INTERACTIONS BETWEEN IONIZATION WAVES AND POTENTIAL STRUCTURE FORMED AT A CONSTRICTION OF THE DC HE POSITIVE-COLUMN

The potential structure formed at a sharp change in diameter of a diff usion-controlled positive column of a low-pressure and low-current de He glow discharge has been experimentally investigated in relation to either externally or self-excited ionization waves. A strong electric field evolved on a typical distance of 1 cm in front of the constricti on to provide the continuity of discharge current through it, when the constricted part of the plasma column is on the anode side. The elect ron energy distribution function is not in equilibrium with the field, showing the formation of a fast electron group that is quickly dumped out towards the anode because of the inelastic processes and the radi al loss. Either an externally or a self-excited ionization wave distur bs the potential structure, by periodically weakening and strengthenin g it. As a result, large fluctuations of the local axial electric fiel d in the structure occur and, related to them, fluctuations of the dis charge voltage and current are yielded, which make the external discha rge circuit a positive feedback loop for the ionization waves.