Proceedings of IBIC2012, Tsukuba, Japan

MOCC03

THE FIRST ELECTRON BUNCH MEASUREMENT BY MEANS OF DASTORGANIC EO CRYSTALS

A pilot user experiment with the seeded FEL havebeen demonstrated at the Prototype Test Accelerator (EUVFEL), SPring-8 from July, 2012. A precise measurement of
the electron bunch charge distribution (BCD) is crucial key
to keep both spatial and temporal overlaps between highorder harmonic (HH) laser pulses and electron bunches.
In addition, R&D of a 3D-BCD monitor with a singleshot detection has been extensively promoted at SPring8. The monitor adopts a spectral decoding based ElectroOptic (EO) sampling technique that is non-destructive andenables real-time reconstruction of the 3D-BCD with temporal resolution of 30 to 40 fs (FWHM). So far, such EO
sampling based BCD monitors have been developed by using inorganic EO crystals such as ZnTe or GaP and theirtemporal resolutions are limited by 110 ∼ 130 fs (FWHM).
As a part of this project, the first BCD measurement with anorganic EO crystal; DAST has been successfully demonstrated at the facility. Signal intensities, temporal resolutions and radiation related issues for both ZnTe and DAST
are discussed.

INTRODUCTION
In general, XFEL (X-ray Free Electron Laser) accelerator drives ultra-short electron bunches such as ∼100
µm (rms) for transverse and ∼30 fs (FWHM) for longitudinal directions. In order to drive such ultra-short electron bunches, it is required not only to measure bunch
length with a few tens of femtoseconds temporal resolution, but also to simultaneously measure transverse bunchcharge distribution (BCD) with real-time, non-destructive,
and shot-by-shot measurement. For example, for SACLA(SPring-8 Angstrom Compact Free Electron Laser), RF de-flectors have been utilized to measure the temporal distribution of electron bunches, so far [1, 2]. However, since
this method is beam-destructive, real-time reconstruction
and shot-by-shot detection of the electron bunch cannot berealized during the operation of SASE-FEL oscillation.
Electrongune-Accelerator

Undulater

Electronbeam dump
X-ray laser

SASE+
HH beam

Electronbunch

SASE FELHigh peak
power laser

HHG process

High-orderharmonic

Seeded FEL

2.0
Intensity (arb. unit)

Abstract( Schematic draw )
1.5
1.0
0.5
0.0
59.0

59.5

60.0

60.5

61.0

Wavelength [nm]Figure 1: A schematic drawing of SASE-FEL and seeded
FEL operation.
Furthermore, for user experiment with HHG-seeded FEL,
both high-order harmonics (HH) laser pulse and electron
bunch are required to be spatially and temporally overlapped at all times as shown in Fig. 1. Therefore, both
longitudinal and transverse BCD measurements, i.e., 3DBCD measurements with probe laser, that is split from the
HHG process, realizes to make shot-by-shot feedback to
the HHG-seeded FEL operation.
The Electro-Optic (EO) sampling is one of the practicaltechniques to realize the 3D-BCD measurements [3, 4]. In
this method, the Coulomb field distribution of a relativistic electron bunch is encoded on to a probe laser pulse as
modulation of polarization due to phase retardation by EO
effect. Intensity spectrum of the probe laser is modulated
after a polarized splitter. In case we measure the intensity spectrum modulation with a multi-channel spectrom-

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Y. Okayasu
JASRI, 1-1-1 Koto, Sayo-cho, Sayo-gun, Hyogo, Japan
H. Tomizawa, S. Matsubara, T. Sato, K. Ogawa and T. Togashi
RIKEN Harima Institute, 1-1-1 Koto, Sayo-cho, Sayo-gun, Hyogo, Japan
E.J. Takahshi
The Institute of Physical and Chemical Research (RIKEN),
Wako Main Campus, 2-1 Hirosawa, Wako, Saitama, Japan
H. Minamide and K. Matsukawa
RIKEN ASI Tera-photonics Laboratory,
519-1399 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi, Japan
M. Aoyama
Japan Atomic Energy Agency, 8-1-7 Umemidai, Kizugawa city, Kyoto, Japan
A. Iwasaki and S. Owada
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan

MOCC03

Proceedings of IBIC2012, Tsukuba, Japaneter, single-shot temporal measurement of electron bunch
can be achieved and this method is well known as the spectral decoding [5].
Temporal resolution of the spectral decoding based EO
sampling is mainly limited and determined following four
factors:
1) A limitation of the temporal resolution (Tres ) in the
spectral decoding is expressed with the Fourier transform limitation (τ0 ) and a chirped pulse width (τc ) ofa realistic probe laser pulse as shown in Fig. 2 andEq. (1):√
Tres ≈ τ0 τc(1)

spatial BCD detection, transverse BCD measurements arealso required. For actual operation in SACLA, electron
bunches should be guided in a straight line at a undulator section with a pointing stability of < 10 µm. Althoughmany RF-beam position monitors (BPMs) with less than 1µm (rms) resolution and screen monitors (SCMs) with 2.5µm (rms) resolution have been successfully developed for
SACLA [6], single-shot measurements with real-time reconstruction of bunch charge distribution, which can simultaneously measure both longitudinal (temporal) and transverse (spatial) Bunch Charge Distribution (3D-BCD), enable fine optimization of electron BCD and energy chirping
conditions during the SACLA operation.
While there are other methods of EO sampling, such as
temporal or spatial decoding based techniques, we adopt
spectral decoding method due to the following two reasons:c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○1) Transverse measurements of a sliced BCD is available
which is direct detection comparing to projected BCD
measurements with BPM and so on. Thus such direct
detection is necessary to precise control of the X-ray
lasing.

Figure 2: EO-encoded spectra with different chirp rate.
2) Dispersion of Pockels EO crystals depend on crystal
thickness and frequency domain in use (both opticaland THz domain). Since the THz pulse is distorted
and broadened inside the Pockels EO crystal, it is difficult to realize femtoseconds order of temporal resolution, especially in THz region.
3) Since electric field of the relativistic electron bunch
has an opening angle of 2/γ, the temporal resolution(Tgeo ) is defined asTgeo =

2r
,
γc

(2)

where γ is Lorentz factor, r the distance between theelectron bunch axis and a probing point of EO media,
and c the light velocity.
4) A velocity mismatch between the electric field of theelectron bunch in vacuum and the probe laser pulse
inside the EO media. Since refractive indices of EO
crystals, especially the Pockels EO crystals, are generally as high as ∼2, which is almost half the light
velocity in vacuum.
These above summarize characteristics of the longitudinal BCD measurements. In order to demonstrate precise

2) Since, in SACLA, the transverse beam size (40 µm
in rms) is larger than the longitudinal one (30 fs
in FWHM, corresponding to 10 µm in the scale), aslight tilt of the beam trajectory can disturb precise
bunch duration measurements. Introducing our nondestructive and real-time 3D-BCD monitor, we cancorrect the bunch tilt and perform precise longitudinal measurements.
Eventually, fine beam tuning can be realized just before theundulator section.

PRINCIPLE OF 3D-BCD MEASUREMENT
Characteristics of the 3D-BCD monitor are 1) nondestructive, 2) single-shot, 3) real-time reconstruction, and4) three dimensional measurement of BCD with 30-fs(FWHM) of temporal resolution. Schematic drawing of
the 3D-BCD measurement based on the spectral decoding
is represented in Fig. 3 (upper). A broadband (∼350 nm in
FWHM @ 800 nm) and linear chirped probe laser pulse are
generated and its intensity spectrum is squarely shaped byan Acoust-Optic modulator (DAZZLER). The probe laser’s
polarization is transferred from linear to radial with a radial polarization converter. And probe laser profile is converted from Gaussian to hollow (anti-Gaussian) beam by
an axicon lens or a mirror pairs. Figure 3 (lower) shows aconceptual design of 3D-BCD monitor. The electron beam
passes through total three Pockels EO detectors. In each
EO detector, eight Pockels EO crystals azimuthlly surroundthe electron beam axis. And the independent three hollowshaped probe laser pulse split from the original laser source
are synchronized to probe the same electron bunch at each
EO detector. The longitudinal distribution of the Coulomb
field associated with the relativistic electron bunch is encoded as a phase retardation on the probe laser spectrum atISBN 978-3-95450-119-92Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, Japaneach crystal. In addition, both incident angles and positionsof the sliced BCD are determined by the set of two EO detectors at both ends. Details of longitudinal and transverse
detections of the BCD are individually discussed as follows.

MOCC03jitter between the probe laser pulse and the electron bunch
is 50 fs (FWHM), τc ≈ 600 fs of linear-chirped probe laser
pulse is required and Tres ≈ 40 fs (FWHM) is expected.
Second, following three spectral transmission characteristics, which affect temporal resolution of the system,
should be taken care:1) absorption in THz range emitted from electron bunch,
2) velocity mismatch inside Pockels EO crystals between
the Coulomb field associated with the electron bunch
and the probe laser pulse,3) dispersion inside Pockels EO crystals in optical range
in addition to THz range.

Transverse Detection

Figure 3: Schematic drawing of element of the 3D-BCD
monitor (upper) and total system of 3D-BCD monitor(lower)Longitudinal Detection
In SACLA, 30-fs of temporal resolution (FWHM) is required to measure longitudinal charge distribution of theelectron bunch. In order to achieve such temporal resolution, 1) broadband and linear chirped probe laser generation and 2) transparency of the Pockels EO crystals in frequency domain of use are essential keys. As we discussed
above, since we prepare ∼350 nm (FWHM) @ 800 nm of
broadband probe laser, the Fourier limited pulse width ofthe probe laser is evaluated to τ0 ≈ 3 fs (FWHM).
First, in case the BCD measurement with one Pockels
EO crystal, temporal resolution is estimated to Tres ≈ 30
fs (FWHM) with τc ≈ 300 − 400 fs (FWHM) of linearchirped probe laser pulse. In the actual 3D-BCD measurement, eight Pockels EO crystals should be probed without
any spatial overlap of each other. When the arrival timing

Next, we evaluate signal intensity modulations for eight
Pockels EO crystals to investigate feasibility of transverse
BCD measurement. In our calculation, we assume 8 GeVof electron beam energy, 100 pC of bunch charge and 30
fs (FWHM) Gaussian distribution of bunch duration basedon a SACLA operation condition. Since Pockels EO crystal (detection point) is expected to be close to the electron
beam axis as much as possible according to Eq. (2), eachdetection point are set as r = 2 mm in both calculation andexperiment. Figure 4 (left) shows a longitudinally sliced
BCD profile with overlays of relative coordinates of eight
Pockels EO crystals. Here, the aspect ratio √of the sliced√
profile is rhol : rver = 2 : 1 ( rhol · rver = 2 × 75 µm≈ 100 µm), where rhol and rver are horizontal and verticalbeam size in rms, respectively.

Figure 4: Longitudinally sliced BCD profile with relative
coordinates of eight Pockels EO crystals (left) and calculation results of signal intensity modulations at each crystal
position (right).

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○So far, inorganic Pockels EO crystals, such as ZnTeand GaP have generally been utilized for BCD measurements in EO sampling. However, since each crystal has
noticeable photon absorption around 5 and 11 THz, respectively, temporal resolution with such crystals is limited by
110 ∼ 130 fs (FWHM) [7]. Therefore, Pockels EO crystals
should be transparent in wide frequency range. Thus we
introduce an organic Pockels EO crystal, DAST (4-N , N dimethylamino-4’-N ’-methyl stilbazolium tosylate) [8].

MOCC03

Proceedings of IBIC2012, Tsukuba, JapanFirst, for the sliced profile is true circle not ellipse, electric field at each detection point (r = 2 mm) are describedasQ
E(r) =,(3)3/2(2π) 0 crσ0
where Q is bunch charge, c light velocity, 0 electric permittivity in vacuum, and σ0 bunch length in sigma. Achange of refractive index (∆n) of isotropic EO media isexpressed as|∆n| =n30 
ξp E(r) + ξk E(r)2 ,2(4)c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○where ξp is Pockels EO, ξk Kerr EO coefficients and n0 refractive index of the EO media. Here, ZnTe with 1 mm
thick is utilized as Pockels EO media and n0 = 2.85
and ξp = 3.97 × 10−12 m/V are adopted for the calculation. Figure 4 (right) compares signal intensity modulations (∆I) at eight Pockels EO crystals while ∆I is definedasIsig − Iorg
∆I =
,
(5)
Iorg
where Isig and Iorg are signal intensities for ellipse andcircle profiles of longitudinally sliced BCD, respectively.
And definition of the signal intensity is described as
 3

π

πn0 ξp EL
22
I = I0 sin∆nL = I0 sin
, (6)
2λ
4λ
where L is crystal thickness and λ wavelength of probe
laser pulse. According to the calculation results as shownin Fig. 4 (right), The EO signal intensity modulation isevaluated to maximum 0.2% with ∼100 µm of beam size.
On the other hand, actual beam size in SACLA is estimated√to rhol · rver ≈ 40 µm, thus transverse BCD measurement with the above criterion is expected to be much difficult. For such a small intensity modulation, we measurea s-wave spectrum instead of p-wave one for higher S/Nratio as described in Fig. 3 (upper).

FEASIBILITY
A feasibility test of the BCD measurement via spectral
decoding based EO sampling was demonstrated at the Prototype Test Accelerator, SPring-8 on February 2012. Experimental setups and conditions are shown in Fig. 5 andTable 1. A linear-chirped probe laser pulse was generated
with 5 nm (FWHM) of band width (central wavelength is795 nm) and 5 ps (FWHM) of pulse duration in a laser
hatch outside the accelerator tunnel. Arrival timing of theprobe laser pulse was adjusted to be the same with electron bunch on the Pockels EO crystal position by a streak
camera for coarse adjustment. Timing signal of the electron
bunch (300 fs in FWHM of bunch duration) and probe laser
pulse were monitored by a CT located behind a EO chamber and a photodiode detector right before the EO chamber,respectively. And precise timing adjustment was demonstrated by a timing delay module (Candox System inc.) for

Figure 5: Experimental setups of the BCD measurement
at the Prototype Test Accelerator, SPring-8 (a and b), andcrystals for beam detection (c, starting from the top, 420,
230, 190 mm thick of DAST crystals, 1 mm thick ZnTe and
Ce:YAG).

Table 1: Experimental Conditions
Electron beam
EnergyBunch charge
Bunch duration
Reputation rate
Peak current

250 MeV
30 ∼ 420 pC
300 fs (FWHM)
30 Hz> 300 A

Probe laser
Bandwidth
Pulse energy
Pulse duration
Linear chirp rate
Reputation rate

5 nm (FWHM) @ 795 nm
5 µJ
5 ps (FWHM)
0.6 ps/nm
30 Hz

EO crystal
3W × 4H × 1T mm3
Distance from e-beam axis
Phase retardation

2 mm∼50 degree

ISBN 978-3-95450-119-9
4Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, Japan

both signal and background. Red lines represent background spectra which is measured without electron beamand blue lines are signal only spectra (black - red). EO
signal intensity derived with DAST crystal is twice higher
than that with ZnTe crystal. In addition, rise time, i.e., temporal response of the signal spectrum with DAST crystal isapparently fast. Electron bunch duration evaluated by the
BCD measurement with ZnTe and DAST crystals were 1.5
and 1.8 ps (FWHM), respectively. On the other hand, EO
signal with DAST crystal was not maintained more than
one hour since we started the measurement. We are investigating DAST crystals on damages due to radiation or laser
illumination.

SUMMARY
We establish the 3D-BCD detection system that realizes
non-destructive, single-shot measurement, and real-timereconstruction with 30 fs (FWHM) of temporal resolution.
This 3D-BCD measurement system is spectral decoding
based EO sampling and consists of 1) eight organic Pockels
EO crystals azimuthlly surrounding electron beam axis, 2)
linear-chirped, broadband, and square-shaped probe laser
with radial polarization and hollow-shape, and 3) opticalcomponents which have extremely higher extinction ratio(1 :∼ 107 ) in one octave wavelength domain. With theseabove powerful configurations, not only longitudinal, butalso transverse BCD measurements can be realized. Feasibility test of the BCD monitor was demonstrated at the Prototype Test Accelerator, SPring-8 on February 2012. The
first observation of the EO signal with an organic Pockels
crystal; DAST was successfully achieved. Further investigation about radiation or laser illumination damages fororganic crystals are extensively proceeded.

Figure 6: Dependency of the EO signal intensities on thebunch charge.
Next, signal intensity spectra (single shot) of the BCD
measurement with an inorganic (1 mm thick ZnTe) and anorganic (230 µm DAST) Pockels EO crystals are measured
and compared in Fig. 7.

REFERENCES[1] O. H. Altemueller et al., Rev. Sci. Instr. 35 438 (1964).
[2] H. Ego and Y. Otake, in Proceedings of EPAC08, Genova,
Italy, 1098 (2008).
[3] H. Tomizawa et al., in Proceedings of FEL2007, Novosibirsk,
Russia, 472 (2007), H. Tomizawa, Japan Patent Application
No: 2007-133046.
[4] X. Yan et al., Phys. Rev. Lett. 85 3404 (2000).
[5] I. Wilke et al., Phys. Rev. Lett. 88 124801 (2002).
[6] Y. Otake et al., in Proceedings of EPAC08, Genova, Italy,
1224 (2008).
[7] G. Berden et al., Phys. Rev. Lett. 99 164801 (2007).
[8] X.-C. Zhang, et al., Appl. Phys. Lett., 61, 3080 (1992).

Figure 7: Signal intensity spectra of BCD measurement via
1 mm thick ZnTe (left) and 230 µm thick DAST (right)
Pockels EO crystals. Average bunch charge is fixed as 0.28
nC.
In Fig. 7, signal intensity spectra directly measured by
the spectrometer are plotted as black lines which include

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○the probe laser. Next, coordinates of the Pockels EO crystal
for electron beam axis was aligned with Ce:YAG phosphor
and remote monitoring with a CCD camera.
Figure 6 shows EO signal intensity dependency on theelectron bunch charge. In addition, calculation results,
which is taken into account only the Pockels EO effectas described in Eq. (6), are overlaid as a solid line on
Fig. 6. Measured EO signal intensity spectrum (average often shots) by a spectrometer (OceanOptics, QE65000) are
fitted with a single Gaussian and their peak values for fivebunch charge sets are plotted on Fig. 6. From Fig. 6, EO
signal intensities are saturated beyond 0.28 pC of bunchcharge. On the other hand, signal intensity evaluation isdifficult for lower bunch charge due to degenerated signal
to noise ratio of the spectra.

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