Gas dependence of paired luminous rings in capacitive radio-frequency discharges

Abstract

Paired luminous rings separated by a narrow dark gap have been studied in pulsed capacitive radio-frequency (rf) discharges using seven gases. These paired rings are clearly observed when H2 or D2 is used. The isotope effect between H2 and D2 is observed in the pressure dependence of the emission profile and different generation time of rings. When He, N2, O2, Ar, and Xe are used, only a few pairs of rings are observed. The atomic-mass dependence of the gas pressure, at which a pair of ring emission is excited (P1) at the electrode separation d=6 cm, is measured. The axial electric field E is calculated at P1 for each gas using two methods: One is the energy-balance method and the other is the ionization-coefficient method. In the former method, the energy-balance equation is solved to obtain E so that an electron is accelerated and reaches ionization energy at 3 cm from the initial position. In the latter method, the equation for the ionization coefficient α/P=Aexp(−BP/E) is solved to find E by substituting P=P1 and 1/α=3 cm. The calculated E using two methods agree relatively well with each other in H2, D2, He, N2, and O2. The empirical scaling-law, P1=(8.61±0.40)/A and E=(0.302±0.0057)BP1, has been obtained from the ionization-coefficient method.