Optogalvanic method for the measurement of negative ions in plasmas

A plasma is irradiated by laser beams (argon ion, YAG, dye and diode lasers) and electrons photodetached from negative ions are detected either by a fixed electrode or by a movable probe immersed in the plasma. A hollow cathode tube (galvatron) is used as a test tube in oxygen. Densities of negative ion species are determined from the intensity and the wavelength dependence of the optogalvanic (OG) signal. In the case of detection from the probe, the maximum sensitivity is obtained at a probe voltage above the space potential; the signal-to-noise ratio (SRN) is more enhanced than in the case with an electrode at a fixed position; the spatial resolution is also better. In the case of pulsed lasers, transient phenomena were observed. The peak height plotted against laser power variation showed a linear increase at low powers and then a saturated profile at high powers. Densities of negative ion species are determined from the peak height and the wavelength dependence of the OG signal.