Instabilität der positiven Säule endlicher Länge im magnetischen Feld / Instability of the Positive Column of Finite Length in a Magnetic Field

Abstract

The instability of a diffusion-dominated positive column of a gas discharge in a longitudinal magnetic field is investigated both experimentally and theoretically. Dispersion relations are derived for the case of an m = l helical instability considering the finite length of the column. Regions of instability are calculated for various wavelengths of the instability.Experiments were carried out in a positive column of variable length, ranging from 8.5 to 40 cm. at varying pressures of about 0.1 Torr of Neon. There is a satisfactory agreement between theory and experiments. The main results are: a) in long tubes instability occurs at a distinct magnetic field, calculated first by Kadomtsev and Nedospasov. Stable intermediate regions are possible at higher fields depending on the relations between the plasma-parameters; b) in short tubes the column is stable again for higher magnetic fields; c) in very short tubes there is no instability of this kind. The observed frequency spectrum agrees with the theoretical values