Impedance behavior in the self-magnetic pinch (SMP) diode on the RITS-6 accelerator

Summary form only given. The RITS-6 accelerator (4-7.5 MeV) at Sandia National Laboratories produces high-power (TW) focused electron beams (<; 3mm diameter) for flash x-ray radiography applications. The beam is generated in a Self-Magnetic Pinch (SMP) diode, which utilizes a hollowed metal cathode to produce a pinched focus onto a high Z metal converter. Optimum radiographic performance requires a small pinch spot size. We have achieved a reproducible performance with a standard cathode diameter of ~ 1 cm. Smaller spot sizes are achievable by reducing the cathode diameter below 1 cm, but performance is compromised by the tendency of the diode to undergo premature impedance collapse in a significant number of beam generation experiments. The cause of this collapse is under active investigation. Besides power flow diagnostics (current and x-ray monitors), we are observing the anode-cathode (A-K) gap with optical diagnostics including high speed (<; 10 ns) framing cameras, optical streak cameras, and spectroscopy. Since the diode impedance decreases with the cathode diameter for a constant A-K gap, the increased current flow occurs within a smaller cathode cross-section, suggesting a possible evolving thermal instability in the A-K gap. We are planning changes to anode-cathode materials, as well as changes to the diode aspect ratio in an attempt to mitigate or eliminate the impedance collapse during the power pulse. Experiments are ongoing, and latest results will be reported.