Calculation of negative ion beam loss for two region arc plasma negative ion source

Abstract

Korea Atomic Energy Research Institute (KAERI) is currently developing a Two Region Arc Plasma (TRAP) negative ion source for neutral beam injection (NBI) systems of a fusion tokamaks which is characterized by two distinct regions with different plasma temperatures without the use of external filter field magnets. The source has recently been designed, manufactured, and installed for testing. Plasma discharge characterization experiments of the TRAP ion source are presently in progress, with planned beam extraction experiments. The beam extractor of the TRAP ion source is capable of accelerating hydrogen ions up to 30 keV and comprises four grids with slit-type aperture structure. Understanding the negative ion beam loss in the extractor is crucial for expecting potential issue within the beam extractor and comprehending the plasma discharge characteristics through negative ion beam. Using a 3-D Monte Carlo program (molflow+), we calculated the vacuum pressure distribution within the TRAP ion source experimental device, and based on these results, we calculated the loss of negative ion beams within the beam extractor and the neutralization efficiency from the beam exit to the target, an effect arising from residual gases. The computational results reveal that approximately 20 ∼ 50 % of the negative ion beam is lost within the beam extractor, and a reduction of more than 60 % in negative current occurs from the beam exit grid to the target within the test chamber due to neutralization of negative ions.