First Beam Extraction Experiments at BATMAN Upgrade

The design concept of the RE driven negative ion sources used in the NBI of ITER has been developed at the BATMAN test facility since 1997 [1]. It is based on the plasma generation with a 1 MHz RE power supply in circular drivers out of which the plasma expands in a large source volume. The negative ions are produced by surface conversion of atoms and ions on the caesium covered plasma grid. During the past two years Batman has been upgraded substantially by a new extraction system with a design closer to that of the ITER source allowing beam extraction with higher acceleration voltage and by a refined beam and source diagnostics. A further improvement concerns the magnetic filter field which was previously generated only by permanent magnets placed at the lateral walls of the source. It can now additionally be produced, like at the ITER source, by a current flowing through the plasma grid (PG). In the first beam extraction experiments significantly higher negative ion currents, much lower currents of co-extracted electron currents and nearly no beam shift was observed with the PG filter field. With the new extraction system a lower beam divergence is achieved by variation of the acceleration voltage up to 32 kV.