On the effect of biased surfaces in the vicinity of the large extraction area of the ELISE test facility

Abstract

The ion source at the ELISE test facility is an intermediate step towards the operation of the ITER NBI source and it demonstrated fulfillment of the ITER NBI requirements for accelerated negative current density in hydrogen. However, in deuterium operation the co-extracted electron current density (je) is higher and much more unstable and limits the source performance. In the standard setup of ELISE, je is reduced by a positive potential applied to the plasma grid (PG) with respect to the source body and the bias plate (BP). To further reduce and stabilize je in deuterium, an alternative scheme by biasing the BP is investigated. Measurements of the BP currents and of the extracted currents, combined with probe diagnostics in the vicinity of the PG for short-pulse in deuterium operation are presented. Biasing the BP, leads to a change in the distribution of the plasma potential in front of the extraction area, affecting the charged particle fluxes towards the BP and the PG and a strong reduction of je.