Emission Characteristics of Vacuum Spark Discharges

Abstract

A study of the total radiation in the visible region from vacuum spark discharges between electrodes of aluminum, magnesium, copper, and carbon has been made. With a capacitance of 21.6 µf charged to potentials up to 20 kv and inductance variable from 2.4 to 45 µh, peak currents up to 65 000 amperes were obtained. Oscilloscope traces of the output voltage from a photocell show that for the underdamped discharge the light output closely follows the current oscillations, dropping to zero between current cycles. The maximum light intensity in a single spark train varies as the third to the seventh power of the peak discharge current, or with the 3/2 to the 7/2 power of the discharge energy, depending on the electrode material and the inductance. For a given electrode material, the exponent increases as the inductance is decreased, while at constant current the maximum light intensity decreases slightly as the inductance is decreased. With aluminum electrodes ranging from 12 to 116 inch in diameter, the maximum light intensity varies inversely with the electrode cross-sectional area. Estimates of current densities as high as 100 000 amperes per cm2 in a 10 000-ampere aluminum spark were made from measurements of the anode spots on the electrodes. Bursts of luminous vapor from the anode, apparently not a part of the current-carrying column, appear to constitute the principal source of light in the critically damped aluminum spark.