Anode plasma dynamics in the self-magnetic-pinch diode

The self-magnetic-pinch diode is being developed as an intense electron beam source for pulsed-power driven x-ray radiography. In ideal high-power operation, the beam electrons desorb contaminants from the anode surface from which positive ions are drawn to the cathode. The counterstreaming electrons and ions establish an equilibrium current. It has long been recognized, however, that expanding electrode plasmas can disrupt this ideal behavior and cause rapid loss of the diode impedance and the radiation pulse. Recently developed numerical techniques are applied to a model of the SMP diode which includes the formation and evolution of anode surface plasmas. Two mechanisms are shown to cause rapid impedance loss, anode plasma expansion into the anode-cathode (A-K) gap and increased ion space-charge along the cathode. The former mechanism dominates for shorter A-K gaps, while the latter dominates for longer gaps. Model results qualitatively reproduce the time-dependent impedances measured for this diode.