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Space Charge Compensation of Negative Ion Beams

M.Cavenago[1], and P. Veltri[2]
[1]INFN-LNL, Legnaro, PD, Italy
[2]Consorzio RFX, Padova, Italy

Neutral Beam Injectors for ITER require development of negative ion sources and beams with carefully reduced beam divergence. In the accelerator this is compensated by the focusing due to the fringe effect of the electric field, while in the drift region, where the beam is propagating without further acceleration, a background positive charge is required to assure beam transmission with small divergence. Inside the ion source plasma space charge density na becomes negligible within lengths comparable to the Debye length. A set of macros called BYPO executed by COMSOL Multiphysics solve the electric field iterating between the Poisson solver and particle tracing. In the original BYPO scope, the positive ion background was represented by a semianalytic model which predicts 100% space charge compensation. In a postprocessing mode, the production of H2+ and other secondaries were computed, finding agreement with the previous semianalytic model. Here a 2D background selfconsistent model is implemented in COMSOL Multiphysics; results are compared to previous model.