Abstract
Intense relativistic electron beams fired into air at varying pressures display a wide range of microwave signatures. These experiments held beam current, energy, and pulse length constant while varying gas pressure. Our observing window is 10 to 40 GHz. At low pressures (<10 mTorr) exponential spectra result, consistent with beam reflexing or virtual cathode oscillations. Above 20 mTorr the spectrum flattens and suggests collective emission at the beam-generated plasma frequencies. Power falls linearly with pressure above 20 mTorr, until electron-Neutral collisions damp the emission at a few Torr. However, weak 10 GHz emission appears at full atmospheric pressure.
Highlights
Though transport of intense electron beams is much studied,1-9 electromagnetic radiation is seldom measured
An approximately 50 nsec relativistic electron beam (REB) of0.8 MV and 120 kA, produced by a relativistic electron beam source,2 is fired into an evacuated stainless steel drift tube (20 cm diameter, 150 cm length)
We expect the movement of a charge cloud beyond the anode will be sensitive to boundary effects and, because the whole cloud must move, to yield radiation in the region of a few harmonics of v b, the beam plasma frequency is approximately 3 GHz
Summary
Though transport of intense electron beams is much studied, electromagnetic radiation is seldom measured. We made absolute power measurements at a wide range of air pressures, and here present tentative explanations of the observations, using ideas developed to explain emission from electron beams in plasma.
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