November Department of Physics, Plasma Fusion Center and Research Laboratory of Electronics. Massachusetts Institute of Cambridge, Massachusetts

Transcription

1 PFC/JA A 35 GH Cyclotron Autoresonance Maser (CARM) Amplifier G. Bekefi, A. DiRieno, C. Leibovitch and B.G. Danly November 1988 Department of Physics, Plasma Fusion Center and Research Laboratory of Electronics Massachusetts Institute of Cambridge, Massachusetts Technology 2139 USA This work was supported by the Air Force Office of Research and the Innovative Science and Technology the Strategic Defence Initiative Organiation. Scientific Office of Submitted for publication in: Applied Physics Letters.

2 A 35 GH CYCLOTRON AUTORESONANCE MASER (CARM) AMPLIFIER G. Bekefi, A. DiRieno, C. Leibovitch and B.G. Danly Department of Physics, Research Laboratory of Electronics and Plasma Fusion Center Massachusetts Institute of Technology Cambridge, Massachusetts 2139 Abstract Studies of a cyclotron autoresonance maser (CARM) are presented. The measurements are carried out at a frequency of 35 GH using a mildly relativistic electron beam (1.5 MeV, 26 A) generated by a field emission electron gun followed by an emittance selector that removes the outer, hot electrons. Perpendicular energy is imparted to the electrons by means of a bifilar helical wiggler. Superradiant measurements give a small signal gain of 9 db/m. Computer simulations are also presented. Introduction The cyclotron autoresonance maser (CARM) has been subjected to extensive theoretical [1],[2] studies and numerical simulations [3]-[5}. However, unlike the gyrotron and the free electron laser, its capabilities as a source of coherent millimeter wavelength radiation remain virtually untested in the laboratory. To the best of our knowledge, only CARM oscillator experiments [6],[7} have been reported in the literature. We present here what we believe to be the first, albeit preliminary, studies of a CARM amplifier. The emission from a CARM results through an interaction between the Doppler upshifted cyclotron wave on the electron beam 1

3 w = no/y + kjivi (1) and an electromagnetic waveguide mode W= kic + we; (2) w and kl are the frequency and axial wavenumber repectively; no = ebi/m is the nonrelativistic electron cyclotron frequency associated with an axial guide magnetic field Bii: -Y = [1 - (Vl/c)2 - (v±/c)2]-1/2 is the relativistic energy factor; and w, is the cutoff frequency of the waveguide mode in question. Maximum gain of the CARM instability occurs near phase velocity synchronism of the two waves. This yields the radiation frequency: ~- ± (c )2]1/2} (3) Here 81 = vil/c,-jj = (1 - # )1/2, and the positive sign refers to the sought after Doppler upshifted CARM mode of operation. Experiment A schematic of the CARM amplifier is shown in Fig. 1. The accelerator potential is supplied by a Marx generator (Physics International Pulserad 11 A) with a maximum capability of 1.5 MV and 2 ka. The electron beam is generated by a field emission gun composed of a hemispherical graphite cathode and conical anode, which also acts 2

4 as an emittance selector. The entire two meter long system is immersed in a uniform solenoidal magnetic field of 7 kg. The 26 A, 1.5 MV beam that issues from the emittance selector has a radius of.254 cm and a measured [8] normalied beam brightness equal to 2.4 x 14 A cm- 2rad-2. This corresponds to a normalied RMS emittance of 4.9 x 1-2 cm-rad and an RMS energy spread A-yl/yl ~.19. We note that considerably higher beam brightness is achieved by sacrificing current. When the radius of the the emittance selector is is reduced to.76 cm, the current drops to 8.4 A, but the brightness increases to 9.5 x 1 4 A cm-2rad-2. The corresponding emittance is now 4.5 x 1-3 and the energy spread An--/ ~.17. The aforementioned 26 A electron beam is injected into a bifilar helical wiggler which imparts perpendicular energy to the electrons. The wiggler has a periodicity of 7 cm and is six periods long. Within the first four periods, the wiggler magnetic field increases slowly and thereby provides an adiabatic input for the electron beam; the last two periods provide a uniform wiggler field with an amplitude on axis equal to 46 G. The resulting transverse electron velocity (v 1 ~.3vil) is estimated from witness plate observations of the beam dimensions. The downstream end of the wiggler is terminated abruptly by means of a metal shorting ring and the spinning electrons are allowed to drift into the 86 cm-long CARM interaction region where they are subjected to the uniform axial magnetic field only. We note that as a result of the wiggler excitation and the abrupt wiggler termination, the energy spread A'y / 1 of the electrons entering the CARM region can be considerably worse than the energy spreads quoted above in reference to electrons leaving the emittance selector. 3

5 The - 2m long,.787 cm radius evacuated drift tube acts as a cylindrical waveguide whose fundamental TE 11 mode has a cutoff frequency w,/27r = GH. Fig. 2 illustrates the time history of the voltage, current and radiation characteristics of the device. The CARM has been operated in the superradiant mode, in which the signal is allowed to grow out of background RF noise. At the output end of the CARM, a mica window transmits the circularly polaried radiation generated in the drift tube, where it is measured by means of standard calibrated crystal detectors, and its spectrum is analyed by means of a 98m-long dispersive line. Two spectral features are observed having differing tuning characteristics with axial magnetic field B 11. The frequency of the stronger spectral line increases linearly with B 11 as predicted by Eq. 1. The weaker line is rather insensitive to B 11. We believe that this line represents the second harmonic of the Doppler downshifted branch (see Eq. 3). In order to determine the growth rate of the wave, the output intensity is measured as a function of the length of the interaction region. This is accomplished by means of an axially movable horse-shoe "kicker" magnet that deflects the electron beam into the waveguide wall at any desired position, thereby terminating the interaction at that point. Fig. 3 shows how the RF power output measured at the far downstream end varies with the "kicker" magnet position. The slope of the curve yields a single pass gain of.9 db/cm, a value that is consistent with computer simulations. In Fig. 4 the predicted linear growth rate F and the efficiency at saturation are plotted as a function of the relative parallel energy spread. The results were obtained from a two-dimensional, self-consistent CARM amplifier code [5]. The experimentally measured growth rate is consistent with theory for a beam energy spread A-1-yl ~-.1. In the superradiant mode the amplifier does not reach saturation, so that the observed RF output power is 4

6 low (~ 1 kw). In conclusion, we have reported what we believe are the first CARM amplifier measurements. The superradiant measurements described here are being followed up by injection of a high power (6 kw) monochromatic, circularly polaried wave from a 35 GH magnetron driver (see Fig. 1b). We note that the Doppler downshifted mode can become absolutely unstable [9] and thereby cause serious deterioration of the CARM amplifier. However, calculations show [9] that this is of no concern in our parameter regime, since we operate well below the critical current (4.2 ka) and the critical magnetic field (B 1 = 11.4 kg) for onset of the absolute instability. Acknowledgments This work was supported by the Air Force Office of Scientific Research and the Innovative Science and Technology Office of the Strategic Defence Initiative Organiation. It was aided greatly by equipment on loan from the Lawrence Livermore National Laboratory. References [1] A.W. Fliflet, Int. J. Electron., 61, (1986). [2] V.L. Bratman, N.S. Ginburg, G.S. Nusinovich, M.I. Petelin and P.S. Strelkov, Int. J. Electron., 51, (1981). [3] K.D. Pendergast, B.G. Danly, R.J. Temkin, and J.S. Wurtele, submitted to IEEE Trans. Plasma Sci., April (1988). [4] A.T. Lin, Int. J. Electron., 57, (1984). [5] B.G.Danly, K.D. Pendergast, R.J. Temkin, and J.A. Davies, Proc. SPIE 873, (1988). 5

7 [6] I.E. Botvinnik, V.L. Bratman, A.B. Volkov, N.S. Ginburg, C.G. Denisov, B.D. Kol'chugin, M.M. Ofitserov, and M.I. Petelin, Pis'ma Zh. Eksp. Teor. Fi., 35, (1982). [7] I.E. Botvinnik, V.L. Bratman, A.B. Volkov, G.G. Denisov, B.D. Kol'chugin, and M.M. Ofitserov, Pis'ma Zh. Eksp. Teor. Fi., 8, (1982). [8] The technique is described by D. Prosnit and E.T. Scharlemann, Lawrence Livemore National Laboratory, ATA Note No. 229, Feb. 22 (1984). [9] J. A. Davies, MIT Plasma Fusion Report No. JA (1988). 6

8 Figure Captions Fig. 1 Schematic of the CARM amplifier experiment. Fig. 2 Oscilloscope traces of the electron beam voltage, current and the radiation intensity. Fig. 3 Radiation intensity at a frequency of 35 GH, as a function of the CARM interaction length. Fig. 4 Computed saturation efficiency and linear growth rate as a function of the electron beam energy spread. 7

9 WAVE LAUNCHER BIFILAR HELICAL WIGGLER CYLINDRICAL DRIFT TUBE TO MARX- ACCELERATOR FIELD EMISSION CATHODE ANODE AND EMITTANCE SELECTOR (a) I SOLENOID GRAPHITE CATHODE RF INPUT PORT I GRAPHITE ANODE ANDr4RPOE - RF POWER EMITTANCE SELECTOR C3Q DRIFT TUBE RF TUNER RF INPUT PORT 2 (b) Fig.1. Bekefi, DiRieno Leibovitch,Danly

10 I I I I I I I I I 1..5 w I I I I I I I I I I I I I I I I I I -25 w cr o I I I I I I I I I I I I I I I I I I r TIME (ns) Fig.2. Bekefi,DiRieno, Leibovitch,Danly

11 I - I I F- H 1 w H I - w 1-4 F~'91 db/m cr 1-5 I I INTERACTION LENGTH Z (cm) I I Fig.3. Bekefi.DiRieno Leibovitch,Danly

12 2 15 I,I E m C Ld C) Ul- U- LuJ 1 - as r 1. ~ F- 'I.5 ENERGY SPREAD 'I.1 (An, ).15 Fig.4. Bekefi,DiRieno Leibovitch,Danly