It has been shown that self-sustained normal dc atmospheric pressure glow discharge (APGD) in helium exists in a large range of current values from 100 µA to 10 A. The plasma of this discharge is weakly ionized (one thousandth part of a per cent) and is non-equilibrium. The plasma non-equilibrium degree in the positive column depends on the discharge current values. Parameters of the cathode region stay constant over the whole current range of normal APGD.

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It is shown that the cathode temperature has a significant influence on the properties of the cathode fall in a self-sustained, normal, dc, atmospheric pressure glow discharge in helium and, in particular, on the electric field distribution, the thickness of the cathode fall layer, the current density and the gas temperature. An increase in the cathode temperature leads to an increase in the gas temperature and cathode fall thickness, a decrease in the current density and a modification in the radial distribution of the electric field close to the cathode surface. A discussion based on a one-dimensional model of the cathode fall region including gas heating is presented.

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The effect of strong anomalous absorption of the X-mode pump wave associated with the two-upper-hybrid-plasmon parametric decay is observed in a specially performed model experiment in a laboratory plasma. The strong microwave absorption takes place at the maximal plasma density higher than the upper-hybrid resonance value for the frequency equal to half of the pump frequency value. Its efficiency is determined at the level of 80% in the initial stage of the parametric decay instability saturation and 45% in the following steady state. The theoretical model of the two-upper-hybrid-plasmon decay in strongly inhomogeneous plasma is developed. The localization, threshold and growth rate of the instability are determined in agreement with experimental observations.

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The influence of nitrogen additions to helium flow on the non-equilibrium plasma parameters and the discharge onset voltage of the self-sustained normal dc glow discharge at atmospheric pressure is studied. The concentrations of the low-excited helium atoms in states 2 1 S, 2 1 P, 2 3 S and 2 3 P are determined in glow discharge in helium (99.98%He) and in helium with a nitrogen admixture using the absorption spectroscopy technique. It is shown that the addition of a small amount of nitrogen into helium (less than 5%) leads to the increase of both interelectrode gap voltage and gas temperature. The drastic reduction of concentration of the low-excited helium atoms (n = 2) in the cathode region even at a nitrogen admixture of 0.5% occurs due to their quenching by the nitrogen excited species. At the same time, concentrations of higher excited helium atoms (n = 3) are essentially unchanged.