CHARGED-PARTICLE FLOW IN PLASMA-FILLED PINCHED-ELECTRON-BEAM DIODES

Plasma-filled pinched-electron-beam diode experiments have been perfor med on the Gamble II (1.5 MV, 800 kA, 60 ns) pulsed power generator at Naval Research Laboratory. These plasma-filled diode (PFD) experiment s show three phases of behavior: a low impedance phase followed by a p hase of rapidly increasing impedance that culminates in a relatively c onstant vacuum impedance phase. The duration of the low impedance phas e as well as the final operating impedance depends on the prefill plas ma density. The charged particle flow in the PFD is studied with one-d imensional (1-D) and two-dimensional (2-D) simulation models. These si mulation models show the formation of growing sheaths at both electrod es during the low impedance phase. The end of the low impedance phase in the simulations coincides with the two sheaths meeting in the cente r of the anode-cathode (A-K) gap. Based on these observations, an anal ytic model was developed that treats the low impedance phase as symmet ric bipolar sheaths. The analytical model adequately predicts the dura tion of the low impedance phase predicted by the 1-D simulation model. Differences between the bipolar model and the experiments or 2-D simu lations can be explained in terms of magnetized sheaths which enhance the ion current over the bipolar level and cause the sheath to grow fa ster than the bipolar model. During the rapidly increasing impedance p hase, the simulations show that the cathode sheath quickly expands to completely fill the A-K gap. At this time, charged particle flow in th e simulation models are consistent with the vacuum gap spacing. Experi mentally, the higher density, longer conduction time, PFD shots exhibi ted a significantly lower final impedances than predicted by 2-D simul ations. This difference is probably caused by expanding electrode surf ace plasmas produced by the interaction of the plasma source with one or both electrode surfaces.