Microwave radiation by a relativistic electron beam propagation through low-pressure air S. Jordan, A. Ben-Amar Baranga, G. Benford, D. Tzach,•> and K. Katob> Department of Physics, University of California, Irvine, California 92717 (Received 12 June 1984; accepted 13 August 1984) 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 wi~h 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. I. INTRODUCTION Though transport of intense electron beams is much studied, 1- 9 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. 10--15 II. EXPERIMENTS The experimental apparatus was detailed elsewhere' and it will be described here only briefly. 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 stain]es. steel drift tube (20 cm diameter, 150 cm length). The matched 7ll vacuum diode (connected to the Marx generator through a coaxial oil pulse-forming line) bas a graphite annular cathode (d = 6.5 cm, 11d = 1 cm). For low neutral gas (air) pressures, p < 20 mTorr, we used a foil-less configuration with a 1 mm flat graphite plate anode, that had a 7 .2 cm diameter hole in the center to match the cathode. For higher pressures we used a 25 µm titanium foil for the anode, this foil separated the vacuum diode from the gas-filled drift tube. The diagnostic probes and microwave horns entered the drift tube through several ports. We performed some shots at atmospheric pressure by firing the beam through an adjustable pressure prechamber and then into air. This prechamber was a stainless steel tube closed at the two ends by 25 µm titanium foil; one of these foils served at the same time as the anode. The microwave spectrum was analyzed by an array of bandpass filters for low frequencies,/< 12 GHz, and by two grating spectrometers for 12 </<.40 GHz. The signals for the detection diodes were recorded from fast oscilloscopes on Polaroid film. The resulting photographs were digitized and computer analyzed. The absolute power at each fre- quency was calculated and normalized to the area of the drift tube by using the calibrations for the detectors, variable at- tenuators, horns, transmitting lines, and spectrometers. Ministry of Defense, Box 2250, Haifa, Israel. b1Presentaddress:Gencra1DynamicsMZ401-IO, Box 2507, Pomona, Cali- fornia 91769. Figure 1 shows the calibrated and normalized micro- wave peak power versus neutral gas pressure for different frequencies. At very low pressures, p < 1 mTorr, there is a very slow decay in power with the pressure, especially at low frequencies such as 7 and 9 GHz. At pressures above 20 mTorr the decay is faster and we can see a linear behavior on the log-log graph, which means a power decay as (pres- sure)-1 for the low frequencies and a (pressure)- 2 decay at the higher ones. The radiation was under our measurable power threshold for pressures above a few Torrs. In between these two pressure domains there is a transition monotonic at low frequencies that peaks around 10 mTorr as the fre- quency goes higher. Qualitatively, we can explain this behavior by reflexing Phys. Fluids 28 (1 ), January 1985 :i PRESSURE •l Present address: mTorY FIG. 1. Emitted power versus p~urefor7, 9, and 21and39 GHz. © 1985 American Institute of Physics
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