Measurement of metastable He*(23S1) density in dielectric barrier discharges with two different configurations operating at around atmospheric pressure

We have measured the density of metastable He atoms in the lowest triplet state (23S1) with a diode-laser absorption spectroscopic technique in atmospheric pressure plasmas produced by dielectric barrier discharge schemes. Two different types of electrode configuration are employed: one is a conventional parallel-plate system and the other is a microdischarge integrated system with stacked metal-mesh electrodes covered by insulating films. We have analyzed the pressure-broadened spectral line corresponding to the 23S1→23PJ (J=0-2) transition to derive the broadening coefficient and to calibrate absolute densities. The measured density ranges from 1011 to 1012 cm-3, but the values in the mesh-type system are larger than those in the parallel-plate system by about one order of magnitude. The density, however, depends strongly on the gas flow rate, showing the influence of quenching by the Penning-ionization process with impurities. Those behaviors are consistent with the variation of the electron density estimated by millimeter-wave transmittance measurement