Time resolved electron density and temperature measurements on a capacitively coupled helium RF discharge

Small capacitively coupled RF plasma sources operated with helium are popular as excitation sources for element detection, especially in combination with a gas chromatograph. The high mobility of helium and the use of relatively low RF frequencies lead to strong time dependences of the electron density and temperature, and hence excitation efficiency. In this study, the electron gas parameters of such a plasma were measured with temporal resolution. This was done by Thomson scattering using a triple-grating spectrograph for stray light rejection and by absolute spectral line intensity measurements. The electron density is measured to vary within a factor of two around 1.5×1019 m-3. The electron temperature found by Thomson scattering varies with the RF signal between 0.5 and 3.8 eV, whereas the temperature deduced from the absolute spectral line intensity ranges from 1.2 to 2.0 eV. This difference suggests the electron energy distribution to deviate from a Maxwellian equilibrium shape, as was observed in a number of Thomson spectra. A model is presented to explain the electron density behaviour on the basis of the measured electron temperatures.