Negative Ion Beams Optics and Beam-Gas Interactions along and after the Acceleration Stage

Negative ions are fundamental to produce intense and high energy neutral beams used to heat the plasma in fusion devices. The propagation of such beams is subject to strong interaction with the background gas, whose presence along the beamline is due to leaks from the plasma source and from the gas neutralizer. The secondary particles created in this interaction have different effects depending on the local intensity of the external electric field: where E is high they are accelerated and dumped on electrodes or transmitted in the drift region with poor optics, while in low field region some of them are accumulated in the potential well induced by the beam space charge, influencing the local space charge distribution and the optics. The modeling of these effects is usually addressed bysplitting the domain into two regions, using specific codes for each of them and assuming special boundary conditions to approximate the presence of the neighbor region. This paper describes a study carried out on these phenomena exploiting a 2D-axialsimmetric PIC code whose domain can extend from the beam extraction region all the way to the drift region.This feature allows to simulate at the same time both aforementioned aspects, highlighting possible interactions among them. The results are discussed and compared with the existing numerical codes, as SLACCAD and BYPO.