Nuclear Instruments and Methods in Physics Research A272 (1988) 147-149North-Holland, Amsterdam

A MICROWAVE FEL AMPLIFIER EXPERIMENT

SU Yi, HUANG Sunren, CHEN Yutao, SHEN Yuanfa, FU Shizhen, LIU Xishan, LI Zhenrongand HU KesongSouthwest Institute of Applied Electronics P O Box 527, Chengdu, Sichuan, PR China

An extremely high power gain true free-electron-laser-amplifier experiment on a pulselme accelerator is described, in which anintense electron beam (700 kV, 500 A) with low axial velocity spread (dß~l~ß1l ~ 0.2%) is used and a narrow band 34.5 GHzmicrowave signal is coherently amplified to several megawatts. An extended interaction oscillator (EIO) is used as the microwavesignal source. The signals are sent into the FEL interaction tube through a quasi-optical cavity . Specific efforts are made to optimizethe experimental parameters in order to obtain higher power gam.

1. Introduction

The free electron laser has become a very importantsubject of intensive research because of its potential tolead to extremely high-power, efficient sources of con-tinuously tunable coherent radiation. In recent years,many Raman-type free electron laser experiments usingIREB accelerators have been reported . We previouslyreported superradiant FEL experiments [1,21 . In thispaper, we report a high gain microwave FEL-amplifierexperiment employing a 700 keV, 500 A solid electronbeam (only 100 A left behind the quasi-optical cavity), a3.45 cm period helical wiggler, and an axial guidingmagnetic field. A tens of watt narrow-band 34 .5 GHzinput signal is coherently amplified to several mega-watts. The total gain is about 47 dB . Output power isdirectly proportional to input power. Saturation doesnot appear .

INPUT SIGNALFROM EIO

SOLENOIDOLENOI

QUASI-OPTICALCAVITY WIGGLER

0168-9002/88/$03 .50 © Elsevier Science Publishers B.V .(North-Holland Physics Publishing Division)

DETECTOR

2. Experiment apparatus

Fig. 1 . Experimental system for FEL amplifier.

The experimental setup for free electron laser ampli-fier configuration is illustrated in fig. 1 . The intenserelativistic electron beam is generated by the EPA-74pulseline accelerator, which uses a field-distortion switchto transfer energy to the diode in order to get goodwaveforms of voltage and current. The voltage of diodecan be tuned from 400 kV to 700 kV . An aperture in theshaped graphite anode is used to collimate the electronbeam field - emitted from a cylindrical graphite cathodewith hemispherical tip . The 4 mm diameter electronbeam (100 A, 50 ns) with low axial velocity spread(Aß111ß1, = 0.2%) is injected into a 16 mm i.d . stainlesssteel drift tube which also serves as the cylindricalwaveguide.

The up to 16 kG axial magnetic field, B, . is pro-duced by a solenoid winding. This magnetic field is

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applied to confine the electron beam and to providegyroresonant enhancement of the effects of the wigglermagnetic field . The wiggler field is a circularly polarizedmagnetic field with a period X,, = 3.45 cm and gener-ated by a 1 m long bifilar helix wound on a nylonframwork . The amplitude of the wiggler field, Br, onaxis of the drift tube is variable up to 1 .5 kG . Toprovide an adiabatic entry of the electron beam into thewiggler, the entrance wiggler field is tailored by flaringthe helix outward over 7 periods. A 5 period adiabaticexit from the wiggler is provided . The pump field isuniform over 17 period long interaction region.

An extended interaction oscillator (EIO) operatingat 34 .5 GHz with an output power of 60 W and pulselength of 10 lts is used as the microwave signal source.The input signal is infected into the FEL interactiontube in a vertically polarized TE11 mode through aquasi-optical cavity coupler without perturbing the elec-tron beam . The input signal can be varied using anattenuator. Measurements show that the insertion loss is3 dB, loss through the drift tube is 6 dB .

The amplified microwave emission is propagatedinto the laboratory via a calibrated 16 dB microwavehorn . The microwave signal is sampled by a 20 dB gainKa-band horn located at 1 m from the output horn andconducted through 18 m of BJ320 standard waveguideinto the screen room. Calibrated crystal detectors areused for radiation diagnostics . A 62 m long Ka bandwaveguide dispersive line and an adjustable bandpassfilter centered at 34 .5 GHz are used to obtain spectralinformation . There are a 20 dB directional coupler anda variable attenuator before the bandpass filter.

3. Experimental results and discussion

For this FEL amplifier experiment, the product k.r bis less than 0.4, where kw = 27r/X �� rb is the beamradius . The electrons are approximately subjected to anessentially pure circularly polarized wiggler field. Theone-dimensional and the three-dimensional orbit theorycan be used to calculate the electron trajectory . Butboth these theories treat the electron trajectory asaxicentered orbits . This is not true for a finite-radiuselectron beam in which the electrons excute approxi-mately helical trajectories about their initial radii, ratherthan about the axis of the wiggler [3] . For this reason,we use the guide-center approximation [41 to calculateelectron trajectories in the combined helical wiggler andaxial guiding field . The parallel and perpendicular com-ponents of the velocity of electrons on an average over awiggler period are given byV

-

V(S2r+S2B)

(1)2(S2o- kwV, )

Su Yi et al /A microwave FEL amplifier experiment

aZ

.5

1 = 2 .34

Br(0) = .8kG

Ao = nc/kwcFig 2. Graph of the average axial velocity vs the axial guide

field .

where Slo, r. e = eB~o,r,e/Ymc, Br and BB are the ampli-tude of the radial and azimuthal components of thewiggler field at the position of the electrons . They stillgive rise to two separate classes of orbits . Fig. 2 showsthe "steady-state" orbits for typical experimentalparameters, and the specified wiggler field, Br and Be,employed in the calculations are an average over thebeam radius .

To obtain large gain and high efficiency, it is decidedto operate in the vicinity of the gyroresonance and onthe group II orbits with the coupling parameter 0 < 0.Previous experiments [1,2] had demonstrated that thisapproximation was in close agreement with experimen-tal results . The amplitudes of wiggler and axial guidingfield are chosen to produce the perpendicular velocityß1 of order 0.3 at a reasonable separation fromgyroresonance, which had proved desirable in the super-radiant experiments [2,5], and then experimentally opti-mized for gain and output power at 34 .5 GHz.

When the injected signal is 30 W at 34 .5 GHz,operating at 700 keV, 100 A, B~ = 8.1 kG, Br = 0.8 kG,40 ns, 1 .5 MW microwave radiation at the samefrequency has been observed. This represents a totalgain of 47 dB and 2% efficiency of conversion . At thesame condition, but in absence of input signal, measure-ment performed by 62 m dispersive line shows the 3MW superradiant emission at 37 .5 GHz. Meanwhile,measurement using the bandpass filter centered at 34.5GHz indicates the superradiant emission through thisfilter is only 3 kW. Radiation at the Doppler shiftedcyclotron frequency does not appear. As shown in fig. 3,the intersection of waveguide dispersion relation withthe Doppler-shifted cyclotron mode is much higher thanFEL resonant frequency.

When the infected signal varied from 3 W to 30 W,the output power is a linear function of the input

3

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O

3

kc( 10 10 sec-1 )

Fig. 3 . Dispersion curves for FEL and cyclotron maser.

107

105

i

/1-4x106 ( W) -

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.

.L

A

I

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1

I

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I

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Pin( W )

Fig. 4. FEL output power vs input power.

power, as shown in fig . 4. This is an important evidenceof the amplifier mode . Obviously, saturation does notappear . If higher power signal could be injected, higheroutput power and higher efficiency should be obtained.

The effect of axial field end taper near the end of the

Su Yi et al / A microwave FEL amplifier experiment

uniform wiggler interaction region is observed again.The effect enhances output power and has been notedin previous superradiant experiments [1,2,6] .

4. Conclusion

A microwave FEL amplifier experiment using anintense electron beam has been carried out. More than1.4 MW radiation at 34 .5 GHz has been observed at aninfected signal level of 30 W. The output power isdirectly proportional to the input power. Saturation hasnot appeared . We are now studing the gain of FEL as afunction of system parameters and preparing to increasethe input power to obtain higher amplified radiationand higher efficiency .

Acknowledgements

The authors wish to acknowledge Prof. TaoZhuchung, Prof. Zhao Weijin for useful consultationand support. We also would like to thank Prof. LinChungwen, M.S . He Yiping and M.S . Wang Qinyuanfor the assistance with the microwave input coupler,and M.S . Wang Pingshan and M.S . Zhang Zhong fortheir experimental assistance .

References

Su Yi, Huang Sunren, Chen Yutao, Liu Xishan, ShenYuanfa, Fu Shuzhen, Li Zhenrong and Hu Kesong, Asuperradiant FEL with an annular electron beam, to bepublished.

[2] Su Yi et al., Experimental investigation on Raman-typefree electron laser, to be published.H.P. Freund and A.K . Ganguly, IEEE J. Quantum Elec-tron. QE-21, No . 7 (1985) 1073 .

[4] Su Yi and Wang Pingshan, Electron orbits of annularelectron beam in helical wiggler and axial guide field, to bepublished.J.A . Pasour and S.H . Gold, IEEE J. Quantum Electron.QE-21, No . 7 (1985) 845.S.H . Gold, W.M. Black, H.P . Freund, V.L . Granatstein,R.H . Jackson, P.C . Efthimion and A.K. Kinkead, Phys .Fluids 26 (1983) 2683 .

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