Plasma-filled rod-pinch diode experiment on Gamble II

Summary form only given, as follows. A plasma-filled rod-pinch diode fielded on Gamble II may represent a breakthrough in electron-beam energy concentration. The injected plasma forms an initial short circuit between the disk cathode and the 0.5-1 mm diameter rod anode. After a short circuit phase of 10-30 ns, the impedance increases and a large fraction of the energy is deposited on the tip of the rod, producing an intense X-ray pulse and a small spot size. The current and voltage (at maximum radiation) vary between 260-770 kA and 1.8-0.45 MV, as the initial plasma density increases. These parameters imply effective gaps between the plasma and rod anode of 10-170 microns, far smaller than can be achieved with metal electrodes without premature shorting. The low impedance (7-0.6 Ohms) allows efficient coupling to the 2-Ohm generator resulting in order-of-magnitude greater dose than obtained using higher-impedance (20-60 Ohm) rod-pinch diodes without plasma fill. We speculate that current is convected to the rod tip by plasma translation instead of e-beam propagation. A gap forms by erosion when plasma is pushed beyond the tip of the rod. These processes are being studied with fluid and particle codes. The plasma-filled rod-pinch may produce record e-beam energy densities and interesting plasma conditions at the tip of the rod. Example parameters include: magnetic field near the rod surface 1-6 MG; electron current density at the rod tip = 50 MA/cm/sup 2/; electron power density incident on the tip = 100 TW/cm/sup 2/; and solid-density tungsten plasma temperature /spl sim/ 100 eV (assuming the electron energy is absorbed in a 0.5 mm diameter sphere). Potential applications for this technique include improved radiography sources, x-ray-matter interaction studies, and high-energy-density plasma generation.