Spatially resolved laser absorption spectroscopy on a micro-hollow cathode He plasma

Abstract

We performed laser absorption spectroscopy on a micro-hollow cathode He plasma with a diameter of 1.0 mm for the two transitions; 23S–23P (1083 nm) and 21P–31D (667.8 nm) simultaneously, at gas pressures of 5–60 kPa. We fitted the two-transition spectra with a superposed Voigt function, and evaluated 23S and 21P atom densities, gas temperature, and electron density. We evaluated the two-dimensional maps of these parameters at respective gas pressures. It is found that the 23S and 21P atom densities are high in the center of the plasma at low pressures, and are high near the cathode-wall at high pressures. In addition, the ratio of the values between the bulk and the sheath for the 23S and 21P atom densities is quite different at the same gas pressure, which may result from the difference in the direct electron-excitation rates.