Thomson scattering measurements on an atmospheric Ar dc discharge lamp

Thomson scattering (TS) experiments have been performed in the region near the electrodes of a dc powered model lamp filled with 1–2 bar argon gas. In order to suppress the false stray light and Rayleigh scattered photons, a triple grating spectrograph was used. In this way the electron density and electron temperature could be measured in the near-electrode region for different arc currents. In order to get the electron density and electron temperature out of the TS spectrum, both collective and non-collective TS fitting procedures were used for the data processing. It was found that the radial profile of the electron temperature is more or less flat in the anode region with a value of about 1 eV. The electron density increases with arc current and is of the order of 1021 m−3. From the result, we can see that the plasma regions investigated are not in local thermal equilibrium in the sense that, locally, the rate of ionization is much larger than that of recombination.