The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

JAERI ERL-FEL: status and future plans
R. Hajima* , M. Sawamura, R. Nagai, N. Kikuzawa, N. Nishimori, E.J. Minehara
Japan Atomic Energy Research Institute, Tokai, Ibaraki 319–1195 Japan
Abstract
An energy-recovery linac (ERL) for a high-power freeelectron laser (FEL) is under development in JAERI. We
completed the construction of the ERL, and demonstrated
the first energy-recover operation and FEL lasing in 2002.
For realizing FEL lasing in 5-10kW average power, further
upgrade of the ERL is carrying on, which includes reinforcement of the injector, replacement of the RF control
system and so on. Future plans for high-power FEL application and basic research towards future ERL light sources
are also described.

INTRODUCTION
A research program of a high-power free-electron laser(FEL) at JAERI (Japan Atomic Energy Research Institute)
started in 1987, and we achieved the initial goal, kilowatt
FEL lasing, in 2000[1]. The program is now proceeding
to the next stage, demonstration of a 5-10kW FEL withan energy-recovery linac (ERL). The energy-recovery technology with a superconducting linac is the most promising device for high-power FELs [2] and next generation
Xray light sources[3]. We consider that the development ofthe energy-recovery linac at JAERI will contribute to future
FELs and light sources.
The ERL was designed by adding a return arc to the original superconducting linac at JAERI-FEL[4]. The original
linac was shut down in the spring of 2001, and the ERL
was completed after a half year construction period. We
demonstrated first energy-recovery operation at 19 February, 2002, and first FEL lasing at 14 August, 2002 [5].
In this paper, we present recent results from the R&D
program at JAERI-FEL, and a future plan.

CONFIGURATION OF JAERI-ERL
The layout of JAERI-ERL is shown in figure 1. An injector, main SCA modules, an undulator and the first arc are
inheritance from the original FEL. An injection merger, ahalf-chicane before the undulator, and the second arc were
newly installed for the ERL.
The injector consists of 230kV electron gun witha thermionic cathode, 83.3MHz subharmonic buncher(SHB), and two cryomodules, each of which contains a single cell superconducting cavity driven at 499.8MHz. An
electron bunch of 0.5nC with length of 800ps (FWHM)
is generated by grid pulser at 10.4MHz repetition, that is5mA average current, and compressed by SHB and following drift. The electron bunch is accelerated to 2.5MeV by* hajima@popsvr.tokai.jaeri.go.jp

two single cells, and further compressed by velocity bunching during a 9m drift. The bunch duration becomes 60ps(FWHM) at the entrance of the injection merger and 15ps(FWHM) after the merger as a designed value.
An electron bunch injected to the main module is accelerated up to 17MeV, and transported to the undulator.
After the FEL interaction, the electron bunch is reinjected
into the main module at deceleration phase for the energy
recovery. The reinjection phase is controlled by changing
recirculation path length. We installed the second arc on
movable tables for this purpose.
The recirculation loop consists of two triple-bend arcs
and a half chicane before the undulator. Each arc has
two families of quadrupoles which enable to vary R 56
while maintaining achromaticity. This variable R 56 is especially required in the second arc, because we need energyspread compression in the return-path. The second arc
also has two families of sextupoles to compensate secondorder aberrations T166 , T266 , T566 arising from large energyspread due to the FEL interaction. The energy acceptanceof the return arc was studied by particle tracking and foundto be 7% (full energy spread) [6].

EXPERIMENTAL RESULTS
Energy-recovery operation was demonstrated at 19,
February 2002. The energy recovery was confirmed froma beam current signal and an RF amplifier signal. Figure
2 is a beam current signal from a current transformer atthe exit of the second main module. We can see both accelerating and decelerating bunches alternatively. In thisexperiment, the bunch interval is 96ns and the recirculation
time is 133ns.
We measured RF forward power from the RF amplifier to the main superconducting cavity for evaluating theenergy-recovery ratio, which is the ratio of recovered RF
power to the beam power. Figure 3 shows RF forward signal for a 100m s beam macropulse. When we interrupt therecirculation beam by screen to turn off the energy recovery, the signal shows beam loading − 105mV from the baselevel, which is − 230mV from the ground level and corresponds to reflected power for beam-off. If we turn on theenergy recovery, the beam loading is almost canceled by
two accelerating and decelerating beams. Although we still
have small fluctuation, 4mV at peak, inside the macropulse
with energy recovery, the energy-recovery ratio is evaluated as 98% assuming the linearity of the envelope detector.
First FEL lasing with the ERL configuration was obtained at 14 Aug. 2002. Figure 4 is an example of cavity length detuning curve for FEL power. The detuning

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Half Chicane
1st ArcUndulator

1-cell SCA

2nd Arc

Beam Dump

SHB

Main 5-cell SCAInjection Merger

Electron Gun

Figure 1: The layout of JAERI Energy-Recovery Linac. An electron bunch generated by 230kV electron gun is accelerated to 2.5MeV and injected into the energy-recovery loop. The electron bunch is accelerated to 17MeV by mainsuperconducting cavities and transported to the FEL undulator. The electron bunch is, then, reinjected to the main cavities
and decelerated down to 2.5MeV and collected by a beam dump.
made gain, loss and spectral measurements yet.
recirculation

100FEL power (W)

80

acc.

dec.

acc.

dec.

60
40
200-40-35-30-25-20-15-10-505∆L (µm)Figure 2: Beam current signal from a current transformer
at the exit of the second main module.

Figure 4: A cavity length detuning curve for FEL power.

UPGRADE FOR A 5-10KW FEL
w/ ERbeam load
w/o ER

Figure 3: RF forward power to the first main module.
length, D L , is relative value from the position at peak, andan FEL pulse is pushed forward by 4 D L every interaction,in the operation of 10.4MHz. FEL macropulses from theHgTeCd detector suggest single supermode lasing for large
D L , and superradiance for small D L , although we have not

To achieve the FEL power of 5-10kW, we need to increase both electron beam power and FEL efficiency. Since
the higher FEL efficiency induces the larger energy spreadin the electron beam, the FEL efficiency is restricted byenergy acceptance of the return arc. Numerical simulations
based on a 1-D FEL code and PARMELA showed that FEL
efficiency of 1.5% is possible with the present arc configuration.
The beam current is now limited by the injector. Eachsingle cell cavity in the injector has been driven by 6kW
RF amplifier, which is designed for 5mA. We are replacing
it by a 50kW klystrode-IOT, which will enable us to accelerate 40mA beam current. The grid pulser has been modified to generate electron bunches with 20MHz repetition,
doubled repetition of the original one [7]. The injection ofelectron beam, 10mA, will be started soon. Design of a83MHz grid pulser for 40mA beam injection is also underinvestigation.
After all the injector upgrade is completed, the electron beam power at the undulator will be 680kW (40mA,

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

17MeV) and will produce 10kW FEL power with 1.5% efficiency. It is, however, noticed that a some amount of FEL
power is lost in the optical cavity due to diffraction. Design of an optical cavity with small diffraction loss is underway[8].
In an energy-recovery linac, it is known that transverse
beam break up (BBU) triggered by higher order mode(HOM) instability restricts beam average current[9]. We,therefore, carried out a HOM stability analysis by using atwo dimensional BBU code together with measured HOM
parameters and designed beam optics. The analysis shows
the instability threshold is about 3A and our design current
40mA is far below the threshold[10]. We conclude that theHOM instability is not a critical issue in our ERL.

OTHER R&D ISSUES
Research towards the high-power FEL is our primary
goal as described above. We have, however, other research
activities on accelerator technology, FEL physics, FEL applications, ERL applications.
In the operation of our superconducting accelerator, variation of RF amplitude and phase due to temperature drift of
the atmosphere has been the most critical problem, which
destroys stability and reproducibility of the accelerator. Ithas been found that long cables of the feed-back loop
( ∼ 100m) should be shortened to a few meters, and modification of the low-level controller is necessary. We decided to fabricate a new RF low-level controller and installit in the accelerator room, where the cables of the feedback loop for each cryomodule becomes less than 2m long.
A special cable with better stability against the temperaturedrift is also prepared to deliver the reference signal to the
low-level controllers[11]. These upgrade of the RF control
system will be completed by the end of this fiscal year. We
also plan to replace the control system of the magnet andthe electron gun. The new system using mTRON will be
installed soon [12].
For an FEL application, we investigate material processing using ultrashort FEL pulses such as drilling, cutting andwelding. By using FEL pulse shorter than pico-second, wecan avoid thermally induced stress and debris generation,
which has been unavoidable in material processing with Qswitched YAG lasers or CO2 lasers.
Application of self-chirped FEL pulses is also studied.
We recently found that frequency chirp is induced in an
FEL pulse, if the FEL oscillator has large gain and is operated at perfectly synchronized cavity length[13]. We
demonstrated generation of an FEL pulse with frequency
chirp of 14.3% and duration of 319fs. A laser pulse withsuch large frequency chirp can be used for quantum control of chemical reaction: the resonant excitation of atomic
or molecular systems, which have an anharmonic potential
ladder[14].
We also explore a future light source based on the ERLtechnology, high-flux photon sources in a wide range ofphoton energy from THz radiation to gamma-ray. In thestudy of ERL light sources, we designed an ERL-loop toaccelerate an electron beam over 100mA in average current
without HOM instability[15], and made cost estimation fora 6GeV ERL light source[16]. Dilution of beam emittancedue to coherent synchrotron radiation in an ERL-loop isinvestigated via matrix approach, which enables us to scan
numerous parameters for the design of achromatic cells ofminimum emittance dilution [17].

CONCLUSION
An energy-recovery linac has been developed for a highpower free-electron laser at JAERI. The energy-recovery
operation has been demonstrated successfully and the linac
is now operated as designed. An R&D program towards
5-10kW FEL is in progress, which includes injector upgrade, a HOM analysis, optical cavity optimization. We
plan to demonstrate material processing and quantum control of chemical reaction using the high-power short-pulse
FEL. Basic research for future ERL light sources is also inprogress.

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