Preliminary studies for a beam-generated plasma neutralizer test in NIO1

The deployment of neutral beam injectors in future fusion plants is beset by the particularily poor efficiency of the neutralization process. Plasma neutralizers, originally proposed in the 80s, promise consistent efficiency improvement with respect to baseline scenarios based on gas neutralizers. Halfway between these solutions, beam-driven plasma neutralizers have been considered by Surrey [1], which might offer an improved neutralization at a minor technological cost. Anyway, since plasma formation is to be obtained by a special configuration of magnetic fields, preferably obtained by permanent magnet only, some trial experiments with reduced-size prototypes are necessary. The test of such a system in the small negative ion beam system NIO1 is discussed in this paper. The proposal also moves from the fact that there are limited experimental measurements of the neutralizer plasma parameters in a negative ion beam systems for fusion. This test would contribute also to study fundamental physics that has direct implications also in beam optics, beam transport, operation of electrostatic residual ion dumps, and long pulse operations with indirect heating of mechanic components not in direct view of the particle beam. The required gas target thickness, and possible magnetic confinement solutions are presented. Integration in NIO1 is also discussed, with the impact on the pumping system and the setup for measuring the residual beam fractions. An estimate of ionization degree and neutral yield is given for this design.