A simple approach to deriving an electron energy distribution from an incoherent Thomson scattering spectrum

A clear picture is given to illustrate how the electron energy distribution in a plasma can be directly related to an incoherent Thomson scattering spectrum when the electron density distribution is spherically symmetric in velocity space. Based on this relationship, a simple approach is outlined to derive the electron energy distribution unambiguously. The method involves plotting the differences in Thomson scattering intensities between adjacent wavelength channels as a function of the square of the wavelength shift. Thomson scattering spectra obtained from microwave-induced helium plasmas sustained at atmospheric pressure at forward powers of 100 and 350 W are used to derive electron energy distributions through this procedure. Significant deviations from a Maxwellian distribution were found under both conditions over the entire wavelength span covered in the experiment, corresponding to an electron energy range of 0.1–6.6 eV. Compared with a Maxwellian energy distribution, the portion of the electrons with energies between 2 and 6 eV appear to be shifted toward both lower and higher energy values in these plasmas. The ratios of the Thomson scattering intensity at the wavelength channel farthest from the center to the corresponding Maxwellian value indicate that the total number of electrons with energies larger than 6.6 eV is at least 1.8 and 3.1 times higher than that predicted by local thermodynamic equilibrium for plasmas at 100 and 350 W, respectively. In contrast to a Maxwellian distribution, electrons in the microwave-induced plasmas are not most concentrated at the center of velocity space, but reach a maximum in a spherical layer at a distance of approximately 4×107 cm s−1 from the space center. In addition, for both plasma power levels, the bulk of the electron velocity distribution function is slightly compressed and the maximum value is shifted toward lower velocity values compared with a Maxwellian distribution.