Anode glows in glow discharges: outstanding problems

Whilst it seems established that uniform anode glows and spots are regions in which there is a space-charge limited current of electrons to the anode, and an oppositely directed current of positive ions, many of their properties are still uncertain. These include the structure of the ionization belt on the anode surface; the rôles of electron reflection and absorbed gas; the nature of the perturbation that initiates the switch from a uniform glow; whether oscillations are absent from apparently stable arrays of spots; details of the plasma parameters and distributions of space-charge in and round the spots; the corresponding multichannel conduction theory; the rôles of negative ions in electronegative gases and of metastable atoms in discharges in which they can be present. Suggestions are made about how some of the problems can be approached. Properties of the anode region in heavy current areas are similar in some respects to those of glow discharges, but are frequently complicated by heating and convection, which are normally negligible in glow discharges.