The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan1993

A 5-MeV ELECfRON INJECfOR AND FELl LINAC FOR FEL FACILITIESCOVERING SPECfRAL RANGE FROM 20 f.1 m to 0.3 f.1 m
T. Tomimasu, Y. Morii, A. Koga, Y. Miyauchi, T. Keishi, E. Nishimura;
K. Saeki, S. Abe, S. Sato, A. Kobayashi, I. Bessho and A. Nagai
Free Electron Laser Research Institute, Inc. (FELl)
2-7-4 Kyomachibori, Nishi-ku, Osaka 550
electron beam in the smaller-energy spread region depends on thedevelopment of stably opemble long-pulsed klystrons and theirmodulators being capable of driving them very stably. Since the FEL
gain is getting smaJler for the shorter wavelength FELs, a considerably
long beam is necessary to ensure saturation of UV and shorter
wavelength FELs and to improve the FEL generation efficiency. The
FELIX group lias succeeded in these developments.

Abstract
FEU is aiming at IR-FEL genemtion by the end of 1994 usinga 75-MeV beam and a 3-m long undulator and UV-FEL genemtion
by the end of 1996 using a 165-MeV beam and a 3-m longundulator. The linac is composed of a 5-MeV electron injector andseven EfL type accelemting waveguides with a length of 2.93m (2 7r/3 mode, linearly tapered type). The injector consists of a 150-kV
DC thermoionic triode gun opemted by a 178.5-MHz, 500-ps pulser,
a 714-MHz prebuncher (SHB), and a 2856-MHz standing wave typebuncher (SWB).
The linac will be operated in three modes of 24 f.l s, 12.5 f.l s
and 0.5 f.l s. With a choice of three modes, the maximum accelemted
energy can be changed from 165 MeV to 310 MeV.
The linac beam will be sent to four undulators using S-type BT
systems installed at 30-MeV, 75-MeV, 120-MeV, and 165-MeVsections at 24- f.l s rf pulse operation. The. ratio of the cavity length
to the undulator length is designed to be less than 3. The beam, once
used for lasing at 30-MeV section or at 75-MeV sec!ioii, can be bent
back to the following accelerating waveguide and is reacclemted andreused for lasing.

5-MeV Electron Injector
The layout of a 5-MeV electron injector is shown in Fig. 1. It
is composed of a 150-kV thermoionic triode gun, a 714-MHz
prebuncher, and a 2856-MHz standing wave type buncher. The. gun
is a Pierce type gridded gun with a thermoelectronic dispenser cathode(EIMAC Y646B model). A cathode voltage of 150 kV for the gun
was chosen because of easy opemtion in air without a tank filled with
SFo gas. The gun is triggered by a 178.5-MHz 500ps pulser(Kentech Instruments, Ltd. England).
In the subnano-pulsed
opemtion, it is expected that the emittance of the gun is less than 5 7rmm•mmd at 1.6A[3]. These parameters are shown in Table 1.
11Ie 714-MHz prebuncher acts as a subharmonic buncher(SHB) and is made from stainless steel to reduce the influence of a
substantial rf field introduced in the cavity by the subnano-beamcurrent from the gun. The rf frequency of 714 MHz is chosen 1) tomake the cavity as compact as possible, 2) to meet the short bunch(<0.5ns) mode opemtion of the gun, and 3) to coincide the micropulse
FEL beam with a micropulse conventional laser genemted with afrequency of 2856 MHz/2n(n is an integer).
The buncher is of a standing wave type (SWB) and its total
length is around 49cm. The peak electric field will be around 10
MV/m for the rf input of 2MW. In order to reduce the influence ofspace-charge fields as much· as possible, the distance from the gun
cathode to the first cavity of the buncher is designed to be less than
80cm for the 0.5 ns-pulse injection mode.

Introduction
A number of approaches have been proposed for the generationof UV and short wavelength FELs[1,2]. One of the key technologies
is keeping the emittance of high-current and long-pulse electron
beam low from a gun to the end of the Iinac, and further to anundulatoi. Of several candidate electron guns at the present time, ahigh DC voltage thermoionic triode gun is the best choice for thecapability of emitting intense microbunches at a repetition mte of 1GHz, during a macropulse of 20 f.l s[3].
Another technology keeping the high-current and long-pulse
CM-GV

GUN

SIP

SHB

VB-APMPMSIP

SHB

VB-B GVQD

SIP PMFig. 1 A 5-MeV Electron Injector-449-0500

Table 1GunPrebuncher

BuncherMain Parameters of the FELl linac injector

Type
Thermoionic triode
EnergyH50keV
0.4~0.5 ns
Micropulse
Microcharge
1.2nC
Normalized emittance <10 1r nun mrad
~24.U sMacropulse
1~10 Hz
Repetition frequency
Type
Re-entrant cavity
Frequency
714 MHz~zooo
Q-Value~5okv
Peak field
TypeStanding wave
~49 emLength
~5 MeV for 2-MW rf
EnergyEnergy spread (FWHM) <130 keY

1000

500

0 o~~~-L~s~o~~~~1o~o~~~~~~~

SHB to the SWB is given by(1)Bs=2V"2 moc/(eVIa) • ../Ti7J"""7tr,
where m o is the electron rest mass, c is the light velocity, Ia is theAlfven current (-17000A). I is the peak current, /3 =v/c is thenominal particle velocity, and r is the Lorentz factor. Themaximum focusing field is designed to be about 0.19T near theentrance of SWB. The field distribution obtained by five focusing
coils is shown in Fig. 2. It is expected that a 0.5-ns injection beamof 120 keY electrons (the corresponding bunch length is 8.7cm) isbunched in a bunched length less than 1.5cm at the entrance of SWB
by the bunching system of SHB and a drift space. Two beam
position monitors are installed in the drift space.

Linac and FEL Facilities
The regular sections of FELl linac are composed of seven E1L
type accelerating waveguides with a length of 2.93m (27r /3 mode,linearly tapered type)[5]. A set of steering coils, focusing coils,

KLYSTRON POWER
P1
pulse length KLY1, KLY2
12.0MW
24.0ps
24MW
17.0MW
12.5ps
34MW
0.5ps
70MW
35.0MW,-17I~~HzP2
6.0 MW
8.5 MW
17.5 MW~T.H~~ SH~

SW!i:Fig. 2 Magnetic Field Distribution
quadrupole magnets, optical and core monitors for beam diagnosis,
and a sputter ion pump, are installed at each accelerating waveguide.
The length of the injector and regular sections including bending
sections for BT systems is 46m. The buncher and these accelerating
waveguides will be powered by two klystrons (E3729, 2856MHz, total
48MW, 24- .Us flat top long pulses). Fig. 3 shows a schematiclayout of the FELl Iinac including rf systems and four undulators.
The klystron used here is a klystron E3729 manufactured byToshiba Corp. E3729 is a modified version of E3712. Since thelatter is usually used in the short pulse mode (4 .U s-80MW, 50pps),
the test operation is necessary at a long pulse mode of 24 .u s. The
linac will be operated in the following modes, that is, 1) 24- .U smode, 2) 12.5- .U s mode, and 3) 0.5- .U s mode, in which the beam isused for the injection to storage rings and slow positron generations.
With a choice of three modes, the maximum accelerated energy canbe changed from 184 MeV to 310 MeV.

Ea
35MeV
42MeV
58MeVEbEcEd

82MeV
97MeV
141MeV

132MeV
156MeV
224MeV

184MeV
216MeV
310MeV

1- - - - - - -I

Fig. 3 FELl Linac including rf Systems and Four Undulators-450-.] .,r~~J -o_~_,_From the Kapchinsky-Vladimirsky equation[4], an llli:ial field
Bs of a focusing solenoid keeping a beam radius r constant from theHigh energy linac rooll!

Low energy linac room

010m

Beamcatcher

20m

FEL experimental room

Visible-UV

Fig. 4 Layout of IR and UV-FEL facilities of FELl including a future SR-FEL project
The klystron is powered by a pulse-forming-netw ork (pfn)
modulator including a stack of optical thyristors and a pulse
transformer. The pfn consists of four-parallel lines so as to optimize
the pulse flatness like FELIX[6]. The pfn signal is fed to the klystroncathode by means of a 1:15 pulse transformer. The cathode voltage
stability of the order of 0.08% is necessary for keeping an rf 0.2%.
Details of the rf system of FELl is presented in this meeting[7].
The BT system for the linac has a quadrupole doublet every 2mon average as' shown in Fig. 4 to keep the emittance and position of
the accelerated beam small and on the axis of the accelerated
waveguide, respectively, from the injector to the end of the linac.
The four BT systems for the corresponding undulators havebeen designed as an achromatic and nearly isochronous (22.5° bend x2) BT systems. The bending angle (a) of 225• and the bending
radius (R) of 0.5m were chosen to reduce the residual
non-isochronicity c6t (c is the light velosity), since cL'-.t isproportional to a 3 R L'-.P/P, where 6.P/P is the relative momentum

Table 2 Beam quality values desired for FEL oscillations
and intended FEL applications at FELl;i;30MeV ~75MeV ;i;120MeV H65MeVElectron energy~100 A
Peak current~l01rmm mrad
Normalized emittance;!;0.3%
Energy spread (FWHM)~5 ps
Micropulse duration
178.5 MHz
Micropulse repetition
frequency~24 ll s
Macropulse dulation
HOHz
Macropulse repetition
frequency<3.06 kW
Avera~ beam nower
20-7/lm U.5J.tm ~0.54/l m ~0.25/lm
FEL wavelength
5W
12W8W
3W
FEL average power
Separation FEL-cr Purification Photosynthesis
FEL applications

spread.
The beam passed through the undulator at 30-MeV and
75-MeV sections can be bent back to the following accelerating
waveguide using the BT (22.5° bend x 2) system and can bereaccelerated, since an energy loss due to lasing is Jess than 1% andthe residual non-isochronicity due to the two BT systems (22.5° bendx 4) is Jess than 0.6ps for a beam with an energy spread of 0.5%.
The reaccelerated beam is reused for lasing at the following undulator
and bent back again for injection to storage rings and slow positron
generations. A lattice design of the BT system of FELl is presented
in this meeting[8].
Table 2 is beam quality values desired for FEL oscillations andintended FEL applications at FELL

Acknowledgements
The authors would like to thank T. Suzuki and S. Nishihara of
Mitsubishi Electric Co. for the injector and accelerator assembly,
H.Yonezawa of Toshiba Co. for the klystrons of E3729 and 1AV88R,
Y. Miyai, H. Ohshita and I. Ito of Nisshin Electric Co. for theS-band, long-pulse modulator, S. Miyake of Daihen Co. and H.
Matsumoto of Nihon Koshulta Co. for the S-band driver amplifier,and M. Yura of Pulse Electric Engineering Co. for the SHB
modulator.

References[1] B. E. Carlsten and K. C. D. Chan, Nucl. Instr. Meth. A272, 208(1988)[2] T. Tomimasu, et al., Proc. HEACC'92 (Hamburg, July 20-24,1992) p.1133[3] R. J. Bakker, et al., Nucl. Instr. Meth. A307, 543 (1991)[4] J.D. Lawson, In Advances inEiectronics and Electron Physics
13C, ed. A Spetier (Academic Press, New York, 1983)[5] T. Tomimasu, IEEE Trans. Nucl. Sci. NS-28(3), 3523 (1981)[6] B. I. H. Meddens, et al., A 50-MW Pulse-Forming Network witha Voltage Stability within 0.03%, to be published in Rev. Sci.
Instr.
[7] Y. Morii, et al., The RF System of FELl, in this proceedings[8] Y. Miyauchi, et al., Lattice Design of the BT System of FELl,in this proceedings-451-