De rerum plasma constricta

A review is presented of the phenomena associated with the constriction that occurs in gas discharges. The extended positive column plasma can become constricted along its axis, even at low to moderate pressures (∼0.1 to 100 torr). At higher pressures and currents an arc discharge is the normal mode unless there is externally sustained ionization or the gas is flowed rapidly through the discharge. The constriction occurs more readily: (i) in molecular gases than in atomic gases; (ii) in the heavier rare gases than in helium; (iii) in electronegative gases than in electropositive gases and (iv) quite dramatically in certain molecular gas-rare gas mixtures (e.g. Nitrogen-Xenon). The constriction effects in electronegative gases occur at low currents and pressures. The cause of constriction has variously been assigned by discharge researchers to several effects including thermal effects, (Seeliger, 1932); to cumulative ionization, (Golabovsky, 1965); to negative ion effects, (Seeliger, 1932); to dissociative recombination (Kenty, 1961); to the presence and influences of clusters and dust, (Langmuir, 1924); and to Coulomb interactions modifying the electron energy distribution. In reality, several of these causes exist simultaneously with the thermal effects becoming more important at high pressures. In the rare gases the current-pulsed onset of constriction from the glow mode is usually accompanied by new waves of ionization. The theoretical condition for these waves is that the electron temperature relaxation occur within the diffusion length of the plasma