The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

DEMONSTRATION OF THE HIGHLY-EFFICIENT AND HIGHPOWER FELS DRIVEN BY A SUPERCONDUCTING RF LINAC
E. J. Minehara, R. Hajima, M. Sawamura, R. Nagai, N. Nishimori, N. Kikuzawa, M. Sugimoto,T. Yamauchi, T. Hayakawa, and T. Shizuma, FEL Laboratory at Tokai, JAERI, JAPAN
Abstract
We need a powerful and efficient free-electron
laser(FEL) for industrial uses, for examples, phannacy,
medical, defense, shipbuilding, semiconductor industry,
chemical industries, environmental sciences, space-debris,
power beaming and so on. In order to realize such atunable, highly-efficient, high average power, high peak
power and ultra-short pulse FEL, the JAERI FEL group
and I have successfully demonstrated the efficient andpowerful FEL driven by a compact, stand-alone and zeroboil-off super-conducting rf linac with an energy-recovery
geometry. Our discussions on the FEL will covermarket-requirements and roadmap for the industrial FELs,
some answers from the JAERI compact, stand-alone andzero-boil-off cryostat concept and operational experience
over these 8 years, our discovery of the new, highlyefficient, high-power, and ultra-short pulse lasing mode,and the energy-recovery geometry.

the original, and added a new path to the old in the third
step to develop and to realize the industrial FELs using
the new lasing mode. The new path using the new
efficient lasing will be discussed in the following.
Possible Growth Rate of the Highly-Efficient High Power

Superconducting rf linac based FEI..s
10 Times Per
Every 2 Years

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1 INTRODUCTION

Year

F;EL(J) Reinvention

A very efficient and powerful FEL has been longrequired to use for almost all industrial applications, forexamples, phannacy, medical, defense, shipbuilding,solid-state physics, chemical industries, environmental
sciences, space-debris orbit control, power beaming, andso on [1] instead of the conventional lasers, and otherlight and heat sources. In the table listed in the last page,
there are summaries of possible academic and industrial
fields, itemized applications and examples. As expectedthat the industrial FELs would become popular in theworld near future, the JAERI FEL group and I have tried
to develop a compact, stand-alone and zero-boil off
superconducting rf linac-based FEL with and without anenergy-recovery geometry [2]. Market requirements forthe industrial FELs, and some replies from the JAERI
compact, stand-alone and zero-boil-off cryostat concept
and operational experiences over these 9 years will be
discussed fully in the text. The JAERI cryogenics will be
explained briefly, and the future directions and plans
discussed shown in fig. I.
Original strategy to develop the industrial FEL at
JAERI consists simply of three steps, the first of making
a highly efficient and high power FEL driver using an rf
superconducting technology, the second of demonstrating
a powerful FEL lasing using the driver [2], and the thirdof increasing an total system efficiency using a beamenergy recovering. After we found the new FEL lasing .
mode of high efficiency last year [3], we modified slightly

197.5

1990

1986

1990

1996

2000

2006

2010

3':>152020

2026

Fig.l, Roadmap for the industrial FELs

2

INDUSTRIAL FELS

2. 1 Market Requirements
Market requirements for the industrial FELs from theusers should be discussed, and itemized for each category
to check how much they can be fulfilled before the FEL
businesses would be started. They are tentatively itemized
here, for examples, costs, reliability, compactness,easiness in the production, operation and maintenance, theoperational and maintenance intervals, fulfillment forradiation safety code, pressure vessel code, other official
regulatory rules and so on.
The capital, operational, and maintenance costs for theindustrial FELs should be minimized as low as the costs
for existing and future conventional laser systems.
Compactness of the FEL is very important because the
FELs used in the factories, schools, hospitals and othersmall facilities must be fitted into a tabletop sized, or a
trailer sized space being available in these small buildings.
In addition to them, we can easily find other important
requirements of readiness to use any time, easiness to use,no specialist required in the operation and maintenance,
safety in operation and maintenance, very long
maintenance intervals, and no regulations from any legal
and official codes and rules. Most of them have been

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

replied positively by the JAERI cryogenics design concept
and others up to now [2].

2. 2 Compactness, Stand-Alone, Zero-Boil Off
Cryostat and Non-Stop Cooling Operation
Once we decide to introduce the stand-alone FEL, we
do not need any huge central liquefier station of He andN2 gas compressors to cool down the FEL driver outside
the room or building. As each module of thesuperconducting rf linac has its own shield cooler andliquid He re-condenser, it stands alone without any
cryogenic liquid coolant outside the module independently.
In short, the stand-alone super-conducting rf linac based
FEL will be run freely and independently in contrast witha parasitic FEL with the central liquefier station.
The zero-boil off cryostat for a superconducting If
linac has been first designed and developed for the JAERIFEL since the beginning of the program in 1989[4]. As
shown in fig.l, the JAERI zero-boil off cryostat has
duplex heat shields, and the 20K/80K shield-cooler and 4K
He-recondenser refrigerators integrated into the cryostat
vacuum vessel.

Figure 2: The JAERI stand-alone, and zero-boil off
cryostat has duplex heat shields, and the shield-cooler and
He-recondenser refrigerators integrated into the cryostat
vacuum vessel.
Unlike superconducting-magnet cryostats, thesuperconducting rf linac cryostat has intrinsically large
heat invasion through many heat bridges, for examples,two beam pipes, main and higher order mode couplers,
support rods, refrigerator or liquid N2 and liquid He
transport pipes and so on. Heat economics in the cryostat
has been optimized to minimize the heat invasion
adopting a finite-element method of temperature
distribution calculation in the cryostat. Calculated andmeasured stand-by losses to be about 4.5W at the JAERI
cryostats are consistent with each other, and the zero-boil
off one usually cuts around 80% or more of the loss inthe conventional one.
A compact 4 K He4 GM-JT (Gifford-McMahon
refrigerator with Joule-Thomson expansion valve) gas
closed-loop refrigerator shown in fig.2 was introduced torealize a stand-alone and zero-boil off superconducting
linac using 500MHz UHF band cavities. A 1.65 K He3
GM gas closed-loop refrigerator has been designed, andtested to realize a stand-alone and zero-boil off

superconducting linac using L and S bands cavities.
Cooling efficiencies of the liquefier is about 30% higher
than the GM-JT recondenser. If the liquefier efficiency
includes transferring losses, both liquefier and recondenser
have nearly the same efficiencies. The capital cost of theliquefier and coolant transferring system is nearly thesame with or slightly cheaper than the GM-JT refrigerator
as long as the system is small.
We have introduced an 8W 4K refrigerator, andmodified it to an 11 W one to cool down our 500MHz
UHF cavity cryostats about 14 years ago. Except for
initial troubles of the recondesers, we could successfully
keep running the whole system over these 9 years. There
have been successfully no trouble and no malfunctioning
in the 4 shield coolers for about 9 years up to now, andno experience to dry up liquid He inside any He vessel ofthe 4 modules since the beginning in 1992. As a typicalexample, we could succeed to run the system without any
trouble for 355days in 1996 Japanese fiscal year andwitho.ut warming up for about 500days from November
1999 to March 2001.
The compact, stand-alone, zero-boil-off cryostat, andnon-stop cooling operation with no warm-up or very long
maintenance interval except for a few hours ofmaintenance each year will completely solve a large
number of operational and maintenance problems. Like asuperconducting magnet based MRI (magnetic ResonanceImaging), we plan to perform a cold maintenance inexchanging a displacer unit of the shield coolers and tokeep the whole cryostat cool without de-conditioning thesuperconducting rf cavities. Because the domesticpressure vessel code does not allow to perform such a cold
maintenance for the liquefier, and actual design or
structure of the liquefier practically makes the cold
maintenance and disassembling impossible, the non-stop
cooling operation is only available for the stand-alone,zero-boil off cryostats like the JAERI FELs, and MRis.

2.3 Novel Ultrashort-Pulsed and HighlyEfficient Lasing Mode
A novel lasing mode has been discovered to realize
ultra-short pulsed and highly efficient lasing in FELs at
the JAERI FEL laboratory in the beginning of the lastyear [3, 5]. As the world-highest 2.34kW average power
and about lGW peak power were obtained at JAERI FEL,
they will be replaced by their new records soon. As wellknown that an FEL conversion efficiency from the beam
power equals with 1/2Nw where Nw stands for the numberof wiggler periods, it is naturally understood that the FEL
efficiency will become large if Nw will become small byanother novel mechanism. There have been expected tobe effectively small number of the period, and efficient
after the FEL saturation because of some pulse-shortening
and spiking mechanisms. As reported that pulse width of
the new mode was measured to be a few cycle lasing of
3.4 cycle and 250fs at 22.4 jlm [3], the high efficiency of
6-9% is consistent with 1/2Ncycie where Ncycie stands forthe number of cycle over the ultrashort pulse FEL width.
If we can find some mechanism and succeed to realize the-151-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

smaller cycle numbered lasing than the 3.4 cycle, thehigher FEL efficiency from the beam power can befeasible to convert almost the whole beam power to the
FEL power. For examples, a single cycle lasing of about
75 femtosecond would be expected to have 50% efficiency
if the FEL efficiency could equal with 112Ncycie· The
brand-new lasing can open up new possibilities in FEL
science and technology that we can drastically increase an
FEL conversion efficiency and the FEL peak and average
power from the electron beam power, to realize a ultrashort and a few cycle FEL pulse, and to obtain a newknowledge in quantum optics understanding the new FEL
lasing mechanism.

2.4 Energy Recovery FELs at JAER/
The figure 3 shows the energy recovery circular loop
at the JAERI FEL under construction from April 2001.

Figure 3: The energy recovery circular loop at the JAERIFEL facility under construction.
Energy recovery concept had been discussed and tried at
Stanford University, Los Alamos National Laboratory andJefferson laboratory since 1980's[6]. First demonstrationof the same-cell energy recovery of the superconducting If
linac has been successfully done in 1999 at Jefferson
laboratory to cut 75% of the needed rf power. Only a
few % or slightly larger rf power of the non-energy
recovering FEL is needed to run the energy recovering
FEL, and the same wall thickness of an ordinary building
is enough to shield very weak and low energy X rays level
being generated. Therefore, we can easily cut most of thebudgets of rf power amplifiers and heavy shielding wallsof the buildings to construct the energy recovering FELs
facility. The 360-degree circular energy recovery
geometry is planned to be used for academic facilities likean X-ray FEL and a light source to produce soft and hard
X-rays ranging from 10 to O.Olnm.
Another energy recovery geometry and conceptual
explanation shown in the fig.4 have a 180-degree
isochronous bending magnet to decelerate the electron
beam anti-parallel with the acceleration direction.
According to the original Canadian patent [7] in 1970's,
the geometry and magnet were used to call a reflextron.
In the 180 degree bending geometry, average or centorid
velocities of the electon pulses in both the accelerationand deceleration are roughly the same along the accelerator
cavity on the contrary to the circular recovering one which
has a large velocity difference around the entrance and exitof the accelerator cavity. The reflextron geometry has arelatively small number of beam optical components inline, and small building space required by the layout. The
reflextron one can accept and recover the lower energy
electron beam than a few MeV because nearly no velocity
difference can be occurred between the deceleration andacceleration. The JAERI plans to make a prototype forthe industrial FELs using the reflextron geometry because
the reflextron has intrinsically several better features
discussed above to realize an ideal energy recovering FELs.

2.5 Industrial FELs near Future
Four industrial FEL models having the reflextron
geometry are illustrated in figs. 5, and 6. Three of them
are infrared FELs, and the forth ultraviolet FEL.

Beam Dump&
Energy Recovety

DCHVPS
1.4MV0.1A

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Figure 4: A Reflextron energy recovery geometry of a 180 degree bending and its conceptual explanation.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

As shown in fig.6, the far-infrared FEL (FIR FEL)
ranging from 200 to 50micron wavelengths uses the
500MHz UHF band cavity of 5-lOMeV electron energy
with the reflextron energy recovery geometry. The
smallest model of the industrial FIR FEL will be made toperform an FEL higher power demonstration than lOkW
or lOOkW, to produce an intense Compton-backscattering
gamma-ray flux of about lOMeVin synchrotron lightsources, to image foreign materials inside foods, grain,
fruits and powder as nondestructive inspection, custom
inspection, and so on.· A mid-infrared FEL (MIR FEL)
ranging from 50 to 8micron wavelengths will use the500MHz UHF band cavity of 12-24MeV electron energy
with the reflextron geometry. Possible and typicalapplications are expected to be large-scaled photochemical
processing, medical, pharmacy, rare-material separationand so on. A near infrared FEL (NIR FEL) ranging
from 12 to 2micron uses the same 500MHz cavity of2448MeV electron beam energy with the reflextron.
0.5GHzUHF
High Power FEL.s
12MeV(FIR)/ 24MeV(MIR)/48MeV(NIR)

r-------,Eacc=4MV/m-8MV/m, 0.5GHz

J6-12MeV(FIR)Jl.5m(Acceleration length)/5Cell
Jl2-24MeV(MIR)

J. 3m /lOCell

124-48MeV(NIR)

16m /20Cell

Figure 5: Three industrial FEL models for lasing in the
FIR, MIR, and NIR wavelength regions with thereflextron geometry. All of them use the 500MHz UHFband cavities of 5, 10 and 20 cells, respectively.
VUV-FEL 200-300MeV
QCW/CWSuperconducting rfLinac Driver

An ultraviolet FEL (UV FEL) ranging from 0.3 to
0.1 micron wavelengths will be planned to use a S or L
band cavity of 200-300MeV electron energy with thereflextron geometry. The FEL will be applied tolithography, photochemical processing, polymer surface
modification, optical mass spectrometer, and so on.

2.6 New Refrigerator and New
Superconductors
The 1.65K He3 GM refrigerator as a recondenser or acooler has been developed at the JAERl FEL to cool
down L or S bands Nb cavities for a tabletop
superconducting rf linac-based FEL, energy recovery X ray
light source and energy recovery X ray FEL near future.
A thermal design of the zero-boil off cryostat for the Sand L-bands linac-based X-ray light source and FELs
looks similar with the 4K high duty, or quasi-CW andUHF linac-based cryostat except for having 3 heat shieldsof 80K, 20K, and 4K and low-duty.
·Whenever a novel superconductor like MgB2 [8]and halogen-based compound doped C60 fullerene [9] will
become usable near future to make a high frequency
superconducting cavity of S-and L-bands, the JAERI
stand-alone and zero-boil off cryostat concept will besuited better, and used to manufacture the novel
supercondutor material-made accelerating cavity cryostats.
Because their critical temperatures have been measured tobe very high, a compact, durable and powerful coolingengine like the GM refrigerator will be usable for thesuperconducting S and L bands cavities. After the newsuperconductor-made cavities could become available, alldifficulties like a vacuum leakage problem relating withsuperfluidity, very expensive capital and operational costs
for the 1.9 K liquefier and so on in the superconducting S
and L bands Nb cavities would be solved by using thehigher critical temperature superconductor cavities andinexpensive and reliable 20K and 80K double-staged GM
refrigerator with a single heat shield.

3 SUMMARY

200-300MeV(VUV) 8m /&Cavities
Eacc=25MV/m, 3GHzll.5GHz

Figure 6: An ultraviolet industrial FEL (UV FEL)
ranging from 0.3 to 0.1 micron wavelengths will beplanned to use aS or L band cavity of 200-300MeV
electron energy with the reflextron geometry and 1.65K
He3 GM refrigerators.
A 1OkW or higher industrial FEL which can lase ataround a fiber-transmittable wavelength of l.3f..lm will bevery useful to transmit their power to a pin-pointed
position in a distant area from the FEL. The FEL will be
widely used in the many factories like a shipyard,
automobile factory, civil engineering and so on.

The FELs driven by the superconducting rf linac
have intrinsically very high average power capability
because the linac driver is highly efficient and powerful.
Relatively low efficiency converted from the electron
beam to FEL power can be overcome, and increased torecover the remained beam power after the lasing. As
discovered the new lasing mode, we could make the FEL
pulse ultra-short and very efficient without the energy
recovering. Both paths of the energy recovery and the
new lasing can be usable to make the FEL efficient
drastically, and to realize the industrial FELs soon. The
reflextron geometry can be applied to make the industrial
FELs compact, powerful, and efficient because an
absolute value of the velocity difference is very small
between the acceleration and deceleration along theaccelerator cavity, and we can recover the beam power at afew MeV or less electron energy.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osakn, Japan, October 2001

REFERENCES[1]J. Hecht and D. Teresi, Laser, Supertool of the
1980s 1982, Ticker & Fields, 383 Orange Street, New
Heaven, Connecticut 06511, U.S.A.
[2]E.J.Minehara et al., Nucl. Instrum. Methods Phys.
Res. Sect.A445, 183 (2000).
[3]N.Nishimori et al., Phys.Rev. Lett. 86, 5707 (2001).
[4]E.J.Minehara et al., pp159-161, in the proceedings ofParticle Accelerator Conference, 1995, Dallas.

E.J.Minehara et al., Free Electron Laser Challenge2(SPIE) Vol.3614, pp.62-71, 1999.
[5]R.Hajima et al., to be published in the FEL
Conference proceedings, August, 2001, Darmstadt.
[6] G. R. Neil et al., Phys. Rev. Lett. 84, 662 (2000).
[7] E.A.Heighway, Magnetic Beam Defection
System .Canadan Patent993124, issued 1976.
[8] J. Nagamatsu et al., Nature 410, pp63 - 64 (2001).
[9) R. F. Service, Science, 2001 August 31, vol. 293,
p1570.

. dustn FEL applications.
r
Table 1a. Summarv of them
Field

ApJ lications
I Uranium Enrichment
Isotope Separation
New Materials
Nuclear Fuel and Nuclear Waste
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Basic Sciences
Photo-electron Spectroscopytn..

0.1:!:1"~ ~-Far infrared spectroscopytnir

New material purification and Isotope
seperation of monoisotopic materials. ~. na:e..soo·...cts:;;!a.~

Biotechnology'1::1~

Photo-excited chemical reaction enhancing
and cutting hydrogen-bonds
THz Sciences and Technologies__ .,., ;:!.
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ExamPlesAtomic Method for Uranium Enrichment, Molecular Method for Uranium Enrichment
Isotope Separation for 9 OZr, 5 <>ri, 5 3Cr and so on.
Atomic Separation for U, Pu and other Radioisotopes
Radioactive components processing ( remote-processing )Nuclear fuel cutting ( remote-processing )Reactor disassembling ( remote-processing )Plasma heating, Current Drive, Plasma diagnostics(Temperature, Density)
Laser Fusion, Muon Catalysis Nuclear Fusion (Laser Enhancer)
High Energy Accelerators, mm Wave Radio astronomy (Local oscillator)
Gravitational Wave Observatory Laser Interferometorv
Spectroscopy for Metal Band Structure. elementary excitation modes. impurity
levels. semiconductors. half-metals and so on.
New superconductor theoretical models, High Tc Superconductor materials,
Superconductor gap energies
Isotope separation of H, B, C, 0, Si and so on.
Improvements of Material properties(heat conductivity etc.)
DNA structual disassembling
DNA, nerve fiber and other biomaterials cutting and processing,
Ultra fast X rav analysis of protein 3D structure
Research and Development for Mesoscopic materials ( <submicron sized materials)
THz fast elements( detector, mixer, laser or oscillator ) utilizing quantum well, quantum
wire, and quantum dot,
Decomposition and ultra sensitive detection of environmentallv-harmful materials

Table 1b. Summary of the industrial FEL applications(continued).
Utilization of non-thermal, andphotochemical reaction processes for
mdeical applications
Utilization of thermal process for
medical applications

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ExamplesApplications

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Remote Energy Beaming

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Ultra-sensitive detection anddecompsition for Environmental
harmful materials--~

Cancer therapy, Medical treatment for Arteriosclerosis, Cholesterol decomposition,bone cutting, Laser surgery, gallstone demolishing
Laser surgery for myopia( short sight )and the cornea
laser florescent endoscope, sterilization, pasteurization
Laser surgery, Photo-coagulation, Birthmark, melanin skin deposition
endoscopic laser surgery
laser
laser functionality recovering, laser rehabilitation, Laser anesthesia.
acupuncture and moxibustion,Laser CT(computer tomography),bodylivingain
Laser measurements of rare element concentration
laser doppler effect measurement of blood flow, diagnosis for abnormal metabolism,
laser dentsitry (therapy, preventive medicine of a tooth decay, tooth processing)laser cell welding and laser cell-bonding, gene-injection
Laser manufacturing, Laser drilling, laser-cutting, laser-welding
Laser surface-modification, laser quenching. laser annelingLaser welding processes(laser alloyng, laser sticking, surface fusing), Laser boring,
Civil engineering, Plant construction,
Photo cemical vapour deposition( laser chemical evaporation)
Lasr initiated etching, Lithography, Laser ablation (thin film deposition), Doping(precise concentration contorl of impurities, high concentration doping) , Laser
marking, laser stamping, sterilization (Wafer, rinsing water) , laser local annelingSpace debris orbit control, High altitude unmanned airship and airplane,
Interplannet probe vehicle, Energy Beaming to communication Satellites, (Electricity
charging, Laser propulsion, Heat source), Orbital Solar Power Station (Laser
Beaming. Heat Beaming)
Remote and widely-range ultra-sensitive detection for Furan. Dioxin
FIR Lidar, Tritium remote sensing, and selective decomposition, vibrational state level
spectroscopy, thermal decompsition through vibrational levels, environmental hormone
and harmful materials, Remote and wide-range laser sensing for chemical agents
depleting of the ozone laver, laser monitoring the effects of (global) warming-154-