Current-Voltage Characteristics of the Plasma Focus: A Deeper Look

Abstract

A capacitor bank discharges a current which is a sinusoidal function, lightly damped by unavoidable circuit resistance. When powering a plasma focus, the current waveform is further damped by the axial motion typically during the rising part of the current. The radial phase, with severe rate of change of inductance due to a rapidly collapsing current sheet to a small radius, is so severely damped over a short period near the current peak that the waveform goes into the sharp dip. This produces the well-known signature current dip of the properly-operated plasma focus. Corresponding to the inductively-caused current dip is a sharp voltage spike which typically rises to a peak value in excess of the voltage to which the capacitor is charged. These features are adequately described by circuit equations coupled to appropriate equations of motion. The loading effect of different gases due to differences in mass, differences in compressibility and differences in radiation also produces differences in the current waveforms particularly in the current dips and voltage spikes. These differences could be subtle or dramatic, as are demonstrated in this paper.