Impedance reduction mechanisms in a magnetically immersed electron diode

Summary form only given. The successful operation of a magnetically-immersed electron diode using inductive-voltage-adder (IVA) technology for radiography requires a stable impedance for >30 ns. In an IVA, the voltages from many acceleration gaps are added in series along a magnetically-insulated transmission line. The final voltage is applied to a thin needle that is immersed in a 10-50 Tesla solenoidal magnetic field. An electron beam is produced in a small spot at the anode target (<2 mm) with high current (30-50 kA) and energy (>10 MV). The electron current flowing off the needle is determined by the space-charge limited flow in a long pipe. Ion-emitting plasmas are produced from direct beam heating of the target and from radiation emitted from the several eV target surface that heats the outer walls of the anode (1-4.5-cm radius). Plasma ions are attracted to the electron beam and provide a degree of charge neutralization. The presence of these neutralizing ions reduces the diode impedance (nominally 300 Ohms), enhancing the electron current. We are studying these impedance reduction mechanisms with the hybrid simulation code IPROP.