3D simulations of 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 high-power operation, the beam quickly heats the anode, generating a surface plasma. Positive ions drawn from this plasma counter-stream with the beam electrons toward the cathode. Although the counterstreaming currents are expected to reach an equilibrium, measurements have shown that the diode impedance steadily falls after peak power is reached. A recent publication described two possible causes of this impedance behavior: anode-plasma expansion into the anode-cathode (A-K) gap and increased ion space-charge near the cathode surface [Phys. Rev. ST Accel. Beams 14, 024401 (2011)]. These effects were illustrated using 2D simulations which included the creation and evolution of anode surface plasmas. Here, we report on a follow-on study which determines the impact of 3D effects on plasma expansion. Results show that while azimuthal asymmetries arise, no instabilities are observed.