High-power pinched-beam diode development for radiographic applications

The negative-polarity rod-pinch diode has been studied at the 6-MV, 150-kA level on the NRL Mercury generator, in both reentrant and non-reentrant geometries. We observe good electrical coupling in reentrant geometry and satisfactory coupling in the non-reentrant case. In contrast to previous results at lower voltage, we see good electrical coupling to thin-walled aluminum anodes We see evidence that the diode geometry away from the rod tip plays a role in diode performance. Our best results to date are 75 rads at 1 meter with a 1.8-mm-diam spot. A spot size as low as 1.3 mm has been obtained, though at lower dose. The measured dose rate agrees very well with that predicted from particle-in-cell/Monte-Carlo modeling. An initial look at plasma-filled operation indicates that this technique is applicable to high-impedance diodes.