Role of plasmas in the operation of a self-magnetically pinched diode

The self-magnetic pinch diode is being developed as an intense electron beam source for high-power x-ray radiography. The diode is comprised of a ∼1-cm diameter, hollow cathode with a rounded tip from which a high-current electron beam is emitted. The beam self focuses in its own magnetic field as it propagates across a ∼1-cm vacuum gap where it deposits its energy onto a planar high-atomic-number bremsstrahlung target. Heating of the anode by the beam quickly provides an ion emitting plasma and bipolar diode operation. The dynamics of expanding electrode plasmas can affect the impedance lifetime of the diode. Realistic modeling of such plasmas is being pursued to aid in the understanding of the operating characteristics of these diodes as well as establishing scaling relations for reliable extrapolation to higher voltages. Here, a hybrid particle-in-cell code is used to study the evolution of electrode plasmas in the self-magnetic pinch diode for a nominal 6-MV voltage and different anode-cathode gaps. The impact of the intense ion beam on the cathode surface can lead to enhancement of the cathode plasma production and faster diode impedance loss.