Microwave emission and beam propagation measurements in a high-power relativistic electron beam-plasma system

Microwave emission was measured from a system consisting of an unmagnetized plasma and a propagating electron beam. A 93-cm/sup 2/ velvet cathode, with an anode-cathode gap of 5.9 cm, injects the electron current into the plasma through an aluminized Mylar anode. Measurements were made of the diode voltage and current in the 6- mu V water dielectric accelerator and net current through the beam-plasma system. The unmagnetized plasma is produced by a 90- mu s, 90-AA current pulse, emitted from a thermionic LaB/sub 6/ electron source, that preionizes argon fill in a 1-m-long, 15-cm-diameter Lucite tube. A microwave spectrometer detects the radio-frequency output in the 2-18, 18-26, and 26-47 GHz bands, filters, and then separates into narrower subbands. The emission takes place in two distinct phases. The 2-GHz output rises promptly with the current pulse and then decays. At 6-GHz and above, a low-level microwave prepulse appears simultaneously with the 2-6 GHz output. This output rises sharply 25 ns after the current pulse begins and includes frequencies out to and beyond 40 GHz. The radio-frequency output falls off before the current pulse ends. The microwave intensity decays monotonically with frequency.<
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