Proceedings of IBIC2012, Tsukuba, Japan

TUPA43

FIRST OPERATION OF THE ELECTRO OPTICAL SAMPLING
DIAGNOSTICS OF THE FERMI@ELETTRA FEL

Abstract
The FERMI@Elettra seeded FEL has demanding specifications in terms of longitudinal properties of the electron
beam. Several diagnostics are installed along the linac. At
the entrance of the FEL1 undulator chain an electro optical sampling (EOS) station based on the spatial encoding scheme is installed. The EOS provides both time jitterand longitudinal profile measurements in a non-destructive
way. The layout of this system is described and the first operational measurement results obtained are reported. The
paper includes also the capability of this diagnostics to perform the temporal coarse alignment of the seed laser to theelectron beam. Finally a discussion on the future developments foreseen for this system is given.

INTRODUCTIONFERMI@Elettra is a seeded FEL operating in the spectral range from VUV to soft x-rays. It is based on a
SLAC/BLN/UCLA type RF-gun and a normal conducting LINAC, currently operated at 1.2 GeV, but designed toreach 1.5 GeV. The longitudinal compression is provided
by two magnetic chicanes BC1 and BC2 (respectively at
300 MeV and 600 MeV). The FEL has two undulator
chains, namely FEL1 and FEL2. The first, FEL1, is a single
cascade HGHG seed system designed to provide hundredsof microjoule per pulse in the wavelength range from 100
to 20 nm. The second, FEL2, is a double cascade seeded
system designed to reach 4 nm as the shortest wavelength.
A deep knowledge of the longitudinal parameters such as:
longitudinal charge distribution, time of arrival jitter, slice
energy spread and slice emittance is needed. For this reason FERMI@Elettra has been equipped with several longitudinal diagnostics to provide the necessary information forthe commissioning and operation of the accelerator. They
are: the Cherenkov low energy profile measurement witha femtosecond streak Camera, the Coherent Bunch Length
Monitors (CBLM), the Bunch Arrival Monitors (BAM), theRF-deflecting cavities at low energy (LERFD) and at highenergy (HERFDs) and finally the Electro Optical samplingstations (EOS). In this paper we present the layout and thefirst operational results from the EOS installed at the entrance of the FEL1 chain of FERMI. The Electro Optical
Sampling provides single shot non-destructive longitudinal profile and time jitter measurements. For this reason
it is installed just before the modulator undulator. In thisposition the EOS system provides also the coarse tempo∗ marco.veronese@elettra.trieste.it

ral alignment of the seed laser to the electron beam. In
general an electro optical sampling diagnostics, makes useof the very large transient electric coulomb field of ultrarelativistic electron bunches to map their longitudinal profile. The electric field induces a variation in the crystal
structure of an electro optic crystal. The refractive index
variation is detected by optical polarization analysis. Since
this effect is very fast, it reproduces the longitudinal distribution of the electron bunch with high fidelity. An ultrafast laser with pulse duration on the scale of 100 fs is
an optimal probe. Several single shot schemes have been
introduced and have been shown to be very effective (e.g.
[1, 2]). For the FERMI FEL1 EOS we have chosen thespatial encoding scheme [3, 4]. This scheme is capableof providing high resolution measurements with the energy
per pulse delivered by a compact, commercial, fiber laser
oscillator.
For ultra-relativistic electrons the electric coulomb field
has a large transverse component. For a single electron theelectric field distance b from electron bunch in the plane(x,y) orthogonal the propagation direction (z) is [5]:Ey=b =eγb4π0 (b2 + γ 2 z 2 )3/2

(1)

For an electron bunch with Ne electrons, the longitudinal
distribution normalized to the maximum current, ρ(z), canbe approximated for the FERMI case to a rectangular distribution that is conveniently modeled by the difference of
two Heaviside functions: ρ(z) = H(z +F W ·/2)−H(z −F W · /2). Then the total field along the axis is then:Z
Ebeam = ρ(z − z 0 )E0,b,z0 dz 0
(2)
From Eq. 1 above , it is clear that the minimum distance
between electron beam and EO crystal is a critical parameter and its proper selection depends on several factors. Verysmall distances are desirable, but this would induce wakefields on the electron beam. Trajectory fluctuations may
cause the e-beam to hit the EO crystal, possibly damaging
it and producing ionizing radiation dose on the downstream
undulators. Finally, in the EOS area, the seed laser travels
collinearly to the electron beam, focusing from its entrance
point upstream the EOS to reach it minimum width at thecenter of the modulator downstream the EOS. The seedlaser has a high energy per pulse (up to tens of microjoules
at a wavelength of 260 nm) that could potentially damage
the crystal in case of an accidental impact. In practice, weshould set the distance b at minimum to 5 mm. For a 500

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○M. Veronese∗ , A. Abrami, E. Allaria, M. Bossi, M.B. Danailov, M. Ferianis,
L. Fröhlich, S. Grulja, M. Predonzani, F. Rossi, G. Scalamera, C. Spezzani, M. Tudor, M. TrovòSincrotrone Trieste S.C.p.A., Trieste, Italy

TUPA43

Proceedings of IBIC2012, Tsukuba, JapanpC electron bunch with a bunch length FWHM of around
1 ps, we get tens of MV/m of peak electric field. To evaluate the amplitude of the EO effect we consider the caseof a laser pulse probing a crystal with zinc-blende crystal structure like ZnTe or GaP, cut along the normal to the(110) plane and a detection based on the balanced detector
scheme. The EO signal is proportional to sin(Γ) where Γis the phase retardation induced by the electric field of theelectron beam as per Equation 3.
Γ=

p
πd 3
n0 Ebeam r41 1 + 3 cos2 (α)λ0(3)

Where d is the thickness of the crystal, λ0 is the laser wavelength in vacuum, Ebeam is the electric field due to the electron beam, r41 is the only independent element of electrooptic tensor for the chosen crystal structure, and finally α
is the angle between the electric field of the electron bunch
and the [−1, 1, 0] direction. This translates for our laser anda crystal of 100µm thickness and α = 0 to values for Γ ofrespectively: 0.3 for ZnTe and 0.8 for GaP, large enough toallow for single shot EO measurement.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○LAYOUT
Figure 1 shows the layout of the EOS installed in the
FEL1 line of FERMI@Elettra. The present setup is an evolution of the initial design [6]. It is based on a Menlosystems fiber laser, model TC780, installed in the accelerator tunnel, close to the EOS vacuum chamber, to minimize
pointing and time stability issues that may be related to along optical transport system. The laser is a frequency doubled 1560 nm femtosecond fiber laser, emitting at 780 nm,
with a repetition rate of 78.895 MHz and an energy perpulse of 0.8 nJ with a pulse duration of 110 fs FWHM. The
laser is phase locked to a pulsed optical phase reference
distributed in one stabilized fiber link of the timing system[7]. A dedicated electronics converts optical pulses to RF
and extracts a 1.499 GHz phase reference which is used by
the phase locking electronics. The coarse time alignment isprovided by phase shifting the 1.499 GHz reference with avector modulator while keeping the laser locked, thus producing an effective delay of the exit time of the pulse fromthe laser. The fine temporal alignment is provided by a 200mm travel delay line. A remotely controlled polarizer anda zero order λ/2 wave plate allow for adjustment of the input polarization. Before entering the vacuum chamber, thelaser beam passes through a cylindrical lens that focus it in
only one direction onto the EO crystals. The vacuum chamber is equipped with a 3 axis high precision manipulator
currently equipped with: an OTR screen (a 1µm aluminumfoil), used for coarse time alignment, a YAG:Ce crystal, forbeam size measurement, two electro optic crystals and acalibration target. For the initial commissioning of the system the setup has been optimized for signal to noise ratio
rather than for maximum resolution. Thus we installed a
1 mm thick ZnTe crystal and a GaP 0.4mm thick crystal.
Figure 2 shows a detailed view of the in vacuum crystals

Figure 1: EOS FEL1 layout top view. Electron beam traveling in the beam pipe from top to bottom.

holder. The crystal EO response has been tested using a
THz emitter before installing them in EOS diagnostics [8].
The EO crystals have been protected from the seed laser
by using 1.5 mm thick fused silica optical substrate witha high reflectivity coating at 260nm placed in front of theEO crystal. The fused silica does not affect the propagation of the 780 nm fiber laser pulses, while we expect a reduction of the effective THz electric field on the EO crystals. THz field reflections are expected only outside thetemporal window of measurement. The laser beam propagation direction presents an angle of 30 degree with thenormal to the surface of the crystal. This angle is needed
to exploit the spatial encoding scheme. The entrance andexit windows are made of fused silica. At the exit of thevacuum chamber the laser passes through the polarization
analysis system usually set in cross polarizers configuration. This consists of two zero order waveplates, a λ/4,
followed by a λ/2 and a finally a Wollaston prism. The
majority of the intensity goes to a dump, while the otherbranch is directed towards intensified gated CCD camera(ICCD) model Dicam Pro produced by PCO. The ICCD
is used with a gate time of 10 ns to select only the laser
pulse in the 78.895 MHz train that has been affected by theelectro optical effect induced by the electron beam. Two
imaging lenses l1 and l2 with focal lengths of respectively
200 mm and 400 mm are installed in the air path at the exitof the vacuum chamber. This lens system produces at theISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

TUPA43one photodiode is involved, the temporal alignment can bemeasured with an accuracy better than few picoseconds.

Figure 2: EOS FEL1 crystal holder. From bottom to top:
OTR, YAG, ZnTe, GaP, calibration target. Laser and ebeam in red and yellow lines respectively.

TEMPORAL ALIGNMENT
The first temporal alignment is that of the ICCD gate to
the laser pulse that modulated at 10Hz. To do this the OTR
emitted from the OTR target is steered by Al mirrors onto
the ICCD and a tuning of the ICCD trigger delay is done
to center the ICCD gate of 10 ns to the OTR signal. Then,a coarse alignment of the EOS laser is performed to about
few tens of ps to allow for a short final search of the EOS
signal time window alignment with the optical delay line.
It is performed with the system depicted in Figure 3. An
horizontal translation stage (MICOS ZVT80) and a verticalactuator (Newport NSA12), both motorized and remotely
controlled, allow to detect with two different fast photodiodes the EOS laser reflected by the OTR screen and the
OTR signal emitted by the OTR screen. The signals are acquired by a Tektronics 6 GHz high bandwidth oscilloscope.
The signals are overlapped in time at the EOS chamber center when a specific temporal distance at the oscilloscope ismet. This distance is defined on the basis of a measureof geometrical optical paths of the two radiations and of
the measured propagation delay induced by the cables (dueto length and propagation speed of the signal). After thealignments the remaining uncertainty is about 100 ps. At
this point the EO signal is looked for by moving the opticaldelay line. This systems is also used for coarse temporalalignment diagnostics of the seed laser arrival time to theelectron beam arrival time. In the case of coarse alignmentof the seed laser, the OTR screen is used both to emit OTR
radiation and to reflect the seed laser radiation. Since they
are collinear both are directed on the same fast photodiode (modified by removing the protection window which
would absorb the seed laser radiation at 260 nm). The photodiode has a small active area of 200 x 200 µm and forthis reason it is mounted on a remotely controlled horizontal and vertical translation system. In this case, since only

Figure 3: EOS FEL1 coarse temporal alignment systemlayout.

LONGITUDINAL PROFILEMEASUREMENTThe ICCD time calibration is performed comparing thecalibrate spot size of the laser on the crystal as seen withan external camera to the size of the laser on the ICCD.
The time windows of the measurement depends on the angle and on the laser transverse spot size at the EO crystal(see Figure 4). The laser spot on the crystal has a FWHM
of 3.3 mm corresponding to a time window of 5.6 ps. On
the ICCD the laser spot has a FWHM of 350 pixels. With
the present setup the calibration factor is of 17 fs/pixel.
Measurements have been performed with both ZnTe andFigure 4: Laser spot on the ZnTe crystal as seen by an external CCD camera.
GaP. In the first case the EO signal is pretty large withgood signal to noise ration and negligible background. In
the second case the background is not negligible, a background subtraction is needed and the SNR is lower. This effect is expected because the rotation induced in the case of
GaP is smaller due to a smaller electro-optic coefficient andsmaller thickness. For both crystals, the best SNR is foundISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○ICCD exit image (ICCD included) a final magnification of
about 0.5.

TUPA43

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○about 3-5 degrees off extinction of the polarizers. The polarization extinction ratio is about 2500 mainly limited by
the residual stress of the vacuum windows. The Figure 5
show a EO signal measured with ZnTe 1mm thick crystal for a moderate compression factor (CF). The left graphshows a series of consecutive shot with average bunch duration of 1.5 ps FWHM. For comparison with high compression the image on the right shows bunch profiles withaverage FWHM of 790 fs. These measurement performed
during a phase fluctuations of the plants upstream of themagnetic compressor show the capability of the system todetect the time jitter and the compression factor jitter induced by jitter in the RF plants. For higher compression

relative phase jumps (or drifts) are sampled at the bunch
repetition rate (10Hz for FERMI) and directly expressed intime units. These measurements, as opposed to the ones
based on single side band phase noise analysis, can bedirectly compared to other systems, like the RF deflecting cavities or the Bunch Arrival Monitor (BAM), also installed on FERMI@Elettra.

Figure 6: EOS signal intensity map over 50 shots. Timejitter of 70 fs rms. Data acquired using a ZnTe 1 mm thickcrystal.

ACKNOWLEDGMENT
Figure 5: EOS longitudinal profile measurement sequences. With CF=3.5 (left) the average FWHM is 1.5 ps.
At CF=8.5 (right) the average FHWM is 790 fs.
factor the crystals chosen for initial operation (ZnTe 1mm
and GaP 0.4 mm) are limiting the temporal resolution of thesystem. We plan the installation of an 0.1 mm GaP crystal
together with new entrance and exit windows to improve
the polarization extinction ratio.

TIME JITTER MEASUREMENT
By using the EOS on FEL1 a time jitter measurementwith good temporal resolution is also possible. As an example we report in Figure 6 a sequence of 50 EOS acquisitions at 1 Hz, acquired during a normal operation of theaccelerator with minimal time jitter. The time jitter is measured as shot to shot displacement of the centroid and it istypically of the order of 70 fs rms for a compression with asingle bunch compressor to a bunch length FWHM of about
1 ps. This measurement is a direct and single shot measurement of the relative arrival time of the electron bunchwith respect to the pulse of the EOS laser, which is phaselocked to the optical femtosecond pulsed phase reference,
distributed to all the laser systems of FERMI@Elettra. The

The system was initially installed at SPARC (LNFINFN) then been moved to FERMI@Elettra for it first
operation. We would like to thank D. Filippetto, A.
Cianci, M. Bellaveglia, G. Gatti, A. Mostacci (INFN-LNF),
L.Giannessi and M. Del Franco (ENEA) for their support
during SPARC activities and S. Jamison (STFC-Alice), P.
Krejcik and D.Fritz (SLAC) for very useful discussions andadvice. This work was partially supported by the Italian
Ministry of University and Research under grants FIRBRBAP045JF2 and FIRB-RBAP-6AWK3.

REFERENCES[1] G. Berden, et al., Phys. Rev. Lett. 93, 114802 (2004).
[2] G. Berden et al. PRL 99, pg 164801 (2007).
[3] A. Cavalieri et al. PRL 94 pg 114801 (2005).
[4] A. Azima et al. MOPCH011 Procs. EPAC (2006).
[5] J. D. Jackson “Classical Electrodynamics”, 2nd ed., Wiley,
Canada (1975).
[6] M.Veronese et al., TUPTPF026, Proc. BIW08, (2008).
[7] M. Ferianis et al., FROAI2, Proc. FEL2011 (2011).
[8] M.Veronese et al., TUPC12, Proc. FEL2009 (2009).

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4Longitudinal Diagnostics and Synchronization