The impedance characteristics of a rod-pinch diode

Summary form only given, as follows. The rod-pinch diode is being developed for use as an intense, high brightness, X-ray radiographic source. The diode geometry is co-axial, with an outer cathode conductor and high atomic number (typically gold or tantalum) inner anode conductor/rod. The beam electrons from the cathode strike the anode and produce bremsstrahlung radiation. Experiments at Sandia National Laboratories and the Naval Research Laboratory have demonstrated reliable diode operation at 1-2.5 MeV with 30-50 kA currents and a corresponding impedance of /spl sim/ 30-40 ohms. The rod-pinch is a bi-polar diode. At 30-50 kA, electrons are magnetically insulated and do not flow directly across the radial A-K gap. The current flows axially and attaches near the tip of the rod. Presently, the diode impedance is modeled by assuming the operating current to be a multiplicative factor of the critical current for insulation, I = aI/sub crit/. Here, ´a´ is an empirically determined scale factor, I/sub crit/ = 8500[(eV/mc2 +1)2 - 1]/ln(rc/ra) amps, V is the voltage, and rc (ra) is the cathode (anode) radius. Details of a new theory describing the operating impedance of the rod-pinch diode will be presented. In particular, the roles of electron backscatter and ion current will be discussed. The theory is an extension to those describing magnetically insulated transmission line operation. It assumes one dimensional laminar flow equilibria for the electrons and includes the effects of a background ion space-charge. Correspondence to the Icrit formula and a discussion of the scaling of ´a´ with voltage and ion current will be presented.