Microwave plasma source for high current ion beam neutralization

A new microwave based plasma source for high current ion beam neutralization is presented. It consists of a tapered WR340 waveguide with a directly coupled 2.45 GHz magnetron and a quartz tube of 40 mm diameter inside the waveguide confined by two graphite electrodes. The upper electrode houses the gas feed, whereas the lower electrode serves as the plasma outlet. Both electrodes can be biased independently. Tuning is done via a magnetically controlled stab inserted into the waveguide. The plasma was characterized by use of the Langmuir probe. The electron concentration of 5x10(11) cm(-3) at 250 W was measured. Typical electron temperature is below 2.5 eV even for pressure as low as 2x10(-3) Pa. The extractable electron current is dependent on microwave power and gas flow. A limitation is caused by the ion saturation current to the negatively biased electrodes. The voltage on the electrodes allows an easy control of electron current. Maximum electron current achieved so far is 400 mA for an argon flow of 5 sccm and microwave power of 300 W. (C) 2000 American Institute of Physics. [S0034-6748(00)65102-X].