Thomson scattering in low temperature helium plasmas of a magnetic multipole plasma source

Abstract

Measurements of electron density and temperature of helium plasmas in a cw running magnetic multipole plasma source by repetitively laser-pulsed 90° Thomson scattering are reported. This is the first experiment in which this technique has been applied to such plasmas. Measurements are performed at a helium gas pressure of pg = 5 Pa, the discharge voltage was Ud = 100 V, the discharge current was 5 A ≤ Id ≤ 30 A, and the cathode heating current was 80 A ≤ Ih ≤ 140 A. Electron energy distribution functions obtained from the Thomson scattering spectra are studied. The obtained plasma parameters are electron temperature 1.5 eV ≤ kTe ≤ 5 eV and density 1012 cm−3 ≤ ne ≤ 4 × 1012 cm−3, respectively. The sensitivity of detection of the experiment is in the range of 109 electrons and the accuracy of the electron temperature and electron density are specified to 15% and 20%, respectively. In addition, the neutral density and helium gas temperature are obtained from the Rayleigh component of the scattered spectra. Langmuir probe measurements are performed under the same plasma conditions and a comparison of the results with Thomson scattering shows good agreement between the two diagnostics.