We present data obtained during tests of the Beam-Emitting Rocket Test (BERT) payload in the Johnson Space Center's Chamber A. During the tests an electron gun on the main payload emitted beams of 0.5, 1, and 2 keV, which stepped through currents of 0.1–20 mA. Simultaneously, an instrumented nose cone, separate from the main payload, measured electric field fluctuations from DC to 5.5 MHz. Abrupt changes in HF wave activity and vehicle potential occurred during 0.5- and 1.0-keV gun operations. Whenever beam current exceeded a threshold level, wave emissions above the chamber electron gyrofrequency, ƒce, were dramatically enhanced and emissions below ƒce were disrupted. At the same time the potential on the electrically isolated BERT payload, which steadily increased with beam current, suddenly ceased to increase and often decreased. The threshold current, Ic, is proportional to the three-halves power of beam energy, ε, as in beam plasma discharge. However, other facts indicate we are observing a different phenomenon. We propose that the Ic ∝ ε3/2 relation arises from the space charge limitation of current flow from the payload to the chamber wall. The sudden change in the payload potential/beam current relationship results when the beam is partially reflected toward BERT by a virtual cathode which forms at the threshold current. The appearance of HF waves can be attributed to electron density enhancement in this virtual cathode or beam stagnation region. A simple one-dimensional, analytical model is presented to explain the Ic ∝ ε3/2 proportionality and the generation of HF waves.
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