Table 1. FEL radiation parameters at K=2.2 (for fundamental radiation)

213

XFEL

1. Status of XFEL Construction

In FY2006, a five-year construction project of an8-GeV XFEL facility was launched at SPring-8. Thisproject was designated as one of the key technologiesof national importance. Over the past three years,the construction has been progressing satisfactorily.Below we summarize our major achievements in2008: (i) a large number of accelerator componentswere produced at factories; (ii) the manufacture ofundulators was started; (iii) the specifications of XFELbeamline were determined; (iv) the electron-beamtransport line from the 8-GeV linac to the SPring-8storage ring was designed; and (v) the acceleratortunnel and undulator hall were completed.

Hereafter, we describe more details of theactivities in XFEL construction. First, we haveperformed the reoptimization of acceleratorparameters to provide a set of high-quality electronbeams. To this purpose, we have performed start-to-end simulations from an electron gun to an 8-GeVbeam dump. In these simulations, resistive wall wakefields in the undulator have been included. Withthese parameters, SASE-FEL radiation propertieshave been precisely evaluated. A summary ofperformance as well as the temporal profile areshown in Table 1 and Fig. 1, respectively.

To facilitate beam commissioning, we startedto investigate beam commissioning-related subjects,i.e., the distribution of electron beam loss includingdark current from the C-band acceleration system, thealignment-error tolerance of the accelerator structure,the effect of accelerator-structure vibration on beamquality, and the introduction of an rf deflector cavity for

PPrrooggrreessss ooff tthhee XXFFEELL PPrroojjeecctt

Fig. 1. Temporal profile of XFEL radiation atK=2.2. The red line shows a guide after smoothing.

FEL parameter

Radiation wavelength (nm)

Photon energy (keV)

Bandwidth

Source size (µm, rms)

Angular divergence (µrad, rms)

Beam size @ sample position (µm, rms, 100 m from ID exit)

Peak power (GW)

Pulse energy (mJ/pls)

Pulse width (fs, FWHM)

Repetition rate (Hz)

Power ratio of higher-order harmonic (2nd:1st)

Power ratio of higher-order harmonic (3nd:1st)

Photons per pulse (phs/pls)

4.4×10-4

1.3×10-4

2.8×10-3

0.13

9.9

9.2×10-4

33

0.73

80

29

30

60 (max) to 1 (min)

0.78

5.0×1011

– 30 – 20 – 10 0 Time (fs)

Pow

er (

GW

)

10

60

40

20

0 20

K= 2.2

30

measuring sl ice emittance and shot-by-shotlongitudinal phase space.

A test bunker for high-power rf components wasconstructed in the SPring-8 site, as shown in Fig. 2,(i) to ensure the performance of the delivered rfcomponents under a high-power condition, (ii) toestablish an aging procedure for stable operationwith design rf power and acceleration gradient and(iii) to accumulate fundamental data on dark currentgeneration and aging period. The first high-power testwas successfully performed: we confirmed to achievea high acceleration gradient of 35 MV/m at a repetitionrate of 60 Hz.

A new field correction procedure for an in-vacuumundulator was established with a manufacturedprototype undulator in order to achieve the high fieldquality required for SASE XFEL. The new procedure

consists of the following steps; the conventionalfield correction is performed first to compensate phaseand multipole-field errors in the field measurementbench with the vacuum vessel removed. To install thevacuum vessel, magnet arrays are detached from themechanical frame and then built again inside it. Thefield distribution is then measured with the developedin situ field measurement system named SAFALI (selfaligned field analyzer with laser instrumentation). Apossible gap deformation during the above assemblyprocess is analyzed with the measured phase errordistribution and corrected by adjusting the ball screwssupporting the magnet array assembly. Figure 3shows the results of the new correction procedure.We found that the final phase errors in the newmethod are sufficiently suppressed down to the samelevel as that obtained in the bench with the vacuumvessel removed.

We achieved three progresses on the beamdiagnostic system. First, a beam position monitorwith a target resolution better than 1 µm has beendeveloped. The detection sensitivity to a transversedisplacement (Fig. 4) has been evaluated. Thisresult, combined with the measured noise-level of anew DAQ (data acquisition) system, shows that aresolution of 30 nm is possible. Second, a screenmonitor has been developed for measuring a smallbeam profile of a few tens of micrometer scale. Asshown in Fig. 5, we have distinguished a 2.5-µmobject. Third, two types of timing monitor system havebeen developed. One is a fast current transformer forthe electron beam, and the other is an in-vacuumphotodiode system for FEL radiation. Both themeasurement systems have pulse responses of

214

XFEL

Fig. 2. Test bunker for high-power rf components.

0

15

10

5

0

– 5

– 10

– 15

– 20

– 25200 400

σφ=3.3° σφ=7.2° σφ=3.3°

Pole Number

Phas

e Er

ror (

degr

ee)

Pole Number Pole Number 600 0 200 400 600 0 200 400 600

(a) (b) (c)

Fig. 3. Phase error distributions (a) before and (b) after reinstallationof magnet arrays and (c) after correction of gap deformation.

XFEL

~100 ps with an arrival timing accuracy of about 1 ps.The R&D of LLRF (low level rf ) and timing

systems was carried out at the test accelerator. Anelectron beam time-jitter of about 46 fs in STD hasbeen achieved at a position downstream of theC-band acceleration system. This result showedthat the LLRF system almost satisfies the stabilityrequirement for XFEL. To apply the developedsystems in the XFEL accelerator on a scale of severalhundred meters, an optical timing and referencerf signal transmission system using the WDM(wavelength division multiplex) method has beendeveloped. The adopted WDM system using oneoptical fiber to transmit the rf signals generatedby the master oscillator can handle five frequencysignals corresponding to each multistage cavity

acceleration unit for the velocity bunching. Theexperiment on optical signal transmission using anoptical fiber of 500 m length revealed a very low noisecharacteristic of the system, as shown in Fig. 6, whichnever enhances the noise of the master oscillatorthrough the transmission.

With respect to the R&D of vacuum technology,a laser beam welding procedure was established for awide and square vacuum vessel that is used in abunch compressor and a chicane where the energy-chirped electron beam becomes wider in thehorizontal plane. A special rf contact for such a vesselwas also developed to reduce impedance.

A remote-I / O card, called OPT-RMT i-DIO,was developed to realize a fast and precise controlof a power supply for electromagnets even under asevere noise environment. High performance wasconfirmed by a test using an engineering sampleof the OPT-RMT i-DIO and a prototype powersupply. The developed FL-net test tool, which checkscommunication with a PLC, was tested in the rf testbunker. We found that the test tool is useful fordebugging the configuration of a FL-net memorymap. This will be used in a factory test before deliveryto avoid a FL-net missconfiguration.

The XFEL facil i ty can contain f ive photonbeamlines. In 2008, the design work for the centralbeamline (BL3) has been completed. The beamlinelayout in the experimental hall as well as a detailedconfiguration are shown in Fig. 7 and Fig. 8,respectively. The main aim of this beamline is tomanage hard X-ray FEL with a photon energy

– 0.1 – 0.2 0 Beam Position (mm)

Out

put V

olta

ge (

V)

0.1

1

0.5

0

– 0.5

– 1

0.2

p0 0.05641 ± 0.00598 – 8.824 ± 0.099 p1

Fig. 4. Output voltages of beam positionmonitor measured at different beam positions.

–200 0

0

5

10

200 400

Differential Intensity (arb. units)

Size

( µµm

)

62.5 µm

15

215

100

– 20

– 40

– 60

– 80

– 100

– 120

– 140

– 160 101 102 103

Frequency (Hz)

Pow

er S

pect

rum

Den

sity

(dB

c/H

z)

104 105 106 107

Signal Source Opt Tx/Rc

Fig. 5. Resolution of optical system of screenmonitor. The dot diameter is 62.5 µm. The graphshows that the sharpness (resolution) of the dotedge is about 4 µm (FWHM). Edge sharpness isobtained by a differential operation of dot contrast.

Fig. 6. Degradation of rf signal quality due to noise ofoptical rf signal transmission system. The increase in thelevel of noise at around 1 MHz corresponds to about 7 fs.

216

XFEL

above several keV; however, softer X-rays can betransported by adopting differential-pumping systems.A double-crystal monochromator and a pair of double-mirror systems are employed as principal beamlineoptics. The former delivers monochromatic X-rays inthe range of 4 to 30 keV (with Si 111 diffraction), whilethe latter works as a low-pass filter below ~8 keV or~15 keV. The beamline components are contained in

the optics hutch. Four experimental hutches forinstalling experimental instruments are set.

Most of foreseen XFEL experiments demandsshot-by-shot measurement to cope with the instabilityinherent in the SASE process. To realize theseexperiments, a feasibility study of the dedicateddata acquisition (DAQ) system has been started.Accordingly, the X-ray two-dimensional detector

Fig. 8. Beamline configuration of BL3. A monochromator (green arrow), and a pair of mirrors1&2a (red arrow) or 1&2b (blue arrow) are exclusively utilized as main optical components.

135 130 125 120 115 110 105 100 95 90 85 80

55 50 45 35 30 25 20 15 10 5 40 0

Distance from upstream wall of experimental hall (m)

Distance from exit of ID18@BL3 (m)

laser booth

EH4 EH3

EH2 EH1

OH 1

OH 2

3000

BL1

BL2

BL4

BL5

BL3

Fig. 7. Layout in experimental hall. Beamline optics are contained in the optics hutch, while experimental instruments areinstalled in experimental hutches. Synchronized lasers are set in the laser booth and distributed to experimental hutches.

217

XFEL

should record frame data at an XFEL repetition rate of60 Hz with a high full-well capacity. The studyincluding that of radiation damage has revealed thefeasibility of the use of the direct detection charge-coupled device (CCD) in XFEL experiments, andyielded the concept of the multiport CCD sensor forXFEL. The finalized design has 64 readout ports with2k×2k 50 µm square pixels, and each pixel has a full-well capacity of 5 M electrons (Fig. 9).

The building construction has progressed onschedule. Figure 10 shows an aerial photograph ofthe construction site. The building frame and exteriorfinish were completed for both the accelerator andundulator buildings. The accelerator tunnel andundulator hall were made of radiation-shieldingconcrete walls whose thicknesses are 2 and 1.5 m,respectively. Soil improvement for the undulatorbuilding was carried out by a replacement methodusing crushed stones. A stable and high-densityartificial rock bed with a maximum depth of 16 m wasformed by rolling crushed stone layers at every 40 cmin depth, as shown in Fig. 11. The completion of boththe accelerator and undulator buildings is scheduled inMarch 2009. A soil survey of the tunnel constructionsite for the beam transport line to the SPring-8synchrotron was carried out in October 2008. Thistunnel construction will be completed in March 2010.

2. Operation Status of SCSS Test Accelerator

The 250-MeV SCSS test accelerator wasconstructed in 2005 to perform basic R&D of theaccelerator systems designed for 8-GeV XFEL. Thefirst SASE FEL saturation was achieved in September2007 (Fig. 12) [1]. Since May 2008, the extremely

Fig. 9. Schematic sensor format of multiport charge-coupled devices (MP-CCDs) with eight readout nodes(left), and a tiled detector with a stepped array of eightdevices with a total pixel number of 2k×2k (right).Each readout node operating a 5 M pixel/sec readoutrate enables synchronization to an XFEL repetitionrate of 60 pps. The tiled detector has a through holeat the center whose size can be controlled by remote.

Side BDead Area, 300 µm width

Side ADead Area, 150 µm width

Readout Side

SiliconImage Area

1024 (v) × 512 (h)50 µm pixels

Package

~4 extra rows

Bonding Pads, etc

Package Bonding Pads, etc

Experimental Facility

Accelerator BuildingUndulator Building

Beam Transport Tunnel

Fig. 10. Aerial photograph of XFEL construction site taken in November 2008.

XFEL

intense FEL radiation in the EUV (extreme ultraviolet)region has been provided for users. User operationhas been conducted over several months withoutany serious problems with the machine.

We introduce the R&D of acceleratorcomponents. A CeB6 cathode assembly, which hadbeen stably used for about twenty thousands hours,was first replaced by a new one in January 2008.Figure 13 shows part of the exchange procedures inthe accelerator tunnel and the old cathode just beforebeing replaced. The cathode assembly was replacedwithout any serious problems. It took only one weekincluding the processes of charging the voltage up to500 kV and raising the temperature up to ~1800 K.We could successfully reproduce the saturation ofSASE FEL power with the new virgin cathode. This

result verified the following issues: (i) the presentdesign of the cathode assembly is suitable for quickand reliable exchange, (ii) the quality control of CeB6

single crystals is sufficiently high for SASE FELreproducibility, and (iii) the beam tuning procedureafter the cathode replacement is well established.

For the C-band acceleration system, the targetperformance was achieved in September 2008: astable SASE FEL saturation has been confirmedwith an acceleration gradient of 37 MV / m at arepetition rate of 60 Hz. The reliability of this high-gradient operation has been confirmed through along-period operation over several months. Faultrate is kept small with an almost the same level asthat in the low-gradient operation.

An event-synchronized data-acquisition systemwith a distributed shared-memory network has beeninstalled and tested in the control system of the SCSStest accelerator. Fifty-four signals from the beammonitoring system such as current transformers, beamposition monitors (BPM), and photodiode intensitymonitors, have been successfully collected with 60 Hz

Fig. 11. Rolling procedure for formingstable and high-density artificial rock bed.

0.6

0.5

0.4

0.3

0.2

0.1

0

102

101

100

10-1

10-2

10-3

10-4 40 30 50

Radiation Wavelength (nm)

Flu

ctua

tion

Pul

se E

nerg

y (µ

J)

60

Fig. 12. Dependences of pulse energy andintensity fluctuation on radiation wavelength.

(a)

(b)

Fig. 13. One scene in cathode exchangeprocedures at CeB6 thermionic gun (a) and gunelectrode with single-crystal CeB6 cathode justbefore the first replacement (b). The central darkpart on the flat-shaped wehnelt is a CeB6 crystal.

218

of beam operation cycles. The accumulated dataare stored on a relational database for pulse-to-pulsecorrelation investigation. The BPM data have alsobeen used for the effective correction of beam orbitand energy drifts.

After the first observation of the power saturationof FEL radiation, the accelerator parameters havebeen continuously optimized in order to improve FELperformance. This effort led us to achieve stableFEL radiation in the wavelength range between 50 to

60 nm during the user operation described below.Figure 14 shows the laser profile at the entrance ofthe experimental station.

Since the beginning of the user operation in May,we have conducted a user operation for 71 days for10 user groups involving approximately 60 scientistsincluding foreign researchers. The targets of theexperiments are categorized into the followingfour groups: technical research preparatory for XFELexperiments; science with gas phase targets asatoms, molecules and clusters [2,3]; imaging anddiffraction; and high-energy-density and warm matter.

In parallel with the user operation, we havecontinued to upgrade the EUV-FEL beamline. Theplan is schematically shown in Fig. 15. The EUV-FELcan be delivered to four experimental stations atbranches D, E, F and G, which is selected by aswitching mirror. In 2008, we have developedbranches B, D and E. In branch B, we have installedcommon diagnostics and optics such as a Ce:YAGfluorescent crystal to monitor beam profile, gas-filledattenuators to control pulse energy, nondestructiveintensity monitors to measure pulse energy, andswitching mirrors to steer the photon beam tobranches D, E (G) and F. At the moment, anunfocused FEL beam can be steered to branchesD and E. A focusing device will be installed in 2009.

219

XFEL

SPring-8 Joint-Project for XFEL

Project-XFEL@riken.jp

Fig. 14. Spatial profile of EUV radiationmeasured with YAG(Ce) crystal.

Fig. 15. Layout of EUV-FEL beamline.

References[1] T. Shintake et al.: Nature Photonnics 2 (2008) 555.[2] T. Sato et al.: Appl. Phys. Lett. 92 (2008) 154103.[3] H. Fukuzawa et al.: Phys. Rev. A 79 (2009)031201(R).

Attenuator by filters Al, Si, Sn, Zr, C

(t = 100 to 500 nm) Optical laser system

for users Optical laser system

for common

Entrance Online photon intensity monitor Online beam position monitor

Attenuator by gas transmittance: 1 to 100%

Beam profile monitor

D-branch (R&D in house)

E-branch (regular focus)

G-branch (tight focus)

Switching mirrors Switching mirrors

Carry-in for instruments Carry-in for instruments

B-branch

F-branch (regular focus)