There has been considerable interest recently in the production of high-power microwaves by the pulsed intense relativistic electron beams which have become available in the last few years. 1 * 2 Experimental observations of high-power microwaves have been made in a variety of beam configurations, including beams injected into a few hundred milliTorr of neutral gas, 3 and magnetically focused annular beams propagating in vacuum (<10" 3 Torr) in the presence of special metal boundaries 4 ' 5 and magnetic field perturbations. 6 Theoretical models have been developed, and calculations have been carried out to try to explain the observations, 7 " 10 but they have been hampered by an incomplete knowledge of the properties of the electron beam. In the present study, we control the transverse energy of beam electrons by varying the static spatial magnetic compression to which the beam, propagating in a straight, metallic drift-tube wave guide, is subjected. We find that if this transverse energy exceeds a certain minimum value, microwave power in excess of that possible by a single-particle mechanism is obtained. Moreover, we find this to be a characteristic of microwave production using the perturbed magnetic-field configuration. A theoret-7 R. to be published, for even stronger cluster properties in the low-fugacity region.ical model based on an interaction between the e observed wave-guide mode and the electron beam gives unstable waves which agree well with observations.The experimental configuration used for the present study is shown schematically in Fig. 1. The electron beam is produced by applying a highvoltage pulse from a 7-12, 50-nsec pulse-forming line to a foilless diode. 11 The diode voltage and current are 350-650 kV and 10-25 kA, respectively. The beam propagates in a 4.7-cm-i.d., thin-walled, stainless-steel drift tube immersed in a quasistatic (10-msec risetime) magnetic I field applied coaxially to the drift tube by a 22-cmdiam, 1-m-long solenoid. By varying the distance d in Fig. 1 between the cathode and the end of the solenoid from -2 cm (i.e., cathode 2 cm inside the coil) to 18 cm, the magnetic field near the cathode relative to that in the middle of the coil is varied from -0.55 to -0.1. Thus, the distance d controls the magnitude of the radial component of the magnetic field near the cathode. The interaction of the beam electrons with the radial field produces the desired transverse energy. Lucite witness plates obtained in the middle of the solenoid for d --2 and 1 cm are shown on the right-The role of the transverse energy of a magnetically focused intense relativistic electron beam in the emission of microwaves is investigated experimentally and theoretic ally " 752
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