We present experimentally obtained rate coefficients for electron-atom collisions for n = 0 transitions of neutral helium. Results for the dipole-allowed transitions 3

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.

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It is shown that, for plasmas in which electron-atom collisions dominate over other collision processes, the electron temperature can be derived from the density of metastable helium states measured with laser atomic absorption spectroscopy. The model describing the electron temperature as a function of the density of the metastable helium states is independent of the electron density and is valid for electron temperatures -10 eV and for electron densities to within an error of 20%. For an extended parameter range a more complex model, additionally requiring knowledge of the electron density, may be used.