60% in the exponential growth regime and then decreases to 20%
in the saturation regime, as shown in this figure. These phenomena
are typical behaviours as predicted by SASE-FEL theory25. Figure 3b
presents results at other operating conditions using l ¼ 72.9 pm(hn ¼ 17.0 keV) with a smaller K, which forces a limitation on thegrowth rate of the intensity. As a result, we achieved the exponential
growth regime, but without reaching saturation in this case.
These experimental results led us to evaluate essential parametersof the electron beam: the normalized slice emittance 1n , the peakcurrent Ip and the bunch duration t. The solid lines in Fig. 3a,b
show simulation results with the code SIMPLEX26 (see http://
radiant.harima.riken.go.jp/simplex), using the common parameters
1n ¼ 0.7+0.1p mm mrad, Ip ¼ 3.5 kA and t ¼ 20 fs (FWHM) with
Gaussian distributions. These curves reproduced the measurements,
indicating that the emittance is kept close to the original value of
0.6p mm mrad at the exit of the electron gun, even after applying
a high compression ratio of over 3,000. The bunch duration t ¼
20 fs agrees reasonably with measurements using radiofrequency
deflectors27. Note that the resulting bunch charge, 75 pC, issmaller than that transported to the beam dump (250 pC). This
discrepancy suggests that only a small portion of the electron
bunch contributes to light amplification, and that further enhancement of the intensity would be possible with different conditions ofbunch compression.
Finally, we investigated the dependence of laser intensity on
wavelength by changing EB and K over wide ranges, as presented
K, -15as ôǴ
in Fig. 4. The intensity
increased 1001Ɲ1ô
for larger values of EB andǳ1ƷǑôŧ== 10
expected from theory13,28. The maximum pulse energy reached0.48 mJ at l ¼ 224 pm (hn ¼ 5.54 keV) with EB ¼ 5.8 GeV andK ¼ 2.1. The solid lines are the simulation results with the same
parameters as used in Fig. 3. Although peak radiation power was
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.141
not measured directly, it was estimated to be at a 10 GW level

between the electron beam and the light required for maintaining
the light–electron interaction (see Methods for details). Second,deviations in the magnetic field strengths in all the undulator
modules were suppressed to DK/K , 2 × 1024, a step that is necessary to maintain the resonant condition. A gradual decrease in beamenergy due to resistive impedance from the small gap of theundulator was compensated by adjustments to the K parameter.
Figure 2a presents a spectrum of laser light generated at l ¼
124 pm (that is, a photon energy hn of 10.0 keV). For comparison,a spectrum of spontaneous radiation (blue line) using a single undulator module is also presented. An increase in the spectral intensity
by over three orders of magnitude, as well as a reduction of theenergy bandwidth from DE ¼ 300 eV to 50 eV (full-width at
half-maximum, FWHM), were observed when we achieved lasing.
Although the spectra are averaged over 10 shots at each sampling
point, a spike feature originating from an inherent shot-to-shot
fluctuation of SASE still remains in the laser spectra (seeMethods). The stability of the SASE central wavelength deduced
from the electron beam energy is "3 × 1024 (root mean square,
r.m.s.). In the inset to Fig. 2a, the single-shot spatial profile of thelaser is presented. We found that the profile is a stable roundshape without any signs of distortion of the wavefront. The
angular divergence was determined to be 2.4 mrad (FWHM),
which is consistent with a design value of 1.8 mrad. Furthermore,
we shortened the wavelength of the light by increasing the beamenergy EB and decreasing K. We obtained an amplification of
intensity at l ¼ 63.4 pm (hn ¼ 19.5 keV), as shown in Fig. 2b,
demonstrating the first laser to operate in the sub-ångström wavelength region. We note that these amplifications were achieved as
the fundamental radiation, that is, n ¼ 1 in equation (1), whereas
theory predicts the existence of higher harmonic components. In

T. Ishikawa et al., Nature Photonics 6, 540 (2012)
T. Hara et al., Nature Communications 4, 2919 (2013)

1

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fact, a third-har
was observed, as
compared with
7.87 keV), which
DF L-CC
C-CAT
S-TWA
the second-harm
SM
EDS
EG
BS L-APS
small as 1024 tim
LETTERS
could be used toaWavelength (pm)has been used
130
C-TWA
C-TWA 12069 pm, hn ¼ 18
1.0
SHB
BC1
BC2
BC3
We next inve
SASE
0.0
50-MeVSR ×100
function of effe
dump
0.8
detuning K in th
at l ¼ 124 pm.
gradual, then s
0.6
exponential chan
FWHM
197 mm
finally reaches s
Figure 1 | Schematic of SACLA. EG, 500 keV electron gun;0.4
DF, deflector
withcollimator;
SHB,
238
MHz
subharmonicbuncher;
BS,
476
MHz
booster;
L-CC,
1.0
1.0
10
10−4
0.4
20
20
0.0
1.0
0.0
1.0
60% in the expo(mm)Horizontal (mm)L-band correction cavity; L-APS, L-band alternating periodic structure (APS)
typed standing-wave cavity; C-CAT, C-bandHorizontal
correction
acceleration tube;
S(C)between the electron beam and the light required for maintaining in the saturation
0.2diagnostic section; SM, switching magnet;
0
10−5
TWA, S(C)-band travelling-wave acceleration tube; BCn (n ≈−5
0.2 1–3), nth bunch compressor; EDS, electron beam 10
the light–electroninteraction (see Methods for details).
Second, are typical behav
0
0
20
40
60
80
NATUREPHOTONICS
DOI: 10.1038/NPHOTON.2012.141
LETTERS0
20 EH, experimental
40
60
80 deviations in the magnetic
field
in all the undulator presents results
UND, undulator line; BLn (n ¼ 1, 3), nth beam line; XDS, X-ray diagnostic system; EHA, experimental hall; OH, opticalhutch;
hutch.
Undulator
length strengths(m)
0.0
Undulator length (m)modules were suppressed to DK/K , 2 × 1024, a step that is necess- (hn ¼ 17.0 keV)
0.0 aFEL beamlines, two beamlines (BL1 and BL3) are
Although SACLA has the capability to install five independent
currently
Alla
9.4
9.6 available.
9.8
10.0 experiments
10.2
10.4 presented
100
100
b
Wavelength (pm)ary to100
maintaintheresonant condition. Ahνgradualdecreasein beam growth rate of th
Pulse
energy−1−1
9.4
9.6 130
9.8
10.0
10.2
10.4
= 17 keV
b
10
10
120
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.141−1 Pulse energy Photon energy (keV)in this Letter were performed at BL3.
energy due toExperimentresistive impedance from the small gap of the growth regime, bhνkeV= 17 keVhν = 1010
1.0
Photon energy (keV)
Experiment
80
Wavelength (pm) Pulse energyb
SASE
These experim
undulator was Simulationcompensatedby adjustments to the 80
K parameter.
0.00.6π mm·mrad
80 10−2
SR ×100
64
63Experiment62aa
100 10−2 65Simulation
0.7π mm·mrad
Wavelength (pm)bFigure2a presents
a spectrum of laser light generated at l ¼ of the electron b
Wavelength2012(pm) | www.nature.com/naturephotonics
NATURE PHOTONICS| VOL 6 | AUGUST
541
0.6π mm·mrad
0.8π mm·mrad
0.8 −1
Simulation
1.0 −2
60
60
10
65
64
63
130
120
10
62
0.7π mm·mrad
current
Ip and t
124
pm
(that
is,
a
photonenergy
hn
of
10.0
keV).
For
comparison,
Fluctuation
0.6π
mm·mradSASE
0.8π mm·mradhν = 10 keV
0.7πSR
mm·mrad1.0
1.0
60 10−3of spontaneous radiation (blue line) using a single undu×100
show simulationa spectrum
8010−30.8π mm·mrad
Fluctuation
Pulse
SASE
SASEenergy
0.6
0.0
0.8
40
40
Fluctuation
radiant.harima.r
lator
moduleisalsopresented.
An
increaseinthespectralintensity
Experiment
SR ×100
SR ×100
10−2
FWHM
10−3
197 mmby overorders of magnitude, as well as a reduction of the 1n ¼ 0.7+0.1p m
0.8
0.8
40 three
Simulation
60
1.0
10−4
10−4
0.4
20
0.6
1.0
0.0
0.6π mm·mradenergybandwidth from DE ¼ 300 eV to 50 eV20(full-width at Gaussian distrib
Horizontal (mm)0.7π mm·mrad
half-maximum,
FWHM), were observed when we achieved lasing. indicating that t
10−3
−4
10
0.8π mm·mrad
20
0.6
0.6
40 0.410−5
0.2
0
10−5
Fluctuation
Althoughthe spectra are averaged over 10 shots at0 each sampling 0.6p mm mrad
FWHM
0
20
40
60
0
20
40
60
80
point, a spike feature
originatingfroman 80inherent shot-to-shot a high compress
197 mmUndulator length (m)Undulator length (m)
1.0
10−4
0
0.4
10−5
0.4
0.0
20 0.2fluctuationof SASE still remains in the laser spectra (see 20 fs agrees reas
1.0
0.0
Horizontal (mm)
0
20
40
60
80 Methods).
9.4
9.6
9.8
10.0
10.2
10.4
100 Figure 3 The
b ǚǲǦ|ƝƷǰǆǩǮǲǌŒλhŷaƞλ=124pm,| Intensity
evolutionof
along
the SASE
undulator.
a,b, Pulse GeV,
energies
stability
thewavelength
deduced deflectors27. No
EBcentral=7.0
Pulse energy
UndulatorDOI:
length(m) hν = 17 keV
NATUREPHOTONICS
10.1038/NPHOTON.2012.141
10−1(left axes) and intensity fluctuations (right axes), plotted as
Photon energy (keV)
Experiment(root ofmean square, smaller than tha
fromthe electron
beam energy is "3 × 1024a function
0.0
0.2
0
0.2
10−5
K=1.8
Ŷŧbƞλ=72.9pm,
E
=8.3
GeV,
K=1.3
effective
undulator
length:
l
¼
124
pm
(hn
¼
10.0
keV)
with
E
¼
7.0
GeV
B
B 20.0 r.m.s.).
80
Simulation
19.2
19.4
19.6
19.8
inset
to pm
Fig.
the
spatialof the discrepancy sug
0
20 Wavelength
40 (pm)
60
80
b
andIn
K ¼the1.8 (a);l ¼ 72.9(hn 2a,¼ 17.0keV)single-shotwith EB ¼ 8.3 GeV
and K ¼profile
1.5
Figure 319.0| Intensity
evolution
along theundulator.
a,b, Pulse
energies
0.6π mm·mrad
PhotonWavelength
energy (keV) (pm)65
64Undulator length63(m)isofForpresented.
Weredfoundthat
profileis a stable round bunch contribut
10−2 and intensity(b).
the pulse References
energy,solid circles
with thelines indicate
the measured
62
0.7π mm·mrad(left axes)
fluctuations(right axes), plotted as alaser
function
0.0
0.0
0.8π mm·mrad
1.blueMadey,J. J.ofM.
Stimulatedof bremsstrahlungin Thea periodicmentmagnetic
results,
and yellow,
and purple
linesdistortion
correspondemission
to simulations
with300 200
100¼ 10.0 keV) with Eshape1.060
cof the inten
without
any
signs
of
the
wavefront.
effective
undulator
length:
l
¼
124
pm
(hn
¼
7.0
GeV
Wavelength(pm)
BFluctuation
field.mm
J. Appl.
42, 1906–1913(1971).of
9.4
9.6
9.8
10.0
10.2
10.4
100 100b0.7p and 0.8p
mrad, Phys.
respectively,
at a peak current
19.0
19.2
19.4
19.6
19.8 SASE20.0(1st)
158 (hn ¼ 17.0
157 keV) with
156 EB ¼
and K ¼ 1.8 (a);159
l ¼ 72.9 pm
8.3 GeV
and1Kn ¼¼0.6p,
1.5
bunch compress
angulardivergencewasdetermined
to ofbeGW2.4
mradpulses(FWHM),
Pulse energy
SRhν
×100
2. circles
Ayvazyan,V. etal. Generation
radiationfrom a VUV free-electron= 17 keV
3.5 kA. Green openwith lines
indicate
intensity fluctuations.
10−3
10−1
Photon energy (keV)
Photon energy (keV)Electronbeam energy
Experiment(b). For the pulse energy, red solid circles withlines indicate
the
laser
operatingin thefemtosecond
regime.
Phys.
Rev. Lett. 88, 104802 (2002).
Finally,
we i
0.8
which
is
consistent
withadesign
value
of
1.8
mrad.
Furthermore,
40measured
GeV
(3rd)
53.0 blue and purple
52.5 lines correspond
52.0 5.8to
results, and yellow,
simulations with
Shintake, T. et ofal. Athecompact free-electron
laser for generating
coherent
radiation
80c
Simulation Wavelength (pm)wavelength
by c
1.0
the3.wavelength
by increasing
the beam
7.0 GeV we shortened
Wavelength (pm)bǮǲǄǲ)ƙĖèr)(×100)ƝǇǠƿǑǭƙ
from the simulations.
again
foundlightreasonableagreementin theWe
extremeultraviolet
region,NaturePhoton. 2, 555–559 (2008).
1n ¼ 0.6p,
at aGeVpeak current of
0.6π mm·mrad−4 0.7p and 0.8p mm mrad, respectively, 8.3
10
(1st)
159
158
157
156
−2
Fundamental
in Fig. 4. The indecreasing
obtained
amplificationenergyEB and65
64
63
betweentheSACLAƝ)Ɲ|Ɲ)gƻǕǭ
experiments
and K.
oursimulations.
It should
benoted laser fromof
20
10
62
0.6
0.7π mm·mrad
4. Ackermann,
W. We
et al. Operationof aanfree-electron
the extreme
3.5 kA. Green open circles with lines indicate 3rd
intensity
fluctuations.
XďǮǲǄǲƝøŔǞǯǜƶƸǭŷ
harmonics
0.8π mm·mradthat small
discrepancies
are(hnfoundin bothFigs 3Natureandshown
4.Photon.
This in
is1, 336–342
expected
from
l
¼
63.4
pm
¼
19.5
keV),
as
Fig.
2b,
intensity
at1.0
ultraviolet
to
the
water
window.
(2007).
60×100ƾǲhŷïďƘXƕƒŞ\ƞŶ(3rd)
53.0 Fluctuation
52.5
52.0
mainlyassumedtheparameters
SASE
Emma,
P. etthatal. operateFirstslice
lasingbeam
andtheoperationof suchan ångstrom-wavelength
free-mJ at l ¼
101 because
demonstrating
the5.wefirst
laser
to
in
sub-ångströmwave- 0.48
aƞEB=7.0 GeV, K=1.8ƘŶŧbƞE
0
1.0B=8.3
10−5
0.4 GeV, K=1.3
as emittance, Twiss
parameters
and Photon.
energy 4,
spreadwouldbe
SR ×100
electronlaser. Nature
641–647(2010).
from0.5
We again found reasonable agreement
10−1the0 simulations.
10−3
20
40
60
80
ǛǲƿřÈƴ3.5kAƙƊƒƙƂƝǅ
K ¼(1960).
2.1. The solengthregion.
Wethe
these
were
achieved
as493–494
constant
alongbunchthat
inT.these
FELamplifications
simulations.
However,this
6. note
Maiman,H.
Stimulated
optical radiationin ruby.
Nature 187,
Fundamental
0.840
between the experiments
andoursimulations.
Itshouldbe
noted
Undulator length (m)simple model andrepresentative
are usefulforas u
the 4.fundamental
thatis,¼ 1 wavelengthin equation
7.radiation,
MacGowan,B. J.slice
et al.nparameters
Short
X-ray(1),laser whereas
research at the parameters
Lawrence
3rd harmonicsbƘä«îÆŒ0.63ÅƝǮǲǄǲƴňŷłxƝƷǰ
that small discrepancies are found in both Figs 3 andThis is ǥǩǮǲǅǪǰƝĉ³ƘŶ#ƙ#
0.2
estimating
the
laser
performance.
Livermore
National
Laboratory.
Phys.
FluidsB
4,2326–2337(1992).
×100
theory predicts
the existence of higher harmonic components. In not measured dmainlyassumedthatundulator.
the slice
parameters
such
Figure 3 because| Intensity weevolution
along thea,b, beam
Pulse energies
10−4
UND

Table 1 | Main parameter list for SACLA.

Pulse energy (mJ)

Vertical (mm)Intensity(a.u.)energy (mJ)
Pulse

Vertical (mm)Intensity (a.u.)

FWHM
195 mmIntensity fluctuation (%)

FWHM
195 mmCǗǰǒ;ŃU ìøq¾
ƷǰǆǩǮǲǌ
3ƖƝƽǲǏƿǖǯǆǲ

Intensity fluctuation (%)

Parameter
Present value
Electron beamBeamenergy8.5 GeV (maximum)
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.141
Bunch charge"0.2–0.3 nC
BL3UND
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.141.3 kA
Peak current, Ipa
100 spread (projected)
Energy
,0.1%
a
a
100
10−1
XDS
Normalized
emittance (projected)
1p mm mrad
Wavelength (pm)
10−1
130
120
hν
= 10 keV Repetitionrate
10 Hz (60 Hz maximum)80
EHhν = 10 keV
1.0
Pulse energy
Undulator
80
Pulse energy
ExperimentSASE
10−2 0.0
Periodic
length, lU
Experiment 18 mm
SR ×100
10−2
OH Simulation
60
Numberofundulator
modules18
0.8
Simulation
608-GeV
0.6π mm·mrad
0.6π mm·mradof periods
4,986
EHA 0.7π mm·mrad Total number
0.7π mm·mrad
dump
10−3
0.8π mm·mrad Maximum
10−3 K
2.2
0.8π mm·mrad
0.6
40
40
Fluctuation
Fluctuation 3.5 mm
Minimum gap
FWHM
197 mm
22 m/22 m
Averagedbetatron function, bX/bY−4

LETTERSřgŏ

BL1

Intensity (a.u.)

Pulse energy (mJ)

Pulse energy(mJ) (mJ)
Pulse energy

Vertical (mm)Intensity
Pulse energy(mJ) (a.u.)

Intensity (a.u.)

Intensity (a.u.)

Intensity fluctuation (%)

Intensity fluctuation (%)

Intensity fluctuation (%)

Intensity fluctuation (%)

FWHM
195 mm

FWHM
195 mm

Vertical (mm)LETTERSSACLAƝ)ƝÚ

Intensity(a.u.)
Pulse
energy(mJ)

Intensity(a.u.)
Pulse energy(mJ)

Intensity (a.u.)

Intensity (a.u.)

c

0

20
159

0.2

19.4
19.6
Photon energy (keV)

19.8

40
60
Undulator length (m)Wavelength (pm)

158

157

156

Peak power (GW)

−5

19.2

ƞƐƲƑƲεn=0.6π, 0.8π mm rad

The future perspectivesof hardX-ray
lasers warrant
consider8. Spielmann,
C. et al.
Generationof coherent
X-rays in the water window using20
as(leftemittance,
Twissfluctuations
parametersand plottedenergywouldaxes) and intensity(right axes),
as aspreadfunction ofation. be
Based on the5-femtosecond
performance verified
here,Science
we can278,
readilyseek (1997).
laser pulses.
661–664
0.0 undulator
effective
length:l ¼ 124(hn ¼FEL
10.0 keV)with EB ¼ 7.0
GeV to shorten
constant
along the
bunch
inpm
these
simulations.
However,
this the 9.laser
wavelength!40
pm (photon
energy Xofray by phase-matched highTakahashi,
E. J. toet al.
Coherent
water window
24.0and K ¼ 1.5
23.6
23.8 E ¼(3rd)
and K ¼ 1.8 (a);23.4
l ¼ 72.9 pm (hn¼ 17.0 keV) with
8.3 GeV
are usefulfor which willB!30
wide generation
applicationinin
investigationsof Rev. Lett. 101, 253901 (2008).
orderfindharmonic
neutralmedia. Phys.
100keV),0simple model and representative slice parameters
(b).
For
the
pulse
energy,redsolidcircleswithlines
indicatethemeasured−2
thecharacteristics
andfunctionsofuniquematerials
withimproved
10.
Seres
J.
et
al.
Sourceofcoherent
kiloelectronvolt
X-rays. Nature 433, 596 (2005).
estimating
the7.8laser performance.
10
7.9
8.0
(1st)
DF L-CC
C-CAT
results, and yellow, blue and purple lines correspond to simulations with resolution and penetration
For thisaS-TWA
combination
11. Elias, L.abilities.
R., Fairbank,
W. purpose,M., Madey,
J. M. J., Schwettman, H. A. & Smith, T. I.
The future
perspectivesof hardX-ray
laserswarrant
considerPhoton 10
energy
(keV)
4
6
8
12
14
16
18
20
1n ¼ 0.6p, 0.7p and 0.8p mm mrad, respectively, at a peak current ofEGacceleratorof thepresenta shorter-period
undulator
with lU ¼by relativistic electrons EDS
Observationof stimulated emissionof radiation
in a
BS andL-APSation.
Basedontheperformanceverified
here,
we
can
readilyseek
Photonenergy(keV)fluctuations.
Greencomparison.
open circles a,b,
withSpectra
lines
indicate
periodictransverse
magnetic
field.
Phys. Rev. Lett. 36, 717–720 (1976).
10 mm and K ¼ 1.6spatiallywould bea promising
approach,witha reasonFigure3.5
2 kA.
| Spectraof laserintensity(SASE, redlines) and

0.0

19.0

Intensity fluctuation (%)ǆǩǮǲǌrƴ100!ƜcƊƒƪƝƙÁĻƊƗƪŶ
0.5ǮǲǄǲ)Ɲ¢ƁǛǲƿ"ƪcƂƃŶǛǲƿyƪsƉż
10
0.4
0.6

Intensity fluctuation (%)

Pulse energy (mJ)LETTERS

20.0

80
(1st)

0.0

to shorten the laser wavelength to !40 pm (photon energy of
Figure 323.4| Intensity evolution
the 23.8
undulator.
Pulse energies
Kondratenko,
A. M. & Saldin,one
E. L.
Generationof coherent radiation by aable saturation 12.
lengthof 80 m. Alternatively,
might
consider23.6 along(3rd)a,b,24.0
spontaneous radiation (SR, blue lines), measured at EB ¼ 7.0 GeV, K ¼ 1.8
(3rd)
53.0
52.5
52.0
!30
which will find wide application in investigationsof(left axes)
and intensity fluctuations (right axes), plotted as a functionofE keV),relativistic
electronbeam
in an ondulator.
Part. Accel.
10, 207–216 (1980). C-TWA
1.0
C-TWA
the machinescale,
simplifying
the construc4the| Intensity
versusphotonPulse
energythe(leftaxis)further
as a reductions of(a)Figure
and
8.3
GeV,
K ¼ 1.3(b), respectively.
The inset
of a shows
B¼
fromsimulations.
We
againenergy.
reasonableagreement
SHBBonifacio,
BC1aC.beam
BC3 andthecharacteristics
and functions
offounduniquematerials
with improved
13.
R., example,
Pellegrini,& Narducci,
L.ofM.
instabilities
effective undulator
length: l ¼ 1247.9pm (hn ¼ 10.0keV) with
7.0function
GeV¼
tion of hard X-ray
lasers. ForenergyBC2
EBCollective
7.8
8.0 EB ¼spatial(1st)
profileof
ofthe
the experiments
laser energy
beam. c, (bottom)
Spectraof third
harmonics(purpleline)
Fundamental
photonandwavelength(top).
Red,
green
betweenand
oursimulations.
Itshouldbenoted
50-MeV
19.0
19.2
19.4
19.6
19.8
20.0
resolution
and penetration
For Kthispurpose,a combination
high-gain
regime inwavelengths
a free-electron
laser.undulator
Opt. Commun. 50, 373–377 (1984).
3.5
could reach
sub-ångström
withand K ¼ 1.8 (a); l ¼ 72.9pm
(hn ¼(keV)
17.0 keV) withEB ¼ 8.3 GeV and
Kthat
¼blue
1.5smallandandfundamental
radiationline)
forabilities.
Eexperimental
2.1.
The
Photonenergy3rd harmonics
B ¼ 7.0
discrepancies
are
found
inGeV,
both¼results
Figs
3 and
is GeVcircleswith(redlines
showatspectra
EB4.¼ This
5.8, 7.0
anddump
Photon energy (keV)
14.
Milton,
S.
V.
et
al.
Exponentialgain
andsaturationofaself-amplified
l
¼
of
the
present
accelerator
andashorter-period
undulator
with
×100
l
¼
6
mm
and
K
¼
1.0.
It
is
worth
notingthatanparametersofwere mainly
averagedbecause
over 10 shotsat each wavelength
thebeamspatialparametersprofile was such U(b). For the pulse energy, red solid circles with lines indicate the measuredU
we assumed
that theand
slice
8.3
GeV, respectively.
andboundariesof the hatchingareas
spontaneous
free-electron
laser.
Science 292, 2037–2041 (2001).
6 mm hasalready beendeveloped
in-vacuum
undulator
with lU ¼emission
10
andshot.
K¼
1.6Upperwould
be lowera promising
approach,with a bereasonbymm
aemittance,
singleFigure 2 | Spectra
a,b, Spectraof laserred lines)
and taken
results,comparison.
and yellow, blue
and purple
lines (SASE,
correspondto simulations
with
as
Twiss
parameters
spreadwouldcWavelength (pm)correspondto simulation
results m.
withand1n ¼energy
0.6p and
0.8pmight
mm mrad,
15. Altarelli, M. et al. (eds) XFEL: the European X-Ray Free-Electron Laser, Technicalable
saturation
lengthAlternatively,
considerspontaneous1nradiation
measuredEB ¼ 7.0 GeV,
¼ 1.8current ofconstant
along thebunchofin80theseFEL
simulations.one
However, this
¼ 0.5
0.6p, (SR,
0.7pblue
andlines),
0.8p mm
mrad, atrespectively,
at a Kpeak
DesignPreprint
DESY 2006-097 (DESY Hamburg, 2006).
(1st)
159
158
157
156
542respectively, at a peak current of 3.5 kA. The right axis is the peak power
NATURE PHOTONICS| VOL Report,
6 | AUGUST2012 | www.nature.com/naturephotonics
further
reductionsofthemachinescale,
simplifying
theconstrucsimple model and representative slice parameters are useful for(a) and EB ¼3.5
8.3kA.
GeV,
K ¼ open
1.3 (b),respectively.
The
inset ofa showsfluctuations.
the
Green
circleswith lines
indicate
intensity
16. Altarelli, M. The European X-ray Free-Electron Laser Facility in Hamburg.
evaluatedwithsimulations.
Figureof1 E| BSchematicof SACLA. EG, 500 keV electron gun; DF, deflector with collimator; SHB, 238 M
estimating
thethelaser
performance.
¼
tionof hardX-ray
lasers. For example, a beam energy
spatial profile of the laser beam. c, Spectra of third harmonics (purple line)
Nucl. Instrum. Methods B269, 2845–2849 (2011).
2色 目
(3rd)
53.0
52.5
52.0
The 2.0
futureof hard X-raywavelengths
lasers warrantwith
consider1色目
2.0
L-bandcorrection
cavity;L-APS,
alternating
structure (APS)
typed Photon.
standing-wave
ca
3.5 GeV
couldperspectives
reach sub-ångström
undulator
andfundamental radiation (red line) for EB ¼ 7.0 GeV,
K ¼ 2.1. The spectra
17. McNeil,
B. L-band
W. J. & Thompson,
N.periodic
R. X-ray free-electron
lasers. Nature
4,
1.0
光パルス
21on the performance verified here, wefromthesimulations.
We
again
found
reasonableagreement
光パルス
ation.
Basedcanreadilyseek(a)(b) BCn (n ≈ 1–3), nth bunch compressor; EDS, elecatSPring-8,
demonstratingthefeasibilityof
constructing
such
a
タ
an
parametersof lU ¼ 6 mm and K ¼ 1.0. It is worth noting
814–821 (2010).
were averaged over
10 shots アンジ
at eachュレーwavelengthand the spatial profile was
TWA, that
S(C)-bandtravelling-wave
acceleration tube;
1.5
1.5
0.0
電子ビーム
アンジュレータ
Fundamental
toshorten undulator
theundulator
laser wavelength!40 haspm already(photontechnologies.
energyofbetween the experiments and our simulations. It should be
noted
short-period
Theline;18.BLn
Huang,Z.
&
Kim,
K.
J. ReviewofXDS,
X-rayX-ray
free-electron
laser theory.
Phys.
Rev.experimenta
STlU ¼to6state-of-the-art
mmbeendeveloped
in-vacuum
withusingtaken by a single shot.
23.4
23.6
23.8
(3rd) 24.0
UND,
undulator
(n
¼
1,
3),
nth
beam
line;
diagnostic
system;EHA,
3rd harmonics!30 keV),will find wide
application(suchin investigationsof unduthat small discrepancies are found in both Figs 3 and 4.development
This
is 1.0 which
Accel. Beams 10,2色目（9.7034801 (2007).
1.0
keV）
of undulator
technologies
as a cryogenic
×100
Although SACLA has
capability
to install fiveindependent
FEL beamlines,
twoCompact
beamlines (BL1 anthesuchcharacteristics and functions of uniquewith improved
下流側
1色目
イン
mainly because
assumedthat7.9the slice
beam
parameters
19.the
Shintake,T., Matsumoto,H., Ishikawa,T. & Kitamura,
H. SPring-8
29 2012materials
シケ
7.8we
8.0
(1st)
シケイン上流側 542
NATUREPHOTONICS| VOL 6 |forces
AUGUST|well
www.nature.com/naturephotonics)this
aspurpose,asaacombination
further reduction
lator
to 0.5enhancemagnetickeV）
0.5 were performed
resolution
and penetration
abilities. For2色目（9.7
SASE Source(SCSS). Proc. SPIE 4500, 12–23 (2001).
2
in this Letter
at BL3.
Photonenergy (keV)
as emittance, Twiss Kparametersand 光パルス
energy spread would
beemittance
K
1
in
the
to
0.1p
mm
mrad
will
play
key
roles
in
these
of theaccelerator and a shorter-period undulator with lU ¼
0.5
シケイン
20. Tanaka, T. & Shintake, T. (eds) SCSS X-FEL Conceptual Design Report (RIKEN
constant along the bunch in these FEL simulations. However,thispresent
00further
10 mmadvances.
and K ¼ 1.6 would be a promising approach, with a reasonFigure 2 | Spectra comparison. a,b, Spectra of laser (SASE, red lines) andHarima Institute, 2005).
simple model and representative slice parameters are usefulfor| VOL 6 | AUGUST 2012 | www.nature.com/naturephotonics
able saturation
length 1色目（12.2
of 80 m. Alternatively,
one might NATURE
consider PHOTONICS
spontaneous radiation (SR, blue lines), measured at EB ¼ 7.0 GeV, K ¼ 1.8
21. Kitamura, H. Recent
trends ofinsertion-device technology for X-ray sources.
-0.5
-0.5
1色目（12.2
keV）
keV）
estimating the laser performance.
Methods
further reductions of the machine scale, simplifying the construcJ. Synchrotron Rad. 7, 121–130 (2000).
(a) and EB ¼ 8.3 GeV, K ¼ 1.3 (b), respectively. The inset of a shows theThefutureperspectives
of
hard
X-ray
laserswarrant
consider-1.0-1.0
of theelectronbeam.
longitudinal
characteristics¼ electrontion of hard
X-ray
lasers.
For Theexample,a beamenergy of EofB the
22. Togawa, K. et al. CeB6 electron gun for low-emittance injector. Phys. Rev. ST
spatial profile of the laser beam. c, Spectra of third harmonics (purple line) Diagnostics
Basedradiationon the(redperformanceverifiedhere,wespectra
can readilybeam
was evaluatedusing sub-ångströmradiofrequency wavelengths
deflectors, which
installed in the
3.5seek
GeV
could reach
withwereundulator
Accel. Beams 10, 020703 (2007).
andation.
fundamentalline) for EB ¼ 7.0
GeV, K ¼
2.1. The
-1.5
-1.5
0.0
electrondiagnostic
sectionin Fig. noting
1. The deflectors
toaveraged
shortenlaserat eachwavelengthpm (photon
ofbeam oflU ¼ 6 mm
and(EDS)
K ¼ after
1.0. ItBC3
is worththat an give aparameters
23. Togawa, K., Hara, T. & Tanaka, H. Electron bunch compression using an
were
overthe10 shotswavelengthtoand!40
the spatial
profile was energy
23.4
23.6
23.8
(3rd) 24.0
time-varying
to lthebunch to transform its temporal distribution
over-correction method for a compact X-ray free-electron laser. Phys. Rev. ST
in-vacuum
undulatorkickwith-2.0into!30
which will find wide application in investigationsof -2.0 transversetaken
by akeV),single shot.
U ¼ 6 mm has already been developedatransverse
spatialprofile.
A
new
C-banddeflecting
structurewasdesignedforhighAccel. Beams 12, 080706 (2009).
the characteristics and functions of unique materials with improved
27
7.8
7.9
8.0
(1st)
-2.0 -1.5 -1.0 -0.5
1.0
1.5 -2.0-2.0 in-1.5
0 2012
0.5 | www.nature.com/naturephotonics-2.0
gradient
operation
SACLA
a1.0
time1.5
resolutionof 10 fs (FWHM)
542
NATUREPHOTONICS| -1.0
VOL 6.-0.5|Consequently,
AUGUST
24. Nuhn, H. D. 0et al.0.5
Characterizationof second harmonic afterburner radiation at
resolution and penetration abilities. For this purpose, a combination
水平位置 (mm)水平位置
Photon energy (keV)
was attained from a transverse profile
on(mm)a YAG screen located 12.5 m downstream.
the LCLS.
Proc. FEL 2010, 690–695 (2010).
of the present accelerator and a shorter-period undulator with
U¼
Theltransverse
emittance was evaluated using the Q-scan method. Transverse beam
25. Saldin, E. L., Schneidmiller, E. A. & Yurkov, M. V. (eds) in The Physics of Free
10 mm and K ¼ 1.6 would be a promising approach, with asizes
reasonFigure 2 | Spectra comparison. a,b, Spectra of laser (SASE, red lines) andon a screen were measured by changing the focusing strength of the upstream
Electron Lasers Ch. 6 (Springer, 1999).
able saturation length of 80 m. Alternatively, one might consider
quadrupole magnet. The emittance was derived from the quadratic dependence
spontaneous radiation (SR, blue lines), measured at EB ¼ 7.0 GeV, K ¼ 1.8
26. Tanaka, T. FEL simulation code for undulator performance estimation. Proc.
of the square of the beam size on the focusing strength.
further reductions of the machine scale, simplifying the construcFEL 2004, 435–438 (2004).
(a) and EB ¼ 8.3 GeV, K ¼ 1.3 (b), respectively. The inset of a shows thetion of hard X-ray lasers. For example, a beam energy Diagnostics
of EB ¼ of X-rays. The X-ray beamline was used to determine the diagnostics of 27. Ego, H. et al. Transverse C-band deflecting structure for longitudinal phase spacespatial profile of the laser beam. c, Spectra of third harmonics (purple line)
diagnostics in the XFEL/SPring-8 ‘SACLA’. Proc. IPAC 2011, 1221–1223 (2011).
3.5 GeV could reach sub-ångström wavelengths with undulator
and fundamental radiation (red line) for EB ¼ 7.0 GeV, K ¼ 2.1. The spectra
the radiation properties of the X-rays. To measure the spectra in Fig. 2, the intensities
28. Kim, K. J. An analysis of self-amplified spontaneous emission. Nucl. Instrum.
anparameters of lU ¼ 6 mm and K ¼ 1.0. It is worth notingofthat
were averaged over 10 shots at each wavelength and the spatial profile was
the monochromatic
beam passing through a double-crystal monochromator
Methods A250, 396–403 (1986).
l
¼
6
mm
has
alreadybeen
developed
in-vacuum
undulator
with(DCM) with silicon(111) diffraction were detected with a pin photodiode. The
taken by a single shot.
U
29. Hara, T. et al. Cryogenic permanent magnet undulators. Phys. Rev. ST Accel.
intensities at each wavelength were averaged over 10 shots. The spatial profile in theBeams 7, 050702 (2004).
inset of Fig. 2a was measured with a monitoring system consisting of a charge542
NATURE PHOTONICS | VOL 6 | AUGUST 2012 | www.nature.com/naturephotonics
30. Tono, K. et al. Single-shot beam-position monitor for X-ray free electron laser.
coupled device (CCD) camera and a fluorescence film of B-doped chemical vapour
Rev. Sci. Instrum. 82, 023108 (2011).
deposition (CVD) diamond, located 84 m from the exit of the final undulator
module. The pulse energies in Figs 3 and 4 were determined with an intensity
Acknowledgements
monitor that detected X-rays scattered from a CVD nanodiamond foil30. The vertical
The authors thank the staff of SACLA/SPring-8 for their continuous support and M.E.
error bars indicate uncertainties in the intensity calibrations.
0.02ǘǭǇŔƝ§ŔvǞǯǲǝ) ǣǰǞ)

垂直位置 (mm)

垂直位置 (mm)Intensity (a.u.)

Intensity (a.u.)2ėǮǲǄǲƘÞıƫEƴ=éƜĤo 2ėǘǭǇƝøŔ1Ř§ŔvƴažƗǇǑǯǢĬ 

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Alignment of the electron beam trajectory. The electron beam trajectory throughall the undulator modules was straightened using spontaneous radiation from each

Couprie for fruitful discussions. The authors also thank H. Matsumoto and H. Baba fordeveloping the basic components of the C-band accelerator system.