Thomson light scattering measurements of electron temperature and density in the transition of a capacitive rf discharge in helium

Previous investigations of capacitive rf plasmas by means of Langmuir probes have shown unusually low mean electron energies of the order of a few tenths of an eV. We have developed the Thomson light scattering technique for its application to such plasmas in order to see whether those unusual plasma conditions can be confirmed by an independent experimental technique. Because impurities can affect helium discharges considerably, the plasma was operated in a UHV set-up. The symmetric electrode arrangement consisted of two aluminium discs of 70 mm diameter each and 40 mm spacing between them. The experimental set-up is described and it is pointed out how the particular problems of investigating such plasmas were solved. From the light scattering spectra of uncorrelated electrons both mean energies between 0.5 and 2 eV and densities in the vicinity of were derived as a function of rf voltage for a pressure of 40 Pa. Comparison with Langmuir probe measurements in the literature shows excellent agreement of values within their error limits including the steep rise and onset voltage of the transition. With respect to values reported from modelling calculations, agreement within error brackets has been found to apply for densities only.