Long-pulse experiment of circular free electron laser

NUCLEAR INSTRUMENTS METHODS IN PHYSICS RESEARCH

Nuclear Instruments and Methods in Physics Research A318 (1992) 749-753 North-Holland

Section A

Long-pulse experiment of circular free electron laser Takahide Mizuno a, Hitoshi Sekita and Tadashi Sekiguchi d

" Faculty of Engineering, Yokohama National h Institute of Space and Astronautical Science,

b,

Kaoru Takagi

d,

Tsutomu Ohshima b, Hirobumi Saito

b

University, 156 Tokiwadai, Hodogaya, Yokohama, Kanagawa, Japan 3-1-1 Yoshinodai, Sagamil:ara, Kanagawa, Japan

Generation of microwave radiation from a relativistic rotating electron beam in a circular wiggler magnetic field (circular free electron laser) has been studied. The beam energy was 280-420 keV, typically, and the axial component of the rotating current was 80 A. Tunable radiation frequency was observed with the grating spectrometer to be typically 11-14 GHz and 36 GHz. This device produced total radiation powers of 115 kW at 12 GHz and 6 kW at 36 GHz. l. Introduction

A novel circular version of a free electron laser, which was proposed by Bekefi (1), has been explored theoretically [2-7]. It has the advantage of being very compact because of the use of a rotating relativistic electron beam. Several other potential advantages of a circular FEL as compared with a conventional linear version of FELs have been reported [5]; for example, lager growth rate, larger saturation efficiency and wider band width for an amplifier. However, few experimental studies [8-13] have been reported . In this device, relativistic electrons are subject to cyclotron motion in a uniform magnetic field. The motion of electrons is modified by an aximuthally periodic wiggler magnetic field, which is generated by an assembly of magnets embedded behind two conducting Rotating electron beam

metal cylinders (fig. 1). The circular rotating beam is generated with a cusped magnetic field. The eV_ x B, Lorentz force acts on a hollow, axially moving electron beam in the cusp region, and converts the axial beam velocity into an azimuthal rotating velocity . As a result the electron spirals in the guide magnetic field downstream of the cusp. The perturbed electron beam interacts with the TM mode electromagnetic wave in the circular wiggler which also acts as a coaxial waveguide. Our previous study [13] reported that the circular FEL with wiggler periodicity N = 24 and wiggler magnetic field Bw = 200 G generated the microwave radiation of 12.7 GHz. In the present study, a circular FEL with larger wiggler magnetic field (N= 12 and BW = 900 G) is tested, and higher radiation power is detected . In section 2, we outline the experimental configuration and describe the rotating electron beam properties . In section 3, a radiation experiment in the superradiant operation is reported including frequency, total power and spatial growth measurement of the radiation microwave. In section 4, we summarize the conclusion of this study. 2. Experimental apparatus 2.1 . Accelerator

Fig. 1. Schematic configuration of the circular free electron laser.

The experimental configuration is shown in fig. 2. The REB diode is energized directly by a Marx generator without a pulse-forming line for a long-pulse operation . Fig. 3a shows the history of the diode voltage. The initial diode voltage was typically 280-420 kV and decayed with a time constant of about 8 p,s. We could

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IX . UNCONVENTIONAL SCHEMES

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T. Mizuno et al. / Long-pulse experirnent of circular FEL

sweep the acceleration voltage in one shot of the discharge. The velvet cloth was used for the cathode emitter since it emitted the brightest electron beam, among the several types of emission cathodes. The inner radius of the hollow velvet cloth was 6() mm and outer radius was 65 mm. Surface flashover along the fibers may play a role in cathode plasma production .

(a) Diode voltage

1200kV/div]

(b)

Axial component of beam current [40A/div]

(c)

Ku band radiation [SOOmV/div]

(d)

Ka band radiation [lOOmV/div]

2 Rotating beatit generation

Cusp and guide magnetic fields are formed with two series of solenoid coils which are installed upstream and downstream of the soft iron, as shown in fig. 2. The cusp field is narrowed by the soft iron. Three pairs of small correction coils are installed near the slit at the cusp region . They reduce the cusp width in order to avoid ripple motion of the electron beam and to increase the current which can pass the slit in the soft iron plate [141. '%Vhen the electron passes through the cusp region, the electron experiences the e,l_ x Br Lorentz force which converts a part of the axial velocity into azimuthal velocity . Downstream of the cusp, the helical motion of the electron is supported by the eVt, x B_ Lorentz force, where B__ is the axial guide field. The velocity ratio, a, of the rotating electron beam is defined as a =1 Here V is the axial component of the electron velocity . The velocity ratio, a, is also expressed as - 1) - 1] a= [(mc1er,B_)2(-y2 t,

-ti=

.

(1)

Here rt,, -e, nt, c and y are the Larmor radius of the rotating motion, the electron charge, the electron rest mass, the light speed and the Lorentz factor, respec-

Time [ l.Otts/div]

Fig. 3. Time history of (a) accelerating voltage, (b) axial component of the beam current. (c) output of the Ku band detector and (d) output of the Ka band detector. 0.(138 T. B_,, = 0.037 T. B = 0.09 T. and N = 12. -

tively . Measured a values agree with eq. (1) in the range up to 5. As the voltage decreases (fig . 3a), the electron spiral motion becomes steep (large a = 1f,lV_) and then the beam current is cut off at the cusp field. Fig. 3b shows the time history of the axial component of the rotating beam. indicating the current cutoff due to the cusp field. An asymmetric cusp is used to cancel out the selfmagnetic field due to the electron beam in the upstream region . The ratio. K, of the upstream guide field B_  and downstream guide field B_a is optimized Spectrum analyzer

Fig. 2. Schematic diagram of the experimental setup.

T. Mizuno et al. / Longpulse experiment of circular FEL

75 1

is 12 . The magnetic configuration is the Halbach-type for N = 12 . The wiggler magnetic field amplitude at the center of the coaxial gap is 900 G which gives the normalized vector potential aw = e8wl CmcN = 0.3 . Fig. 4b shows the cross-sectional photograph of the rotating electron beam with the wiggler field. It is observed that the rotating beam is modified by the wiggler field. 3. Radiation generation In the circular FEL the wiggling velocity in the axial direction an couple with the axial electric field of the TM radiation mode . This interaction is expected at the crossing point of the beam mode and the TM waveguide mode in the dispersion relation . The bunched electron beam supports a "synchronous mode" described by

w=(p+N)D~/y+kzV,

(2)

where Dc = e&lrn, p, k- and V_ are the cyclotron frequency, the harmonic number in the azimuthal direction, the axial wave number, and the axial component of the electron velocity, respectively . The waveguide mode is expressed as

w, = (0 à p, q) +c-k2 (3)

Fig. 4. Cross-sectional photograph of the rotating electron beam (electron energy 344 keV, N = 12) (a) at the entrance of the wiggle and (b) at the end of the wiggler (N = 12). experimentally, and selected to be 1 .05, with K defined as K = Bz /B_d. The axial component of the rotating beam current injected into the wiggler is 80 A and a current of 40 A passes through the wiggler (fig. 3b). The rotating current density is measured with Bz pickup coil to be 560 A/m (per unit axial direction) before the wiggler. The cross-sectional photograph of the rGtating electron beam is shown in fig. 4a . 2.3. Circular wiggler field The circular wiggler field is generated by an assembly of Nd-B-Fe permanent magnets (fig . 1), whose remanence is 1 .2 T. The number of wiggler periods, N,

where q is the radial harmonic number and wj p, q) is the cutoff frequency of the TM(p, q) mode calculated with Bessel functions. Both modes are synchronized at the crossing point in the dispersion relation. For electron energy of 344 keV and the guide field B_  = 0.038 T, B-d = 0.037 T (a > 4), we observed the radiation with Ku and Ka band crystal dtectors . The dispersion relation is shown in fig. 5, assuming axial wavenumber k z = 0 in eqs. (2) and (3). From this dispersion relation, we estimate that electron synchronous mode can couple to TM(6, 1) mode with

N

Lu u

c

L b~r

Harmonic number p

Fig. 5. Dispersion relation between waveguide TM modes and synchronous wave supported by the rotating electron beam . Electron energy 344 keV, Bz = 0.037 T, a = 70 mm, b = 55 mm, N =12 and k z = 0. IX . UNCONVENTIONAL SCHEMES

T. Mizuno et aL / Long-pulse experiment ofcircular FEL

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Table 1 Experimental configuration and results

80 .0 344 80 10 0.038/0.037 0.09 12 70 55

Electron energy [keV] Axial beam current 1, (A] Divergence angle 60 [mrad] Guide field B_  /B-d [T] Wiggler field B,,, [T] Wiggler periodicity N Waveguide outer radius a [mm] Waveguide inner radius b [mm]

Radiation frequency [GHzJ Radiation power [kWJ Efficiency

Lower mode

Higher mode

12 115 0.42

36 6 0.02

60 .0 ü û 40 .0 C

40 C

e

0 .00 ' 1 .0

' 1 .2

' 1 .4 1 .6 1 .8 2.0 E ectron energy y

2 .2

2 .4

Fig. 7 . Voltage tuni g of the radiation frequency .

frequency 11 GHz, and TM(37. 1) mode with frequency 30 GHz. The waveforms of the Ku and Ka band detector output are shown in fig. 3c and 3d. In fig. 3 we can see that, radiations of both bands, Ku and Ka, were generated simultaneously just before cutting off the beam current due to the cusped magnetic field and their pulse width was - 300 ns. We measured this radiation frequency using a grating spectrometer which is shown in fig. 2. As a result, we found that the_ lower made (Ku band) frequency was 12 GHz and the higher mode (Ka band) frequency was 36 GHz. The experimental configuration and results are summarized in table 1. The total power was measured by integrating angular distribution of the radiation over the solid angle at the distance of 1 m away from the end of the coaxial waveguide with the wiggler. The total power was measured to be 115 kW in the lower mode (12 GHz) and 6 kW in the higher mode (36 GHz). Thus,

the efficiency was 0.42% in the lower-mode operation and 0.02% in the higher-mode operation. The spatial growth of the radiation power in the axial direction was measured and the result is shown in fig. 6. An acrylic plate was inserted in the wiggler region in order to obstruct the interaction of electrons with the electromagnetic wave . The role of the acrylic plate is the same as the kicker magnet . Fig. 6 suggests that the radiation power reached saturation after 10 cm from entrance of the wiggler, and then the power decreased after the saturation point. The power level reached saturation when electrons experienced about 15 wiggler periods. The total power at the saturation point was expected to be higher than 115 kW which is measured for the full wiggler length (20 cm). We varied the electron energy from 280 to 420 keV, as shown in fig. 7. As a result the radiation frequency was changed between 11 and 14 GHz in the lower mode. These results almost agree with theoretically predicted values. 4. Conclusion

E

0

c.

0 L O w

20 .0

10 2

0

4

Axial

8

12

distance

[cm]

16

20

Fig . 6 . Spatial growth of the rf power in the axial direction . Electron energy 344 keV, B Z  = 0 .038 T, B ed = 0 .037 T, N = 12 and B,, = 0 .09 T.

We observed superradiant radiation at 12 GHz with 115 kW power and at 36 GHz which 6 kW from a circular free electron laser with strong wiggler pump aW/y = 0.16, using the rotating electron beam whose energy and axial component of the beam current were 344 keV and 30 A, respectively. The energy conversion efficiency was found to be 0.42% in the lower-mode operation (12 GHz), and 0.02% in the higher-mode operation (36 GHz). The spatial measurement of the radiation indicates that the power level reached saturation and then decreased. In the lower mode, the radiation frequency was varied between 11-14 GHz adjusting the electron energy .

T. Mizuno et al. / Longpulse experiment of circular FEL

References [11 G. Bekefi, Appl . Phys . Lett . 40 (1982) 578. [2] R.C. Davidson, W.A . McMullin and K. Tsang, Phys . Fluids 27 (1984) 233. [3] C.L. Chang, E. Ott, T.M . Antonsen and A.T. Drobot, Phys . Fluids 27 (1984) 2937. [4] Y.Z. Yin and G. Bekefi, Phys. Fluids 28 (1985) 1186. [5] H. Saito and J. Wurtele, Phys. Fluids 30 (1987) 2209 . [6] Y.Z . Yin, R.J . Ying and G. Bekefi, IEEE Trans. J. Quantum Electron . QE-23 (1987) 1610. [7] Y. Kawai, H. Saito and J.S . Wurtele, Phys. Fluids B3 (1991) 1485. [8] G. Bekefi, R.E . Shefer and W.W. Destler, Appl . Phys . Lett . 44 (1984) 280 .

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[9] F. Hartemann, G. Bekefi and R.E. Shefer, IEEE Trans. Plasma Sci. PS-13 (1985) 484. [10] W.W. Desgler, F.M . Aghamir, D.A. Boyd, G. Bekefi, R.E . Shefer and Y.Z. Yin, Phys . Fluids 28 (1985) 1962 . [11] E. Chojnacki and W.W. Destler, IEEE J. Quantum Electron . QE-23 (1987) 1605 . [121 F. Hartemann and G. Bekefi, Phys . Fluids 30 (1987) 3283. [13] H. Sekita, T. Mizuno, H. Ohta, M. Kitora and H. Saito, Nucl. Instr. and Meth . A304 (1991) 137. [14] T. Mizuno, H. Sekita, H. Saito and T. Sekiguchi, Jpn. J. Appl . Phys . 30 (1991) 1128.

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