REB focusing in high aspect ratio diodes

Typical current-voltage “trajectories” in pinched electron beam diodes have been characterized by a sequence of phases beginning with cathode plasma formation and expansion, a Langmuir-Childs phase, transition to and operation in a focussed beam made (where the cathode plasma is contained in the B_θ fields and forced back toward the cathode), and a final impedance collapse phase [1]. Recent experiments with tapered hollow cathodes have addressed the transition phase and indicate that formation of an anode plasma is required (in addition to the current exceeding the Friedlander and Jory critical current, I_c) before the impedance deviates from a Langmuir-Childs trajectory. Time correlated measurements of beam pinch collapse initiation, proton beam generation, and pinch arrival on axis are given. Once the pinch is formed the impedance is found to be within ±15 percent of the saturated parapotential value for the remainder of the driving pulse. Comparison with previous data indicates that a 300 to 450 J/gm dose is required to form the anode plasma (brass and copper anodes) and suggests that ≈ simultaneous achievement of I ≈ I_c and anode plasma formation produces a longer-lived, more stable pinched flow phase.