On the influence of secondary electrons in RF discharges in helium

Abstract

The transition from the alpha to the gamma mode in a low-pressure helium RF discharge between parallel plates is investigated by spatio-temporally resolved plasma induced emission spectroscopy. The time development of several levels of atomic helium and of the n=4 level of ionic helium is presented over a wide range of discharge pressure (40-300 Pa) and power input (75-300 W). Single photon counting is applied to resolve the time dependence. The transition to the gamma or secondary electron dominated state is observed directly in space and time dependent excitation of the helium ion level. The effect of the extreme lowering of the electron temperature resulting from the non-local sustaining mechanism is observed in the light of atomic helium lines, which show substantial differences between the singlet and the triplet systems due to spin changing collisions (singlet to triplet) coupling the metastable levels. The cross section of these collisions increases with a decrease of electron temperature below the threshold value of 0.79 eV. In addition, probe measurements in a similar, but symmetrically driven parallel plate discharge optimized for use with Langmuir probes are presented to give independent evidence to the rapid decrease of electron temperature when the transition occurs.