Voltage Hash in Low-Current DC Vacuum Arcs

The nature and mechanism of the high-voltage pulses in the anode region of copper-vapor arcs have been investigated. Arcs are drawn between a 1.3-cm-diameter anode and a 5-cm-diameter cathode in the dc range of 30-400 A. Prior to anode spot formation and at constant current, the arc voltage exhibits HF pulses superposed on the dc voltage. The amplitude and frequency of repetition (= 500 kHz) of these pulses is highly erratic, and the total instability is evidenced as voltage hash. The hash amplitude increase with both arc current and contact spacing and, at 2.5 cm, varies from several tens of volts at 30 A to several hundreds of volts at 400 A. The hash is attributed to fluctuations in vapor and plasma density in the anode region. Although these fluctuations appear unrelated to the gross motion of the cathode spots, streak photographs indicate a time-variant cathode evaporation that may contribute to the hash phenomenon. A second contributory mechanism is suggested from experience of hash in the anode region of fluorescent lamps, and involves quasi-periodic emission from the anode.