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Contribution Oral

H– and D– sources for fusion, accelerators and other applications

The RF H$^-$ Ion Source Project at RAL


  • Dr. Olli TARVAINEN

Primary authors



A Penning-type surface-plasma ion source source has provided H$^{-}$ beam for the ISIS spallation neutron and muon facility at the Rutherford Appleton Laboratory (RAL) for nearly 35 years. The source delivers 55 mA of H$^{-}$ beam current with a beam duty factor of 1.5 $%$ at 50 Hz repetition rate and a transverse 4-rms emittance $<1.0$ $pi cdot $mm$cdot$mrad. Only 25 mA of H$^-$ current is actually required for ISIS operations, so poor beam transport efficiencies in the front-end are tolerated but are inefficient in terms of ion source performance overhead. The ion source is fundamentally lifetime-limited by material erosion, redistribution and flaking, through sputtering processes. Sputtering is mostly attributed to the use of cesium: mandatory for sustaining the discharge and for H$^{-}$ surface production. The limited lifetime affects the facility operational programme, necessitating a scheduled ion source replacement every 2-3 weeks in the middle of user cycles, plus very occasional unscheduled changes. It also requires significant technician effort to maintain a fleet of ten spare operational ion sources.

An RF-driven volume-production H$^-$ ion source project was approved and its funding secured in April 2018. It is proposed that the ion source will be coupled to the spare ISIS RFQ and a new medium energy beam transport (MEBT) line to form a completely new front-end. The front-end will have much improved transport efficiency and phase space matching into the ISIS linac, reducing the ion source output requirements to around 30 mA: within the realms of cesium-free operation. The front-end will undergo long-term performance benchmarking off-line, before installation on ISIS in the 2020s. In this paper we present the overview and current status of the RF ion source project. The conceptual design of an adjustable permanent-magnet filter field is introduced, together with preliminary simulations of the H$^{-}$ ion beam extraction and co-extracted electron dump. Technical solutions for the ignition element, guaranteeing reliable breakdown of the main discharge at 50 Hz repetition rate at the optimum gas pressure for H$^{-}$ production, are pursued with LPSC Grenoble and are also described.