Laser absorption spectroscopy for measurement of He metastable atoms of a microhollow cathode plasma

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 2³S₁-2³P₀ 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 2³S₁ 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 2³S₁ atomic densities were ∼10¹⁹ 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.