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

A plasma neutralization efficiency of high-energy negative ion beams was measured in a practical neutralizer with substantial dimensions and compared with a gas neutralization efficiency for fusion application. Hydrogen negative ion beams of 200 keV, 4 mA were neutralized with hydrogen and argon arc plasmas in the plasma neutralizer of 2 m in length and 0.6 m in diameter. To clarify the difference between the plasma and the gas neutralizations, the high-density plasmas were produced at low operating pressure. By means of ∼30 G transverse magnetic fields that were applied locally at openings for beam path, leakage of the primary electrons to the outside of the neutralizer was suppressed sufficiently. The plasma densities were 1011-1012 cm-3 at low operating pressures of 0.002-0.05 Pa for both plasmas while an arc power was ∼40 kW. Neutralization efficiency was increased by using plasmas whose ionization degree was 10%-15%. The maximum neutralization efficiency for hydrogen plasma increased from 55% of the gas neutralization to 65%. The maximum neutralization efficiency for argon plasma increased from 51% of the gas neutralization to 55%. The line densities maximizing the plasma neutralization efficiencies decreased to 65% of those for the gas neutralizations. Neutralization efficiencies in the plasmas and the gases were in good agreement with those calculated from the cross sections. It is concluded that a practical plasma neutralizer with substantial dimensions can increase neutralization efficiency at a low line density as predicted from the numerical analysis based on the cross section data