Cavity Ringdown Technique for negative-hydrogen-ion measurement in ion source for neutral beam injector

The Cavity Ringdown Technique (CRD) is applied for negative hydrogen ion (H-) density measurement in H- source for the neutral beam injector. The CRD is one of the laser absorption techniques. Nd:YAG pulse laser was utilized for negative-hydrogen-ion photodetachment. The H- density related to extracted H- beam was successfully observed by a fixed position CRD. A two-dimensional movable CRD has been developed to measure the H- density profile. Measured profiles were consistent with expected profiles from the H- production area in pure hydrogen and cesium seeded plasmas. By applying absorption saturation in the optical cavity, negative hydrogen ion temperature was evaluated and was confirmed as being a similar value measured with other diagnostics.