Nature and Role of Ionizing Potential Space Waves in Glow-to-Arc Transitions

Abstract

The transition from a glow discharge to a transient arc induced in Al cathode tubes 145 cm long by shorting of a series resistor has been studied for a variety of gases by observing currents at the electrodes, the potentials at various points in the tube using probes, and the movement of luminous pulses using photomultiplier tubes and fast oscilloscopes. By applying a dc power supply with up to 2600 volts and a large capacity parallel to the discharge tube, and with due regard to proper impedance matching, relatively pure gases ${\mathrm{N}}_2$, A, ${\mathrm{H}}_2$, ${\mathrm{O}}_2$, and mixtures of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$, were studied from 50 microns Hg up to several hundred microns. In addition, shorter tubes with Cu, Ni, Pt, Hg, W cathodes and filled with He, or ${\mathrm{N}}_2$, at intermediate pressures up to 7.2 mm Hg, were briefly studied. The decrease of the series resistance increased the very low current in the normal glow mode to higher values up to 50 ma usually in the abnormal mode. With slightly oxidized cathodes, there followed, after times varying from ${10}^{-3\mathrm{}}$ second to many seconds, catastrophic breakdowns to a transient power arc of the order of 10-20 or more amperes. The transition was initiated by a burst of electrons from the cathode yielding a cathode current increase to several amperes in ∼(2 to 10)×${10}^{-9\mathrm{}}$ second. This burst sends a luminous pip or pulse to the anode at velocities varying from 5×${10}^8$ to 8×${10}^9$ cm/sec, depending on pressure, voltage, and gas. On arrival at the anode, a luminous return arc plasma front moves towards the cathode with speeds from ${10}^8$ to ${10}^9$ cm/sec or more. When it arrives at the cathode, the conductivity is sufficiently homogenized along the column so that the current and luminosity increase along the whole column until the drain on the capacity lowers the supply voltage. The luminous pip, and to a less clearly defined degree the return plasma front, are accompanied by potential space waves which can be established by cross-plots yielding luminosity and potential distributions across the tube at various times. The transition appears to be initiated by a sudden breakdown of the oxide layer on the cathode as it thins by ion bombardment, causing a small fraction of the oxide molecules to be ionized within millimicroseconds, thus yielding the initial current burst. These electrons, accelerated by nearly the full potential drop across the dark space, move as ionizing potential fronts, accompanied by luminosity, with velocities in most cases dictated by the cathode fall potential. In ${\mathrm{O}}_2$, when pressures permit effective photoionization within the tube length, the velocities of the front are increased up to 8×${10}^9$ cm/sec.