Microwave plasma source as an ion beam neutralizer

A 13.56 MHz radio-frequency (rf) driven multicusp ion source has been developed at the Fast Neutron Research Facility. An argon ion current density of 29 mA cm−2 can be obtained for argon gas at a pressure of 0.4 Pa, rf power of 500 W and extraction voltage of 3 kV. For such a low energy and high current of several milliampere ion beam, additional low energy electrons are needed to suppress the unwanted beam broadening by the low energy Ar+ ions. This can be achieved by injecting additional electrons into the Ar+-ion beam. An in-waveguide microwave plasma source, operating at a frequency of 2.45 GHz, has been constructed based on the design of Korzec et al. [D. Korzec, A. Muller, and J. Engemann, Rev. Sci. Instrum. 71, 800 (2000)], at Wuppertal. An electron current of up to 250 mA can be extracted from the source, at an absorbed microwave power of 90 W, a pressure of 8×10−2 Pa, and an extraction voltage of 50 V. The neutralization source is installed downstream close to the beam line. By electrostatic interaction the plasma electrons are attracted to the positively charged ion beam forming a beam envelope. To investigate the neutralization effect, the ion extractor is modified to produce a parallel beam. A fluorescent beam profile monitor is used to measure the beam size in both cases; with and without the neutralizer. The profile of the beam can give information about the effect of the neutralizer on the ion beam. The beam current was also measured by using a Faraday cup. Results of the measurements will be presented and discussed.