Modeling of impedance collapse in high-voltage diodes

Electron-beam diodes driven by fast-rising, high-voltage pulses often opera te with cold cathodes for which the presence of a plasma adjacent to the ca thode surface is essential to obtain adequate electron emission. A conseque nce of such surface plasma, however, is closure of the interelectrode gap b y plasma motion, Resistive heating of the plasma competes with work perform ed in expanding the plasma and heat transfer to the cold-cathode boundary, The resulting closure speed is calculated, using an MHD code, and found to agree web with results of experiments using organic-cloth Cathodes at 35 kV , Computed plasma speeds are typically 8-12 km/s, and are relatively insens itive to the applied voltage. Gap closure due to the plasma motion calculat ed numerically corresponds to estimates based on impedance collapse in the experiments.