COMPARISON OF PROBE MEASUREMENTS AND EMISSION-SPECTROSCOPY IN A RADIOFREQUENCY DISCHARGE

Langmuir probe measurements were performed in connection with optical emission spectroscopy to study the alpha to gamma transition in a symm etric radiofrequency (f = 13.56 MHz) discharge between parallel plates in helium. The measurements were carried out over a wide range of pre ssures (30-300 Pa) and discharge voltages (20-400 V). The correlation between these two diagnostic methods is investigated. The alpha to gam ma transition can be seen in the sudden change both in the measured el ectron-energy distribution function and in the line intensities with i ncreasing discharge voltage. The observed dramatic lowering of the ele ctron temperature in the gamma mode (the secondary-electron-dominated mode) results in extreme differences in the optical emission between s inglet and triplet helium lines due to spin-changing collisions of ele ctrons and metastable atoms. it is deduced, using two different electr ode materials (aluminium and stainless steel), that secondary electron s and metastable atoms play the dominant role in the formation of the two discharge modes. Furthermore, the probe measurement data are shown to be in very good agreement with those recently reported by Godyak e t al.