Speaker
Ms
Klara Mitosinkova
(1. Institute of Plasma Physics, Czech Academy of Sciences, 2. Faculty Mathematics and Physics, Charles University in Prague)
Description
Two identical Neutral Beam Injectors (NBI) were designed and manufactured in the Budker Institute, Novosibirsk for the COMPASS tokamak at IPP CAS, Prague. Each of them generates a neutral atom beam with energy up to 40 keV and total power up to 350 kW. The injector is composed of the ion source, the accelerating and focusing grids and the neutralizer. The ion source is filled with the working gas (deuterium or hydrogen) from a reservoir, and subsequently an RF discharge is ignited. Formed ions are accelerated and focused by the system of four grids. Finally, the ions are neutralized by charge-exchange collisions with a residual working gas in the neutralizer.
Different ions are created inside the ion source, mainly D+, D2+, D3+ and D2O+ ions. All of them are accelerated to the same energy Eb. However, the molecular ions dissociate in the neutralizer and resulting neutral atoms have fractional energies, Eb/2 Eb/3 and Eb/10. These atoms have a shorter mean free path inside the plasma, therefore, they cannot penetrate as deep to the plasma as the atoms with the full energy Eb. In view of this fact, the knowledge of the beam composition is essential for understanding of NBI plasma heating.
The beam composition is determined from intensities of Doppler shifted $D\alpha$ lines measured by a high resolution spectrometer. The neutral beam is observed in the beam duct located at the entrance of the tokamak vessel. A significant fraction of neutral atoms in the beam is exited and radiates. They propagate with a high forward speed causing the radiated $D\alpha$ line is shifted due to the Doppler effect. As a consequence of different discreet energies of atoms in the beam, we observe several differently shifted $D\alpha$ lines. The ratio of the intensities of the shifted $D\alpha$ lines is proportional to the ratio of the different extracted ions.
The ratio of the formed ions inside the ion source depends on plasma conditions in the ion source. In the case of the NBI installed on the COMPASS tokamak, only the pressure inside the gas reservoir and RF power applied to the ion source plasma can be varied. This contribution presents the method how to determine the beam composition and its dependence on conditions in the ion source.
Primary author
Ms
Klara Mitosinkova
(1. Institute of Plasma Physics, Czech Academy of Sciences, 2. Faculty Mathematics and Physics, Charles University in Prague)
Co-authors
Mr
Jan Stockel
(Institute of Plasma Physics, Czech Academy of Sciences)
Mr
Jozef Varju
(Institute of Plasma Physics, Czech Academy of Sciences)
Mr
Vladimir Weinzettl
(Institute of Plasma Physics, Czech Academy of Sciences)
