Development of large RF driven negative ion sources for neutral beam injection

The development of a large-area RF source for the negative ion beam production has been continued concentrating on investigations of volume-mode Cs-free operation. In contrast to most arc discharge sources the extracted H− current density can be significantly improved in the RF source by the addition of argon. Probe measurements close to the plasma grid show that this enhancement is due to a dramatically increased plasma density with the addition of argon. Variation of the number of extraction holes and of the effective transparency showed no dependence of the current density on the extraction area or on the transparency, however, still at a rather low level. The H− current density that has been reached at present amounts to about 5 mA/cm2 at 100 kW and 0.6 Pa without cesium as published previously. In order to investigate the role of the magnetic filter, a large electromagnet has undergone first tests. Preliminary results show no strong dependence on the magnetic field.