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

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Abstract
The composition of the plasma on the axis of the positive column of a helium glow discharge is 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. Rates of excitation to the metastable level 23S and of step ionisation were calculated. The distribution at energies epsilon lower than the excitation energy epsilon 1 of level 23S is near maxwellian with a temperature Te, while for epsilon > epsilon 1 it decreases much faster. The relative number of atomic ions is maximal at the pressure p=5 Torr, increases with current and decreases with pressure. At the pressure p=5 Torr and currents i>or=100 mA practically all ions are atomic. The proportion of molecular ions decreases with current and increases with pressure. At the pressure p=40 Torr and currents i<or=40 mA practically all ions are molecular.