IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 0 2

STUDY OF VISIBLE SYNCHROTRON RADIATIONMONITOR ON SOLEIL BOOSTER
A. Moutardier∗ , G.Cauchon, M. Chevrot, Z. Fan, N. Hubert, S. Kubsky, M. Labat, M. Thomasset
Synchrotron SOLEIL, Gif-Sur-Yvette, France
MRSV DESIGN

Abstract

INTRODUCTION
Fourth-generation synchrotron light sources are presently
emerging all around the world. Those machines target ultralow emittances, below hundred of pm.rad, enabling the delivery of photon beams with unprecedented brilliance.
In this context, the French synchrotron light source
SOLEIL is planning an upgrade for the late 2020s, referred
to as SOLEIL-II. The targeted emittance in the storage ringis 88 pm · rad in the horizontal plane and 35 pm · rad in thevertical plane. To reach such a low emittance, all SOLEIL’s
accelerators will have to be upgraded, i.e. the storage ring
but also the linac and the booster. The requirements on thebooster beam quality will be significantly increased, and anemittance measurement along the energy ramp in the booster
will be mandatory.
This emittance measurement could be done using a Visible Synchrotron Radiation Monitor (MRSV). In order toconfirm this strategy, preliminary tests are ongoing on thepresent SOLEIL booster, using the MRSV system installed,
but barely used, since 2006.
This paper traces back the issues encountered with thisdiagnostic and presents the solutions considered to achieve
an emittance measurement along SOLEIL’s booster ramp.
∗ alexandre.moutardier@synchrotron-soleil.frAn MRSV is an electron beam diagnostic which enables
to image the synchrotron radiation emitted inside a dipole atits source point. It usually simply consists of an extraction
mirror, to collect and deflect the synchrotron radiation outof the accelerator vacuum chamber, an imaging system anda camera. SOLEIL’s booster MRSV setup is shown in Fig. 1.
The extraction mirror is a simple Pyrex glass, silver-coated,
flat mirror, mounted at 45° to deflect the synchrotron radiation in the vertical plane. The imaging system consistsof three lenses allowing it to reach a magnification of 0.24.
The camera is a CCD camera (Model acA1920-50gm fromBasler [1]). A UHV viewport of fused silica between theextraction mirror and the first lens ensures the transition
from vacuum to air.

1

1.132 mSilver mirror
Photon1.0405 m

In the scope of SOLEIL II, the booster must also be upgraded to reduce from 130 to 5 nm · rad the emittance ofthe beam delivered to the ring. Control of the emittancein the booster will become crucial to ensure the nominal
performance of the storage ring injection. The SOLEIL
I booster is already equipped with a Visible Synchrotron
Radiation Monitor (MRSV). This equipment, made of an
extraction mirror and a simple optical system, was originallyplanned to be used only for beam presence verification but
has not been used routinely for operation since the commissioning in 2005. The control and acquisition systems had
to be refreshed to be usable again and allow the beam size
measurement along the booster energy ramp. The extraction
mirror was replaced due to unexpected degradation leading to a second spot appearing on the camera. This paper
traces back the MRSV upgrades from understanding thecause of mirror degradation until mirror replacement andthe first proper beam visualisation, achieved at the beginningof 2023.

UHV window
Lens 1
Lens 2
Square slit

ElectrondipoleDensity
Lens 3

CameraFigure 1: Experimental setup of the MRSV implemented on
the booster of SOLEIL.
On many synchrotrons, MRSVs are simply used to checkbeam presence and its rough stability. But MRSVs canalso be used to measure the electron beam size inside thesource dipole [2]. In this case, the Point-Spread-Function(PSF) must be accurately considered because dipoles create alongitudinally extended source point. The PSF corresponds
to the light distribution of one electron (or zero emittanceand energy spread beam) in the image plane which can not
be resumed to an infinitesimal point. The light distributionof the real beam (finite emittance and energy spread) resultsfrom the convolution of this PSF with the electron beam
distribution at the source point magnified by the imaging
system. Both the PSF and the light distribution from thewhole beam in the image plane can be accurately simulatedusing SRW [3] as shown in Fig. 2.

WEP00203 Transverse Profile and Emittance Monitors

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3331

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 0 2
A first series of machine studies enabled to exclude thepossibility of beam instability, two orbits, or even orbit shifting, confirming that the diagnostic itself was malfunctioning.
Then, the position of each component of the MRSV was carefully checked and compared with the theoretical design.

Extraction Mirror Degradation
Figure 2: Simulation of the synchrotron radiation in theimage plane of the booster’s MRSV. (a) Single electron case,i.e. PSF, (b) Finite emittance case. Simulation with SRW at
the end of the booster ramp using a beam of 2.75 GeV, 8 nC,
130 nm · rad horizontal emittance and 4.5 nm · rad verticalemittance.
Once deconvoluted from the simulated PSF, the image
measured on the MRSV camera can provide the electron
beam size in the image plane, allowing retrieval of the electron beam size at the source point, and finally, assuming that
optics functions are known, the electron beam emittance.

MRSV ISSUE
Double Beam Observation
A few months after the commissioning of SOLEIL in2006, the booster’s MRSV monitor started to exhibit two
spots, as shown in Fig. 3 (b), instead of the single one as
expected from Fig. 2. However, since this MRSV was onlyused for beam presence verification and no major failure atring injection was noticed, this anomaly was not thoroughly
addressed, and the double spot remained for more than adecade.

Eventually, the guilty was found to be the extraction mirror. The inspection of this mirror through the UHV window
without breaking the booster vacuum is not easy. However,it seemed that a black strip was covering part of the mirror.
Following this observation, new SRW simulations were attempted, including the presence of an obstacle equivalent tothe black strip that had been observed. But such an obstacleon the mirror only led to some diffraction features in theimage plane, not to two spots. We therefore simply decided
to replace the mirror with a new one, though exactly of thesame type and geometry.
In January 2023, the extraction mirror has been dismounted. As suspected, the mirror was strongly damaged,
as shown in Fig. 4, with the presence of a thick dark strip all
across the mirror (at the location of the synchrotron radiationfan) and a clear iridescence besides the dark strip. The dark
strip is attributed to carbon deposition. This hypothesis hasbeen confirmed by placing the mirror for ten minutes in an
𝑂 2 plasma chamber [4] to remove all visible black traces.
Iridescence is a well-known phenomenon which could beeasily attributed to a strong heat-up.

Figure 4: Picture of the booster’s MRSV extraction mirror,
after nearly 18 years of operation.

Figure 3: Image of SOLEIL’s booster MRSV recorded at
the end of 2022, i.e. while the double spot issue was still
unsolved.
But in the scope of SOLEIL-II, since a similar system
could become a key diagnostic for the booster and downstream storage ring operation, it was decided a year ago totackle this issue.

Once cleaned, the mirror surface was scanned using awhite light interferometer (WLI [5]) at SOLEIL’s surface
Laboratory. This technique enabled to map the 3D surfaceof the mirror. As shown in Fig. 5, it clearly appeared thatthe mirror presented a groove, exhibiting a 5 µm deep valley
all along the synchrotron radiation fan location. In addition,
the upper and lower parts of the mirror, with respect to thehorizontal/synchrotron radiation fan axis, were found to betilted by an angle of roughly 1 mrad. Lastly, some drops
could also be noticed, attributed to a local melting of thesilver coating. The main result of this measurement is theobservation of an angle between the mirror’s lower and upper

WEP002
332

03 Transverse Profile and Emittance Monitors

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 0 2Figure 5: Results of the laser interferometry performed on
the extraction mirror of the booster’s MRSV after 18 yearsof operation. (a) 3D map of the mirror surface. (b) Mirror
surface height along the vertical axis, i.e. line profile along
the top image.
parts. Indeed, such an angle can induce a different pointing
for the upper and lower parts of the synchrotron radiation
fan (the middle part not being reflected due to the dark strip)
towards the image plane, which could easily explain thedouble spot previously observed.

MRSV Issue Solving
Those observations indicate clearly that the synchrotron
radiation has strongly damaged the extraction mirror. Dueto a lack of time for a new design, a similar extraction mirror
has been immediately mounted to replace the damaged one.
During the following beam time at low injection rate, anice single spot very close to the prediction was observable(Fig. 6 (a)). A survey of the MRSV image reveals that only
a few weeks of operation lead to visible deformation of theMRSV spot (Fig. 6 (b)), hence a very quick degradation of
the new mirror.

lation between those observations and numerical thermal
simulations must be established.
First, an estimation of the average power deposited on themirror has been made relying on SRW simulations and assuming the booster to be running at 3 Hz in the higher charge
mode (8 nC). The 2D average power density distribution wascomputed in the plane of the extraction mirror. The total
average power was estimated to be ≈ 6 W, and the average
power density to be ≈ 0.26 W/mm2 . The thermo-mechanicalbehaviour of the extraction mirror has been simulated using
the synchrotron radiation 2D power density map and a 3Dmodelling of both the extraction mirror and its environment
in the Ansys software [6]. According to this work, the mirror
surface temperature at equilibrium could reach 500°C, as
shown in Fig. 7 (a). Without silver coating, this temperature
could even heat up to 1000°C. Hence any defect in the mirror coating could easily lead to a local melting of the silver(silver fusion point: 961.8°C).
The reason for such high temperatures is the poor thermal
conductivity of Pyrex glass (≈ 1.2 W m−1 K−1 ). The heatload from the synchrotron radiation is locally stored by themirror instead of dissipating within the whole mirror substrate and then through the mirror holder, finally leading toserious damage. To increase the lifetime of our extraction
mirror, a first simple solution will be to change the mirror
substrate material from Pyrex glass to Copper, offering amuch higher thermal conductivity (≈ 400 W m−1 K−1 ). The
performances in terms of final temperature expected using
such a mirror are presented in Fig. 7 (b). Thanks to a more
efficient heat diffusion inside the mirror and towards its support, the final equilibrium temperature could drop down to200°C.

Figure 7: Surface temperature of the extraction mirror andits support simulated with Ansys using a mirror made of (a)Pyrex, i.e. actual mirror, (b) Copper, both silver coated.
Figure 6: Image of SOLEIL’s booster MRSV recorded (a)just after, (b) two weeks after, the replacement of the old
extraction mirror (March 2023).

NEXT STEPS
Dark strip, iridescence and melted silver drops on the mirror were intuitively linked to an over-heat of the extraction
mirror. But in the scope of SOLEIL-II, a reliable corre-

Following these encouraging results, a new mirror (Copper substrate with UV-enhanced Aluminium coating) has
been ordered. It has been mounted during last summer breakdown and commissioned on the 1st of September 2023. As
expected, resulting image is a beam spot with similar intensity as Fig. 6 (a) in the same conditions. A regular survey isscheduled to check the presence of spot deformation (characteristic of mirror degradation) but with lower operating
temperature, no degradation of the surface is expected in theWEP002
03 Transverse Profile and Emittance Monitors

333

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 0 2future. Beam size measurement is now possible along theRF booster ramp and will hopefully be available for daily
operation in the coming months.

[2] Å. Andersson et al., “Determination of a small vertical electron
beam profile and emittance at the Swiss Light Source”, Nucl.
Instrum. Methods Phys. Res., Sect. A, vol. 591, 2008,
pp. 437–446. doi:10.1016/j.nima.2008.02.095

CONCLUSION
We presented our first steps towards a future reliable beam
size/emittance measurement along SOLEIL’s booster energy
ramp. After nearly twenty years of operation, we understood that the extraction mirror of our MRSV diagnostic wassuffering a high heat load, not compatible with beam size
measurement. We now expect to be able to achieve a reliable emittance measurement by the end of the year. All thiswork will be especially useful in the frame of the SOLEIL-II
booster and associated diagnostics design.

[3] O. Chubar and P. Elleaume, “Accurate and Efficient Computation of Synchrotron Radiation in the Near Field Region”,
in Proc. EPAC’98, Stockholm, Sweden, Jun. 1998, paper
THP01G, pp. 1177–1179.

ACKNOWLEDGEMENTS[5] firebolt, (GBS) mbH, Ilmenau, Germany, https://www.gbs-ilmenau.de/index.php/en/pages-2/370-123-35

Also thanks to S. Morand, M.-A. Tordeux (SOLEIL).

[4] DIENER Pico, Diener Plasma GmbH & Co. KG,
Ebhausen, Germany, https://pdf.directindustry.
com/pdf/diener-electronic-gmbh-co-kg/pico/50802-157239.html

REFERENCES[1] acA1920-50gm Basler ace GigE camera, https://docs.
baslerweb.com/aca1920-50gm[6] Academic Research Mechanical, Ansys, https://www.
ansys.com/products/structures/ansys-mechanical

WEP002
334

03 Transverse Profile and Emittance Monitors