPGA platform with custom analog based interfaces. The unit has been integrated without using any specific commercial system for the desired goal.
This paper presents the details regarding the design of interfacing electronics of LPD with some test results. In addition, the FPGA firmware, the control software and signal acquisition process are presented.
References:
[1] K Pandya et. al., “First results from negative ion beam extraction in ROBIN in surface mode”, AIP Conference proceedings 1869 (2017), p. 030009
[2] J. Soni, et al., ” Conceptual design, implementation and commissioning of data acquisition and control system for negative ion source at IPR”AIP Conference Proceedings, 1390 (2011), pp. 624-633

Speaker: Mr Himanshu Tyagi (ITER-India, Inst. for Plasma Research)

Thursday, 10 September

08:00 → 10:00 P3

08:00 Analysis of plasma characteristics of high-power radio frequency negative ion source based on Langmuir probe

The plasma parameters of the radio frequency (RF) negative ion source directly affect the density of negative ion and the uniformity of the plasma in the extraction zone. In order to understand the behavioral characteristics of the plasma inside the RF negative ion source, a set of Langmuir probe system (plate probe and cylindrical probe) was developed and tested on the RF ion source test facility at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The Langmuir probe was used to measure the spatial distribution of the plasma parameters in the extraction zone and the axial distribution of the plasma parameters in the expansion zone. In the experiment the relationship between the density parameters and the RF power, source pressures is explored. Experimental results show that the plasma parameters presents better uniformity and the electron temperature is maintained at a lower level (Te<2eV) due to the filter magnet field in the extraction zone and the electron density can reach up to $10^{17}/m^3$ at the 60kW RF power. Considering the production of negative ions in the extraction zone, the Electron Energy Distribution Functions（EEDF) is also given at different operational parameters. This article helps us to understand the plasma characteristics and provide technical support for experimental study of negative ion production and extraction.

Speaker: Dr Yongjian Xu (Institute of Plasma Physics, Chinese Academy of Sciences)

NIBS2020_24_Xu.pptx

08:00 Analysis of the DEMO-FNS magnetic field passive reduction and neutral beam injectors shielding methods

Steady-state operation mode of a fusion neutron source (FNS) will require plasma heating and maintaining the current in it by fast atoms beam injecting. The DEMO-FNS project [1] assumes the use of six injectors providing additional heating power up to 30 MW at an atomic energy of 500 keV. As a prototype for the DEMO-FNS injector, an injector developed in detail for the ITER project can be used, with the injector layout retained, but changes in individual components, which is caused by the difference in beam energy and power [2]. Inside these components, there are very strict restrictions on the magnetic field magnitude (the flux density should be below a certain value along the path of ion movement and even lower in the neutralization region) [3]. To achieve these characteristics in an environment with a high scattered field due to the magnetic system of the facility, which includes the coils of the poloidal and toroidal fields, the central solenoid and the plasma itself, additional shielding of the injectors is provided. At this stage, we expect that the proposed design will allow obtaining the required magnetic field values only by passive injector(s) shielding due to a case made of a ferromagnetic material with a high magnetic permeability index. An electromagnetic analysis of the effectiveness of such a screen was performed using 3D modeling using the ANSYS code. For this, a computational finite-element model DEMO-FNS was created, which includes a vacuum volume in which an electromagnetic system is located and one of 6 heating injectors. The magnetic field components values on the injection axis were calculated without shielding the injector region. It was shown that the vertical field component Bz in the injector region is maximum and is in the range of 300 G at the input (from the side of the torus) to 150 G at the other end of the shielded case. Variants of single-layer shielding using various materials, and multilayer ones: two-, three- and four-layer with different layer thicknesses and vacuum gaps between them were considered. By choosing the optimal thicknesses of layers and gaps, the projection of the magnetic induction vector on the plane perpendicular to the beam direction was suppressed to acceptable values in the region of the injector components. The BTR code was used to calculate the motion of particles in the conditions of the obtained magnetic fields, taking into account reionization along the entire injection path length. The distributions of loads and power losses on the injector components and the transverse beam power dynamics are obtained. Loads calculations have shown that for a given magnetic field distribution, the loads on the duct are significantly uneven, which together with the reionized particles fluxes focusing in magnetic fields, is the greatest danger for heat removal. These results will be used later in the injector case and the duct engineering design.
1. Y. S. Shpanskiy, “Progress in the design of the DEMO-FNS hybrid facility,” Nucl. Fusion, vol. 59, no. 7, p. 076014, Jul. 2019
2. S.S. Ananyev, E.D. Dlougach, A.I. Krylov, A. A.Panasenkov and B. V. Kuteev, Concept of Plasma Heating and Current Drive Neutral Beam System for Fusion Neutron Source DEMO-FNS — Physics of Atomic Nuclei, 2019, Vol. 82, No. 7, pp. 981–990, DOI: 10.1134/S1063778819070020
3. S.S. Ananyev, E. D. Dlugach, B.V. Kuteev, A.A. Panasenkov, Modeling and optimization of neutral beam injectors for fusion neutron source DEMO-FNS - VANT. Ser. Thermonuclear fusion, 2018, vol. 41, no. 3, DOI: 10.21517 / 0202-3822-2018-41-3-57-79

Speaker: Sergey Ananyev (NRC Kurchatov institute (НИЦ Курчатовский институт))

NIBS2020_P3_Ananyev.pdf

08:00 BTR Application for Beam Slowing-down Analysis

BTR code, which is generally used for NBI beamlines design and optimizations, is applied to calculate the injected beam stopping in plasma, beam deposition and shine-through power in a fusion neutron source DEMO-FNS (R = 3.2m, a = 1m, k = 2, B = 5T, Eb = 500keV, PNBI ≈ 30MW). Beam-plasma model calculates the detailed beam spatial and angular distributions with account of injector and tokamak operation parameters. The cross-section fits by Janev-Suzuki are used for beam ionization, and beam power deposition in axial and normal beam planes, including shine-through power in the far wall (FW) plane are obtained for different geometries. The influence of beam size and beamlets angular distribution (focusing and divergence) is found essential. Thin, rectangular and focused beam geometries are compared. It is shown that neutral power decay aberrations in toroidal plasma target lead to power profiles asymmetry, which is high enough for tangentially injected thick and divergent beams. BTR beam model allows more precise evaluation of beam losses in plasma, shine-through power profiles and beam power and current deposition in plasma.

Speaker: Mrs Eugenia Dlougach (NRC Kurchatov Institute, Russia)

NIBS2020_P3_Dlougach_9.pdf

08:00 BTR code Recent Modifications for Multi-Run Operation

BTR code has been used for many years in the design and performance optimization of ITER HNB/DNB components and other NBI systems based on negative or positive ion sources. BTR beam formation and transmission along the beam line is simulated by a simple and comprehensive 6D beam model, which accounts for beam transport losses and power deposition on the injector surfaces. Beam particles tracks are followed in deterministic manner (no random values) with electromagnetic fields deflection, with transforming on gas and plasma targets (neutralization, ionization in gas/plasma). In BTR power flux and power deposition at the injector components are calculated along the entire beam track. Each single BTR run in “conventional” BTR session is started by the input manual tuning procedure for a specific scenario, which is next followed by the code restart. This input routine required the User's extensive efforts and time to get the final result for multiple operation scenarios. Recent modifications in BTR-5 code engine and I/O have made it possible to run automatically multi-parametric scans of different scenarios by a single click, with a preset list of scenarios input records. This reduces much of the User’s handwork, and makes the total design routine shorter and more efficient (including the post-processing efforts). NBI geometry input has become more flexible, allowing the User to choose between the “standard” NBI geometry (PDP - compatible) and free-surfaces input. Memory optimization in BTR-5 allows independent tracking of each sort of particles - to obtain the optimum statistics for maps calculation, without restrictions (in earlier versions the maximum was ~10000 particles per beamlet). BTR-5 multi-run version has been used for parametric scans of DLM power loading, which resulted in the DL “worst case” scenario definition, and maximum power load for each DL surface across all the range of scenarios. Power maps resolution in BTR-5 (unsmoothed 2D profiles) on average is higher than in BTR-4, due to the increased number of model particles of each sort, the standard cell for each surface is varied between 1mm and 1cm depending on the specific surface dimensions.

Speaker: Eugenia Dlougach (RRC "Kurchatov Institute")

NIBS2020_P3_Dlougach_26.pdf

08:00 Design of gas flow control of RF negative hydrogen ion source based on ASIPP NBI test facility

The gas pressure during plasma discharge affects the density and temperature of its electrons, which is essential to realize the steady-state operation of the radio frequency negative hydrogen ion source used in the nuclear fusion auxiliary heating experimental device.In order to maintain the density and uniformity of the plasma and improve the operating efficiency of the ion source.It is necessary to keep the gas pressure during plasma excitation of the ion source and the gas pressure during beam extraction within a certain range.By simulating the dynamic changes of vacuum in the ion source with different gas flow rates, gas intake times and pumping speeds, hardware circuits are designed to control solenoid valves and mass flow controllers (MFC) to achieve the precise control of gas flow.The experimental results in the RF test facility of ASIPP NBI show that the precisely controlled gas flow strategy designed has better repeatability and more stable density and uniformity of the plasma .At the same time, gas flow is also an important reference for the calculation of the real-time pumping speed performance of cryopumps and the analysis of the reasons for the failure of the beam extraction experiment.

Speaker: Dr Jinxin Wang

NIBS2020_P3_Wang.pdf

08:00 Estimation of inter conductor stray capacitance for HV dc transmission line of negative neutral beam injector

Neutral beam injectors inject multi megawatt neutral beams, several tens of amperes and energies from few 100 kV to MV, into the tokamak for heating and diagnostic purposes. The neutral beams are produced through the route of neutralisation of ion beams. The ion beams of machines like ITER shall use large area RF based negative ion sources, for plasma production, coupled to multi grid (3-7), extractor and accelerator systems. Depending on the energy requirements and the beam optics the gaps between the extractor and accelerator stages can range between a few mm to few tens of mm. The multi aperture multi grid extractor accelerator systems also provide the route for the gas being fed in the ion source for the plasma production to escape to the surrounding. As a result the gas density in the gaps is high and can lead to breakdowns often referred to as Paschen breakdowns. Another source of stored energy could be the inter conductor stray capacitance of the high voltage transmission line. These breakdowns could lead to damage of the grid segments and thereby considerable down time of the injector. One of the possible route to reduce the stored energy could be to reduce the inter conductor stray capacitance by increasing the distance between the conductor and the outer ground cover. This will result in complex geometry of transmission line and direct estimation of inter conductor stray capacitance of such complex geometry is not possible. Hence a technique is proposed to estimate the inter conductor stray capacitance of complex geometry transmission line.

A study has been carried out to estimate the inter conductor stray capacitance for various configurations of the transmission line using the method of stored energy in COMSOL platform. The estimates for one such configuration have been validated experimentally from measured values of capacitances for a 1 m long prototype element. The results of these studies and the experimental observations shall be presented and discussed.

Speaker: Vishnudev M N (ITER India, Institute for Plasma Research)

NIBS2020_P2_MN.pdf

08:00 Hydrogen negative ion beam extraction with Cs

A negative ion beam source of 200 keV / 0.5 A has been under development at National Fusion Research Institute (NFRI). Recently, a Cs dispenser was equipped to feed the Cs to enhance the source performance. To see the Cs seeding effect, preliminary short-pulse beam extraction experiments were carried out with 50 keV accelerator prior to the installation of 200 keV accelerator. It was confirmed that the performance of Cs dispenser was maintained for more than 2 hours and the beam currents were increased by about 2 times. The co-extracted electron currents were drastically decreased. It is necessary to find optimum operational conditions such as the current waveform of Cs dispenser to control the Cs feeding rate and particularly the plasma grid (PG) temperature which is the main factor for the deposition rate or the thickness of Cs on the PG surface.

Speaker: Min Park (National Fusion Research Institute (NFRI))

2020NIBS_poster.pdf

08:00 Recent progress in the RF Hydrogen Negative Ion Source in NFRI

A prototype radio frequency (RF) negative hydrogen ion source is under developing in the national fusion research institute (NFRI) in pursuing of the negative ion beam extraction of 200 keV, 0.5 A. The machine has extracted negative hydrogen ion beam since 2018, and recently two major upgrades have been implemented. The first upgrade is in the inductively coupled plasma (ICP) source. The ICP antenna is modified to supply a maximized RF current at the LC resonance frequency by installing additional capacitors. The RF power supply delivers 50 kW of RF power by active control of the RF frequency. A tungsten filament provides seed electrons just before the RF power is applied, and the plasma is generated by the RF power supply within 200 us. The second upgrade is in the series of the stackable high voltage power supplies. The output of the power supply is 20 kV / 1 A, and each power supply is floated and powered by batteries. So far, stacking is successfully applied to two modules, and will be extended. In this presentation, the details of the above upgrades and beam extraction result will be shown. In the near future, more upgrades will be followed: the plasma grid heater using hot oil circulation system will be used for Cs conditioning, and diagnostics using triple probe, CR based OES, and laser photo-detachment will be prepared.

Speaker: Dr Byungkeun Na (National Fusion Research Institute)

NIBS2020_P3_NA.pdf

08:00 Study of population dynamics of excited atomic hydrogen in negative hydrogen ion sources base on collisional radiative model

Collisional radiative (CR) models are useful to analyze the dynamic process of excited states and are in favor of optical emission spectroscopy (OES) in negative hydrogen ion source plasma. In this paper, a CR model of atomic hydrogen containing excitation of H, dissociative excitation by H2 and dissociative recombination by H2+ is developed. The above three channels account for the main contribution for an excited H in cesium free negative hydrogen ion source. Then, the population and depopulation mechanisms of excited atomic hydrogen states are presented for each channel. Furthermore, the influence of electron energy distribution function with a depleted or overheated tail is discussed. Finally, above CR model is used to obtain plasma parameters by OES in hydrogen ICP discharge, with electron density 4.9E16 m-3 and electron temperature 2.2 eV at a source pressure of 2.3 Pa and power of 600 W.

Speaker: Zengshan Li

NIBS2020_P3_Li.pdf

08:00 Study of response of negative ion beam to bias voltage in phase space

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Mechanism of the meniscus formation in negative ion sources for fusion has not been clarified yet since negative charges in the plasmas consist of multiple components, electron and negative ion, and a magnetic field in the vicinity of extraction holes has 3D complicated topology. In our previous study, correlation between the negative ion beam optics and plasma parameters was examined by varying a bias voltage, where the beam width was measured by means of beamlet monitor consisting of CFC tiles and an IR camera. It was observed that the beam width changes along the same curve with respect to the negative ion density for different bias voltages while the negative ion-to-electron density ratio and the plasma potential significantly changed depending on the bias voltage[1]. This implies that the meniscus shape in the negative ion extraction could be controlled by just a perveance matching as usually adopted in the positive ion extraction. On the other hand, a specific feature in the negative ion beamlet was observed in the phase space. The phase space of negative ion beamlet is occupied by multi-Gaussian components[2], and this feature has not been reported for the positive ion beamlet. Then, in this study the beam measurement in the phase space was conducted at different bias voltages (3 V and -10 V) with the same negative ion density in order to clarify the impact of the bias voltage on the meniscus formation in more detail. As a result, it was observed that a beamlet is composed of three Gaussian components in both cases, and that the phase space structure does not depends on the bias voltage. This suggests that the electron is not involved with the meniscus formation at least in caesiated plasmas. In addition, the effect of an electric field estimated from the difference between the bias voltage and plasma potential on the meniscus shape was not clearly identified in this study.

1. M. Kisaki et. al., Rev. Sci. Instrum. 91. 023503 (2020).
2. Y. Haba et. al., New J. Phys. 22 (2020) 023017.

Speaker: Masashi Kisaki (National Institute for Fusion Science)

NIBS2020.pdf

10:00 → 10:20 Coffee break

10:20 → 11:20 O5

10:20 NIBS'20 Award Lecture

Speaker: Mieko Kashiwagi and QST N-NBI Team

NIBS2020_Award_D-Pace.pdf

NIBS2020_O5_QST_NIBSAwardLecture.pdf

10:40 NIBS'20 Summary

NIBS2020_O5_ Belchenko.pdf

NIBS2020_O5_Dan_Faircloth.pdf

NIBS2020_O5_Tsumori-san.pdf

NIBS2020_O5_Wada-san.pdf

11:00 Welcome to NIBS'22

NIBS2022_Neil Broderick.mp4