Proceedings of IBIC2012, Tsukuba, Japan

MOIC02

ELECTRON BEAM DIAGNOSTIC SYSTEM FOR THE JAPANESE XFEL,
SACLA

Abstract
We present the design and performance of the beamdiagnostic instruments for the Japanese x-ray free electron
laser (XFEL) facility, SACLA. XFEL radiation is
generated by self-amplified spontaneous emission (SASE)
process in SACLA, which requires a highly brilliant
electron beam with a normalized emittance of less than
1 mm mrad and a peak current of more than 3 kA. To
achieve this high peak current, 1 A beam with 1 ns
duration from a thermionic electron gun is compressed
down to 30 fs by means of a multi-stage bunch
compression system. Therefore, the beam diagnostic
system for SACLA was designed for the measurements ofthe emittance and bunch length at each compression
stage. We developed a high-resolution transverse profile
monitor and a temporal bunch structure measurement
system with a C-band rf deflecting cavity etc. In addition,
the precise overlapping between an electron beam andradiated x-rays in an undulator section is necessary toensure XFEL interaction. Therefore, we employed a Cband sub-Pm resolution rf cavity BPM to fulfill thedemanded accuracy of 4 ȝm. All the performances of ourdeveloped beam monitors reached the demanded
resolutions. By using these beam diagnostic instruments,
the first x-ray lasing at a wavelength of 0.12 nm wasachieved and SACLA has been stably operated for user
experiments since March, 2012 in the wavelength region
from 0.08 nm to 0.25 nm.

INTRODUCTION
The x-ray free electron laser (XFEL) facility, SACLA(SPring-8 Angstrom Compact Free Electron LAser) [1],was successfully commissioned and the first x-ray lasing
was observed in June, 2011 at an x-ray wavelength of
0.12 nm. SACLA has been stably operated for various
user experiments since March, 2012 in the wavelength
region from 0.08 nm to 0.25 nm. In SACLA, XFEL
radiation is generated by a self-amplified spontaneousemission (SASE) process. The SASE process in the x-ray___________________________________________

region requires a high peak current of more than 3 kA anda small normalized emittance of less than 1 mm mrad [2].
To achieve these requirements, we designed andconstructed a low-emittance injector, an 8 GeV C-band
accelerator and a short-period in-vacuum undulator
beamline, as shown in Fig. 1. An electron beam isgenerated by a thermionic electron gun with a CeB6
cathode. The normalized emittance of the electron beam
is 0.6 mm mrad, the initial current is 1 A and its pulse
width is 1 ns (FWHM) formed from 3 Ps (FWHM) by ahigh-voltage chopper. The beam is accelerated to 8 GeVby the following series of rf accelerator cavities:
238 MHz pre-buncher, 476 MHz booster, L-band(1428 MHz), S-band (2856 MHz) and C-band(5712 MHz) accelerators. In the meantime, the bunch
length is shortened from 1 ns to 30 fs by using a velocity
bunching process through the sub-harmonic acceleration
cavities and a bunch compression process by means of
three magnetic chicanes. The peak current is finally
boosted up to 3 kA without substantial emittance growth.
The electron beam is then fed into in-vacuum undulatorswith a period of 18 mm and the maximum K-value of 2.2,
and XFEL light is finally generated.
In order to maintain the high gain SASE process at an
x-ray wavelength, we need to monitor a beam position, atransverse beam profile, beam arrival timing and atemporal bunch structure at each acceleration stage. The
resolution of the beam-position monitor in the undulator
section is required to be less than 1 ȝm so as to maintainthe overlap between the electron beam and radiated x-rays
within 4 ȝm precision [3]. The transverse beam profile
should be measured with a spatial resolution of less than
10 ȝm in order to measure a normalized emittance lessthan 1 mm mrad. The required resolution of the temporalbunch structure measurement is 10 fs at a position after
the full compression, since the bunch length becomes
30 fs. In addition, since the initial bunch length is 1 ns,
temporal profile monitors with a wide time scale from
1 ns to 10 fs are demanded. Therefore, we developed

Figure 1: Schematic layout of SACLA.

# maesaka@spring8.or.jp

ISBN 978-3-95450-119-9
Overview and Commissioning of Facilities1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○H. Maesaka#, H. Ego, C. Kondo, T. Ohshima, H. Tomizawa, Y. Otake,
RIKEN SPring-8 Center, Kouto, Sayo-cho, Sayo-gun, Hyogo, JapanS. Matsubara , T. Matsumoto, K. Yanagida,
Japan Synchrotron Radiation Research Institute, Kouto, Sayo-cho, Sayo-gun, Hyogo, Japan

MOIC02

Proceedings of IBIC2012, Tsukuba, Japanbunch length monitors of several types.
In this paper, we describe the design and performanceof each diagnostic instrument and demonstrate someresults from the XFEL commissioning.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○BEAM DIAGNOSTIC SYSTEM
The beam diagnostic instruments for SACLA can beclassified into three categories: beam position monitor(BPM), transverse profile monitor and temporal profile
monitor. In the following sections, the details of thesethree categories are described together with theexperimental results of performance tests from the XFEL
commissioning. In this section, we summarize theoverview for each category. The number of monitors foreach diagnostic instrument is listed in Table 1.
For the BPM, we employed rf cavity BPMs (RF-BPM)[4] for both the accelerator section and the undulator
section in order to achieve sub-ȝP UHVROXWLRQ. Although
the accelerator section does not need such a highresolution BPM, the same RF-BPM system is employedso as to reduce design and maintenance work. Only forthe dispersive part of the three bunch compressors (BC1 –3), multi-stripline BPMs [5] are used, since the beamwidth becomes wider than the aperture of the RF-BPM. In
addition to the beam position measurement, RF-BPM has
capability to detect the beam arrival timing [6]. Sincesome of the user experiments, such as a pump and probe
experiment, require precise arrival timing, the timing
information from the RF-BPM is provided for users.
For the transverse profile monitor, a screen monitor(SCM) [7] is employed. Optical transition radiation(OTR) or scintillation light from a target screen is focusedby a lens system into a CCD camera, which records abeam. We employed a Desmarquest fluorescent screen forthe low-energy beam part of the accelerator, a stainlesssteel foil for OTR or a YAG:Ce screen for a high-energy
part. After full bunch compression, intense coherent OTR(C-OTR) was observed and the beam profile could not be
obtained in this case. Therefore, we tried to mitigate the
C-OTR effect [8].
For the temporal characteristic measurement of theelectron beam, we designed several types of monitors,
since the bunch length is compressed from 1 ns to 30 fs,
which is too wide to measure the bunch length by one
method. In the injector part, we use coherent transition
radiation (CTR) from a screen monitor target and a fastdifferential current transformer (DCT) [9] in order toobserve a bunch length of around 100 ps. The CTR
monitor detects the intensity of the CTR, since the CTR
intensity has a correlation with the bunch length. With the
DCT, the bunch length is estimated from the pulse
duration of the signal waveform. Coherent synchrotron
radiation (CSR) monitors [10] are also utilized to measure
the bunch length at the BCs. The intensity of CSR emitted
from the fourth dipole magnet of each BC is detected,
since the CSR intensity has bunch length information in
the same way as the CTR monitor.

In order to directly measure the temporal beam profile,a streak camera and a C-band transverse rf deflector(RFDEF) [11] are employed. The streak camera detects
the OTR from the metal foil. The streak camera system
can measure the bunch length from sub-ps to 100 ps. The
C-band RFDEF is installed downstream of BC3 in order
to confirm the 30 fs bunch length after full bunch
compression.
Table 1: List of beam diagnostic instruments and their
numbers
Number of Monitors
RF cavity BPM

57

Multi-stripline BPM

4Screen Monitor

43

Differential Current Transformer

30

CTR monitor

5

CSR monitor

3

OTR screen for Streak Camera

3

Transverse RF Deflector

1

BEAM POSITION MONITOR
In this section, the design and performance of the RFBPM and multi-stripline BPM (MS-BPM) are described.
After the design overview of the RF-BPM, position
resolution of the RF-BPM along the undulator beamline isanalyzed and the orbit control for the beamline is also
described. In addition, the resolution of the beam arrival
timing and the trend of the timing during the SACLA
operation are discussed. Finally, the technical details of
the MS-BPM and the energy measurement performanceare described.

RF Cavity Beam Position Monitor
A schematic drawing of the RF-BPM cavity is shownin Fig. 2. The beam position is detected by using a
TM110 dipole mode cavity, which is tuned to be aresonant frequency of 4.76 GHz. In order to obtain both X
and Y positions, the TM110 cavity has four coupling
ports, two X ports and two Y ports. These four ports are
symmetrically arranged so as to prevent field asymmetry.
The signal amplitude from a TM110 cavity port can be
written asܸ = ܸ଴ ‫ ݁ݔݍ‬௝ఠ௧ି௧/்
where ܸ଴ is a proportionality constant, ‫ ݍ‬is the beam
charge, ‫ ݔ‬is the beam position, ߱ is the resonant angularfrequency and ܶ is the time constant of the cavity. The
amplitude of the beam-induced field in the TM110 cavity
is proportional to the beam position. The sign of the beam
position can be obtained from the rf phase of the signal.
The RF-BPM has additional TM010 monopole mode
cavity with the same resonant frequency of 4.76 GHz.
Since the TM010 mode is almost independent of the beamISBN 978-3-95450-119-9
2

Overview and Commissioning of Facilities

Proceedings of IBIC2012, Tsukuba, Japanposition and the signal amplitude is only proportional tothe beam charge. Furthermore, the phase of the TM010
mode signal is a normalization reference to the TM110
signal. By using both TM110 and TM010 signals, thebeam position can be obtained.
The RF-BPM signal is processed by an IQ (In-phaseand Quadrature) demodulator. The baseband signal fromthe IQ demodulator is recorded by a 12-bit or 16-bit VME
A/D converter. The rf amplitude and phase of the TM110
and TM010 cavities are calculated from the baseband
signal voltages at the specified timing and the beam
position is calculated from these values.

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Figure 2: Drawing of the RF-BPM cavity.

In order to confirm the sub-ȝPUHVROXWLRQGHPDQGHGE\
the lasing condition in the undulator beamline, position
resolutions of 20 RF-BPMs in the undulator section were
measured [4]. The electron beam energy was 7 GeV andthe bunch charge was 0.1 nC in this measurement. The
beam position at a given RF-BPM was estimated from theother 19 RF-BPMs and the detected position wascompared with the estimation. The estimated position was
calculated by the least squares method. The details ofanalysis are described in Ref. [4].
Figure 3 shows an example of the correlation plot
between the measured position and the estimated position.
The estimation is almost the same as the measurement.
We define the position resolution as the rms of thedifference between the measured value and the estimatedone. The position resolutions of 20 RF-BPM along theundulator section are plotted in Fig. 4. The resolution was
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Steering Magnet Current Setting for Variable
Gap Undulators Based on RF-BPM Data
In SACLA, variable-gap in-vacuum undulators are used
for XFEL generation. Since the error magnetic field of theundulator depends on a gap width, a trajectory perturbed
by the error field must be corrected by steering magnets
in order to maintain the straightness of the beam orbit. For
this purpose, each undulator is equipped with two steering
magnets, one of which is placed at the entrance of theundulator and the other is at the exit.
The current setting of the steering magnet is determinedby using RF-BPM data as a function of the undulator gap.
Details of the analysis can be found in Ref. [12]. The orbit
distortion was obtained from the RF- BPM data at each
undulator gap. The current of the steering magnet as afunction of the gap width is calculated from these data so
as to reproduce the almost straight reference orbit.
The horizontal orbit reproducibility is shown in Fig. 5.
In this data, the gap widths of all the undulators werechanged at the same time and the steering magnet currents
were set to appropriate values for individual gap widths.
The beam orbit error from the straightness was within
10 Pm for the entire gap variation region. By using thecurrent setting data or the steering magnet, the XFEL canbe generated with any K-value from 1.5 to 2.1

Overview and Commissioning of Facilities

Figure 3: Scatter plot of the measured position of a BPM
versus the estimated position from other BPMs.

Y㻌X

Figure 4: Position resolutions of 20 RF-BPMs along theundulator beamline.

Figure 5: Reproducibility of the beam orbit after changing
the undulator gap. The K values of 1.5, 1.8 and 2.1
correspond to the gap width of 5.2 mm, 4.4 mm and3.7 mm, respectively.
ISBN 978-3-95450-119-9
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Copyright ○Position Resolution Measurement

MOIC02

Proceedings of IBIC2012, Tsukuba, Japan

Injection Orbit Stabilization for the Undulator
Beamline

Multi-stripline BPM
A schematic drawing of the MS-BPM used at thedispersive part of the bunch compressor is shown in
Fig. 9. The cross-section of the beam aperture of the MSBPM has dimensions of 70 (x) x 10 (y) mm2. On each top
and bottom surface, five stripline electrodes are arranged
with a 10 mm interval. Since the vertical opening of the
MS-BPM is 10 mm, most of the electric field from asingle electron is confined to within 10 mm. Therefore,
we can roughly obtain a horizontal beam profile with aresolution of about 10 mm. The horizontal beam positionis calculated to be the center of mass of the charge

Figure 7: Phase difference between the reference cavitiesof the two adjacent RF-BPMs.
0.5

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As described previously, the beam arrival timing can bemeasured by the phase of the TM010 resonator of the RFBPM. The resolution of the arrival timing was measured
at the SCSS test accelerator by using the two neighboring
RF-BPMs [6]. Figure 7 shows the distribution of thephase difference between the two TM010 cavities. The
standard deviation is 0.0459 degree at 4760 MHz,corresponding to 27 fs. This value is ξ2 times larger than
the timing resolution of each RF-BPM, if we assume theresolution of each RF-BPM is the same. Therefore, thearrival timing resolution is approximately 20 fs.
In order to estimate the long-term reliability of thearrival timing measurement, we analyzed a long-term
variation of the beam arrival time by using the RF-BPM
data [13]. Figure 8 shows a trend graph of the phase of theTM010 cavity of the RF-BPMs at the entrance and theexit of the undulator beamline. The distance from theentrance and the exit of the undulator line is about 110 m.
The phase of 1 degree corresponds to 600 fs. The timingof the averaged data was varied about 600 fs pk-pk. Thisdrift was caused by a parameter drift of the injector partof the accelerator. Nevertheless, the RF-BPMs themselvesdetected the beam arrival timing correctly, since the twographs show a same tendency. Although a small
difference can be seen, this difference is consistent withthe thermal expansion of the optical fiber for the reference
rf signal transmission due to ambient temperature drift.

3/22/12 0:00

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Copyright ○Beam Arrival Time Measurement

Figure 6: Contour plot of the XFEL intensity as a functionof the injection beam orbit. The X and Y axes of this plot
are the horizontal beam positions of the RF-BPM 7 m
upstream from the first undulator and that of 1 m
upstream of the first undulator, respectively. The Z-axis(color) shows the XFEL intensity.

phase (degree)

In order to constantly maintain the XFEL intensity, thebeam orbit in the undulator beamline should be stabilized.
In SACLA, an initial orbit drift is measured by two RFBPMs upstream of the undulator beamline and the orbit isstabilized by a feedback control of two steering magnets
upstream of these RF-BPMs. The period of the feedback
control is a few seconds. A contour plot of the XFEL
intensity as a function of these RF-BPM data is shown in
Fig. 6. The XFEL intensity depends on the injection orbit.
Therefore, the target point of the feedback control is set tothe peak of the contour plot of the XFEL intensity. By
using this control sequence, the XFEL intensity drift issufficiently reduced.

Figure 8: Trend graphs of the phases of the TM010
cavities of the RF-BPMs measured at the entrance (1-2-2)
and the exit (5-1-2) of the undulator section for two days.
The data of each beam shot is plotted by a dot and the 16shot moving average is shown by a solid line.
distribution. The length of the stripline electrode is
160 mm, which corresponds to Ȝof 476 MHz.
The signal from the MS-BPM is converted to a pulsed
rf signal with a 476 MHz band-pass filter. Five rf signals
are sequentially multiplexed into one channel by using agroup delay of another band-pass filter. The multiplexed
rf signal is detected by a 476 MHz IQ demodulators andthe baseband signals generated with the detector areISBN 978-3-95450-119-9
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Proceedings of IBIC2012, Tsukuba, Japan

MOIC02

recorded by a VME waveform digitizer board. The detailsof the MS-BPM system are described in Ref. [5].
We confirmed the position sensitivity of MS-BPM at
the SCSS test accelerator [5]. The horizontal and verticalsensitivities were less than 0.1 mm and 0.01 mm. This
horizontal sensitivity is sufficient for energy measurement
at each bunch compressor, which is demanded to be lessthan 0.1%, since the dispersion of each bunch compressor
is more than 100 mm. Since the beam energy at eachbunch compressor is strongly correlated with a bunch
compression ratio, the data from the MS-BPM iseffectively used for beam tuning.

In order to obtain a transverse beam profile, we use ascreen monitor (SCM) with a fluorescent target and an
optical transition radiation (OTR) target made of stainlesssteel. For a short bunch length beam of less than 100 fs,
coherent OTR (C-OTR) was detected. Therefore, we tried
to remove the C-OTR by using a YAG:Ce scintillation
target and the spatial property of OTR. The emittancemeasurement result and the comparison of the position
measurement result with the RF-BPM data to check the
performance of the SCM are also described.

Figure 9: Schematic drawing of the multi-stripline BPM.

Figure 10: Photograph of the screen monitor.

Screen Monitor
The SCM consists of a screen movement mechanism invacuum and an imaging system [7]. The screen can beremotely inserted into the beam path with a pneumatic
actuator. The radiation from the screen is focused by acustom-made lens system to a CCD camera, as shown in
Fig. 10. The optical resolution of the lens system wasconfirmed to be 2 ȝm in case of the maximum
magnification of x4.

Figure 11: Setup of the screen monitor to mitigate C-OTR
with a mask.

Mitigation of Coherent OTR Effect
The SCM works well for a bunch length longer than
100 fs. However, intense C-OTR was observed from thestainless steel target when the bunch length was less than
100 fs and the beam profile image was no longer
obtained. Therefore, we changed the target to the 0.1 mmthick YAG:Ce in order to use scintillation light.
Nevertheless, C-OTR was still observed from the
YAG:Ce screen. We put an OTR mask in front of the lens,
as illustrated in Fig. 11, since OTR is emitted in theforward direction with a radiation angle dependence of
1/J [8]. After that, an appropriate beam profile image is
obtained, as shown in Fig. 12.
Although the C-OTR mask method fairly works well,
but non-negligible stray light of the C-OTR is often
observed. Furthermore, even if the beam is scattered by
the YAG:Ce screen, C-OTR is still generated at a mirror
behind the YAG:Ce screen. In this case, it is difficult toremove C-OTR by using a mask. Therefore, we tried touse a perforated mirror to reduce the C-OTR effect [8]. A
schematic drawing of the SCM with the perforated mirror
is shown in Fig. 13. The C-OTR light generated at the
YAG:Ce goes through the hole of the perforated mirror.

Masked region
㹼 5 mmScintillation㻌

COTR㻌

Figure 12: Beam profile image observed by the beamprofile monitor with the YAG:Ce screen and OTR mask.
The electron beam also passed through the hole and hence
the C-OTR is not generated at the mirror. Only the
scintillation light from the YAG:Ce screen is reflected andthe beam image can be obtained.
An example of the results is shown in Fig. 14. The holediameter on the mirror is 3 mm in this measurement. The
beam image was appropriately obtained and scattered
light from the edge of the mirror hole can also be slightly
seen. When the beam is moved near the edge of the holeby using a steering magnet, C-OTR radiation wasISBN 978-3-95450-119-9

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5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○SPATIAL BEAM PROFILE MONITORMOIC02

Proceedings of IBIC2012, Tsukuba, Japandetected together with the scintillation beam image. Thus,
C-OTR is sufficiently removed by this method.
In order to generate intense XFEL radiation, the beamemittance should be as low as possible. Therefore, wemeasured the beam emittance at each stage of bunch
compression. The emittance is measured by thequadrupole scan method [14]. We scanned the current
value of the quadrupole magnet just upstream of a screen
monitor and took the transverse beam size by using thescreen monitor. The square of the beam size satisfies thefollowing formula:ଶߪ௫ଶ = ߚߝ൫‫݈ܮ‬ொ ‫ ܭ‬+ ߙ‫ܮ‬/ߚ െ 1൯ + ߝ‫ܮ‬ଶ /ߚ,where ߪ௫ is the rms beam size measured by the screen
monitor, ߙ and ߚ are the Twiss parameters, ߝ is theemittance, ‫ ܮ‬is the distance between the quadrupole
magnet and the screen monitor, ݈ொ and ‫ ܭ‬is the effective
length and the K value of the quadrupole magnet,
respectively. This formula is fitted to the data and theemittance and Twiss parameters are obtained.
One of the results is shown in Fig. 15. This data wastaken after the bunch compression by BC3, and the screen
monitor at about 15 m downstream from the exit of BC3.
In this screen monitor, the YAG:Ce screen with the COTR mask was used. The data points were appropriately
fitted by the analytical formula. The normalized emittanceof this beam was 1.09 mm mrad, which was sufficient for
XFEL. In this data, the minimum beam size was 30 ȝm
rms and the profile of such a tiny beam can beappropriately obtained by the screen monitor with the
YAG:Ce screen and C-OTR mask.

Figure 13: Setup of the profile monitor to mitigate C-OTR
effect with the perforated mirror.

Beam

Edge of the mirror holeĳ 3 mmFigure 14: Beam profile image observed by the beamprofile monitor with the YAG:Ce screen and perforated
mirror.

Comparison between Beam Positions Measured
with RF-BPM and Screen Monitor
In order to confirm the validity of the measurements of
the RF-BPM and the SCM, we compared the beam
positions from the two monitors. We used a screen
monitor in the undulator section and this screen monitor
has the YAG:Ce screen with the C-OTR mask. The beam
position from the SCM is calculated from the center ofmass of the beam profile image and compared with thebeam position data from the RF-BPM adjacent to theSCM. In order to change the beam profile, we scanned thebeam energy from 3.8 GeV to 7.4 GeV, while the fieldstrength of the quadrupole magnet was kept constant.
Figure 16 shows the comparison between the RF-BPM
and SCM position data. Even though the beam profile
was changed, the beam positions measured with both the
SCM and RF-BPM were consistent each other within theerror of less than 10 Pm (rms). Thus, the RF-BPM and theSCM with C-OTR mask are working well as expected.

TEMPORAL PROFILE MONITOR
A temporal beam profile is monitored with several
methods depending on the bunch length. A fastdifferential current transformer (DCT) and a coherent
transition radiation (CTR) monitor are used for theISBN 978-3-95450-119-9
6

Figure 15: Emittance measurement result. The strength of
the quadrupole magnet was scanned and the square of thebeam size was plotted. This graph was fitted by theparabolic function and the emittance was calculated.

BL3-2 SCM vs BPM X
SCM [mm]

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Copyright ○Emittance Measurement

0.45

3.772GeV

0.40

4.445GeV

0.35

5.263GeV
6.067GeV

0.30
0.25
-0.05

6.667GeV
0.00

0.05
0.10
BPM [mm]0.15

7.437GeV

Figure 16: Correlation between the beam positions
measured by an RF-BPM and a screen monitor. Even
though beam energy was changed in order to vary thebeam shape, the beam position was consistent each other.

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Proceedings of IBIC2012, Tsukuba, Japan

injector part, where the bunch length is around 100 ps.
For a non-destructive monitor of the bunch length at eachbunch compressor, a coherent synchrotron radiation(CSR) monitor is utilized. For the direct detection of thetemporal profile less than 10 ps, a streak camera and a Cband transverse rf deflector (RFDEF) are employed.
We developed a high-speed DCT [9], as shown in
Fig. 17. This DCT has four outputs: two are positive andthe others are negative. By subtracting a negative signal
from a positive one, we can remove any common-mode
noise. A single-turn pickup coil and a high-frequency
magnetic core (Finemet, Hitachi Metal, Ltd.) are used so
as to achieve the short rise time of a DCT output pulse.
The rise time of the raw signal was measured to be 200 ps.
Therefore, this CT has a capability to measure the bunch
length around 500 ps.
The bunch length of the injector part is measured withthe two DCTs (CT-238 and CT-476), as shown in Fig. 18.
The raw signals from these DCTs are taken by a 12 GHz
band-width high-speed oscilloscope. An example of theraw signal waveform of CT-476 is shown in Fig. 19. In
this data, an input rf power into the 238 MHz subharmonic buncher was changed and an rf phase was set tothe bunching condition. The rf power for the othercavities were turned off. The pulse width and amplitude at
CT-476 depend on the acceleration voltage and the bunch
length is the shortest at an input power of 5.6 kW. Thus,
we can obtain the bunching property of the injector partby using DCTs.

Positive port
Ceramic insulator
Magnetic core
Positive port
Negative
port

SMA connector
Pickup coil (1 turn)
Negative port

Bean pipe (I22)

Figure 17: Schematic view of the differential CT.

Figure 18: Schematic Drawing of the sub-harmonicbuncher part.

Coherent Transition Radiation Monitor
We use a CTR from a fluorescent screen in order toestimate the bunch length in the injector part, as shown in
Fig. 20. Since the wavelength of the CTR is the same or
longer than the bunch length, the bunch length can beobtained from the spectrum of the CTR. The CTR fromthe screen is reflected by a retractable mirror and detectedby a CTR detector, as depicted in Fig. 21. The CTR
detector has successively connected 5 rectangular
waveguides and each waveguide has an antenna with a
50 GHz band-width rf detector. Since each rectangular
waveguide has different cutoff frequency, we can obtain arough spectrum of CTR. The cutoff frequencies of the 5
waveguides are 3, 6, 12, 24, 48 GHz.
Figure 22 shows a CTR intensity for each cutoff
frequency as a function of the acceleration voltage of the
238 MHz sub-harmonic buncher. The CTR monitor usedin this data is SCM-476 in Fig. 18. The CTR intensity hasa peak around 175 kV, where the bunch length is theshortest at SCM-476. This is consistent with a 1D
simulation result. Thus, the bunch length information wasappropriately obtained with the CTR monitor.

Coherent Synchrotron Radiation Monitor
For a non-destructive bunch length monitor, a CSR
monitor was developed [10]. A schematic view of the
CSR monitor is illustrated in Fig. 23. CSR from the 4th

Figure 19: Waveforms of the DCT (CT-476). In thismeasurement, the acceleration voltage of the 238 MHz
sub-harmonic buncher was changed.

Figure 20: Schematic view of the CTR monitor attached
to a screen monitor.
dipole magnet of the bunch compressor chicane isreflected and extracted from the vacuum chamber. The
CSR is then focused with a THz lens and detected with apyro-electric detector.
Figure 24 shows a CSR intensity depending on thebunch length. The bunch length was varied by the rf
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Proceedings of IBIC2012, Tsukuba, Japan

phase of the S-band accelerator upstream of BC2. In thisdata set, bunch lengths measured by the RFDEF systemwere 200 fs, 300 fs and 400 fs (FWHM) at -1, 0 and +1
degree, respectively. Here, the rf phase of 0 degree means
the lasing condition of SACLA. The sensitivity of the
CSR monitor to the S-band phase was 0.1 degree, which
is sufficient for SACLA, and the linear response to the rf
phase is suitable for the feedback control of the bunch
length.

To A/D converter

RF detector㻌
Antenna㻌

Streak Camera
In order to measure a bunch length from 300 fs to
100 ps, a streak camera system is used. OTR light emitted
from a stainless-steel foil is transmitted to the klystrongallery and detected with a FESCA-200 streak camera[15], which is located downstream of BC2 or BC3.
Figure 25 shows an example of a measured bunch length
after BC2. The bunch length was 0.54 ps (FWHM) in thiscase. Thus, the streak camera system has capability tomeasure the bunch length less than 1 ps.

Rectangular waveguide㻌

Figure 21: Schematic drawing of the CTR detector.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Transverse RF Deflecting Cavity
We use a C-band (5712 MHz) transverse rf deflecting
cavity (RFDEF) [11] to measure the temporal bunch
structure of the electron beam with a 30 fs bunch length
after the full bunch compression. A schematic setup of the
RFDEF system is shown in Fig. 26. The electron beam ispitched by a transverse deflecting rf field at a zerocrossing phase and the temporal structure is converted toa transverse profile. The transverse profile is observed by
the SCM with the YAG:Ce screen with the OTR mask.
The rf mode excited in the RFDEF is HEM11 and thepeak deflecting voltage of the RFDEF system is 60 MV.
The drift space between the RFDEF and the SCM isapproximately 10 m. In this case, the temporal sensitivity
o RFDEF is approximately 50 fs/mm at a beam energy of
1.4 GeV, which was confirmed with measurement. Since
the beam radius is about 0.1 mm, the temporal resolution
is estimated to be 10 fs, which is limited by the emittanceand orbital beta-function of the beam.

Peak-current and E-t Phase-space Measurement
Figure 27 shows a measured temporal bunch structureof a 1.4 GeV beam. The electron beam is verticallystretched by RFDEF. The bunch length of approximately
30 fs (FWHM) is appropriately obtained. The peakcurrent is estimated to be more than 3 kA by the projectedcurve a graph of the beam image to the vertical axis.
Figure 28 shows an E-t phase space image of theelectron beam. In this measurement, a quadrupole magnet
at the dispersive part of BC3 was turned on and somedispersion was leaked at the SCM position. This E-tphase-space data is useful to obtain the bunch
compression property.

SUMMARY
We developed and constructed high-resolution beamdiagnostic instruments for the measurements of a beam
position, a transverse profile and a temporal profile. A

Figure 22: CTR intensity as a function of the acceleration
voltage of the 238 MHz sub-harmonic buncher. Eachsolid line represents the CTR intensity for each cutoff
frequency. The line with solid circles shows a 1D
simulation result.
Mirror with㻌
Beam Hole㻌

Mirror㻌Viewing Port㻌
Dipole Magnet㻌
THz Lens㻌
Pyro-electric Detector㻌

Figure 23: Schematic setup of the CSR monitor.

Figure 24: CSR intensity at BC2 as a function of the Sband acceleration rf phase. The origin of the horizontal
axis is the phase at the XFEL lasing condition. Error bars
represent the standard deviations of intensity fluctuations.

ISBN 978-3-95450-119-9
8

Overview and Commissioning of Facilities

Proceedings of IBIC2012, Tsukuba, Japan

Figure 25: Example of a streak camera image. The
temporal structure is stretched vertically. The full scale ofthe time range is 20 ps. The red line shows the projectionof the image, which has the FWHM value of 0.54 ps.

Figure 26: Schematic setup of the RFDEF system.

RFDEF OFF㻌

RFDEF ON㻌Beam㻌
100fs㻌
Stray lightof COTR㻌
Figure 27: Beam profiles taken by the RFDEF system.
The RFDEF is off in the left figure, and the RFDEF is onin the right one.

t㻌E㻌Figure 28: E-t phase-space image taken at thedownstream of the RFDEF. A horizontal dispersion at thescreen monitor was generated by a quadrupole magnet atthe dispersive part of BC3.

REFERENCES[1] T. Ishikawa, et al., Nature Photonics 6, 540-544(2012).
[2] T. Shintake and T. Tanaka (eds.), “SCSS X-FEL
Conceptual Design Report”, RIKEN Harima Institute(2005).
[3] T. Tanaka, H. Kitamura and T. Shintake, Nucl.
Instrum. Meth. A 528, 172 (2004).
[4] H. Maesaka, et al., Nucl. Instrum. Meth., doi:
10.1016/j.nima.2012.08.088, in press.
[5] H. Maesaka, et al., “Development of a MultiStripline Beam Position Monitor for a Wide Flat
Beam of XFEL/SPring-8”, Proceedings of IPAC’10,
p. 954 (2010).
[6] H. Maesaka, et al., “Development of the RF Cavity
BPM of XFEL/SPring-8”, Proceedings of DIPAC’09,
p. 56 (2009).
[7] K. Yanagida, et al., “Development of Screen Monitor
with a Spatial Resolution of Ten Micro-Meters for
XFEL/SPring-8”, Proceedings of LINAC’08, p. 573(2008).
[8] S. Matsubara, et al., “Improvement of Screen
Monitor with Suppression of Coherent-OTR for
SACLA”, in these proceedings.
[9] S. Matsubara, et al., “Development of High-speed
Differential
Current
Transformer
Monitor”,
Proceedings of IPAC’11, p. 1227 (2011).
[10] C. Kondo, et al., “CSR Bunch Length Monitor for
XFEL/ SPring-8–SACLA”, Proceedings of IPAC’11,
p. 1224 (2011).
[11] H. Ego, et al., “Transverse C-band Deflecting
Structure for Longitudinal Phase Space Diagnostics
in the XFEL/SPring-8 ‘SACLA’”, Proceedings of
IPAC’11, p. 1221 (2011).
[12] K. Togawa, et al., “Laser Wavelength Tuning by
Variable-Gap Undulators in SACLA”, Proceedingsof FEL’12 (2012).
[13] T. Ohshima, et al., “Variation of Beam Arrival
Timing at SACLA”, Proceedings of FEL’12 (2012).
[14] S. Y. Lee, Accelerator Physics Second Edition, p. 62,
World Scientific (2004).
[15] Hamamatsu Photonics K. K., http://www.hamamatsu.
com/
ISBN 978-3-95450-119-9

Overview and Commissioning of Facilities

9c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○20 ps

RF-BPM with a sub-ȝP UHVROXWLRQ was successfully
utilized for the XFEL commissioning of an undulator
beamline. A MS-BPM has a sufficient energy sensitivityof less than 0.1% at each bunch compressor. A SCM also
works well even under the condition of C-OTR irradiation.
A spatial mitigation technique of C-OTR with a YAG:Cescreen was developed and the beam profile wasappropriately obtained for a beam size of around 30 ȝPA temporal bunch structure was monitored with a DCT
and a CTR detector in an injector part and with a CSR
detector at each bunch compressor. A sub-ps bunch length
was directly measured with a streak camera system and a
30 fs bunch length was confirmed by using a C-band
RFDEF. By using these beam diagnostic instruments,
XFEL radiation in the wavelength region around 0.1 nm
was successfully obtained and SACLA is stably operated
for user experiments..

MOIC02