Metastable atoms and the electron distribution function in a helium discharge positive column

The influence of chemo-ionization processes on the shape of the electron distribution function (EDF) is investigated both by solving the Boltzmann equation and by measurements in a dc low-pressure (0.3 Torr) low-current (i = 10 - 80 mA) helium discharge. Probes were used to measure EDFs, electron densities and the axial and radial electric field strengths. The concentrations of metastable helium atoms in triplet and singlet states were measured using an optical absorption method. For calculating distribution functions the kinetic equation was solved analytically in local and nonlocal approximations. Good agreement is achieved between experiment and the nonlocal approximation for electron energies above 5 eV. Chemo-ionization is found to influence the high-energy tail of the EDF and thus the rates of electron impact excitation and ionization.