Proceedings of IBIC2012, Tsukuba, Japan

TUCC03

DESIGN AND EXPECTED PERFORMANCE OF THE NEW SLS BEAMSIZE MONITOR*Abstract
The vertical emittance minimization campaign at SLS,
realized in the context of the TIARA WP6, has alreadyachieved the world's smallest vertical beam size of
3.6 μm, corresponding to a vertical emittance of 0.9 pm,
in a synchrotron light source. The minimum value
reached for the vertical emittance is only about five times
larger than the quantum limit of 0.2 pm. However, theresolution limit of the present SLS emittance monitor has
also been reached during this campaign, thus, to further
continue the emittance minimization program theconstruction of an improved second monitor is necessary.
In this paper we present the design and studies on theperformance of this new monitor based on the image
formation method using vertically polarized synchrotron
radiation in the visible and UV spectral ranges. This newmonitor includes an additional feature, providing thepossibility of performing full interferometric measurement by the use of a set of vertical obstacles that can bedriven on the light path. Simulations results are used toinvestigate the possible sources of errors and their effects
on imaging and the determination of the beam height. We
also present the expected performance, in terms ofemittance accuracy and precision, and discuss possible
limitations of this new monitor design.

MOTIVATION
The main objective of the TIARA (Test Infrastructureand Accelerator Research Area) work package 6 [1, 2] isthe achievement and control of ultra-low verticalemittances. This is of large interest for present and futurestorage ring based light sources in order to utilize small
period / gap undulators, which provide higher photon
energies, and for the design of damping rings for future
linear colliders to obtain their desired high luminosities.
In this context, a vertical emittance tuning andoptimization program has been launched at the Swiss
Light Source (SLS) at Paul Scherrer Institut (PSI) in
Villigen, Switzerland [3]. The main activities include (a)the suppression of betatron coupling and verticaldispersion by beam-assisted realignment of the storage
ring magnets and the subsequent application of correction
schemes using skew quadrupoles, (b) the measurement ofsmall vertical beam sizes by means of a high resolution
beam profile monitor and (c) the determination of intra
beam scattering contributions to the emittance.

SLS Vertical Emittance Minimization
In case of ideal flat storage ring lattices, extremely low___________________________________________

vertical equilibrium emittances can be obtained, which
are only limited by the direct recoil of the emitted photons[4]. For SLS this so called quantum limit of the verticalemittance is at 0.2 pmrad. In reality, however, magnet
errors and alignment tolerances (typically in the order offew tens of ms) as well as beam position measurement
errors lead to betatron coupling and vertical dispersion
causing a subsequent emittance growth to several pmrad
or even tens of pmrad. Thus, as a first step towards
emittance minimization, a beam-assisted re-alignment of
the SLS storage ring magnets (lattice) was carried out inApril 2011, leading to a substantial reduction of the rms
vertical corrector kick from initially ~ 130 rad to ~ 50
rad. On the wa to a systematic vertical dispersion
correction, the BPM roll errors were determined andregarded as “fake” vertical dispersion contributions, whenthe model-based dispersive skew quadrupole correction
was applied. As a result, the spurious vertical dispersion
could be reduced by using the 12 dispersive skew
quadrupoles to < 1.3 mm rms. In a final step, a SVD
model-based betatron coupling correction was performed
with the 24 non-dispersive skew quadrupoles, which
reduced the coupling part of the orbit response matrix byalmost a factor of 2.5. Several iterations of this procedureincluding a final random optimization lead to a verticalbeam height of 3.6 ± 0.6 m and a corresponding verticalemittance of 0.9 ± 0.4 pmrad [5]. Figure 1 shows thisfinal “random walk” optimization of the SLS verticalbeam height, measured with the existing -polarization
monitor [6], which reached its resolution limit during thisemittance minimization campaign.

Figure 1: Random optimization of the SLS vertical beamheight. The resolution limit of the existing -polarization
monitor has been reached at 3.6 ± 0.6 m.

*The presented work has received funding from the European
Commission under FP-7-INFRASTRUCTURES-2010-1/INFRA-20102.2.11 project TIARA (CNI-PP). Grant agreement no. 261905.

ISBN 978-3-95450-119-9
Transverse & Longitudinal Emittance Measurements

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○N. Milas, M. Rohrer, A. Saa Hernandez, V. Schlott, A. Streun , PSI, Villigen, Switzerland
A. Andersson, J. Breunlin, MaxLab, Lund, Sweden

TUCC03

Proceedings of IBIC2012, Tsukuba, JapanTHE NEW SLS BEAM SIZE MONITOR
The new beam size monitor at SLS will provide twodifferent modes of operation: (a) the imaging of thevertically polarized synchrotron radiation (SR) with the socalled π-polarization method [7] and (b) the interferenceof the vertically polarized synchrotron radiation with theso called interferometric method [8].

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Working Principles and Monitor DesignBoth, the π-polarization and the interferometricmethod, allow the use of visible or ultra-violet SR for thedetermination of beam sizes well below 10 m.
In a -polarization mode of operation, the two lobes ofthe vertically polarized SR from the BX08 bending
magnet of the SLS storage ring are imaged onto a CCD
camera (Basler scA1300-32gm) at the end of the beamline. In order to avoid damage of the optical elements andheat-load induced distortion of the image, the hard X-ray
part of the synchrotron radiation, which has a small
vertical opening angle (~ 1/) and contains most of theenergy, is absorbed by a water cooled horizontal “finger”
absorber (4 mm height) without obstructing the well
separated vertical lobes of the SR. In case of the new SLS
beam size monitor, the imaging of the -polarizedcomponents will be achieved by a toroidal mirror,
generating the characteristic destructive interference
pattern, caused by the 180° phase difference of the twovertical SR lobes. The resulting peak-to-valley ratio canbe related to the source size (vertical beam height). In
addition to the beam height and width, the -polarization
method reveals also information about the local verticaldispersion by determining the tilt of the image.
The interferometric method will be implemented on thesame beam line by introducing a double slit in the path of
the synchrotron radiation. The working principle of thismethod follows the van Cittert-Zernike theorem [9] and isbased on the fact that the measurement of the spatial
coherence of synchrotron light in the vis-UV region isrelated to the beam size.
A sketch of the new monitor is presented in Figure 2,
and more details on the design can be found in [10]. The
main characteristics and improvements of the newmonitor in respect to the existing one are:
 the beam line ends outside of the SLS storage ringtunnel, thus being accessible at any time - also
during machine operation
 the larger magnification ratio of M = -1.45 increases
the measurement precision of almost a factor two the use of the toroidal mirror as a focusing element
provides a free selection of the SR wavelength
without moving the image plane the potential use of shorter wavelengths (e.g.
266 nm) enables an increase of the resolution tosmaller beam sizes
 two measurement methods – π-polarization andinterferometric – enable cross-checking of results
and extended measurement ranges
 calibration and alignment with the use of a laser
setup provides an online performance check of themonitor

Expected Performance
The simulations of the SR propagation along the newbeam size monitor have been performed using the SRW(Synchrotron Radiation Workshop) code [11].
The -polarization monitor resolution has been studied
for three different wavelengths (266, 340 and 498 nm).
Figure 3 shows the expected increase of the monitor
sensitivity for shorter wavelengths as the slope of thepeak-to-valley curve becomes steeper. Although theproximity of the two lobes in the image plane increases atshorter wavelengths as well, this will not limit theFigure 2: Schematic design of the new SLS beam size monitor applying the -polarization and interferometric methods(top). Layout of the X08DA beam line – calibration and alignment set-up and imaging station are schematic (bottom).
ISBN 978-3-95450-119-92Transverse & Longitudinal Emittance Measurements

performance, since the smaller pixel size of 3.75 m of
the Basler CCD camera and the enlarged magnification of
the new monitor compensate for this effect.

Figure 3: SRW simulation of peak-to-valley resolution forthe -polarization method and for different wavelengths.
The sensitivity of the -polarization method to beamheight changes is well matched to the nominal user
operation mode of the SLS with a vertical beam size of~ 10 m at 0.13 % coupling. In case of further emittanceminimization studies in the SLS storage ring, theinterfometeric method will be applied, providing superior
resolution of < 2 m as shown in Fig. 4.

Figure 4: SRW simulation of the fringe visibility for theinterferometric beam size measurement method as afunction of slit separations (15, 20, 25 mm). The polarization method is shown as a reference.

Tolerance Studies
The toroidal mirror is the most critical component of
the new beam size monitor concerning surface quality,offsets and misalignments. Mirror imperfections weremodelled according to [12] and simulated with SRW,
resulting in specifications for the mirror fabrication interms of slope errors < 0.2 arcsec, roughness of /30 (at633 nm) and waviness from the polishing procedure to beallowed in vertical or radial directions. The toroidal
mirror will be mounted in a gimbal mount, a pivoted
support which allows for the rotation of the mirror aroundthe horizontal and vertical axes, while keeping its centre

TUCC03fixed. In order to define the constraints on mechanicalalignment necessary to utilize the monitor at its full
resolving power of < 2 m, all possible misalignments ofthe toroidal mirror from its ideal position have been
included in the SRW simulations. The results show thathorizontal and vertical offsets of up to ± 50 m are notaffecting the determination of the vertical beam size. A
vertical tilt, however, turns out to be quite critical andresults in an overestimation of the beam size from thevalley-to-peak ratio of 11%, 100% or even 270% for tiltsof ± 1, ± 3 or ± 5 mrad respectively. In order to avoid
such overestimation of the beam size due to vertical tilts,
the gimbal mount will be adjusted by using a laser-basedalignment and calibration set-up until a symmetricinterference pattern is observed at the CCD camera.
Horizontal tilts of the same amount as above, will lead toan underestimation of the beam size in the order of 6%,
33% and 46% in case of decreasing incident angles of thelight on the toroidal mirror. Increasing horizontal incident
angles lead to a less critical overestimation of the beam
size by 4%, 6% and 10%. These effects will becompensated by adjusting the gimbal mount of thetoroidal mirror until the distance between the two peaks isminimal. Finally, rotation around the normal axis of thetoroidal mirror by ± 1, ± 3 or ± 5 mrad results in beam
size overestimations of 1.5%, 17% or 49%.

REFERENCES[1] For more information on TIARA, see: http://www.eutiara.eu/[2] For more information on the TIARA work package 6 see:
http://www.eu-tiara.eu/rtd/index.php?id=42[3] For more information on SLS, see: http://www.psi.ch/sls/[4] T. O. Raubenheimer, “Tolerances to Limit the VerticalEmittance in Future Storage Rings”, SLAC-PUB-4937,
Aug.1991.
[5] M. Aiba M. Böge, N. Milas and A. Streun, “Ultra low
vertical emittance at SLS through systematic and random
optimization”, NIM A 694 (2012) 133.
[6] Å. Andersson, M. Böge, A. Lüdeke, V. Schlott and A.
Streun, “Determination of a small vertical electron beamprofile and emittance at the Swiss Light Source”, NIM A
591 (2008) 437.
[7] O.V. Chubar, “Resolution Improvements in Beam Profile
Measurements with Synchrotron Light”, IEEE Particle
Accelerator Conference, Washington, 1993, p. 2510.
[8] T. Mitsuhashi, “Measurement of small transverse beam size
using interferometry”, DIPAC 2001, Grenoble, p. 26.
[9] P.H.van Cittert, Phisica, 1, 201(1934) and also see M. Bornand E. Wolf, "Principles of Optics", chapter 10, P491,
Pergamon press. (1980).
[10] TIARA/ WP6 Report “Specifications for the new SLS
beam size monitor”.
[11] O. Chubar and P. Elleaume, “Accurate and efficientcomputation of synchrotron radiation in the near fieldregion”, proc. EPAC98 (1998) 1177.
[12] M. Sanchez del Rio and A. Marcelli., “Waviness Effects in
Ray-Tracing of Real Optical Surfaces”, NIM A 319 (1992)
p. 170.

ISBN 978-3-95450-119-9
Transverse & Longitudinal Emittance Measurements

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Proceedings of IBIC2012, Tsukuba, Japan