Long Pulse H[sup −] Beam Extraction With A RF Driven Ion Source With Low Fraction Of Co-Extracted Electrons

Abstract

IPP Garching is developing H−/D− RF ion sources for the ITER neutral beam system. On the MANITU testbed the experiments are focussed on long pulse H−/D− beam extraction with a 100 kW prototype source. The negative ion production is based on surface conversion of atoms and positive ions on Caesium layers. In long pulses with H− beam extraction the ion currents were stable but with too high fraction of co‐extracted electrons. The electron current could be lowered considerably by avoiding copper impurities from the Faraday screen in the plasma which was achieved by coating of the inner surfaces of the source with Molybdenum. A positive bias potential with respect to the source applied to the plasma grid, the bias plate or to a metal rod installed near the plasma grid enables regulation of the electron current during long pulses. In this way low values consistent with the ITER requirements can be achieved without significant loss of ion current.