Characterization of the time-evolution of a microsecond electron beam diode with anode effects

Summary form only. The time evolution of a low-aspect-ratio, microsecond-pulse-length, electron beam diode with anode plasmas, operating at initial current densities of approximately=100 A/cm/sup 2/, has been studied using a variety of experimental and empirical techniques. Of particular interest in this study was the observation of a transition to a roughly constant impedance regime. Very rapid cathode plasma closure velocities from 8-11 cm/s were observed. This expansion led to rapid self-filling of the diode, principally on-axis, causing dense plasma formation from the anode and resulting in the observed impedance characteristics. Due to the low-aspect-ratio diode design, many kiloamperes of electron current were still emitted, which interacted with the plasma that filled the diode from both the cathode and anode. Before self-filling, a low-effective-charge cathode plasma was observed, consisting almost exclusively of components derived from the hydrocarbon surface impurities. Evidence for cathode plasma instabilities was also noted. After self-filling, evidence for a beam-plasma interaction leading to RF emission, beam filamentation, and collective effects was suggested.<>