Anode plasma formation and expansion in pulsed high-voltage diodes

Plasma formation and expansion in pulsed high-voltage diodes are investigated using a relativistic one-dimensional particle-in-cell Monte Carlo collision simulation. A 600 kV pulsed DC voltage applied across a 1 cm anode–cathode gap over 100 ns generates quasineutral plasma near the cathode during ramp-up, which subsequently expands toward the anode. A relativistic space-charge-limited (SCL) electron beam is extracted from the expanding cathode plasma front and collected at the anode, resulting in neutral desorption (both stimulated and thermal) from the electrode surface. In the present model, a fraction of the neutrals desorbed from the anode is ionized, producing anode plasma. The simulations show that the current within the gap is initially SCL due to cathode plasma expansion, which later transitions to a bipolar SCL flow as the anode plasma forms. The results demonstrate that anode plasma formation plays an important role in establishing bipolar flow when thermal desorption dominates over stimulated desorption. The extracted current in the simulation is consistent with the relativistic SCL bipolar flow theory, which depends on the electron and ion currents, as well as the distance between the anode and cathode plasma fronts.