The Role of Plasma Evolution in the Operation of a Self Magnetically Pinched Diode

Summary form only given. The self-magnetic-pinch (SMP) 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 team 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 target from which bremsstrahlung is produced. The evolution and dynamics of expanding electrode plasmas has long been recognized as a limiting factor in the impedance lifetimes of high-power vacuum diodes. Accurate 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 SMP diode for a nominal 4 MV voltage and different cathode configurations. A low density plasma front is calculated to quickly evolve towards the anode producing an effective 1 mm anode-cathode gap. The simulated plasma motion is consistent with that expected from jxB acceleration. The small sheath enables a high intensity electron spot at and rapid heating of the anode. The interplay of the cathode plasma with the subsequently expanding anode plasma will determine the overall diode impedance.