Beam species mix as a function of DIII-D neutral beam ion source operation parameters

Abstract

By measuring the Balmer-alpha emission for deuterium, Doppler-shifted spectroscopy (DSS) can give relative fractions of the ions D/sup +/, D/sup 2+/, D/sup 3+/ (species mix) produced by the DIII-D neutral beam ion sources. The species mix varies with beam energy, arc power, gas flow rates, and time within a beam pulse. The full energy species fraction gets larger and tends to become constant with increasing beam energy while the third energy species tends to decrease. Varying the neutralizer flow rate between 0 and 20 torr-l/sec causes the full energy species fraction to decrease by /spl sim/0.04 in favor of more third energy particles. Measurements made at various times within a beam pulse showed that the species fraction normally varies by about 0.02 but can have a variation of as much as 0.14. The effect of the beam parameters on the species mix varies slightly between ion sources. Each of the beam species has a different neutralization efficiency. Thus, the neutral beam power injected into the tokamak depends on the species mix. The neutralization fraction (and the total injected power) obtained using beam species as measured by DSS at a beam energy of 80 keV is 4% lower than previously believed. This difference becomes smaller with decreasing beam energy until they converge at 50 keV.