Fig. 2-10 Comparison of neutral gas and plasma neutralizers In the plasma neutralizer, D+ ions and electrons, which have higher neutralization cross-sections, can effectively neutralize D- ions.

Fig. 2-11 Neutralization efficiency as a function of line-integrated H2 density The measured neutralization efficiencies (, ) agreed well with the theoretical values (lines). It is foreseen that neutralization efficiency of higher than 70% can be obtained at an ionization degree of 30%.

It is essential to inject powerful neutral beams of D0 to achieve or maintain thermonuclear fusion. In a neutral beam injector (NBI), the neutral beams of D0 are produced by passing high-energy D- ion beams through a neutralizer cell. In the existing conventional gas cell, the electrons of the D- ions are detached by collisions with the D2 gas molecules. The neutralization efficiency, defined as the conversion efficiency from ions to neutral atoms, is limited to 60%. This neutralization efficiency results in a NBI system efficiency of 40%. However, higher NBI system efficiency (> 50%) is required for future fusion plants. Theoretical analysis shows that a higher neutralization efficiency (~ 80%) can be obtained by using a highly ionized plasma cell as a neutralizer instead of the conventional gas cell (Fig. 2-10). A key condition to realize such an efficient plasma neutralizer is to suppress plasma leakage from the large openings for transiting beam without degrading the beam optics. To suppress the plasma leakage, weak transverse magnetic fields (~ 30 gauss) have been applied locally at both openings. The magnetic fields enhanced plasma density by a factor of 1.5, and the ionization degree increased to 10%. The neutralization efficiency was experimentally examined to confirm the theoretical analysis. Fig. 2-11 shows the neutralization efficiency as a function of the line-integrated density of gas molecules. The maximum neutralization efficiency increased to 65% from 60%, which was obtained with the gas neutralizer. Further, the measured neutralization efficiencies agreed with the theoretical values. These results show that plasma can improve the neutralization efficiency as predicted by theoretical analysis. The theoretical analysis shows that plasma with an ionization degree of 30% improves the neutralization efficiency to > 70%. This higher neutralization efficiency can enhance the system efficiency to > 50%, which is a requisite for future fusion plants. Further, this lead to a reduction in the size and cost of the neutral beam NBI system.

Reference M. Hanada et al., Experimental Comparison between Plasma and Gas Neutralization of High-Energy Negative Ion Beams, Rev. Sci. Instrum., 75(5), 1813 (2004).

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