The positive column with negative ions in a magnetic field at low pressure

The effect of an axial magnetic field on the structure of the positive column at low pressures is examined in situations where negative ions are present and where they may dominate in density over electrons. In particular, when the electrons are magnetized to the extent that their cross-field mobility is severely reduced and their effective mass is greater than that of the negative ions the structure of the discharge becomes quite different from the conventional Langmuir-Tonks description at low pressures or the Schottky one at medium pressures. The results of computations in both plane and cylindrical geometries are given, demonstrating that in such circumstances the negative ion density increases towards the outer boundary. This has been carried through for parameters appropriate to discharges where the dominant processes for negative ions are attachment-detachment, for example O-2, and also attachment-recombination, for example CCl4. Experimental results in CF4 which simulated the present work are consistent with the general conclusions. Furthermore, the nature and relevance of the Bohm criterion for sheath formation is examined, with the conclusion that at moderately strong magnetic fields it is no longer of significance because other criteria take over.