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 γ-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.

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We investigate optical emission from a low-pressure ‘beam mode’ inertial electrostatic confinement glow discharge, created using cylindrical grid electrodes, having cathode inside a grounded, concentric anode. Spectral line distributions were measured in helium from this and a hollow cathode mode, with four lines emitted from singlet and triplet states with n = 3 and L = P and D compared against simple models for impact excitation by electrons and by ions and neutrals, suggesting much beam mode emission is caused by fast neutrals, and a significant component also caused by electrons. Beam mode emission occurs in a pattern of radial lines, that pass through aligned electrode apertures and extend across the chamber. Shadows cast by the electrode wires indicate electrically-neutral, beam-like sources of excitation, that cross the electrode assembly from bright radial lines in the opposing inter-electrode space, and calculated potential distributions show lensing surfaces around the anode grid apertures, focussing for inward-drifting ions. These indicate the emission pattern is caused by radially-convergent ion beams, and co-linear beams of fast neutrals produced in these. Emission was also observed with various alterations made to the electrode arrangement: with the anode removed, no beams were observed; with the cathode apertures rotated out of phase with those of the anode, bright radial beams between the electrodes followed the distribution of anode apertures; similar beams appeared with the grid cathode replaced by a solid version. In these experiments, progressive obstruction of pathways through the cathode caused increasing reduction in the beam pattern of emission beyond the anode radius, and in discharge perveance at similar pressure and voltage. Beam-like emission was also observed for a parallel-planar configuration, in which a cathode grid was held between two identical anode grids. The beam mode is shown to be a convergent, anode-focussed ion beam discharge, distinct to the star mode described elsewhere.

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