Simulations of a plasma-filled pinched electron beam diode

Plasma-filled diode (PFD) experiments on the Gamble II pulsed-power generator show three phases of PFD behavior: a low impedance phase followed by a phase of rapidly growing impedance which culminates in a vacuum impedance phase. One-dimensional (1-D) and two dimensional (2-D) simulations of the low impedance phase and the transition to high impedance in the PFD have been performed which show good agreement with the electrical measurements. The 1-D simulations show the formation of multiple virtual cathodes in the diode along with the trapping and heating of the emitted electrons during the low impedance phase. Similar behavior is also observed in the 2D PFD simulations during the low-impedance phase. In the 1-D simulations, the low impedance phase ends when the position of the virtual cathode exceeds the anode-cathode gap size. This is accompanied by a growing bipolar sheath which is responsible for the rapid voltage growth observed in the 1-D simulations. In the 2-D simulations, the low impedance phase seems to end when the diode current exceeds critical current. The transition to high impedance occurs as the electron flow becomes strongly pinched.