Progress in rod pinch electron beam diodes as intense x-ray radiography sources

Rod-pinch electron beam diodes were reintroduced at BEAMS´98 as small-diameter intense x-ray sources after a twenty-year hiatus. Much progress has been made recently in both analytical and particle-in-cell numerical modeling of the diode behavior and in obtaining intense x-ray sources for pulsed radiography. Rod-pinch diodes utilize a thin annular cathode surrounding a small-diameter anode rod extending through and beyond the plane of the cathode. Extensive experimental data have been obtained on the NRL Gamble facility between 1 and 2 MV at 20 to 80 kA with 50 ns FWHM. Circuit analysis has been used to study electrode-plasma evolution which dominates the time-dependent diode-impedance behavior, and PIC simulations have been used to study electron and ion dynamics. These are providing a complete picture of diode operation from initial space-charge-limited flow, to magnetically-limited flow, to ion-induced electron beam propagation and, finally, to a tightly focused pinch on the rod tip. Use of the rod pinch for x-ray radiography provides substantial improvement over previously available sources. Typically, < 1-mm-diam spot sizes are measured in the forward direction with doses in the 1 to 3 Roentgen range, 1 meter from the source.