H- BEAM CHARACTERISTICS FROM A CESIATED RF-DRIVEN MULTICUSP SOURCE

A special dual function collar and getter strip holder subassembly was placed around the aperture of an rf-driven multicusp source. With the source at 35 kV, and the rf pulsing at 24 kW for 1 ms at 1 Hz, the ce sium in the getter strips was dispensed gradually over a half hour. No high-voltage reconditioning time was needed. After the cesium was dis pensed, the strips were used to maintain the collar temperature at alm ost-equal-to 150-degrees-C, which optimized the H- current enhancement and electron current reduction. An electrostatic analyzer was used to measure beam emittance and divergence with and without gas neutraliza tion (using argon or xenon). The neutralizing gas reduced the beam div ergence by as much as a factor of 2, but had little effect on emittanc e. Effects on H- and electron currents of pulse length (300 mus vs 1 m s) and bias voltage on the first grid were studied. The source pressur e was lower by almost a factor of 2 for the shorter pulse, and the H- yield was higher. The highest H- currents were obtained for short puls e, negative bias. The effects on electron current were not as pronounc ed.