Composition of the positive column of a helium glow discharge at intermediate pressures

The composition of the plasma on the axis of the positive column of a helium glow discharge has been investigated. Four rate equations for the concentration of atomic and molecular ions and metastables can be written. For conditions of gas pressure between 5 and 40 Torr and currents between 10 mA and 200 mA, the equations can be simplified and solved. Rate coefficients for different processes were taken from afterglow measurements. The dependence of these coefficients on electron and gas temperature was taken into account. Rates of excitation to the metastable level 23S and of step ionisation were calculated with the help of the electron energy distribution, obtained from the Boltzmann equation. The distribution at energies epsilon lower than the excitation energy epsilon1 of level 23S is near maxwellian with a temperature Tsub(e), while for epsilon > epsilon1 it decreases much faster. (author)