Proceedings of IBIC2012, Tsukuba, Japan

MOPB82BUNCH-COMPRESSOR TRANSVERSE PROFILE MONITORS OF THE
SwissFEL INJECTOR TEST FACILITY

Abstract
The 250 MeV SwissFEL Injector Test Facility (SITF)
is the test bed of the future 5.8 GeV SwissFEL linac thatwill drive a coherent FEL light source in the wavelength
range 7-0.7 and 0.7-0.1 nm. Aim of the SITF is to demonstrate the technical feasibility of producing and measuring
10 or 200 pC electron bunches with normalized emittancedown to 0.25 mm.mrad. A further goal is to demonstratethat the electron beam quality is preserved in the acceleration process, in the X-Band linearizer and the magnetic
compression from about 10 ps down to 200 fs. The SITF
movable magnetic bunch-compressor is equipped with several CCD/CMOS cameras for monitoring the beam transverse profile and determining the beam energy spread: a
YAG:Ce screen and an OTR screen camera at the mid-pointof the bunch compressor and a SR camera imaging in thevisible the Synchrotron Radiation emitted by the electron
beam crossing the third dipole. Results on the commissioning of such instrumentations, in particular in the low
charge limit, and measurements of the beam energy spreadvs. the compression factor will be presented.

INTRODUCTION
The SwissFEL project aims at the construction of a
5.8 GeV electron linac driving a coherent FEL light sourcein the wavelength regions 7-0.7 and 0.7-0.1 nm. The relative compact size of the facility (the SwissFEL total lengthis about 700 m, linac+undulators+experimental area) being
a constraint for the linac energy performance, high brilliance features are imposed to the electron source by thelasing condition. Projected emittance 0.65/0.25 mm.mrad
and longitudinal length of 25/2 fs (RMS) at the undulator
are indeed the design parameters of the 200/10 pC electron
bunches of the SwissFEL [1]. In the two different charge
operation modes of the SwissFEL, sequences of two electron bunches separated in time by 28 ns will be produced
at a repetition rate of 100 Hz by a photocathode gun andaccelerated by S-band Travelling Wave (TW) accelerating
structures (injector section) up to 330 MeV and, finally,
by a C-band TW linac up to the final energy of 5.8 GeV.
Thanks to a magnetic switchyard and a further accelerating
section (up to 3.4 GeV), two different undulator lines will
be simultaneously electron-supplied at 100 Hz: the hard Xray undulator line (ARAMIS) and the soft X-ray undulator∗ gianluca.orlandi@psi.chline (ATHOS). The electron beam, produced at the cathodeof the RF gun by a Ti:Sa laser with a flat-top longitudinal
profile (3.6/10 ps FWHM), will reach the undulator after a
two-stage longitudinal compression by means of two magnetic chicanes. The magnetic compression scheme foresees
an energy chirping of the electron beam, achieved in the Sband injector, and a longitudinal phase space linearization
performed by a couple of X-band cavities upstream the firstmagnetic chicane.
The preparatory phase of the future SwissFEL is carriedout at the 250 MeV SwissFEL Injector Test Facility (SITF).
This is composed of, see Fig. 1: a Copper photo-cathodeand a Standing-Wave (SW) S-band 2.5-cell RF gun accelerating a 10/200 pC electron bunch up to 7.1 MeV/c at a repetition rate of 10 Hz; four S-band TW RF structures accelerating the beam up to a maximum final energy of 250 MeV;
a compression section composed of a X-band cavity (under
installation) and a magnetic chicane; finally, downstream a
SW S-band 5-cell Transverse Deflecting Cavity (TDC), a
FODO section and an energy spectrometer where both thetransverse and the longitudinal phase spaces of the electron
beam are experimentally characterized. Goal of the experimental activity so far performed at the SITF is to checkthe reliability of the key components of the future accelerating machine, to optimize the procedures to experimentally characterize the electron beam parameters, to demonstrate the feasibility of an electron source with the required
high brilliance quality and, finally, to show that the highquality features of the electron beam are preserved by the
X-band linearizing scheme of magnetic compression. The
campaign of measurements carried out so far at SITF established and consolidated several measurement techniquesof the beam projected and slice emittance measurements
for both the charge operation modes and of the longitudinal phase space, for instance. The outcome of this experimental work [2, 3] confirmed that the extremely high beam
quality constraints required by the SwissFEL design can beachieved. The bunch compressor is one of the key components of the future SwissFEL whose main diagnostics havebeen recently commissioned or is under commissioning as
in the following described.

BUNCH-COMPRESSOR DIAGNOSTICS
The vacuum chamber of the magnetic bunch compressor (BC) is a flexible structure composed of two arms anda central part whose rigid components are joined together
ISBN 978-3-95450-119-9

Transverse Profiles, Screens & Wires Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Gian Luca Orlandi∗ , Masamitsu Aiba, Simona Bettoni, Bolko Beutner, Helge Brands,
Rasmus Ischebeck, Eduard Prat, Peter Peier, Thomas Schietinger, Volker Schlott, Vincent Thominet
Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
Christopher Gerth, DESY Hamburg, Germany

MOPB82

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 1: Schematic layout of the 250 MeV SwissFEL Injector Test Facility (SITF).
by vacuum-bellows (see Fig. 2). The central part of the BC
vacuum chamber, mounted on a movable girder together
with the two central dipoles of the chicane, can be horizontally shifted with respect to the accelerator beam axisby means of a micro-meter stepping-motor. The BC acceptance of the dispersive trajectories ranges continuously
from 0 to about 404 mm, corresponding to a BC bending
angle between 0◦ and 5◦ . The BC is equipped with diagnostics measuring charge, center-of-mass, transverse andlongitudinal distribution of the electron bunch and phasejitter of the beam. Charge and position in the transverse
plane of the electron beam can be measured by Beam Position Monitors (BPM): Resonant Strip Line (RSL) (7 µmresolution 5-500 pC) are located upstream and downstream
the first and the fourth dipoles of the chicane [4]; Cavity
BPM, also installed in the BC arms, are under commissioning [5]. Bunch Arrival-time Monitor [6] (BAM, resolution
better than 5 fs, SwissFEL specifications) are installed upstream and downstream the BC to measure, respectively,
the phase jitter of the beam due to both the gun and thelaser arrival time and to the S-band accelerating structure
in off-RF-crest operations. A single shot monitoring of
the electron bunch length can be provided by pyrodetector monitors detecting in the THz the synchrotron radiation(CSR) emitted by the fourth dipole of the chicane and thecoherent diffraction radiation (CDR) produced, just after
the BC, by a 1 µm thick Ti foil with a circular hole of diameter 5 mm. A couple of Electron Optical (EO) monitors
are also installed upstream and downstream the BC. They
will be used for on-line monitoring the compression and thebunch shape with a resolution of 200 fs. At the BC, bunch
length measurements can be obtained from a spectroscopic
analysis of the CSR performed by a Martin Puplett Interferometer equipped with a bolometer.
The transverse profile of the electron beam can be monitored at the entrance of the third bending dipole of the chicane and in the mid symmetry plane of the BC. The visible
Synchrotron Radiation (SR), emitted by the electron beamat the third bending dipole of the BC, can be imaged by
a CMOS-Camera located below the vacuum chamber between the third and the fourth dipoles thanks to a couple
of in-vacuum and out-vacuum mirrors forming a verticaltranslation stage for the SR light. The set-up of the SR
out-coupling mirror guarantees the transmission of the full
SR light spot within a range of the BC bending angle between 3◦ and 5◦ . The CMOS-Camera, equipped with a
300 mm focal length lens (AF-S Nikkor 300 m f/2.8G ED
VR II), can image the SR light spot with a projected pixel
size resolution of 0.040 mm. A YAG:Ce and an OTR viewscreen allows the monitoring of the beam transverse profileat the mid symmetry plane of the BC. Both view-screenscan be vertically inserted in the central vacuum chamberof the BC by a step-motor feed-through. The YAG:Cescreen (0.3 mm thick and 45◦ vertically tilted with respect
to the machine axis) can be imaged by a vertically oriented
CCD camera that is located below the BC central vacuum
chamber. This CCD camera, equipped with a 45 mm focal length lens (Nikkor 45 mm f/2.8D ED), can resolve thebeam transverse profile with a projected pixel resolutionof 0.115 mm. The BC OTR screen, normally intercepting
the electron beam, emits a backward light pulse that can beimaged by a second CMOS camera whose optical set up
is almost identical to the SR one and specularly symmetric to it with respect to the BC mid plane. The backward
OTR light spot can be imaged by a CMOS camera witha 300 mm focal length lens and projected pixel size resolution of 0.049 mm (under commissioning). At the nominal bending angle (4.07◦ ) of the SITF magnetic chicane,
the BC transverse profile monitors are designed to measure the relative energy spread (∆E/E) with an expected
resolution of: 1.2 × 10−4 (SR-monitor); 1.5 × 10−4 (OTRmonitor); 3.5 × 10−4 (YAG-monitor). The sCMOS cameras (PCO.Edge) are connected via a CAMERLINK-Fiber
bridge to a PC with a 64-bitWindows operation system.
The CCD camera (Basler SCOUT) for the YAG screen isconnected via a direct GigE-Link to a similar PC described
before. The cameras synchronization is done with a TTLpuls generated by the event based timing-system (Micro
Research Finland). The software (IOC/EPICS) controls
the camera via the vendor software interface and delivers
the picture over EPICS or saves the pictures directly to afilesystem.

ISBN 978-3-95450-119-92Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, Japan

YAG & OTR screen
feed-through

MOPB82

SR monitor box-

OTR screen
camera box

YAG screen
camera box

Figure 2: The SITF Bunch Compressor layout with Transverse Profile Monitors

MEASUREMENTS AND RESULTS
2.2
12 pC
165 pC185 pC

Bunch-Length (ps,RMS)

2
1.8
1.6
1.4
1.2
1
0.8

-5

0

5

10

15

20

25

OFF-RF-crest phase (Deg)

Figure 3: Bunch-Length measurements vs. compression at
12 pC, 165 and 185 pC.
The BC transverse profile monitors of SITF were commissioned in both the 10 pC and the 200 pC operation
modes. A Ti:Sa laser pulse with a nominal flat-top longitudinal profile (3.6/10 ps, FWHW) is the photo-cathode laser
used at SITF for 10 and 200 pC operations. A Nd:YLF

laser with a nominal Gaussian longitudinal profile (2.7 ps,
RMS) is also available for machine operations at SITF. According to the compression scheme designed for SITF, theelectron beam is chirped in energy by running off-RF-crest
the last two TW-structures of the injector and operating themagnetic compression at a nominal bending angle of 4.07◦of the magnetic chicane, corresponding to a horizontal dispersion function of about ηx =331 mm. In order to preserve
the matching of the magnetic optics with the beam during
the compression scan, the beam energy is maintained to a
constant value lower than the nominal one (230 MeV instead of 250 MeV) so that the forward RF power to thelast two TW accelerating structures can be accordingly increased as a function of the off-RF-crest phase. During thecompression operations, the transverse size of the electron
beam in BC can be measured on-line and continuously by
the SR monitor and in a destructive way by the YAG and
the OTR monitors. The relative energy spread can be finally determined from the measured horizontal beam size
σx from:√()2
2 + η ∆E
σx = σx,0(1)xE√
where σx,0 = ϵx βx is the natural betatron size of theelectron beam.
A campaign of measurements aiming at studying the perISBN 978-3-95450-119-9

Transverse Profiles, Screens & Wires Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Courtesy P.Heimgartner

MOPB82

Proceedings of IBIC2012, Tsukuba, Japan

1.4

2.2
HE Spectrometer
BC-YAG Monitor
BC-SR Monitor

1.2

HE Spec
BC-YAG Screen
BC-SR Monitor

2
1.8
1.6
1.4

s (mm)

0.8

1.2

X

X

s (mm)

1

0.6

1
0.8

0.4

0.6

(a)

0.2

4

x 10

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Relative Energy Spread (D E/E)

3.5
3

0

5

10

15

20

25

-3

7
HE Spectrometer
BC-YAG Monitor
BC-SR Monitor
ASTRA

2
1.5
1

(b)

0.5

0

5

x 10

10

15

20

0

5

10

15

25

25

5

4

3

2

(b)

1

OFF-RF-crest phase (Deg)

20

OFF-RF-crest phase (Deg)-3

HE Spec
BC-YAG Screen
BC-SR Monitor
ASTRA

6

2.5

0
-5

0.2
-5

OFF-RF-crest phase (Deg)

Relative Energy Spread (D E/E)

0
-5

(a)

0.4

0
-5

0

5

10

15

20

25

OFF-RF-crest phase (Deg)

Figure 4: Beam Energy 230 MeV, bunch charge 12 pC. (a)Horizontal beam size vs. off-RF-crest phase measured in
the High Energy Spectrometer, BC-YAG Monitor, BC-SR
Monitor. (b) Resultant Relative Energy Spread in comparison with numerical results.

Figure 5: Beam Energy 230 MeV, bunch charge 165 pC.
(a) Horizontal beam size vs. off-RF-crest phase measured
in the High Energy Spectrometer, BC-YAG Monitor, BCSR Monitor. (b) Resultant Relative Energy Spread in comparison with numerical results.

formance of the compression scheme was carried out at
SITF. As a function of the compression factor (i.e., as afunction of the off-RF-crest phase of the last two accelerating structures of the injector), the transverse profile ofthe electron beam was measured in the BC by means of
the YAG and SR monitors. In parallel, the electron bunch
length was measured by means of the TDC as well as thebeam energy spread in the high energy (HE) spectrometer. TDC bunch-length measurements were done streaking the beam onto a view screen where the vertical betatron phase advance maximizes the TDC resolution. In thebunch-length measurements as well as in the HE spectrometer measurements, a YAG:Ce screen (0.2 mm thick and45◦ horizontally tilted with respect to the beam axis) and a
CCD camera with a projected pixel size of about 0.023 mm
were used.
Bunch compression measurements were performed witha bunch charge of 12 pC (Ti:Sa laser, flat-top profile) and
165 and 185 pC (Nd:YLF laser Gaussian profile). As afunction of the off-RF-crest phase of the last two accelerating structures of the injector (compression), the beamtransverse profile at the BC, the bunch length by meansof the TDC and the energy spread at the HE spectrometer
were measured in a row. Results of the measurements ofthe bunch length (Fig. 3) and of the horizontal beam size
(BC YAG and SR monitors and HE spectrometer) are reported in Figs. 4(a), 5(a), 6(a). From the measured valuesof the beam transverse size (BC YAG and SR monitors andHE spectrometer), the corresponding values of the relative
energy spread of the beam were determined via Eq.(1) andcompared with numerical simulations of a beam tracking
code (ASTRA, [7]), see Figs. 4(b),5(b), 6(b). In the analysis of the experimental data, Eq.(1) was used neglecting thenatural betatron size of the beam except in the case 12 pC.
In the 12 pC operations, the signal-to-noise ratio of the SR
monitor was checked by reducing the light intensity with a
10% density filter with a performance of appreciable sensitivity of the monitor in the low compression regime.
The measured results of the beam transverse size andbunch length, reported in Figs. 4(a), 5(a), 6(a) and (Fig. 3),
respectively, are RMS values. These values were obtainedby using a noise-cut routine of the camera images of theISBN 978-3-95450-119-9
4Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, Japan
2.2

value of the projected pixel size (0.115 mm) of the YAG
monitor in the BC strongly affects the resolution and theprecision of such a device. Finally, numerical simulations
and experimental data are in a good agreement, in particular, in the 12 pC case, where the role of space-charge effects in the numerical model of the beam tracking is lessimportant compared to the high charge case. Further measurements and studies on modeling the beam dynamics areforeseen.

HE Spec
BC-YAG Screen
BC-SR Monitor

1.8
1.6

1.2

X

s (mm)

1.4

1
0.8
0.6

(a)

CONCLUSIONS

0.4
0.2
-5

7

x 10

Relative Energy Spread (D E/E)

6

0

5

10

15

20

25

OFF-RF-crest phase (Deg)-3

HE Spec
BC-YAG Screen
BC-SR Monitor
ASTRA

5

4

3

2

(b)

1

0
-5

0

5

10

15

20

25

OFF-RF-crest phase (Deg)

Figure 6: Beam Energy 230 MeV, bunch charge 185 pC.
(a) Horizontal beam size vs. off-RF-crest phase measured
in the High Energy Spectrometer, BC-YAG Monitor, BCSR Monitor. (b) Resultant Relative Energy Spread in comparison with numerical results.
beam profile. An adaptive region-of-interest is used tomask the noise not related to the beam spot. This isachieved by an intermediate smoothing of the image andthen a comparison with a certain threshold. Pixels above
the threshold in the intermediate image are considered tobe beam related in the original image [8]. For the beamtransverse profile analysis, a 10% noise-cut threshold wassettled on a basis of a check of different threshold values
and of a comparison with results of a Gaussian fit whenpossible. For the bunch-length analysis, a cut-threshold of
6% was defined. In the numerical simulations of the beamenergy spread, the values of the laser profile at the cathodeof the RF gun were set according to the TDC measurementsof the bunch length (see Figs. 3): i.e., 1.1 ps (RMS, 12 pC);
1.9 ps (RMS, 165 pC); 2.2 ps (RMS, 185 pC).
On the basis of a comparative analysis of the experimental characterization of the relative energy spread vs. compression, the resolution performance of the BC SR monitor can be evaluated. In particular, in the case 12 pC, theSR monitor shows to have a very good sensitivity to small
variations of the energy spread energy. The relatively high

The BC transverse profile monitor were successfullycommissioned in SITF. The commissioning results confirmed the high resolution performance of the SR monitor
in determining the relative energy spread of the electron
beam in comparison with instrumentation already in operation at SITF. The experimental characterization of thebeam energy spread vs. compression is also in agreementwith expected results from numerical simulation of beam
tracking codes. Moreover, the sensitivity of the SR camera in monitoring the beam transverse size of the beam has
been checked to extend far below the 10 pC charge valueof the SwissFEL specifications. Further steps towards routine operations of the BC transverse diagnostics will be
the commissioning of the OTR monitor and more extensive comparative studies of the energy spread monitors of
SITF. The improvement of the energy spread resolution of
the BC YAG camera by replacing the 45 mm focal lens witha 85 mm focal lens is also foreseen.

ACKNOWLEDGMENT
The authors wish to thank the SITF operation crews
for support during measurements. The precious work ofMarkus Baldinger and Martin Heiniger in setting up the
BC hardware is also acknowledged.

REFERENCES[1] SwissFEL Conceptual Design Report, PSI Bericht Nr. 10-04
April 2012.
[2] M. Pedrozzi et al., FEL2011 Free Electron Laser Conference,
Shanghai, China, 22-26 August 2011.
[3] T. Schietinger, LINAC12 XXVI Linear Accelerator Conference, Tel-Aviv, Israel, September 9-14, 2012.
[4] B. Keil et al., “Commissioning of the Resonant Stripline
BPM System of the SwissFEL Test Injector”, Proc. FEL2010,
Malm, Sweden, p. 429.
[5] M. Stadler et al., “Beam Test Results of Undulator Cavity
BPM Electronics For The European XFEL”, these proceedings.
[6] F.Loehl et al., Phys. Rev. Lett., Volume 104, Issue 14, 144801(2010).
[7] K. Flottmann (Desy, Hamburg).
[8] Bolko Beutner, FEL2009 Free Electron Laser Conference,
Liverpool, UK, 2009.

ISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○2

MOPB82