We generated a 0.3-mm-diameter DC, hollow-cathode helium discharge in a gas pressure range of 10-80 kPa. In discharge plasmas, we measured position-dependent laser absorption spectra for helium 2 3 S 1 -2 3 P 0 transition with a spatial resolution of 55 µm. From the results of the analysis of the measured spectra using Voigt functions and including both the Doppler and collision broadening, we produced two-dimensional maps of the metastable 2 3 S 1 atomic densities and gas temperatures of the plasmas. We found that, at all pressures, the gas temperatures were approximately uniform in space with values in the range of 400-1500 K and the 2 3 S 1 atomic densities were >10 19 m %3 . We also found that the two-dimensional density distribution profiles became ring-shaped at high gas pressures, which is qualitatively consistent with the two-dimensional fluid simulation results.

We apply laser absorption spectroscopy to direct-current hollow-cathode helium plasma generated in a 0.3mm-diameter cylindrical hole at gas pressures of 10-80 kPa. The measured transitions are 2 3 S-2 3 P and 2 1 P-3 1 D. We evaluate the helium metastable 2 3 S density and gas temperature using the former transition and the 2 1 P atom density and electric field strength from the latter one. The observed lateral distributions of the 2 3 S and 2 1 P atom densities depend on the gas pressure, which results from the sheath length estimated based on the spatial distribution of the evaluated electric field strength.

We performed laser absorption spectroscopy on a micro-hollow cathode He plasma with a diameter of 1.0 mm for the two transitions; 2 3 S-2 3 P (1083 nm) and 2 1 P-3 1 D (667.8 nm) simultaneously, at gas pressures of 5-60 kPa. We fitted the two-transition spectra with a superposed Voigt function, and evaluated 2 3 S and 2 1 P 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 2 3 S and 2 1 P 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 2 3 S and 2 1 P atom densities is quite different at the same gas pressure, which may result from the difference in the direct electron-excitation rates.