Influence of cathode grid geometry upon mode structure of a transparent cathode discharge

We measured current-voltage and optical emission for self-sustained discharges obtained using two forms of cylindrical wire grid cathodes, having either an enclosed or an open end. Enclosure of the open end extended the low-pressure range for a "cathode-confined" or CC mode, from around 12.5 Pa to below 3.5 Pa, conditions at which a "beam" mode discharge otherwise occurs. The modification also caused a dark space to envelop the glow within the cathode, bridged by a bright emission resembling plume for the CC mode, and an electron beam for the beam mode. We explain these results by treating the CC mode as a hollow cathode discharge, for which only gamma-electrons that suffer inelastic loss before escaping the cathode grid are significant. For the two cathodes, respective degrees of electron confinement possible for the different sheath configurations predict the low-pressure ranges, and the calculated values of cathode fall for self-sustenance by the hollow cathode effect agree approximately with the experimental voltages across a range of pressures. Plume- and beam-like forms of emission indicate inherently different electron energies, consistent with bulk transport across potential distributions characteristic to the mode. Where these features bridge the enclosed cathode boundary, this shows existence of an otherwise closed potential surface within the cathode, confirmed by the geometry of the plume-CC mode configuration, where the relationship between the main glow and plume interface surfaces indicates the arrangement to self-organize in a state of non-ambipolar current flow. Similarities in the mode structure reported elsewhere for related discharges indicates the findings to be relevant for these also.