First Beam Extraction Experiments at BATMAN Upgrade
- Dr. Werner KRAUS
- Dr. Werner KRAUS (Max-Planck-Institut für Plasmaphysik)
On the BATMAN testbed prototypes of RF driven negative ion sources of about one eight of the size of the ITER source have been developed and tested since 1997. The positive results had led to the decision to equip the ITER NBI with RF sources. During the past 1.5 years this test facility has been upgraded in order to have an extraction system and source design closer to that of the ITER source and to improve the beam as well as the source diagnostics. In the new extraction system 70 apertures of 14 mm diameter are arranged similar like in one segment of the ITER NBI. The total extraction area was enlarged from 70 cm2 to 108 cm2 and the maximum beam energy from 33 kV to 50 kV. An additional feature is the implementation of a fourth electrode upstream of the grounded grid. By this “repeller electrode”, which is on positive potential, the current of the back streaming positive ions can be reduced. In this way the space charge compensation of the beam downstream the grounded grid is expected to be changed. The filter field, previously generated only by permanent magnets placed at the lateral walls of the source, can now additionally be produced, like at the ITER source, by a current flowing through the plasma grid which enables to vary the field strength. In this paper the results of the first beam extraction experiments are reported and compared to that achieved with the previous design of the source and the extraction system with emphasis on the impact of the improved filter field.