Experimental comparison between plasma and gas neutralization of high-energy negative ion beams

Hanada, Masaya; Kashiwagi, Mieko; Inoue, Takashi; Watanabe, Kazuhiro; Imai, Tsuyoshi

A proof-of-principle test on plasma neutralizer, that is capable of enhancing a system efficiency of neutral beam injector for future fusion reactors, has been carried out. A 2 m long and 0.6 m diamater neutralizer with multicusp magnet line was used, improving the confinement of primary electrons flowed from both ends of the neutralizer by a pair of magnets. This improvement produced relatively high density Ar plasma of 10 - 10cm at low operating pressure of 0.002 Pa - 0.03 Pa. In the neutralization experiment, 200 keV H ion beams were neturalized with the plasms and gas. Compared with the gas neutralization, the maximaum neutralization efficiency by the plasma was 6% higher than that by the gas. Further, an optimum Ar gas line density for maximizing the neutralization efficiency was 30% lower than that by the gas. These results are in good agreements with results analyzed from the cross-section data for neturalization. Thus, it was experimentally verified that the neutralization effiency can be enhanced at relatively low line density by using the plasma.