ELECTRICAL IMPEDANCE OF ELECTRODES IMMERSED IN MOVING PLASMA

Abstract

The electrical impedance of two cold electrodes immersed in moving argon plasma has been measured. The plasma is formed by a shock wave moving at 4–8 × 10⁵ cm/sec into argon at an initial pressure of 0.01–1 mm Hg. It is found that a pronounced discontinuity in electrode impedance exists under all experimental conditions at electrode voltages in the neighborhood of 50 volts. An appreciable capacitance, which is dependent both on electrode area and on plasma density, is detected in the electrode impedance with electrode voltages of less than about 50 volts. This is ascribed to displacement currents in a sheath at the surface of one or both electrodes. Values of sheath thickness calculated from the observed "capacities" show order-of-magnitude agreement with the calculated plasma Debye length. Since the sheath electric fields corresponding to an electrode voltage of 50 volts and these sheath thicknesses are in the range 10⁶–10⁸ volts/cm, it is thought that the observed impedance discontinuity arises from the onset of electron emission from the cathode.