Design of the new extraction grid for the NIO1 negative ion source

Abstract

NIO1 is a compact source of negative ions jointly developed by RFX and INFN, to study the physics of production and acceleration of H- beams. Negative ions, up to 120mA of current, are extracted from a radiofrequency driven plasma, bymeans of a gridded electrode (plasma grid, PG) featuring 9 apertures arranged in a 3x3 square lattice. The same aperture pattern is replicated in the following electrodes, allowing ion acceleration up to 60 keV. All electrodes are realized in copper, by electro-deposition technique, leaving empty slots in themetal to placemagnets and to flow water for the grid cooling. The first set of electrodes was completed, installed in the source and tested. At the same time, an upgrade of the extraction system was carried out, in order to optimize the beam optics and to explore alternative electrostatic configurations. In particular, the accelerator will bemodified by completely replacing the EG grid, exploiting themodularity of NIO1. The new electrode will feature other slots in between apertures, to place additionalmagnets. This allows testing differentmagnetic configurations, to optimize electron filtering and residual ion deflection. The present paper describes the theoretical activities driving the design of these new extractors, carried out withmost updated numerical codes, and exploiting the synergy with the refinedmodeling of the 40 A ITER negative ion sources, under development at Consorzio RFX. Beam simulations are performed both with tracing codes (SLACCAD and OPERA) and with particle in cell codes (ACCPIC)