Electron and Ion Density Hysteresis Effects in a Low-Pressure Mercury-Vapour Plasma carrying a Current of Varying Instantaneous Value

Abstract

AN experimental single-anode mercury arc valve was constructed containing a low-heat-resistance pool cathode of the molybdenum-cup type¹ to anchor the cathode spot and a plasma probe inserted with its tungsten tip at the axis of the valve at a distance about one-third from anode to cathode. The valve was operated in a resistance-loaded single-phase half-wave rectifier circuit, thus carrying a half-period sinusoidal current at a frequency of 50 c/s. The cathode temperature of the valve, and hence its saturated vapour pressure, was adjusted by a control of air flow from a fan below the cathode, and the pressure was thus varied over a range of 2.5–17.5 µm mercury. The crest value of current through the valve was altered by changing the load resistance. Further on, a modified version of plasmo-graph² was constructed to obtain on a cathode-ray tube screen a trace in semi-logarithmic form of the probe characteristic at any chosen instant of the wave-shape of the current pulses passing through the valve. Probe characteristics were obtained to construct sets of characteristics of the plasma parameters as functions of the instantaneous values of current i_a for different values of load resistance at a fixed cathode temperature, and for different values of cathode temperature at a fixed value of load resistance.