A compact free-electron laser forgenerating coherent radiation in theextreme ultraviolet region

A list of authors and their affiliations appears at the end of the paper

Published online: 27 July 2008; doi:10.1038/nphoton.2008.134

Single-pass free-electron lasers based on self-amplifiedspontaneous emission14 are enabling the generation of laserlight at ever shorter wavelengths, including extremeultraviolet5, soft X-rays and even hard X-rays68. A typical X-rayfree-electron laser is a few kilometres in length and requires anelectron-beam energy higher than 10 GeV (refs 6, 8). If suchlight sources are to become accessible to more researchers, asignificant reduction in scale is desirable Here, we reportobservations of brilliant extreme-ultraviolet radiation from a55-m-long compact self-amplified spontaneous-emissionsource, which combines short-period undulators with a high-quality electron source operating at a low acceleration energy of250 MeV. The radiation power reaches saturation atwavelengths ranging from 51 to 61 nm with a maximum pulseenergy of 30 mJ. The ultralow emittance (0.6pmmmrad) of theelectron beam from a CeB6 thermionic cathode

9 is barelydegraded by a multiple-stage bunch compression system thatdramatically enhances the beam current from 1 to 300 A. Thisachievement expands the potential for generating X-ray free-electron laser radiation with a compact 2-GeV machine.

Synchrotron radiation10 is widely used as a source of light,with wavelengths ranging from the infrared to hard X-rays.Third-generation synchrotron facilities have been providingbrilliant, short-wavelength radiation for various applications at anumber of beamlines surrounding large storage rings. The self-amplified spontaneous-emission free-electron laser (SASEFEL)has been described as a post third-generation synchrotron sourcethat produces much brighter radiation with a subpicosecondpulse duration. In 1984, Bonifacio, Pellegrini and Narducciproposed the basic concept1. Subsequent extensive studies at theStandard Linear Accelerator Center (SLAC) and the DeutschesElektronen-Synchrotron (DESY) opened up the possibility ofoperating SASEFEL in the hard X-ray region using large-scaleaccelerators originally developed for high-energy physics6,8.

Both third-generation and SASEFEL sources commonly useaccelerators to produce high-energy electron bunches andundulators to generate radiation. However they have onesignificant difference in their basic configurations: the former is astorage-ring-based source, whereas the latter is a linear-accelerator-based source in which only a single beam port isavailable at one time (a similar limitation to that of conventionalvisible lasers). The number of machines available is thus a basicparameter in controlling the development of scientificapplications8,11 for SASEFEL.

An effective means for making FEL sources more widespread isa reduction of machine scale. In pursuit of this goal, recent

proposals include high-gradient accelerators driven by alaser-plasma wakefield12 or an FEL seeded with high-order laserharmonics13. Another option, a compact SASE source thatsuppresses the acceleration energy, is a simple, reliable and stablescheme with the capability of being used in the hard X-rayregion. For such a low-energy accelerator, a decrease in theperiodic length lU of the undulator magnets is a crucialrequirement for generating short-wavelength radiation, becausethe radiation wavelength l is expressed as

l lU1 K2=2=2ng2; 1

where n, K and g are the order of harmonics, the magnetic deflectionparameter and the g-factor (proportional to the beam energy),respectively. For this short-period undulator, the peak magneticfield Bo should be increased to avoid decreasing the integratedmagnetic field along the unit period. The recent development of in-vacuum undulator technology14, which encloses the undulatormagnets inside a vacuum chamber, has successfully increased Bo bydecreasing the gap between the magnets.

Although this scenario appears simple, low-energy operationimposes a serious limit on the electron-beam parameter. Thenormalized emittance 1n, which defines the intrinsic beam extensionin the transverse phase space, should be significantly reduced.

Vertical

Horizontal

Horizontal position

5.6 mm

Inte

nsity

Figure 1 Profile of the electron beam. The spatial distribution of the 500-keV

electron beam was measured at a position 0.5 m downstream from the CeB6gun. The inset shows the cross-section along the vertical centre. A flat-top

profile was achieved within the circular boundary.

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According to a SASEFEL theory1,15, in the linear regime (LU/LG.15),radiationpowerexponentially increaseswith the ratioLU/LG.Here,LUisthe undulator length, and LG is the gain length given by

LG lU

4p3

pr

2

The denominator r, termed an FEL parameter, is given by

r g2l2reIP

16p2ceb1nF1 K

13

3

where IP, b, re, c and e are the peak current, the average betatronfunction, the classical electron radius, the speed of light and theelectron charge, respectively. The function F1(K) is provided by

F1K K2

1 K2=22 J0K2=4

1 K2=2

J1K2=4

1 K2=2 2

with the Bessel functions Jv(x). From equation (3), a desirableincrease in the FEL parameter r, which is the reciprocal of theperiodic number LG/lU for one gain length, is accomplished byincreasing the electron density by having a small emittance 1nand a high peak current IP. For lower-energy operation, therequirement of higher electron density is emphasized in order tocompensate for the decrease of g in equation (3).

It is particularly difficult to suppress the normalized emittance1n, because this requires precise control of all processes for beamemission and the subsequent evolution at a low-energy region,which are influenced in a complicated manner by the many-bodyelectromagnetic interactions among all electrons. However, anexceptionally simple situation arises for a laminar-flow, uniformlydistributed beam within the cylindrical beam boundary16. Here,evolution under internal forces is analytically given by a simpleenvelope equation. The beam dynamics under weak external fields(that is, focusing and acceleration) are easily predictable, and theunique characteristics allow a dramatic enhancement of the beamcurrent by a gradual compression, even when the emission currentis low, while maintaining the initial normalized emittance.

For a simple approach to this laminar-flow beam, weinvestigated the possibility of using a thermionic gun, which is

widely used in electron accelerators as it has good stability andreliability. Although a typical emittance in a conventional designis as large as 1n 30pmmmrad, a new design based on asingle-crystal, smooth-surface CeB6 cathode with a 3-mm-diameter rod has successfully achieved an ultralow emittance of1n 0.6p mmmrad, which is close to the theoretically expectedvalue of 0.4p mmmrad for 90% of the core bunch charge9

(Fig. 1). A beam current of 1 A, which is designed to suppressunwanted spacecharge repulsion at the initial beam energy of500 keV, should be enhanced by orders of magnitude for a SASEFEL arrangement. To perform a proof-of-principle verification ofthe total SASEFEL system, a SPring8 Compact SASE Source17

(SCSS) test accelerator was constructed18 with an accelerationenergy of 250 MeV. The parameters are listed in Table 1.

Figure 2 shows a schematic of the accelerator. The core of theelectron beam from the gun is extracted with a 1-ns beamdeflector and a 5-mm circular collimator. The beam is graduallyaccelerated and compressed with a multiple-stage bunchcompression system to enhance the peak current up to Ip 300 A.The purpose of this adiabatic compression scheme is to suppressan unwanted increase of the space-charge force in the course ofcompression; the compression and acceleration parameters havebeen designed to stabilize the longitudinal density of the electronson the rest frame. The 45-MeV electron beam from this injectorsection is further accelerated to 250 MeV with a normal-conducting C-band linac that applies a high-gradient field of

Table 1 Parameters of the SCSS test accelerator.

Parameter Present value

Electron beamBeam energy, EB 250 MeVBunch charge 0.3 nCPeak current, Ip 300 ARepetition rate 10 Hz

(60 Hz max.)Undulator and photon beamPeriodic length, lU 15 mmNumber of periods 600Maximum K 1.5Minimum gap 3 mmAverage betatron function, bX/bY 7.2 m/1.6 mRadiation wavelength, l 30261 nm

EG DF PB BS S-APS S-TWA

BC C-TWA1 Chicane

UND 1 UND 2 250 MeVdump

Injector

Injector detail

50 MeVdump

Photondiagnostics

C-TWA2

Figure 2 Schematic configuration of the SCSS test accelerator. EG, 500-keV electron gun; DF, deflector with collimator; PB, 238-MHz pre-buncher; BS, 476-MHz

booster; S-APS, S-band standing-wave alternating periodic structure; S-TWA, S-band travelling-wave accelerating structure; BC, bunch compressor; C-TWA, C-band

travelling-wave accelerating structure; UND, undulator. The core of the 500-keV electron beam from the electron gun is extracted by the deflector. The PB provides

energy modulation along the electron bunch, BS increases the beam energy to EB 1 MeV and enhances the peak current to Ip 50 A at the entrance of the S-APS.S-TWA further increase these values to EB 45 MeV and Ip 80 A. The peak current is finally boosted by the bunch compressor up to Ip 300 A. This beam isaccelerated to 250 MeV with a couple of the C-band acceleration units. After the beam halo is removed by the chicane, the two undulators emit SASE radiation.

The electron beam is sent to the 250-MeV beam dump, and the photon beam is transported to the diagnostics section.

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35 MV m1. The beam is transported into two short-period in-vacuum undulators, which generate SASE radiation. The radiationwavelength is rapidly tunable by adjusting K by controlling theundulator gap. We note that this high tunability is essential forspectroscopic research in the EUV region.

We have evaluated the radiation properties of SASE (seeMethods). Figure 3a shows the measured pulse energy Ep as afunction of the radiation wavelength l, tuned by controlling thegap of the two undulators. The observed wavelength range,30 l 61 nm, corresponds to a range of the deflectionparameter, 0 K 1.5, in equation (1). The maximum pulseenergy reaches Ep 29+9 mJ at l 60.8 nm. For comparison, asimilar measurement was performed using only the upstreamundulator. In both cases, Ep monotonically increases for larger l(that is, larger K). This behaviour qualitatively agrees with thatof the one-dimensional FEL theory in equation (3): an increaseof K from zero raises the FEL parameter r. More importantly,FEL theory states that the initial exponential power growth in thelinear regime is finally saturated at LU/LG &15 due to the balance

of the energy exchange between the radiation field and theelectrons. The power fluctuation, which is initially small in thespontaneous radiation regime (K 0), is expected to be muchmore significant in the linear regime, and finally to be suppressedunder this saturation regime. In Fig. 3a, the increasing rate of thefunction Ep(l) is found to be saturated for l 50.6 nm using thetwo undulators. Simultaneously, the fluctuation of the pulse energy,which is plotted in Fig. 3b, drastically decreases to sP 10%.The results are explicit proof of saturation being reached at50.6 l 60.6 nm. Note that this fluctuation level sP 10%agrees well with predictions made by a three-dimensional FELsimulation code, SIMPLEX19 (http://radiant.harima.riken.go.jp/simplex): the fluctuation originates mainly from the SASEstartup process, not from instabilities of the accelerator system.

We present other essential properties of SASE radiation. Figure 4ashows plots of a typical single-shot spectrum and also one averagedover 100 shots. The observed spikes in the former are acharacteristic feature of SASE radiation, and arise from noise20.Figure 4b shows the sequential spectrum distribution over the 100

102

101

100

101

102

103

104

Puls

e en

ergy

( J

)

60

30 40 50 60

0

10

20

30

40

50

Pow

er fl

uctu

atio

n (%

)

Wavelength (nm)

30 40 50 60Wavelength (nm)

Figure 3 Pulse energy and fluctuation of the SASE radiation. a, Pulse energy

measured using two undulators (red filled circles) and one undulator (black open

squares). The simulation results are shown as solid lines with normalized

emittances 1n 0.5 (green), 0.7 (orange) and 0.9 (blue) p mm.mrad,respectively. The error bar indicates the uncertainty (30%) for the pulse energy

calibration. b, Power fluctuation measured using one undulator (black open

squares with a solid line) and two undulators (red filled circles with a solid line).

With one undulator, the fluctuation increases with larger wavelength; the

radiation process transfers from the stable spontaneous radiation regime to the

highly fluctuating SASE linear regime. With two undulators, the fluctuation for

l 40 nm increases with l, similarly to the one-undulator case. However, thefluctuation decreases for l . 40 nm, and is drastically suppressed for

l . 51 nm, which indicates a movement into the saturation regime.

Inte

nsity

(a.u

.)

0

1

2

1

50

100

Wavelength (nm)

Shot

num

ber

58 60 62

Wavelength (nm)

58 60 62

Figure 4 Spectra for SASE radiation. a, A single-shot spectrum (blue solid

curve) and averaged spectrum over 100 shots (red solid curve), measured at a

fixed K of 1.45 with the two undulators. The typical spike width of 0.05 nm inthe single-shot spectrum corresponds to a pulsewidth on the order of 100 fs, if

we assume a non-chirped pulse. The averaged spectrum width is 0.58% in

FWHM. b, Stability of the spectra over the 100 sequential shots. Every single-

shot spectrum is vertically arranged with the shot number. The root-mean-

square (r.m.s.) jitter of the mean wavelength was as low as 0.1%. The white

dashed line corresponds to the shot shown in a.

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shots. The jitter of themeanwavelength is sufficiently smaller than theaverage spectrum bandwidth. The beam divergence was measured tobe 200 mrad in full-width at half-maximum (FWHM), which isconsistent with simulated results. The shot-to-shot pointing stabilityis excellent; the position jitter is less than 5% of the FWHMbeam size.

Evaluation of electron-beam emittance is the final objective. Toachieve this, we analysed the measured results of the radiationenergy Ep(l), because this function strongly depends on theelectron beam density IP/1n through the FEL parameter r, as isessentially expressed in equation (3). Additional informationrequired for the analysis was as follows. The temporal distributionof the electron beam was measured to be 300 A by the r.f. zero-phasing method (see Methods) and the slice energy spread wasassumed to be 0.05%. Using these data, an FEL simulation wasperformed using SIMPLEX, varying the emittance. It was foundthat an emittance of 1n 0.7pmmmrad excellently reproducesthe experimental result, as shown in Fig. 3a. Although, as statedabove, we assumed that the emittance is invariant along thebunch, the emittance at both edges can be slightly broadened dueto a nonlinear spacecharge effect. The value 1n 0.7pmmmradis thus regarded as being the upper limit. In conclusion, theoriginal emittance 1n 0.6pmmmrad from the gun waspreserved even with an extremely high compression ratio of 300.

Two issues relating to the accelerator and the undulator featureprominently in the development of the compact FEL (seeSupplementary Information). The first is a need for a furtherincrease in the peak current of the electron beam. The present value,IP 300 A, is limited by nonlinear components in the bunchcompression process. This degradation can be compensated by usingthe well-established method of introducing additional correctioncavities (see Chapter 7 in ref. 6). We simulated the evolution of IPwith an accelerator configuration including such cavities. Weobtained two typical results: (a) IP 1 kA at a beam energyEB 450 MeVand (b) IP 6 kA at EB 2 GeV. For condition (a) itis possible to generate EUV radiation at l 13 nm (the targetwavelength of EUV lithography) using the present undulator withlU 15 mm and K 0.9. The pulse energy reaches 70 mJ with asmall undulator length of 13.5 m. We emphasize that this isachievable simply by using a combination of existing technologies.

The second target is more challenging: a further reduction of theperiodic length lU of an undulator. This requires greatly increasingthe peak magnetic field. A promising technology involves theintroduction of cryogenic cooling to the magnets to enhance themagnetic field21: K. 0.7 is achievable at lU 5 mm by settingthe gap value to 1 mm. By combining the 2 GeV parametersrepresented in condition (b), the undulator will generate hardX-ray FEL radiation at l 0.2 nm with a saturation length within40 m. In this case, a strong wakefield induced at the small gapdiminishes the radiation power in the tail of the electron bunch.Thus the pulse energy is reduced to the moderate level of severalmicrojoules. However, a GW-level peak power is achieved with ashort pulse duration of several femtoseconds.

METHODS

PHOTON DIAGNOSTICS

The radiation properties were investigated with a spectrometer, a photodiodeand fluorescence monitors22. The system was placed downstream of a gold-coated deflecting plane mirror that eliminated background g rays. Thereflectivity was calibrated using synchrotron radiation at BL5B of UVSOR(http://www.uvsor.ims.ac.jp/) with an uncertainty of 10%. The spectrometercomprised an incident slit, a concave grating and a charge-coupled device

(CCD). The spectrum of the incident beam was horizontally dispersed on theCCD plane for single-shot detection. The resolution at l 60 nm was designedto be 0.05%. The pulse energy was measured with a photodiode. The efficiencywas calibrated at National Institute of Standards and Technology (NIST) andUVSOR BL5B with an uncertainty of 15%. Thin metal foils were used asattenuators for the high-intensity region. The transmittance of the foils wascalibrated using the spectrometer with an uncertainty of 10%. The totalambiguity in the pulse energy measurement was estimated to be 30%.A supplemental measurement of the pulse energy was performed with thespectrometer. The efficiency was calibrated by comparing the simulated intensityof spontaneous emission with that measured during a debunching operation.We confirmed that the results from both methods agreed.

RADIO-FREQUENCY ZERO-PHASING METHOD

The temporal profile of the electron bunch was evaluated to be Ip 300 Awith aFWHM width of 0.7 ps by the r.f. zero-phasing method23. Here, the electronbunch was linearly chirped with the second C-band acceleration unit,which operated at the zero-crossing r.f. phase for beam acceleration.The temporal distribution was given by the energy spectrum observed atthe dispersive chicane. The ambiguity of the measurement was estimated tobe as low as 15%.

Received 19 February 2008; accepted 16 June 2008; published 27 July 2008.

References1. Bonifacio, R., Pellegrini, C. & Narducci, L. M. Collective instabilities and high-gain regime in a free-

electron laser. Opt. Commun. 50, 373377 (1984).2. Milton, S. V. et al. Exponential gain and saturation of a self-amplified spontaneous emission free-

electron laser. Science 292, 20372041 (2001).3. Ayvazyan, V. et al. Generation of GWradiation pulses from a VUV free-electron laser operating in the

femtosecond regime. Phys. Rev. Lett. 88, 104802 (2002).4. Murokh, A. et al. Properties of the ultrashort gain length, self-amplified spontaneous emission free-

electron laser in the linear regime and saturation. Phys. Rev. E 67, 066501 (2003).5. Ackermann, W. et al. Operation of a free-electron laser from the extreme ultraviolet to the water

window. Nature Photon. 1, 336342 (2007).6. Arthur, J. et al. Linac Coherent Light Source (LCLS) Conceptual Design Report SLAC-R593

(Stanford, 2002).7. Tanaka, T. & Shintake, T. (eds) SCSS X-FEL Conceptual Design Report (RIKEN Harima Institute,

Hyogo, Japan, 2005).8. Altarelli, M. et al. (eds) XFEL: The European X-ray Free-Electron Laser, technical design report,

preprint DESY 2006-097 (DESY, Hamburg, 2006).9. Togawa, K. et al. CeB6 electron gun for low-emittance injector. Phys. Rev. ST Accel. Beams 10,

020703 (2007).10. Elder, F. R., Gurewitsch, A. M., Langmuir, R. V. & Pollock, H. C. Radiation from electrons in a

synchrotron. Phys. Rev. 71, 829830 (1947).11. LCLS. The First Experiments, SLAC-R611 (Stanford, 2000).12. Schlenvoigt, H.-P. et al. A compact synchrotron radiation source driven by a laser-plasma wakefield

accelerator. Nature Phys. 4, 130133 (2008).13. Lambert, G. et al. Injection of harmonics generated in gas in a free-electron laser providing intense

and coherent extreme-ultraviolet light. Nature Phys. 4, 296300 (2008).14. Kitamura, H. Recent trends of insertion-device technology for X ray sources. J. Synchrotron Rad. 7,

121130 (2000).15. Saldin, E. L., Schneidmiller, E. A. & Yurkov, M. V. The Physics of Free Electron Lasers

(Springer, Berlin, 1999).16. Reiser, M. Theory and Design of Charged Particle Beams (Wiley, New York, 1994).17. Shintake, T., Matsumoto, H., Ishikawa, T. & Kitamura, H. SPring8 Compact SASE source (SCSS).

Proc. SPIE 4500, 1223 (2001).18. Tanaka, H. et al. Low-emittance injector at SCSS. Proc. FEL 2006, 769776 (2006).19. Tanaka, T. FEL simulation code for undulator performance estimation. Proc. FEL 2004,

435438 (2004).20. Bonifacio, R., Salvo De, L., Pierini, P., Piovella, N. & Pellegrini, C. Spectrum, temporal structure

and fluctuations in a high-gain free-electron laser starting from noise. Phys. Rev. Lett. 73,7073 (1994).

21. Hara, T. et al. Cryogenic permanent magnet undulators. Phys. Rev. STAccel. Beams 7, 050702 (2004).22. Yabashi, M. et al. Photon optics at SCSS. Proc. FEL 2006, 785792, (2006).23. Wang, D. X., Krafft, G. A. & Sinclair, C. K. Measurement of femtosecond electron bunches using a rf

zero-phasing method. Phys. Rev. E 57, 22832286 (1998).

Supplementary Information accompanies this paper at www.nature.com/naturephotonics.

AcknowledgementsThe authors express special thanks to the staff of SPring8, in particular to T. Hasegawa and S. Tanaka formachine operation, S. Kojima, S. Indo and K. Nakashima for their technical support, H. Tomizawa for hisoutstanding insights, and H. Suematsu, N. Kumagai and H. Ohno for their continuous encouragement.We also thank H. Matsumoto and H. Baba for developing the C-band accelerator system.

Author informationReprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/.Correspondence and requests for materials should be addressed to M.Y.

TSUMORU SHINTAKE1, HITOSHI TANAKA1, TORU HARA1,2, TAKASHI TANAKA1,2, KAZUAKI TOGAWA1, MAKINA YABASHI1*, YUJI OTAKE1,YOSHIHIRO ASANO1,2, TERUHIKO BIZEN2, TORU FUKUI1, SHUNJI GOTO2, ATSUSHI HIGASHIYA1, TOKO HIRONO2, NAOYASU HOSODA1,

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TAKAHIRO INAGAKI1, SHINOBU INOUE1, MIHO ISHII2, YUJONG KIM1, HIROAKI KIMURA2, MASANOBU KITAMURA1, TOSHIAKI KOBAYASHI2,

HIROKAZU MAESAKA1, TAKEMASA MASUDA2, SAKUO MATSUI1, TOMOHIRO MATSUSHITA2, XAVIER MARECHAL2, MITSURU NAGASONO1,

HARUHIKO OHASHI2, TORU OHATA2, TAKASHI OHSHIMA1, KAZUYUKI ONOE1, KATSUTOSHI SHIRASAWA1, TETSUYA TAKAGI2, SUNAO TAKAHASHI2,

MASAO TAKEUCHI2, KENJI TAMASAKU1, RYOTARO TANAKA1,2, YOSHIHITO TANAKA1, TAKANORI TANIKAWA1, TADASHI TOGASHI1, SHUKUI WU1,

AKIHIRO YAMASHITA2, KENICHI YANAGIDA2, CHAO ZHANG2, HIDEO KITAMURA1,2 AND TETSUYA ISHIKAWA1

Affiliations for authors: 1RIKEN, XFEL Project Head Office, Kouto 1-1-1, Sayo, Hyogo 679-5148, Japan; 2Japan Synchrotron Radiation Research Institute, Kouto 1-1-1,

Sayo, Hyogo 679-5198, Japan; Present address: Free Electron Laser Laboratory, LaSalle Street Extension, Duke University, Durham, North Carolina 27708-0319, USA

(Y.K.); Nichizou Electronics and Control Corporation, Electronics Systems Division, Nishikujyo 5-3-28, Konohana-ku, Osaka 554-0012, Japan (M.K.); ULVAC, Head

Office, Hagisono 2500, Chigasaki, Kanagawa 253-8543, Japan (K.O.); Takagi Accounting Firm, Minami-Karasuyama 4-28-7, Setagaya, Tokyo 157-0062, Japan (T.T.);

*e-mail: yabashi@spring8.or.jp

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