Proceedings of IBIC2012, Tsukuba, Japan

MOPA39INTRODUCTION OF PHOTON BPMS IN SOLEIL GLOBAL ORBITFEEDBACK SYSTEMS
N. Hubert#, L. Cassinari, L. Nadolski
Synchrotron SOLEIL, Saint Aubin, BP 34, 91192 Gif-sur-Yvette, France

SOLEIL global orbit feedback systems (slow and fast),
based on 122 electron Beam Position Monitor (e-BPM)
readings, are in operation since 2008 and give verysatisfying performances (0.1Hz-500Hz vertical noise
below 300 nm RMS and long term (8h) drifts below 1µm
RMS).
Whereas each straight section is equipped with an
upstream and downstream e-BPM, there is no e-BPM
next to a dipole magnet. For that reason, photon BPMs (xBPMs) in the dipole beamline frontends give additional
information that can be used to better stabilize the source
point in the dipoles. In fact x-BPMs provide also a better
position angular measurement resolution, as they arelocated at 4 meters from the source point.
Results presented in this paper show that vertical position
stability on bending magnet beamlines can be improvedby including their x-BPM measurements in the global
orbit feedback systems.
As a first step x-BPMs have been introduced in the Slow
Orbit FeedBack system (SOFB) that corrects the orbit
with a repetition rate of 0.1Hz. In a second step x-BPMs
will be introduced in the Fast Orbit FeedBack system(FOFB) running at a repetition rate of 10 kHz.

INTRODUCTION
SOLEIL synchrotron is a third generation light sourcein operation since 2006. Orbit stability is a key parameterfor the beam quality delivered to the 26 beamlines. Very
good performances have already been achieved for longterm and short term stability as reported in Ref. [1].
Combined slow and fast orbit feedback systems [2] keep
the horizontal and vertical position and angle within thestandard stability requirement: less than 10 % of the beam
size and divergence, respectively. Nevertheless,continuous effort is done to improve even more the
SOLEIL beam orbit stability taking into account new
demands for stabilizing the closed orbit. Currently,
integration of the photon beam position monitors (xBPMs) in the feedback loops is being studied. Two xBPMs are located several meters away from the source
point and have a better sensitivity than the electron beam
position monitors (e-BPMs). Moreover, bending magnet
beamline x-BPMs can give additional information for theorbit stability, since the SOLEIL lattice does not have any
e-BPM next to a bending magnet source point (Fig. 1).
____________________________________________# nicolas.hubert@synchrotron–soleil.fr

Figure 1: Vertical beam position measured by a dipole xBPM at 4.7m from the source point (red curve). As acomparison, the projection of the position calculated fromposition and angle of the electron beam at source point isplotted in blue. The stability observed on the x-BPM is
not as good as the one observed on e-BPMs.

CONTEXT
Orbit Feedback Systems
The SOLEIL beam orbit is stabilized using two
interleaved orbit feedback systems based on the readingof the 122 e-BPMs of the machine. The slow system acts
every 10 seconds on a set of 57 corrector coils located in
the sextupole magnets (arcs). The fast system acts every
100 µs on a different set of 50 air-coil correctors locatedupstream and downstream of every straight section. Bothsystems are global, that is to say that the information fromall e-BPMs is taken into account to compute the setting
values for each corrector. An interaction process [2]allows having both systems efficient on a common
frequency range: from DC to 0.05Hz for the slow orbit
feedback (SOFB) and from DC to 200 Hz for the fast
orbit feedback (FOFB).

Photon BPMs
SOLEIL beamlines frontend are equipped with blade xBPMs, manufactured by FMB [3] after a design fromBESSY. The position measurement given by this kind of
x-BPM is based on the beam halo detection and thus isstrongly dependent on the photon beam shape. On
bending magnet source points, the beam shape is constantwhereas for the insertion devices, it hugely depends on its
configuration (gap, phase, photon polarisation …). For
this very reason, only x-BPMs on bending magnet
beamlines can be included at the moment in the orbit
feedback loops (vertical plane only).

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPA39

Proceedings of IBIC2012, Tsukuba, Japan

Two photon BPMs are installed in each of the 4
bending magnet beamline frontends. They are located at
4.7 and 6.73 meters after the source point, respectively. A
shutter, controlled by the beamline users, is placed
between the two x-BPMs. Therefore, the first x-BPM canbe included in the feedback loops whereas the second one
will be used as an independent observer (Fig. 2).

Figure 2: Schematics of a SOLEIL bending magnet
beamline frontend (extraction angle = 1°). A shutter isplaced between the two x-BPMs.

SOLEIL Latticec 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Orbit stabilisation of a source point depends mainly on
the readings of the 2 closest e-BPMs. In SOLEIL lattice,several magnetic elements are located between bending
magnets (source point) and the closest e-BPM: one
sextupole and one quadrupole at each side (Fig. 3).

Figure 3: SOLEIL lattice detail around one dipole
magnet. Bending magnet and closest BPM are separated
by one sextupole and one quadrupole.
In this case, e-BPMs do not give a direct measurementof the source point in the dipole and orbit stabilization is
not optimized for the source point stabilization.
Four bending magnet beamlines have two x-BPMs intheir frontend as described in the previous paragraph.
Those beamlines are located symmetrically around thestorage ring (Fig. 4)

Figure 4: Bending magnet beamlines location around thestorage ring. Four bending magnet beamline frontends areequipped with x-BPMs than can be included in orbit
feedback systems.

INTEGRATION INTO GLOBAL ORBITFEEDBACK SYSTEMS
Global Feedback Systems
SOLEIL slow and fast orbit feedback systems are bothglobal, that is to say that the information from all e-BPMs
is used to compute the new setpoints of the corrector
coils. The RF-frequency is used as an extra knob in thehorizontal plane to correct for circumference slow
variation of the storage ring in the SOFB loop. Correction
algorithms are based on SVD method.
To be integrated into feedback loops, x-BPMs are
considered as additional BPMs in the response matrix
with specific weighting factors.
The first step has been the integration into the SOFB inorder to see the feasibility and also to check the efficiency
on the beam stability of such the introduction.

Integration into the Slow Orbit Feedback System
First, a simulation has been done, to check that thecorrection process was converging when x-BPMs areincluded. Loop has been first closed using a theoreticalresponse matrix, and later with an experimental one.
Correction was immediately converging. It is performed
using 57 singular values in both planes. The weights foreach e-BPM and x-BPM are all equal to one, but can be
set independently for future optimisation.
Experimental tests have been carried out over medium
term periods (~5 hours). In order to compare the stability
with and without x-BPMs in the SOFB loop, beamline
shutters are opened, and observations are done on thesecond x-BPM which is not in the feedback loop (Fig. 5).
For the four bending magnet beamlines, vertical beam
position stability is improved. Vertical position variations,with and without x-BPMs in the feedback loop, are given
in Table 1. Introduction of x-BPMs in the slow feedback
loop improves the stability by about a factor 2.
Table 1: Variation (RMS) of the vertical position on xBPM N°2 with and without x-BPM N°1 in the slow
feedback loop (experiment duration = 6 hours).
Beamline
namew/o x-BPMs

w/ x-BPMs

Reduction
factorODE

1.25 µm rms

0.37 µm rms

3.3

METRO

0.46 µm rms

0.24 µm rms

1.9

SAMBA

0.38 µm rms

0.23 µm rms

1.7

DIFFABS

0.33 µm rms

0.16 µm rms

2.1

In the same time, vertical orbit stability on the othersource points (insertion device beamlines) has not been
deteriorated.

ISBN 978-3-95450-119-9
2

Feedbacks and Beam Stability

Proceedings of IBIC2012, Tsukuba, Japan

MOPA39Those tests have been carried out with the slow
correction loop alone. Now that it has been shown that
introduction of those x-BPMs was beneficial for thevertical orbit stability, it will be shortly put in operation
with the FOFB running in parallel (without x-BPMs in a
first time).

In the future, we can also expect to integrate x-BPMs
from insertion device beamline frontends. Neverthelessthe issue of position measurement dependence with theinsertion device configuration is complex and has still tobe solved.

Integration into the Fast Orbit Feedback System

AKNOWLEDGMENT

Integration of x-BPM data into fast orbit feedback
system requires additional precautions on the datasynchronization point of view:e-BPM and x-BPM data sampling rates must be
identical (10079Hz at SOLEIL)
e-BPM and x-BPM data latency must be sensibly
equal (less than 20 µs difference)e-BPM and x-BPM data must be synchronized anddistributed over the same dedicated network
This has been done by replacing the existing electronics
of the first x-BPM by Libera Photon modules [4] on thefour bending magnet beamline frontends.
Moreover the FOFB algorithm has been modified toaccept additional inputs.
Benefits of including x-BPMs in the FOFB have still tobe demonstrated, but as almost all elements are ready, it
will be under commissioning in the coming months.

The authors would like to thank the operation groupthat has been involved in the medium term tests during
dedicated nights. They also thank N. Leclercq from thecontrol group and all the staff from the diagnostic groupinvolved in the project.

REFERENCES[1] N. Hubert et al., « SOLEIL Beam Orbit Stability
Improvements », IPAC’11, San Sebastian, September
2011, TUPC068, http://www.JACoW.org.
[2] N. Hubert et al., « Global Fast Orbit Feedback System
Down to DC Using Fast and Slow Correctors »,DIPAC’09,
Basel,May
2009,
MOOC01,
http://www.JACoW.org.
[3] http://www.fmb-berlin.de/[4] P. Leban et al., “Photon Beam Position Measurements by
Libera Photon Using Copper Blade Sensors at SOLEIL
Synchrotron”, Santa Fe, May 2010, TUPSM023,
http://www.JACoW.org.

CONCLUSION
Bending magnet source point vertical stability benefits
from the introduction of dipole-based x-BPM in the slow
orbit feedback loop. First tests have been carried out andare very encouraging. After some additional work to
adapt this new configuration to the control system, it canbe shortly adopted during user operation mode.
Electronics and algorithm are ready for the integration
into the FOFB, and first tests will be done in the next
months.

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 5: Vertical beam position measured on second x-BPMs (not included in the feedback loop) at the METRO,SAMBA and DIFFABS bending magnet beamlines. First x-BPM is alternatively included or excluded from the slow
feedback loop. Improvement on vertical stability can be seen when the SOFB runs with the x-BPMs.