Nuclear Instruments and Methods in Physics Research A 507 (2003) 382–387

Status of SPring-8 compact SASE source FEL project

T. Shintake*, T. Tanaka, T. Hara, K. Togawa, T. Inagaki, Y. J. Kim, T. Ishikawa,H. Kitamura, H. Baba, H. Matsumoto, Shigeru Takeda, M. Yoshida, Y. Takasu

SPring-8, RIKEN, Harima Institute, Hyogo 679-5148, Japan

Abstract

To develop the fourth generation light source, RIKEN has started research program on an X-ray FEL source based

on SASE in 2002. For the first stage of the program, in order to develop key technology specially required for the X-ray

FEL, and to study technique for handling of high-peak-power radiation, RIKEN is developing the SCSS-machine:

SPring-8 Compact SASE Source, which aims to generate FEL radiation at VUV region using low-emittance electron

beam at 0.5–1 GeV energy. SCSS uses three key technologies (1) low-emittance electron injector with 500 kV thermionic

pulsed gun, (2) high-gradient C-band (5712MHz) RF system and (3) in-vacuum type short period undulator. This

paper reports the current R&D status of the SCSS project.

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 41.60.C

Keywords: FEL; SASE; X-ray laser; Low emittance; Undulator; C-band

1. Introduction

An X-ray FEL based on the SASE conceptrequires a large-scale accelerator and a longundulator. It can provide X-ray beam in a fewbeam lines only, therefore construction cost perone beam line becomes quite high compared tothat in the conventional SR machines. This is thereason why the X-ray FEL is designed as aparasitic facility attached to an existing electronaccelerator, such as the LCLS project at SLAC,which aims to use the two-mile Linac, or TESLA

project which is designed as an integrated facility intoa future electron and positron linear collider. There-fore, the opportunity to have an X-ray FEL facility isquite limited and will be regionally localized.

Since the X-ray FEL will open a wide range ofnew science, many FEL-machines will be requiredall over the world in the future, similar to today’sSR source facilities. To realize this dream, themachine cost should be minimized. Reasonablecost would be the construction cost of a few X-raybeam lines in the existing SR facilities. To do this,the most important point is to make the machinesize compact, because the building as well as themachine itself becomes inexpensive. Therefore, wenamed our machine as SPring-8 Compact SASESource (SCSS) [1].

*Corresponding author. Tel.: +81-791-58-2929; fax: +81-

791-58-2840.

E-mail address: shintake@spring8.or.jp (T. Shintake).

0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0168-9002(03)00949-5

In the SCSS project, the following three keytechnologies help to realize the compact machineas illustrated in Fig. 1.

(1) Low-emittance beam injector based on ther-mionic single crystal CeB6 cathode, whichmakes the FEL gain higher, and saturationlength shorter.

(2) High-gradient C-band accelerator. The accel-erating gradient is as high as 40 MV/m, whichenables the main linac length being only 30 mto reach 1GeV.

(3) In-vacuum undulator, to allow short undula-tor period, thus the beam energy becomes

lower, and smaller accelerator size. It alsocontributes to shorten the FEL gain length.

This paper describes the basic machine design,and current status of hardware R&D on theelectron gun, C-band RF-system and the short-period undulator.

2. SCSS machine

2.1. Machine layout

Fig. 2 shows the machine layout at the finalstage of the SCSS project, whose parameters aresummarized in Table 1. SCSS consists of a low-emittance electron injector, the C-band main linac,the bunch compressors and an undulator sectionfor FEL interaction.

In order to saturate FEL at shortest wavelengthin SCSS: lx ¼ 3:6 nm within 20 m, we need a shortbunch of 0.5 ps. FWHM, peak current of 2 kA,and 1 nC charge.

Note that, in this paper, we describe the bunchlength in FWHM value rather than rms vale ðszÞ:This is because in our system the longitudinalcurrent profile has a flat shape rather than aGaussian shape.

In Fig. 2 showing the final stage, we will use fourC-band accelerator units, in which the beamFig. 1. What makes SCSS compact?.

E = 0.5 MeVq = 1 nClpk = 3A

∆t = 0.33 nsec

E = 230 MeVq = 1 nC

lpk = 2 kA∆t = 0.5 psec.FWHM∆t = 0.15 mm.FWHM

E = 1 GeVq = 1 nC

lpk = 2 kA∆t = 0.5 psec.FWHM∆t = 0.15 mm.FWHM

∆E/E = 2×10-4

εn = 2 πmm.rad

20 MeV1 = 1 nC

250 A4 psec1.2 mm

λx = 3.6 nm

ρ =8.9 × 10-4

Lg = 0.94 mLsat = 20 m

Psat =1.9 GW

500 kVPulse Gun

476 MHzBuncher-200 kV

L-bandLinac

C-band Linac C-band Linac

EnergyFilter

20 MV

476 MHzBooater Cavity~600 kV

Unit-1 Unit-2 Unit-3 Unit-4BC

×1/8

X-bandCorrection Cavity~70 MV

40 Mv/m1.8 m × 4/unit288 MeV/unit

40 Mv/m1.8 m × 4/unit288 MeV/unit

λu =15 mmKmax = 1.3

gap.min = 3.7 mm<βx,y> = 10 m

L = 4.5 × 5 = 22.5 m

Undulator

Fig. 2. SCSS beam line layout at final stage (beam energy 1 GeV, radiation wavelength 4 nm).

T. Shintake et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 382–387 383

energy will reach 1GeV. For the first phase, wewill start FEL experiment at 230 MeV using oneunit of the C-band system, and observe FELperformance at UV region first, from which wewill investigate the electron beam quality.

It should be noted that the beam parameters inTable 1, and the system diagram in Fig. 2 are still atentative design. In a recent study, it has beenunderstood that the emittance dilution due to theCSR effect in the bunch compressor dominates thebeam quality. To avoid this, we are designing aninjector system to compress the electron bunchshorter, and relax the compression factor in thebunch compressor.

2.2. Electron injector

In the SCSS project, we chose a high-voltagepulse-gun with thermionic-cathode, instead of RF-Gun. As known in SASE-FEL theory, the qualityof the ‘‘internal’’ structure of the bunched beamdominates FEL performance, that is, the sliceemittance and slice energy spread directly affectthe FEL gain. We believe a single crystal LaB6 orCeB6 cathode will provide a high performancebeam with uniform density, with a laminar flowwithout turbulence.

In the HV pulse gun, 3 A beam with 300 ns flat-top pulse will be generated from a single-crystalCeB6 cathode of 3 mm in diameter (Fig. 3). TheR&D issues related to this design are (1) develop-ment of reliable 500 kV pulsed power supply, (2)reliable heating mechanism of the cathode (nom-inal operation temperature is 1450�C). We havecompleted the high-voltage test, and high-tem-perature test in 2002.

As seen in the injector layout of Fig. 4, rightafter the gun, a beam chopper will be prepared,which cuts out the rising and falling parts of thepulse, and forms a 2 ns pulse beam. The 476 MHzpre-buncher provides energy modulation of400 kV peak-to-peak, followed by energy filter tocut the energy tail (top and bottom). After drifting800 mm, due to the velocity difference, electronsform a short bunch. Before the space chargebreaks the beam, we accelerate to 1MeV in thebooster cavity.

The L-band accelerator captures the bunch, andaccelerates to 20 MeV. Since the bunch length atthe entrance is still long, we chose L-bandfrequency, this is the lowest frequency band wherea high-peak power klystron is commerciallyavailable and also the fabrication of the accelerat-ing structure is relatively easy. The first section ofL-band accelerator compresses the bunch lengthby off-crest acceleration. In the second-half L-band accelerator, the beam energy reaches to20 MeV [2].

Table 1

SCSS design parameter at final stage: 1 GeV

Bunch charge Q nc

Normalized emittance enx;y 2 pmmmrad

Final electron energy E 1 GeV

Final rms energy spread sd 0.02 %

Final FWHM bunch length Dz 0.15 mm

Dt 0.5 ps

Peak current Ipk 2 kA

Undulator period lu 15 mm

Radiation wavelength lx 3.6 nm

Minimum gap g 3.7 mm

Maximum K-parameter K 1.3

Undulator unit length L1 4.5 m

Total undulator length 22.5 m

Beta function b 10 m

FEL parameter r 8.9 � 10�4

Gain length Lg 0.94 m

Saturation length Lsat 20 m

Saturation power Psat 2.0 GW

Note that the bunch length is denoted by FWHM value.

Fig. 3. Newly designed cathode assembly, using CeB6 single

crystal cathode and graphite heater.

T. Shintake et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 382–387384

2.3. C-band main linac

Fig. 5 shows the C-band RF system of the mainaccelerator. In one unit, two klystrons of 50 MWpeak power drives four accelerating structures of2m length each, capable of 35 MV/m accelerationgradient for multi-bunch and 40 MV/m for singlebunch. Each unit is capable of acceleration of288 MV, and totally four units provide 1GeVbeam.

The C-band (5712 MHz) accelerator technologyhas been developed at KEK Japan as the mainlinac of the future 500 GeV e+e� Linear Colliderproject. The SCSS will provide a best string test ofthe main linac system for the Linear Colliderproject.

Among the recent R&D items, the inverter typeHV power supply for charging the PFN capacitorof the klystron power supply (klystron modulator)is very important in order to realize stableoperation of a short-wavelength FEL. By meansof a two-step charging scheme, the newly devel-oped HV power supply has achieved voltagestability better than 0.14% full-width (see Fig. 6).

Other related R&D items are:

(1) Compact closed type modulator for klystron.

(2) C-band 50 MW PPM klystron.(3) Stable HV voltage monitor using ceramic

divider.(4) Pulse-to-pulse energy and phase feedback

system using digital vector detector/modula-tor.

(5) Stable support girder R&D for acceleratingstructure and other accelerator component,which uses hard concrete and precision moverof rotary cam design.

For detailed information on related aboveitems, please refer http://www-xfel.spring8.or.jp.

2.4. Bunch compressor

In the chicane bunch compressor, the bunch iscompressed to 0.5 ps FWHM. Since the coherentsynchrotron radiation (CSR) effect breaks thetransverse emittance, we need careful design. Arecent new finding was made at DESY SLACcollaboration: the CSR amplifies the micron-scaledensity fluctuation on the incoming bunch in thebunch compressor, it is now called ‘‘CSR instabil-ity’’ [3]. To avoid CSR instability and emittancebreak-up, we are trying to shorten the bunch

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1428 MHz10 MV/M × 1 mTraveling Wave

1428 MHz10 MV/M × 2 mTraveling Wave

EnergyAnalyzer

3000

Low Emittance Injector for SASE-FEL

HV Pulse GunSub-harmonic

BuncherEnergy Filter

476 MHzBooster Cavity

L-BandBuncher

L-BandAccelerator

CathodeCeB6 single-crystal

φ3 mm1450 deg.C

εn 0.4 × π µrad

Vp 500 kVIp 3A

Filter 400~600 kVIp 3A

tFWHM 1.5 µsec

K 0.01 µA/V1.5fr 60 pps

Vp 20 MVIp 100 AtFWHM 10 psecQ 1 nC

tFWHM 0.3 nsec

ToMain Linac

Fig. 4. Injector layout.

T. Shintake et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 382–387 385

length in the injector by proper modulationvoltage, and reducing the compression factor atthe downstream bunch compressor. For designdetails, refer Ref. [4].

2.5. Undulator

We use ‘‘in-vacuum type’’ undulator. The meritsof this design are:

(1) Since there is no vacuum envelop, we can bringthe magnets closer to the beam, thus it generateshigher field with short undulator period.

(2) Since there is no vacuum envelop of fixedheight, for the machine tuning, we can widelyopen the aperture, which enables a bigemittance beam to pass at the beginning ofmachine tuning.

Fig. 5. C-band RF System for high-gradient beam acceleration.

Fig. 6. Voltage stability of inverter HV power supply for the

klystron modulator.

T. Shintake et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 382–387386

(3) Tapering the gap and the K-value along theundulator. This is useful to recover a part ofthe energy loss due to the resistive wake-field,or to enhance the power conversion efficiencyat the final radiating section.

The SCSS undulator consists of five segments,each 4.5 m long. The total active length is 22.5 m.Fig. 7 shows one segment of the prototypeundulator, whose parameters are summarized inTable 1. We chose the operational gap size to be3.7 mm at the minimum; at this aperture theresistive wake-field effect does not deteriorate theFEL performance.

One big technical issue is ‘‘the longitudinalphase matching’’ and ‘‘transverse alignment’’between the undulator segments. With analyticalmodel and 1D simulation, we found that when asegment length is longer than a few times of thegain length, SASE-FEL does not fully requireperfect overlapping of optical wave to the electronbeam [5]. With 3D simulation, we found that thetransverse alignment is important to match theradiation wavefronts. Therefore, in case the onesegment length is larger than a few times the gainlength:

(1) Longitudinal phase matching is not required.(2) Transverse alignment is required to keep

orientation of the radiation wavefront.

The transverse alignment tolerance for a 4.5 mlong undulator segment in SCSS is about 50 mm.We will install focusing quadrupole magnet in eachgap between undulator segments, and attach RF-BPMs onto the quadrupole magnet. By means ofthe beam-based alignment, we can minimize thetrajectory error at Q-magnet, which ensures theminimum transverse kick from the Q-field, andmaximize the FEL gain. The detailed hardwareconfiguration is under design.

3. SCSS milestone

From 2002 to 2003, we have been developingkey technologies required for the SCSS machine.From 2004, we will start machine construction,aiming to start FEL experiment in 2005 at 50 nmwavelength using 230 MeV electron beam, deliv-ered by one C-band accelerator system. After that,by installing successive C-band accelerator units,we will test FEL operation at shorter wavelength,ultimately 4 nm wavelength, and start providingsoft-X-ray beam to users.

References

[1] T. Shintake, SPring-8 Compact SASE Source, SPIE2001,

San Diego, USA, June 2001.

[2] Y.J. Kim, et al., Beam Parameter Optimisation for the

SPring-8 Compact SASE Source, proceedings of the XXII

International Linear Accelerator Conference (LINAC2002),

Kyongju Korea, Aug.19–23, 2002.

[3] CSR Workshop 2002, January 14–18, 2002 at DESY-

Zeuthen (Berlin, GERMANY). http://www.desy.de/csr/.

[4] http://www-xfel.spring8.or.jp.

[5] T. Tanaka, H. Kitamura, T. Shintake, Phys. Rev. ST Accel.

Beams 5 (2002) 040701.

Fig. 7. Prototype in-vacuum undulator for SCSS using NdFeB

magnet assembly. One segment length is 4.5 m.

T. Shintake et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 382–387 387