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Proceedings of the 20th Linear Accelerator Meeting in Japan ( September 6—8,1995 Osaka Japan)

叙23
RECENT DEVELOPMENT OF THE JAERI FEL DRIVEN BY A SUPERCONDUCTING
RF LINAC ACCELERATOR
E.J.Minehara, M.Sugimoto, M.Sawamura, R.Nagai and N .K ikuzawa
Free Electron Laser Laboratory, Department o f Physics,
Japan Atomic Energy Research In stitu te
2-4 Shirakata-shirane, Tokai-mura, Naka-gun, Ibaraki-ken, 319-11 Jap anABSTRACT
A prototype for a quasi-cw, and high-average power free electron laser(FEL)
driven by a 15 MeV super-conducting rf linac has been developed, and constructed at
Tokai, JAERI. Expected cryogenic( stand-by loss<3.5W at 4.5K) and accelerating fields*
perform ances( Eacc 〜 7MV/m and Q 〜 2x10+ 9) of four JAERI su p e r c o n d u c tin g
accelerator modules have been demonstrated, and installed them in the FEL a c c e le r a to rvault. A beam test and com m issioning of the JAERI superconducting rf linac as an FEL
Driver have been successfully performed to get an electron beam ranging from 10 to 20
MeV with nearly full transmission.
FELopticals and beam transport elements arou n d
the undulator, which have been already assembled, are now under co m m issio n in g .
Spontaneous emission in the wavelength of 20 从m to 80 \im or longer have b e e nobserved by using the Ge(Cu) detector and fast current amplifier system, and 20m lo n glight pipe.
1 . In tr o d u c tio n
As well known, a laser consists o fthree major parts, i.e., a laser d riv e r,
gain media, and an optical reso n a to r.
Since the invention of the laser i n
1950's, their efficiency, and a v e r a g e
power level have been limited to v e r y
low by their huge heat losses in the la s e r
driver and gain media. If we could u se
an FEL, we could neglect the heat loss i n
the gain media. Unfortunately, as lo n g
as the normal conducting a c c e le r a to r
were used to produce high e n e r g yelectron beam as the laser driver, w e
could not realize the high efficiency andhigh average power level due to the h e a tlosses in the accelerator as the FELdriver.
In order to make a h i g h l y efficient, and high average powered FEL,
we resultantly have to minimize the h e a tlosses in the a cce lera to r .
A developmental program [l,2] o f
the free electron laser(FEL) system for a
far-infrared region from th ew avelength of 20 |im to 80 |im or lo n g e r
has been undertaken at Japan A tom ic
Energy Research Institute(JAERI), Tokai.

The purpose of the present JAERI FEL
program lies in constructing a very lo n gpulse or quasi-continuous w a v e (c w )superconducting rf linac e le c tr o n
accelerator, and demonstrating a h i g h average power FEL in the fa r -in fr a r e d
wavelength region.
Because wall losses and re q u ir e d
rf power become minimal in t h esuperconducting accelerator cavity, w e
may realize a quasi-cw and h ig h - c u r r e n t
rf linac driver, and hence a h ig h average power laser. Each major part o f
the program including future plans h a s
been reported in other papers [3-8] i ndetail.
2. Status of the JAERI FEL F a cility( l) I n j e c t o r
The injector of the JAERI FEL
consists of a thermionic cathode e le c tr o n
gun with a pulsing grid, a su b -h a r m o n ic
buncher(SHB), and a buncher. T heaccelerating voltage in the single g a pelectron gun is typically around 230KV,
and the gun is usable from 200 to 250 KV.
The cathode is mounted horizontally in astainless-steelpressurizedvessel w ith

SFg gas to 2 k g /cm ^ in order to p r e v e n tbreak down across a 45 c m -lo n ginsulating ceramic tube of the gun. The
accelerating gap electrodes a refabricated in a re-entrant geometry toincrease the accelerating gradient.
(2)Superconducting RF L in ac
The JAERI superconducting r flinac consists of two p r e -a c c e le r a to r
modules of the single-cell cavity ty p e
and two main modules of the 5-cell c a v ity
type. The resonant frequency of th ecavities is 499.8MHz which is exactly th esame with the buncher, and the six th
harmonic of SHB in the injector. D esign
values of the accelerating field s tr e n g thand Q-value for the cavities are 5M V /m , 、and 2 x 10^, respectively. In 1992
4Japanese fiscal year, we h a v e
successfully demonstrated exp ected
cryogenic( stand-by loss<3.5W at 4.5K)
and accelerating fields’ p e r fo r m a n c e s (E acc 〜7MV/m and Q - 2 x \ 0 +9) of fo u r
JAERI superconducting a c c e le r a to r
modules, and installed them in the FEL
accelerator vault.
(3)Cryostat and R e fr ig e r a to r
We have newly developed a m u ltirefrigerators system[5] integrated in to
the super-conducting accelerator m odule
cryostat to realize a h ig h ly - e f f ic ie n tsystem without any liquid coolant. A 4K
closed-cycle He gas refrigerator m ou n ted
just above a liquid-He supply tower o f
the module was adopted to cool down andto recondense cold vapor of liquid He
around a heat exchanger in the liquid He
container. A 40K/80K two-stage clo sed cycle He gas refrigerator, which w as
mounted in a vacuum vessel of th e
module was adopted to cool down the 40K
and 80K heat shields and other m ajor
components of the cryostat. C ooling
capacities of the 4K and 40K/80K
refrigerators are about 11W at 4.5K and40W/120W at 40K/80K, respectively.

- 6 7 -(4)RF Power S ou rce
One of the largest merit of asuperconducting accelerating cavity isvery low power loss, which makes it
possible to use all-solid-state RF p o w e r
amplifiers for all of the cavities[4]. Twosets of all-solid-state 50kW RF p o w e ramplifiers for the main a c ce lera to r
modules have been already installed, andhave been ready to use at th e
experimental area. Performance of th e
rf power supplies has b e e npreliminarily measured to be better th a n
1% of amplitude and within 1 degree o fphase stability at an rf power level o f
50kW or m ore.
(5) Electron Beam Transport System
The energy of electron b eam s
accelerated by the linac ranges fromabout 10 to 20 MeV. A conceptual d e sig nof the transport system was done b yusing the beam optics code TRACE-3D [9].
High current beams have to be fed to th e
undulator under isochronous andachromatic conditions for e f f ic ie n t
lasing of FEL. Because of the la r g eamount of charge density, space c h a r g eeffects would become serious in a lo n gtransport line and a beam waist. S in ce
the code could take into account p a rtia lspace charge effects, the tra n sp o rtsystem has been investigated by u s in g
the code.
3. Present Status and S ch ed u leSince 1992 Japanese fiscal y ea r,we have successfully d em onstrated
expected cryogenic( stand-by loss<3.5W
at 4.5K) and accelerating f ie ld s ’performances( Eacc >5MV/m and Q>2x l0 +9) of four JAERI su p e r c o n d u c tin g
accelerator modules, and installed th em
in the FEL accelerator vault. In 1994,
Optical resonators and beam tra n sp o rtsystems, which have been alreadyassembled, are now under the beam test.
In 1994, a preliminary beam testof the JAERI superconducting rf lin a cFEL has been successfully performed toget an electron beam of ten and se v e r a lamperes of peak current after the m a inaccelerator at around 15 MeV. M easured
energy resolution of a p r e -a c c e le r a te dbeam is about 3% of FWHM , and that of afully-accelerated beam about 0.8% o rless. Maximum transmission of the beamfrom the gun is now at around 100%. We
are now active in transporting e le c tr o n
beams in the JAERI FEL and i nperforming some lasing experiments i nthe optical resonator.
FELopticals andbeam transport elements around th eundulator, which have been alread yassembled, are now u n d e r
com m issioning. Spontaneous em issio nin the wavelength of 20 jum to 80 |im o r
longer have been observed by using th eGe(Cu) detector and fast c u r r e n tamplifier system, and 15m long lig h t
pipe.

REFERENCES[ 1 ] M.Sawamura et. al. ,Nucl. In stru m .
MethodA318 (1992)127.
[2] M.Ohkubo et. al. ,Nucl. In stru m .
Methods A296 (1990)270.
[3]R. Kato, et al., in the Proceedings o fSixteenth International Free E lectron
Laser Conference, 1994，San Francisco.
[4]M. Sawamura, et al., ibid.
[5]N. Kikuzawa, et al” ibid.
[6]R. Nagai, et al., ibid.
[7]M.Sugimoto, et al., ibid.
[8]K. Sasaki, et al., ibid.
[9]K. R. Crandall, et al., TRACE 3-D
Documentation, LA-1054-MS, UC-32 andUC-28, 1987.