Multi-shot performance of a 1-MV, 15-kA rod-pinch-like diode for radiography

Summary form only given. Multiple-shot operation of a rod-pinch-diode radiography source without breaking vacuum and replacing diode components is desired. Multi-shot operation is useful for checking alignment and testing detector responses in radiography measurements. Single-shot operation of the rod pinch with a 0.5-mm diameter tungsten rod on the TriMeV pulser at 1.1-MV, 35-kA, and 15-ns FWHM pulse provides a 0.5-0.6 mm source diameter and 0.5-0.6 rad(Si) dose at 1-meter in the forward direction. For multiple shots, this diode is scaled to operate without pinching, but with the same voltage and current, by using a short-length cathode (3 mm) and a few mm diameter anode. A larger diameter cathode, 14(16)-mm, is used to give similar impedance. For this diode, the anode is a 3.2(4.8)-mm diameter tungsten-rod tapered to a blunt end of 2(3)-mm diameter over 15-mm. Radiation is measured from this rod pinch for at least nine shots without breaking vacuum. Near peak radiation, the current is 15-20 kA and the voltage is 1.0-0.8 MV, corresponding to 60-40 Ohms. The dose averages 0.7-1.0 rad(Si) at 1-meter, depending on the diode geometry, and exhibits +/- 15% shot-to-shot variations. The radiation is emitted over 80-ns because the impedance holds up during multiple reflections of the main pulse. However, 50% of the dose is emitted within the 20-ns main pulse. The source diameter decreases with multiple shots, changing from 4.1 to 3.2-mm for the 4.8-mm rod and from 2.7 to 2.1-mm for the 3.2-mm rod. Shots on a rod are terminated after the tip of that rod is extensively damaged. Tungsten surface melt is apparent after one or two shots, and the extensive damage after six to nine shots is consistent with convergence of a shock wave onto the rod-tip axis.