JAERI-Conf 95-021

PA13
DEVELOPMENT OF A COMPACT LINAC FOR FEL OSCILLATION
Tsuyoshi HAGA, Kunihiko Tsumori, Tsuyoshi Shinzato, Katsuji EMURA and Hioshi TAKADA
Sumitomo Electric Industries, Ltd. Harima Research Laboratories
1431-12, Harima Science Garden City, Kamigori, Hyogo, 678-12 Japan
Abstract

2-1. Long macropulse duration

We developed a compact linac as both an injector forsynchrotron radiation (SR) ring and an electron source forfree electron laser (FEL). The linac has a high accelerating
gradient of 22MeV/m and is very compact. Recently, we
proceeded with studies on FEL using this linac. The beam
acceleration of lOJ.ls macropulse duration at SOMe V wasachieved and energy spread and peak current were 0.6% and28A, respectively.

When we discussed the macropulse duration of the electron
beam for FEL experiments, the results of FELIX [4] wasreferred. As the intensity of FEL saturates after 3J.1s or sofrom the pulse rising of the beam, the macropulse duration ofmore than 10J.1s is necessary for the infra-red FELoscillation.
A klystron used in our linac is the TH 2146 by Thomson
Tubes Electroniques. It is capable of producing the twodifferent microwaves of 1 J.1S duration (45MW) to inject theSR-ring and of lOJ.ls (22.5MW) for FEL. The mode of
SR/FEL can be changed automatically.

1. Introduction
Recently, it gets much attention that synchrotron SR
light has potential for the industrial applications, thus acompact SR machine has been awaited. We have continued
our R&D efforts to develop compact SR machines. The SR
machine consists of a linac and an SR ring. Both of themrequire compact design. We first launched the development ofa compact SR ring (NIJI-III) using superconducting
magnets. It has already completed in 1991.
For the linac, we completed the development of a compact
linac by ourselves in 1993 (Fig.1) and achieved the electron
beam acceleration at 100MeV-100mA in July 1994. The
compact linac has two special features. One is a highaccelerating gradient (22MeV /m) and another one is the twomode accelerating (SR and FEL mode).
Recently, we proceeded with studies on FEL using ourlinac[1 1[2]. On this paper, the beam characteristics of ourlinac and the experimental results of FEL are described.
2. Compact linac for FEL oscillation
Since very severe qualities of electron beam, described asfollows, are needed for FEL oscillation, we make someefforts in our linac.
(l)long macropulse duration(2)narrow energy spread(3 )low emittance(4 )high peak current
We aim for FEL oscillation in an infra-red region. In order toobtain small signal gain of 10%, the peak current of 25A andthe energy spread of 0.6% are necessary. The gain wascalculated in accordance with the reference of [3].
Experimental conditions are summarized in Table I [1].
Table I. Experimental conditions of FEL
Beam energy
Energy spread
Peak current
FEL wavelength
Gain

50MeV
0.6%
25A
2.8Jlm10%

2-2. Narrow energy spread
In order to achieve the energy spread of 0.6%, weattempted to stabilize the temperature of an accelerating tube
and the output voltage of a klystron modulator.
Temperature variation of the accelerating tube causes
thermal expansion/compression of the oxygen-free copper,
the major structural material for the accelerating tube, andcauses the resonance frequency to shift by approximately 50
kHz I "C. Since the accelerating tube has a quite high Q of
12,500, even a slight shift of resonance frequency can cause
impedance mismatching, and the microwave power that canbe injected into the accelerating tube is consequentlyreduced. As a result, beam energy varies. To further narrow
the energy spread, the temperature stability of the coolingwater must be improved. Some time ago we developed ahigh-precision cooling water system for optical fibertesting. For the present study we developed, with assistancefrom Sumitomo Densetsu Co., Ltd., a cooling water systemwith a very high temperature stability of less than ±0.02"C.
With conventional cooling water systems, the beam energy
spread caused by temperature variation of the acceleratingtube is 0.7%, whereas it is below 0.1% with !the our coolingwater system.
Suppressing beam energy fluctuation requires stable output
pulse voltage for the klystron modulator. A high-stability
klystron modulator was developed with cooperation from
Nissin Electric Co., Ltd. The klystron demands a high-power
driving pulse with a voltage of 305 kV and current of 340 A(short pulse mode). The klystron modulator has several
features as follows:
1)PFN (Pulse Forming Network) consisting of parallel
networks of21 capacitors and 21 reactors
2)capacitors with very low self inductance (<lOOnH)
3 )steppjng '?Ot<?rs to simplify a reactance adjustment ofthe PFN ctrcmt
The output voltage stability of less than 0.3% was achieved
as shown in Fig.2.

Accelerating tube No.20!.Om

f,p Tel tel.!.!dsJs!

Fig.1. Compact linac for FEL oscillation-58-JAERI-Conf 95-021

Q)

~0>

"5c.
"50TimeHelmholtz Coil Number
Fig.3. The longitudinal magnetic field distribution

Fig.2. Pulsed voltage waveform of the klystron modulator(ordinate : 44kV/div, abscissa : 2J.ls/div)

3. Experimental results and discussion

2-3. Low emittance

3-1. Measurements of micropulse shape

The electron gun is a thermionic triode gun based on adispenser cathode assembly (EIMAC model Y646B). A highD.C. voltage of 200KV is applied to anode-cathode gap tp
obtain low emittance. The shapes of the electrodes wereoptimized through simulations. A normalized emittance of
7rt mmmrad ancf beam current of 1.5A were obtained in theelectron gun testing.
2-4. High peak current
To achieve high peak current (25A), an SHPB system (anSHPB cavity and a 476MHz rf amplifier) was installed in thecompact linac. The SHPB is a standing-wave cavity with aresonance frequency of 476MHz. Short filling time and highshunt impedance are necessary to reduction of the rf
amplifier load. In order to optimize both Q-value and shunt
impedance of the cavity, a portion of the inner surface of thecavity was coated by OFHC.
To supply rf signals to the SR ring (158.6MHz) and thegrid pulsar of the burst-mode electron gun (79.3MHz), we
chose a frequency multiplier system which can generate themicrowaves of 79.3, 158.6, 476, 2856MHz. In general thelowest frequency is chosen as a fundamental one. On theother hand, the frequency stability for acceleration(2856MHz) is most important. In consideration of thefrequency-limitation of IC-countcr, the harmonic relation offour microwaves, the results of KEK and so on, we chose
467MHz as the fundamental frequency. The rf power
amplifier for the SHPB is all-solid-state one and its
maximum rf output power is 4.6kW.
The electron beam emitted from a cathode is focused by thefocusing electric field of the Wehnelt-anode gap. Also thebeam is defocused by the space charge effects in thebunching section. Thus, according to the increase ofelectron density, the beam focusing of the solenoid ischanged.
An ideal magnetic field of nine helmholtz coils arranged
along the bunching section is given by KapchinskyVIadimirsky equation[ 1]. If electron beam energy is lower
than 5MeV, using the V-K equation, the magnetic field Bs tofix a radius of an electron beam can be calculated from:The SHPB is effective for the increase of the peak current.
So it is important to understand the characteristics of theSHPB.
In order to measure a micropulse shape of the electron
beam, Cherenkov radiation which electron beams emit in airare. detected by the hi-planar phototube (HAMAMATSU
PHOTONICS : R-1328U-02 ; Rise time : 60ps). The
measurement set-up of Cherenkov radiation is schematicallydepicted in Fig.4. A wire monitor (secondary-emissionmonitor) was installed to the end of the beam line to obtain acharacteristic of the micropulse shape.
We measured the bunching ratio for various rf power
outputs to obtain optimized rf power of SHPB. The bunchingratio was obtained by the ratio to a pulse height at bunchedbeam to non-bunched one that measured by the bi-planar
phototube or a wire monitor. The rf power dependence ofbunching ratio is shown in Fig.5. When the rf power wasoptimized (about 3kW), the peak cunent increased from 6Ato 28A. The pulse shape ofthe beam bunched at 476MHz isshown in Fig.6. The transformation of micropulse shape forvarious rf phases is shown in Fig.?. These behaviors can be
explained that the tf phase and power to the SHPB cavity
was optimized.

Bs(T) = 3.69xi0·5ji(A)/~y /r(m)where I is peak cunent of electron beam, r is beam radius,~=v/c ,and y is Lorentz factor.
The longitudinal magnetic field distribution along thebunching system at present is shown in Fig.3. The field
optimized at the experiment is identical with the calculation.

-59-Fig.4. Optical guiding _an_d measurement system ofCherenkov radiation

JAERI-Conf 95-021

63-2. Energy spread measurement
Small signal gain depends on the energy spread of thebeam. The energy spectrum at 50MeV is shown m Fig.8. Theenergy spread (FWHM) of 0.6% was achieved.

-------------,
0·g

4~bll]

g:I

r:Q2

••••
~=0.6%
2

03~-

RF Power (kW 'j1nFig.S. Rf power dependence of bunching ratio
32.8

41.4

50.0

58.6

67.2

Beam Energy (MeV)
Fig.8. Energy spread at 50MeV
4. Conclusions

Fig.6. Micropulse shape(abscissa : lns/div)

We have developed the compact linac as both the injector
for NUl-III and the electron source for FEL oscillation. Theaccelerator tube cooling system with high temperaturestability and the klystron modulator with a output pulse
voltage stability of less than 0.3% help the achievement ofnarrow energy spread and the stable micropulse cycle. As aresult, the acceleration of 50MeV with energy spread of0.6% was achieved.
The SHPB system was installed and we tested its
performance. As a result,the SHPB system was successfullyoperated and the peak current increased from 6A to 28A .
We achieved the experimental condition of FEL. Also we .
intend to accomplish FEL oscillation .
Acknowledgements
The authors wish to thank T.Hori of JAERI SPring-S
project team for his advice.
References[1 ]K.Emura et al.,Proceedings of The 20tti Linear Accelerator

Meeting in Japan. (1995) 63[2]T.Shinzato et al.,in this symp.
[3]P.W.van Amerdfoort et al., The FELIX project status
reRort April 1988,Rijnhuizen Report 88-176[4]B.J.Bakk:er,"Free electrons as a versatile source ofcoherent radiation." Doctoral Thesis in FELIX 1993.
[5] T.Tomimasu et al., Introductory Book on Free Electron
Laser. (1995) 103. (Atomic Energy Society of Japan)

Fig.7. Rf phase dependence of the micropulse(abscissa : lns/div)-60-