We discuss the properties of a dc discharge which occurs at the interfaces between a disc of fused glass beads and parallel electrodes. The discharge consisted of a large number of microdischarges. The discharge current had a large number of pulses superimposed on a dc background current. The background current increased with decreasing bead size and decreased as the separation between the disc and the electrode increased. The current had an extremely slow and nonlinear transient behaviour when the dc high-voltage source was switched on. The frequency of the current pulses decreased with increasing bead size and disc-electrode separation. The average amplitude of the pulses, in contrast, increased with increasing bead size and disc-electrode separation. A significant dependence of the properties of the current pulses on the polarity of the applied voltage was observed, although the dc current was almost independent of the polarity. It was found that the enhancement of the electric field near the contact points between the electrode and the beads, where the current concentrates, initiates the discharge. The density of ozone produced by the discharge was also measured. Damage to the discs by the discharge was also observed, thereby altering the properties of the discharge.

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The axial distributions of the optical emission intensity and metastable atom density are shown experimentally for DC and RF helium discharges. Significant differences in the cathode glow adjacent to the cathode sheath are perceived between the DC and RF discharges. These differences seem to lead to differences in the consistencies of their cathode sheaths. It is found that the RF glow is caused by a direct excitation of non-thermalized high-energy electrons; on the other hand, the DC glow is caused by a dissociative recombination of old molecular ions and thermalized low-temperature electrons in addition to the former process. A theoretical model of metastable atoms in the cathode glow region is proposed, and the theoretical curves are compared with the experimental results of metastable density distribution. The validity of this model is then assured, at least qualitatively.

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