Electron Scraping and Electron Temperature Reduction by Bias Electrode at the Extraction Region of a Large Negative Ion Source

The neutral beam test facility (NBTF), built in Consorzio RFX (Padua, Italy), aims at optimizing the ITER neutral beam injectors through dedicated devices. The facility hosts the full-size prototype of the ion source of the ITER injectors, source for the production of ions of deuterium extracted from a radiofrequency plasma (SPIDER), a radio frequency-based, cesium-assisted negative ion source. The negative ion beam is extracted by a triode with total acceleration voltage up to 108 kV; each grid is provided with 1280 apertures. On the plasma side, the plasma electrode [plasma grid (PG)] is partially covered by an additional electrode [bias plate (BP)] featuring wide openings of the same size as the beamlet groups. The PG and BP can be independently biased with respect to the ion source, influencing the uniformity of the beam and the amount of coextracted electrons. This contribution aims at studying the scraping effect of the electrons by the BP along the direction parallel to the filter field through numerical simulations. The possible role of the BP bias in reducing both the electron density and temperature in front of the extraction apertures and the correlation of the electron temperature with the bias of the electrodes are also discussed.