Proceedings of IBIC2012, Tsukuba, JapanTUPB76INTRA UNDULATOR SCREEN DIAGNOSTICS
FOR THE FERMI@Elettra FEL

Abstract
The FERMI@Elettra seeded FEL poses demanding requirements in terms of intra undulator diagnostics due tothe short wavelength of its FEL radiation and to the coexistence of the electron and photon FEL beams. An advanced multi-beam screen system has equipped both FEL1
and FEL2. The system has been designed for transverse
size and profile measurement on both the electron beamand the FEL radiation. Challenging design constrains arepresent: COTR suppression, seed laser suppression, FEL
wavelength range and minimization of the ionizing radiation delivered to the undulators. This paper describes
the novel design and the obtained performance with the
FERMI intra undulator screen system (IU-FEL).

INTRODUCTIONFERMI@Elettra is a seeded FEL operating in the spectral range from VUV to soft x-rays [1]. It is based on
a SLAC/BLN/UCLA type RF-gun, a normal conducting
LINAC, currently operated at 1.2 GeV (up to 1.5 GeV).
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 hundreds of microjoule
per pulse in the range from 100 to 20 nm. The second,
FEL2, is a double cascade seeded system designed to reach
4nm at the shortest wavelength. On both chains, in between
the undulators several devices are installed: phase shifter,
cavity BPM, a quadrupole and the intra undulator screen(IU-FEL). The IU-FEL screen is a multipurpose screen designed to provide transverse beam size measurements ofthe electron beam, the seed laser beam and the FEL beam.
At present, seven stations are installed on the first FEL1line and eight on the FEL2 line. The initial design of thesystem was performed in collaboration with Argonne National Laboratory. The setup presented in this paper has
been deeply upgraded based on specific operational experience. Similar intra undulator diagnostics have been developed and tested at sFLASH at DESY [2].

GENERAL LAYOUT
The FEL1 and FEL2 chains are depicted in Figure 1.
The FEL1 chain starts with the modulator where interac∗ marco.veronese@elettra.trieste.ittion with the seed laser occurs. The modulator is followedby the dispersive section that converts the energy modulation in current modulation (bunching). The first IU-FEL
station is installed downstream the dispersive section before the first radiator. This station is used, in conjunction
with a standard multi-screen upstream the modulator, toperform a transverse alignment of the seed laser with theelectron beam in order to guarantee the spatial overlap of
the two beams in the modulator. This means that the IUFEL screen should have the capability to image the electron
beam and the seed laser at the same time. The other six
IU-FEL stations are installed at each break in between theradiators. They provide imaging of both the electron beam
and of the FEL electron beam with high resolution. The
seed laser beam has a waist in the modulator, from there
on the beam size increases due to its natural divergence.
Moreover the vacuum chamber of the undulators has a vertical aperture of only 7 mm. This means that already after
the second radiator the seed laser transverse profile is dominated by the reflection and diffraction from the vacuum
chamber. In FEL2 the first cascade consisting of the modu-

Figure 1: FERMI FEL1 and FEL layout. The IU-FEL station are marked as red dots.
lator, the first dispersive section and the first two radiators,
basically replicates the FEL1 chain. Downstream, the delay line is installed. It is a magnetic chicane that is used todelay the electron bunch with respect to the FEL radiationfrom the first stage. Zero delay would mean seeding with
the FEL radiation emitted by the first stage on the same
portion of the electron beam used for the first which has
corrupted electron beam properties (emittance and energy
spread). The delay line allows to seed on a fresh portion of
the electron beam with unperturbed properties (fresh bunchtechnique). Downstream the delay line, a second modulator
is installed and then a the second dispersive section. From
there the radiators of the second stage will amplify thebunching by the second dispersive section after the seeding
process which now has also a shorter wavelength. The final
ISBN 978-3-95450-119-9

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1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○M. Veronese∗ , A. Abrami, E. Allaria, M. Bossi, A. Bucconi M. De Marco, L. Frölich, M. Ferianis,
L. Giannessi, S. Grulja, R. Sauro, C. Spezzani, M. Tudor Sincrotrone Trieste, Trieste, Italy
T. Borden, Facility for Rare Isotope Beams, Michigan State University, East Lansing, MI, USA
F. Cianciosi, ESRF, Grenoble, France

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Proceedings of IBIC2012, Tsukuba, Japan

radiation wavelength of FEL2 is basically the wavelengthof the seed laser (260 nm) divided by the harmonics number of the first stage and by the harmonic number of thesecond stage. In the final configuration, with an electron
beam energy of 1.5 GeV this will allow the FEL2 chain
to lase at wavelengths as short as 4 nm. IU-FEL stations
are installed upstream and downstream the first modulator,
downstream the delay line, the second dispersive section
and at each second break between the second stage radiators, with a final station downstream of the last radiator.

lic scheme, so that suppression or mitigation strategies had
to be adopted. Finally the same detection system had tomeasure both the electrons and the FEL radiation. YAG:Cescintillators are used for both e-beam and FEL radiation
but they have a different yield of visible photons, leading
to an expected ratio of about 200. This constraint imposed
to use full aperture of the iris of the lens objectives for the
FEL, while using one inch neutral density filters with optical densities ranging from 1 up to 3.

VACUUM LAYOUT

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○DESIGN REQUIREMENTS AND
CONSTRAINTS
The main electron beam parameters in all the stations areat present: energy of 1.25 GeV, charge of 500 pC, a projected emittance of about 2 mm mrad, a transverse electron
beam size below 100 microns rms. The seed laser wavelength is 260nm with Ti:Sa (from 235nm to 265nm withthe optical parametric amplifier). Seed energy per pulse
is of several tens of microjoule. The FEL1 output energy
downstream the last radiator has recently be measured as
300 microjoule at 52nm and has exceeded 20 microjoule at
20 nm. The energy per pulse emitted from the first radiator
is a factor of 100 smaller than the one emitted at the lastradiator.
For FEL1 several design constraints had to be considered designing the IU-FEL system. The requirements were
multiple and often conflicting. The first requirement was toprovide high resolution transverse imaging of the electron
beam. The second task was to provide a transverse imagingof the FEL radiation. The third was to provide few stations
with the capability of imaging at the same time the seedlaser and the electron beam. The fourth goal was to provide pinholes in the IU-FEL screen which could be used
to align by optical means all the emission from multiple
undulators on the same line, to make easier the definitionof the reference trajectory. The constraints in the design
were multiple. First, a limited space was available since thewhole intra-radiator section should be as short as possible,
and a very limited space was available on the sides. This
forced us to keep the vacuum chamber as short as possible
and to develop the optics part below it. The second constrain, a machine protection issue [3], is to limit the radiation dose deposited in the undulators to few kGy over their
lifetime to avoid demagnetization. This imposes the useof screen as thin as possible with small impact angles withthe electron beam so that the effective thickness is kept as
small as possible. Moreover materials with as low density
and atomic number (Z) as possible were adopted. Finallycoherent optical transition radiation (COTR) has been an
issue too. Metallic screens could not be used for OTR diagnostics because the COTR would dominate and corrupt
the information on the transverse distribution as seen alsoin LCLS [4] and SACLA [5]. Also using a scintillator likeyttrium aluminum garnet doped with cerium (YAG:Ce),
COTR is present although much weaker than for a metal-

The vacuum part consists of a small optical stainlesssteel tablet, on which all relevant component are mounted
as shown in Figure 2. The assembly is moved transversely
to the direction of propagation of the electron beam by an
external stepper motor actuator. In Figure 2, three components are distinguishable. The first component top rightis the setup used for e-beam diagnostics, the blue bottom
component is a pinhole, the top left component is used
for FEL diagnostics. Both the electron beam and the FEL
beam generate fluorescence on a YAG:Ce. This visible radiation is imaged by an external detection system. The

Figure 2: Vacuum optical tablet with the electron beam
mount, the FEL mount and the pinhole.
electron beam setup is based on a new scheme that wedeveloped. The standard YAG:Ce scintillators mounted at
45deg with respect to the electron beam and making a direct imaging of the scintillator surface have several limitations. The new scheme, shown in Figure 3 adopts a
YAG:Ce 100 µm thick at quasi normal incidence. The scintillator surface is not imaged directly but by means of an in
vacuum mirror which is displaced from the electron beam
an set at an angle so that the optical axis is at normal incidence with respect to the surface of the YAG:Ce scintillator. This scheme has several advantages: the first is thatthe effective thickness is of only 105 µm reducing both thedose and the effect on the resolution due to the standard
45 degree layout. Moreover, there is no depth of field issue allowing for best performance of the optical also withfully open iris. Finally, since the nearest edge of the mirror
is set at a distance above 10/γ no COTR radiation emitted by the YAG:Ce is collected. The FEL setup is depicted

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Proceedings of IBIC2012, Tsukuba, JapanTUPB76

Figure 3: Electron beam setup scheme.
in Figure 4. The first mirror decouples the FEL radiationfrom the e-beam. The second mirror deflects the FEL radiation towards the YAG:Ce scintillator. The scintillator isimaged by means of an in vacuum mirror. Here the angles

and the materials technology play a crucial role. The angle
should be kept as large as possible to minimize the effective thickness of the material, but at the same time as small
as possible allow for high reflectivity in the largest possible
spectral range down to 10nm. The material used for the firstmirror should have both low density and low Z to minimize
its dose yield. To quantify the possible choices compatible
with the available technological capabilities we have studied the dose deposited on the undulator by means of thecode Fluka [6] for different materials (Cu, Be, Si SiO2),
thickness and angle. Figure 5 show an example calculationfor Si of different thickness. The difference in the dose at
the undulator is use huge (about a factor of 100) going from
0.1 mm thick copper at 2.5 degree of grazing angle, to 0.35mm silicon at 10 degree. The reflectivity of the mirrors was
also studied as a function of material (Cu, Be, Si, Au, anddiamond) and angle (from 2.5 degree to 20 degree) using
the CXRO code [7]. In Figure 6 we show the reflectivity ofgold in the range from 10 to 40 nm for gold at 2.5 degree
and at 10 degree. In both cases the reflectivity is pretty flatin the whole range and the relative difference is of 30%. In
general, small angles would be preferable from the reflectivity point of view, while larger ones would be preferablefrom the machine protection point of view. The best compromise was found using silicon as a substrate, with a gold
coating with a thickness 50 nm over a 5 nm bonding layerof chromium. The angle of incidence chosen is 10 deg tominimize the effective thickness but at the same time allow
for good reflectivity down to a wavelength of 10 nm. Thereis an ongoing activity to test the performance of ultra-thinmirrors. Silicon 50µm mirrors have been installed withplans to test diamond ultra-thin mirrors. The second mirror

Figure 5: Simulation of dose generated by Si 300 µm thick(upper plot) and by Si 50µm thick (lower plot).

Figure 6: Simulation of dose generated by Si 300µmthick(upper plot) and by Si 50µm thick (lower plot).
is made of the same material and coating. The YAG:Ce has
been coated with 90nm of aluminum to suppress the seedlaser radiation co-propagating with the FEL and the COTR
emitted by the first mirror.

OPTICAL LAYOUT
The detection part design is also based on some constraints: ideally a magnification (M) of about 1 is desired.
The active area of the YAG:Ce is of 6mm. Together withM ≈ 1 this defines the size of the CCD. The closest format commercially available is the 1/3 format. The design
resolution of the system is 12 µm rms, this requirement toISBN 978-3-95450-119-9

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Copyright ○Figure 4: FEL beam setup scheme.

TUPB76

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○gether with the magnification leads to a pixel size of below
10 µm. The distance from the first lens to the targets is ofabout 210 mm. All this constrains have been met by usinga Basler Scout 1300-32gm CCD combined with a Sigma
MACRO 150 APO OS HSM lens used with fixed focus
and full aperture iris installed with an extender tube of 1
inch to reach correct magnification. Since there is a focal
distance difference of about 10 mm between the YAG:Ceof the electron beam setup and the YAG:Ce of the photon setup, the CCD and lens assembly has been installed
on a motorized translation stage that is used to adjust thelens while keeping the magnification unchanged. As canbe seen in Figure 7 the detection part is below the vacuum
chamber and is composed of a rotating filter wheel, a 45
degree mirror, the lens attached to the CCD and mounted
on the translation stage. The filters are of neutral density
absorptive type, the diameter is of 1 inch. Each station isequipped with three filters with optical densities: 3, 2, and1.3. Together with a clear aperture slot they provide thenecessary dynamic range to the system to cope with the intensity difference of the image in the e-beam and FEL case.

Figure 8: Transverse alignment of the seed laser to the electron beam. The left image shows unaligned beams. The
seed laser spot being the dimmer spot on the left while the
e-beam is the brighter spot on the upper right. The rightimage shows seed laser and the e-beam transversely overlapped.
the two beams overlapped. The seed laser intensity is attenuated to the level of 1 microjoule to avoid possible damage
to the YAG:Ce crystal.

ELECTRON BEAM MEASUREMENTS
The main goal of the electron beam side of the system is
to check whether the electron beam optics provides the desired FODO to keep the transverse dimension of the electron beam small all along the undulator chain to guarantee optimal overlap of the FEL radiation with the electron
beam. For this reason a good resolution is needed. Figure 9 shows the image of the smallest horizontal size of the
FERMI electron beam in the last IU-FEL station of FEL1
acquired during a resolution test of the diagnostics withspecific manual tuning of the quadrupole. The Gaussian
fitting of the measure spot size profile has a sigma of 15µm. It should be noted this is well below the standard operational setting of the FEL. This result is in line with themodulation transfer function test on the detection systemmeasured in the optical laboratory to be about 120 lp/mm.
The COTR mitigation strategy worked as expected since

Figure 7: Layout of the optical in air detection part.

SEED VS EBEAM TRANSVERSE
ALIGNMENTThe IU-FEL stations installed downstream the first dispersive magnets of FEL1 and FEL2 are used in conjunction
with another multi-screen upstream of the modulator findthe transverse overlap of the electron beam and the seedlaser. This means that both beams should be imaged at thesame time on a YAG:Ce. The typical situation before andafter alignment is shown in Figure 8. The image on the left
shows the two beams on the YAG:Ce not overlapped. The
e-beam is the bright upper right spot while the seed laser isthe dimmer lower left spot. The image on the right shows

Figure 9: Image of electron beam on YAG:Ce with beam
optics optimized to minimum horizontal electron spot size.
The horizontal profile has a Gaussian rms width of 15 µm.
none of the usual COTR effects (image pattern distortion
like donut shape or other and intensity fluctuations) wererecorded.

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FEL BEAM MEASUREMENTS
FEL radiation transverse pattern measurement can bemade shot by shot downstream of the last three radiators,
while for the first three a multi shot average is needed since
the FEL power is pretty low in the first part of the FEL
chain. As an example of the system capabilities we reportin the figures 10 and 11 the FEL radiation patters measured
downstream of the last undulator with FEL1 tuned at 43
nm. The first image shows the pattern with the last undu-

TUPB76PINHOLES
The optimal alignment between FEL and electron beam
is not a trivial task, and beam based alignment (BBA) is
needed but not always sufficient. To define a straight trajectory, as done in other labs (see [5]), we decided to install also pinholes in the IU-FEL vacuum system as shownin Figure 2. Since the seed laser divergence and diffraction makes it very quickly unusable as an alignment laser
as the distance from the waist increases, we will use pinholes. The idea is to use each pinhole in the Nth station asa diffraction source and image the patter on the downstream
YAG:Ce screen in station (N+1). To ease the work we decided to use only the central lobe of the Airy diffraction
patter of a pinhole. Given the pinhole-screen distance, the260 nm seed laser wavelength the resulting pinhole diameter is 1.5 mm. The operational experience so far with thepinhole has shown their capability of highlighting inconsistencies in the alignment performed with the laser tracker.

Figure 10: FEL radiation at 43nm as measured downstream
the last undulator of FEL1 with the last radiator open.
lator open, the second one with the last undulator closed.
Not only the central cone of radiation is more intense in
the latter case as expected, but also the fringe structure ischanged. We attribute the fringes to interference from thedifferent undulators.

We would like to thank J.Noohnan and D.Walters of Argonne National Laboratory, as well as G. Ciani for the
initial design of the system. M. Lazzarino of CNR-IOM
laboratory for the thin film coating of both mirrors and
YAG:Ce. R.Tamaro for very accurate preparation of highprecision components . This work was supported in partby the Italian Ministry of University and Research under
grants FIRB-RBAP045JF2 and FIRB-RBAP-6AWK3.

REFERENCES[1] E. Allaria et al., “Highly coherent and stable pulses from the
FERMI seeded free-electron laser in the extreme ultraviolet”,
Nature Photonics advance online publication, 23 September
2012 (doi:10.1038/nphoton.2012.233).
[2] J. Bödewadt et al., MOPD54, Proceedings of DIPAC2011,
Hamburg, Germany (2011).
[3] L. Fröhlich et al., The machine protection system for
FERMI@Elettra. Proc. FEL’10, pp. 667670, Malm, Sweden,
August 2010.
[4] J. Frisch et al. THBAU01, Proceedings of FEL2008,
Gyeongju, Korea (2008).
[5] T. Hara et al., TUPA26, Proceedings of FEL2011, Shanghai,
China (2011).
[6] “FLUKA: a multi-particle transport code”, A. Ferrari, P.R.
Sala, A. Fasso‘, and J. Ranft, CERN-2005-10 (2005),
INFN/TC 05/11, SLAC-R-773.
[7] CXRO, http://henke.lbl.gov/optical_constants/

Figure 11: FEL radiation at 43nm as measured downstream
the last undulator of FEL1 with the last radiator set on resonance.

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Copyright ○ACKNOWLEDGMENT