The Negative Glow and Cathode Spots in Low Pressure Arcs†

Diffusion conditions in the negative glow (or cathode spot) of low pressure thermionic and pool arcs are investigated. It is shown that both plasma density and potential maxima occur and the cases of planar, cylindrical and spherical symmetry are considered in detail. The ion current at the cathode can be an appreciable fraction of the electron current with an upper limit of half the ratio of the cathode foil to the ionization potential in the most favourable case. The cathode fall can exceed the anode potential by several volts in addition to the anode fall. Two modes are possible for arcs with no positive column : one in which the negative glow fills the whole inter electrode space and another in which the glow is confined to the cathode spot region and the rest of the space is filled with a dark plasma corresponding to the Faraday dark space. Investigation of the dark plasma shows that cathode spots or ‘ lines ’ must form for this mode to occur. In this case the current-carrying electrons pass through the sheath dividing the two plasmas in the retarding direction and the conditions for the existence of such a sheath are derived