A laser photodetachment technique for the measurement of H- in a high frequency traveling wave discharge

A probe based laser photodetachment system has been developed to measure th e H- density in a high frequency hydrogen discharge sustained by a travelin g surface wave. Problems associated with application of this technique in h igh frequency discharges in general are pointed out. The influence of the t ime varying voltage across the plasma probe sheath on the probe measurement has been minimized by applying a passive method of compensation. An emissi ve probe, placed inside the launcher, has been used as a reference electrod e in order to minimize the discharge disturbance and to reduce the addition al resistance in the measuring system. A filtering technique has been incor porated in the detection system to separate the photodetachment signal from the signal at the discharge stimulating frequency. The big heating of the probe, set at electron saturation condition, has been avoided by using a co nvenient time sequence of the detection system operation. The photodetachme nt H- density measurements are confirmed by applying a high sensitivity sec ond derivative probe technique as a second reference method. Axial variatio n of H- density, as measured, correlates with the axial distribution of hyd rogen atoms. Experimentally obtained radial H- density profile confirms the well known fact that the negative ions are confined in the existing radial potential well. (C) 2001 American Institute of Physics.