Effect of plasma dynamics on impedance behavior in magnetically immersed diodes

Summary form only given. The magnetically immersed diode is being developed as an intense electron beam source for high power X-ray radiography. Electrons are emitted from a needle cathode of 0.5-5 mm diameter and confined in a 10-50 Tesla solenoidal field. These electrons, striking a high atomic number anode converter, can potentially deliver a high brightness source of X-rays for flashradiography. In recent experiments on the 150-kA, 4-MV RITS-3 accelerator at Sandia National Laboratories, using <4-mm radius needles these diodes exhibit anomalously fast gap closure velocities that limit the X-ray pulse length. In typical pulsed-power diodes, electrode plasma velocities are of order 1-3 cm//spl mu/s. For an immersed diode with a small cathode radius, the effective speed associated with the rate of impedance loss is roughly 600 cm//spl mu/s-consistent with that of 2-4 MeV, 10-20 amu ions. The hybrid particle-in-cell code LSP is used to study the evolution of both cathode and anode plasmas for the RITS-3 electrical input and different cathode configurations. In particular, we examine the relative effects of charge exchange acceleration of anode neutrals, the effect of ion-ion in-flight stripping and cathode plasma motion. Mitigation techniques are also studied.