Electron energy distribution reconstruction in low-pressure helium plasmas from optical measurements

Electron energy distribution functions (EEDFs) in electron-cyclotron resonance heated low-pressure helium plasmas were reconstructed form-free from eight helium spectral line intensities. The extreme ill-posedness of the inversion problem due to the small amount of noisy data is tackled with Bayesian data analysis by quantification of all the relevant information and combining them within a probabilistic theory. The line intensities are calculated with a stationary collision-radiative model including de-excitation of the metastable states by wall collisions. The uncertainties of the cross sections of electron impact excitation are taken into account as they correspond in size to the errors of the measured line intensities. The most probable form-free EEDF and its confidence interval describing only the significant information content in the data shows a distinct non-Maxwellian form with a depressed tail in the range of inelastic collisions.