Magnetic field modification of 18 GHz electron cyclotron resonance ion source at the Japan Atomic Energy Research Institute

An 18 GHz electron cyclotron resonance ion source under development since. 1994 has a solenoid coil between a pair of mirror coils to vary the mirror ratio over a wide range. The solenoid coil of the original design formed a small bump in the bottom of the mirror field profile when the source parame ters were optimized for high charge states. Source performance in generatin g highly charged ions improved with a number of minor changes on the vacuum , the gas feed, and the beam extraction system. The maximum beam current of Ar16+ was 2 enA, which was much lower than that expected from the microwav e frequency and the magnetic field strength. Little effect of a biased disk on intensities of highly charged ions and dependence of the charge state d istribution on the solenoid coil current suggested that the bump divided pl asma into two regions and limited the source performance. The bump was remo ved by halving the solenoid coil length, and the beam current of Ar16+ incr eased to 1.3 e mu A as a result. (C) 2000 American Institute of Physics. [S 0034-6748(00)60502-6].