Enhanced Microwave Emission Due to the Transverse Energy of a Relativistic Electron Beam

Abstract

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

Theoretical and Experimental Research on a Millimeter-Wavelength Free-Electron Laser

“…An interesting correlation is found between the electrons with large initial transverse velocities can support 2 cm band emission and the millimeter wavelength emission. stimulated emission as a free electron laser oscillator at millimeter wavelengths [2]. One would not expect Raman emis-II.…”

“…1' •-2 modified by optical guiding as it occurs in our overmoded waveguide system./ , Further studies of optical guiding, including new results on the effects of guiding on the sidebands, are to be published in Physical Review A this year, and represent a follow-on of the above research continued under further ONR and additional NSF sponsorship. [2]. Signal growth oc-Marx Bank and a four meter coaxial transmission line serving curs when an intense, relativistic electron beam, propagating as a pulse forming cavity.…”

“…Notice that both stimulated emission and absorption width of this FEL oscillator to be the spread of unstable fre-can occur with relatively small changes of order Wp in the beat quencies in the three-wave interaction, which, for our system, frequency wo. This fact imposes stringent conditions on variayields Awcow, -I percent (1], [2]. Decreased pump ampli-tions of -in time (ZA7), since wi is a sensitive function of 7. tude reduces the spread of unstable frequencies [6] and Note that it is the pump frequency which varies during the narrows the bandwidth, while feedback permits power to grow FEL pulse; the scattered frequency is established by ambient over many bounce times of the cavity radiation.…”

“…We adopt the standard L\ VoZ form for the helical wiggler field, neglecting any radial dependence [2]. Thus, we consider the equilibrium potentials lbo = 0 + go) …”

“…A 10 ki Marx generator lel (P.) and transverse (PI) to the guide magnetic field, the charges a 75 ns, 20 S1 coaxial pulse-forming line to a typical dispersion relation for the TEO, mode was found [2] propagates through a drift tube with ID 4.7 cm. The inter7m J + (R,)" yin 2 action region consists of a 60 period pulsed bifilar helical undulator with adiabatic transitions (field taper) at each end; * k(k + 1k.) -(2 (2) its transverse field is variable up to 1.3 kG and the spatial The field compression technique was used to generate a Bessel function and primes denote differentiation with respect beam with appreciable transverse motion.…”