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

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R. Bronès†, A. Bence, J. Bisou, N. Hubert, D. Pédeau, G. Pichon
Synchrotron SOLEIL, Gif-sur-Yvette, France
Abstract

SOLEIL is upgrading its Fast Orbit Feedback platform
to withstand coming obsolescence of electronic BPM andfuture evolutions of the machine. This new platform has
to be compatible with current boundary devices such as
BPM electronics or corrector power supplies, but it also
shall evolve to interface future versions of these systems.
A MTCA based platform was designed and installed. It is
integrated in the control system by mean of a OPCUA
server, and care has been taken to seamlessly toggle theclosing of the feedback loop on the former or new FOFB
platform. This paper will present the first tests and results
conducted to commission this new system.

CONTEXT AND SPECIFICATIONSSOLEIL is working on upgrading its accelerators to thefourth generation of synchrotron. SOLEIL II will bringoutstanding performances, with a new beam lattice,renewed systems and cutting-edge technics. As presented
in [1], the renewal of BPM electronics will be conductedprior to the long shutdown and machine upgrade. The
current Fast Orbit FeedBack (FOFB) system, which isstrongly embedded in these electronics, is to be ported ona new platform.
The evolution of the FOFB spreads on several years
and must follow the numerous modifications andimprovements that will bring SOLEIL II. The roadmapfor this system can be summarized in Table 1, while
evolutions of key points are underlined in Table 2.

2024
2025

Table 1: FOFB Roadmap
Evolution
Impact

FOFB running on
new platform
Added features

2026

New BPMelectronics
2028 SOLEIL II, newPSC
2028 + New correction
algorithm

Same performances

Faster lattice identification.
Loop latency reduced, data rate increased,correction bandwidth increased
Loop latency reduction, dimension
reduction, SOLEIL II performances
Increased performances

Table 2: FOFB Performances Evolution
Actual FOFB

Future FOFB

# BPM

122

180~200# Corrector

50 H & V

44~60 H & V

Data rate

10 kHz

100 kHz

Correction BW

200 Hz

1 kHz

Loop Latency

350 µs100 µsStability

10 % of beam size
20 µm H ; 0.8 µm V

2-3% of beam size
50 nm H & V______________________________________________†romain.brones@synchrotron-soleil.fr

FOFB NEW PLATFORM INSTALLATION
A versatile platform was elaborated and is presented
in [1]. We will detail last additions and comment on
the installations.

Hardware Platform
The basis of the platform is a MTCA chassis, equippedwith a SOC-FPGA board that host a SFP-FMC. The
overall system is composed of five of these chassis,
connected in a star topology. For denomination: a Central
Node connected to four Cell Nodes. Communication
between these nodes is a simple custom packet protocol,
encapsulated in Ethernet frames and transported overoptical fibers at 10 Gbps. This network was installed in
the first months of 2023. Three full duplex fiber pairs arelayed between Central and Cell Nodes, with one used andtwo as spare links.
The Cell Nodes are installed near BPM electronics infour different cells. For now they are connected as data
spy to the running communication ring transporting theposition data. After the upgrade of the BPM electronics,
each Cell Node will be connected directly to a subset ofthem. Position data are grabbed, before being packeted
and forwarded to the Central Node.
Correction packets are received back and transmitted toPower Supply Controller (PSC). A 1.25 Mbps RS422
UART link connects a Cell Node to each PSC. This limits
the link length to 80 m and great care has been taken toposition the Cell Nodes, each one serving PSCs in its own
cell cabinet and to three neighbour cells. Current PSC canonly receive command from one driver. We selected andinstalled very simple RJ45-CAT6 cables, each one
transporting 4 differential pairs. To ease commutationfrom actual drivers (BPM electronics) to Cell Nodes,
electromechanical relays were temporarily installed totoggle the differential lines from one driver to the other.
Each relay can toggle 2 differential pairs. All relays are
powered from a central location, activated by a single
switch lever. Switched off relays connect the PSC to theBPM electronics. For the new platform tests, we simply
have to toggle the lever to switch on all relays andconnect PSC to the Cell Nodes. This feature was wellreceived and shortened greatly the set-up times for
dedicated machine shifts. Ultimately, relays will beremoved.
To provide the RS422 UART commands to each PSC,
Cell Nodes are equipped with a custom made RTM board
named CACTUS. These very simple boards receive
LVCMOS single-ended signals from the AMC FPGA andtranslate it to RS422 pairs, transported to RJ45 outlets,
allowing us to put eight outlets on the RTM, hence
providing signals to eight groups of four PSCs. Except
from the MTCA management, no other functionality is
MO2C0408 Feedback Systems and Beam Stability

11

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SOLEIL NEW PLATFORM FOR FAST ORBIT FEEDBACK

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embedded on the CACTUS. These board will bedecommissioned with the PSC and replaced with a newinterface compatible with SOLEIL II new PSC.

The latency from the position packet departure to Central
Node to the corrector packet reception has been measured
on field with a maximum value of 4.5 µs.

FPGA Firmware

The Central Node receives and aggregates all thepositions data from the Cell Nodes. The correction
processing scheme is kept identical to the previous
implementation [2]: (1) Remove reference orbit to get theposition error at each BPM, (2) space change from BPM
position errors to PSC correction errors using the inverted
response matrix, obtained from singular value
decomposition
andcorrectedwith
Tikhonov
Regularization and (3) filter corrections signals with a
8 Hz low-pass filter given in Eq. (1), before broadcastingcorrections to the Cell Nodes. Note that with the closedloop, perturbations will be attenuated by the inverse ofthis correction filter.

Fast or interface-specific functionalities are supportedby the FPGA matrix present on the AMC. From theposition data grabbing to the correction commandsending, the data passes through FPGA logic blocs:
Ethernet core, BPM communication decoder, correction
computation, UART for PSC, etc. The overall firmware
project is built with the help of FWK, the frameworkprovided by DESY MSK. This framework allows toseparate functionalities in deployable blocs, organizedand versioned to prepare two slightly different systems:
Central and Cell Nodes firmware. Further features of thisframework are an easier integration with software controland process script to rebuild. This will keep the firmware
project clean and allow further evolutions.

H ( z)=64−1
4096−4075z(1)

The Cell Nodes grab the position data exchanged on theBPM dedicated network. Every 100 µs (10 kHz),122 positions packets are exchanged, composed of (1) theBPM identification, (2) X and Y position data and (3) aframe number incremented at each iteration. Each Cell
Node selects around ¼ of these packets and forwards
these to the Central Node. On the way back 100
correction packets are received from the Central Node,composed of (1) the PSC identifier, (2) the correction dataand (3) the same frame number used to compute thiscorrection. Each Cell nodes selects in the stream thecorrection data for the PSC it is connected to and forward
it with the proper format on the correct output interface.

Data can be captured before and after the correction
processing chain. On user demand, a few tens of secondsof data can be captured and pushed in a dedicated DDR
memory for the on-board software. A sine waveformgenerator is also added on the data path. It can add a sine
waveform, with programmable frequency, phase,
amplitude and offset to the correction signal. Thesefeatures are heavily used for the systems tests andcommissioning. The global data processing scheme in theCentral Node is pictured in Fig. 1.

From Cell Nodes

To Cell Nodes
Sine
generator

Position
depacketer

MatrixMultiplication

Correction
controller

Data capture

Correction
packeter

Serializer

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Data capture

Figure 1: Overview of data processing inside the Central Node.

On-board Software
The FPGA-SOC provides a CPU which is running aLinux distribution built by Yocto. Regular operation canbe performed: SSH connection, NFS mount,configuration scripting, logging, etc. To access the FPGA
bloc functions programmed, we use a ChimeraTK genericserver, which access FPGA register using Linux UIO andrun an OPCUA server to provide access to remotecontrol. The available FPGA registers are automaticallyunderstood by the software with a MAP files generated
alongside the firmware. This feature allows name accessto registers, from the OPCUA server. Identically, it ispossible to access from python scripts and prompt with aChimeraTK DeviceAccess python bindings. This features
proved to be very helpful to conduct tests.

Integration in the Control System
SOLEIL II control system will still be Tango. Controlof the new platform must initially be compliant withcurrent Matlab Script and GUI, with features and accesstools gradually added. Base access to the on board
functionalities will use a Tango Server / OPCUA client
which will bring registers up to the Tango Level. Expert
users or applications will be built over this server.

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Transfer Function Identification
This identification aims at measuring the frequency
response, or Transfer Function (TF) from the PSC
command to the BPM Fast Acquisition (FA, 10 kHz) dataoutput. This identification is performed open-loop, thecorrection algorithm is disabled and outputs zeros and asine waveform is generated for a single PSC. Data iscaptured for several frequencies in the band of interest,
capture length is stretched to be able to record at least 100
periods of the sine.
Data capture and sine generator are synchronized on thesame clock reference. The sine generator is composed of a
16 bits Numerically Controlled Oscillator with aprogrammable phase increment Φi. Equation (2) showsνsin the normalized frequency to the data-rate which will
be used for frequency analysis, preventing scalloping
losses. Equation (3) shows how for each capture, we
compute the complex Discrete Time Fourier Transform(DTFT) of all the BPM position at νsin before dividing itby the PSC DTFT. With this, we have the TF gain andphase of the selected PSC to every BPM, at eachfrequency point captured.
1616f sin =f datarate Φi / 2 → νsin =Φ i / 2
DTFT [ bpm n ] ( νsin )TF m, n (ν sin )=DTFT [ pscm ] ( νsin )

(2)
(3)

Each BPM has a different TF gain, depending on theOrbit Response Matrix (ORM). As we are interested in
the overall shape of the TF, we first select the ten mostsignificant TF and further normalize each one by its value
at a given frequency (210 Hz) before aggregating the databy mean. The result is displayed on the Bode plot inFig. 2. It shows an expected drop at 1.5 kHz coming fromthe notch filter, and a constant delay of 354 µs and 332 µsfor X and Y planes respectively. We suspect that thisdifference comes from the vacuum chamber geometry atthe corrector magnets location, resulting in a different
latency from Eddy currents.

Figure 2: Bode plot of the measured TF. Blue and orange
are one horizontal and one vertical PSC respectively.

This identification aims at measuring the ORM of thefast corrector magnets to BPM. Usually, this is performedby taking each of the PSC and apply a constant current
step. ORM coefficients are deduced from the positiondifference induced on each BPM. Another approach,presented in [3] is to apply sine waveforms and perform afrequency analysis. This brings the possibility offrequency multiplexing to quicken the measure.
We tried this latter approach with our new platform.
PSC in both planes were randomly selected by group often. One sine waveform is allocated and generated
simultaneously by each PSC, from 223 Hz to 423 Hz,
with 20 Hz gaps. For each group, 1.5 seconds of position
data is captured. The measure has been repeated 4 times,
with different groups and frequency distributions,allowing us several identifications for further corrections.
For each capture, the DTFT at the ten sine frequencies arecomputed. We used a Blackman window on the data toreduce frequency leaking from one sine to the other, but
in the future a better selected frequency gap will be used
to minimize the leaking.
Two corrections are applied to the DTFT result before
obtaining the ORM coefficients: (1) the phase is correctedby the identified constant delay, different to each plane,
leading to real values. And (2) the module is corrected byestimating the TF shape on the measurement frequency
band, for each PSC. Indeed, the results showed a ±2 %module variation over the 200 Hz, different for each PSC.
Once these corrections done, we observe the repeatability
over the four repetitions in Fig. 3. We hope to improve
this by performing a better identification of the TF shapefor each PSC.

Figure 3: ORM measured coefficients. Each point is a
PSC to BPM coefficient, its location gives the mean andpeak to peak values from the 4 measure repetition. The error distribution is given for convenience.
MO2C0408 Feedback Systems and Beam Stability

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Orbit Response Matrix Identification

NEW IDENTIFICATION FEATURES
With the on-board sine generation and the local data
capture, it is now possible to conduct easily identificationof the system.

12th Int. Beam Instrum. Conf.
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MACHINE TEST RESULTS
A few machine shifts have been dedicated to test thenew platform correction. After some struggles with thecorrector algorithm gain, we succeeded in closing theloop and achieving similar orbit stability compared to theprevious system. We measured performances by assessing
the integrated position PSD, and measuring the FOFBefficiency.

Position PSD

FOFB running. By repeating this measure at several PSC
frequencies, we have the efficiency over the band ofinterest, as plotted in Fig. 5. It shows an attenuation factorof 5 at 100 Hz and a resonance crossover frequency at
400 Hz. Comparison to previous system efficiency is notpossible as that one is outdated. The new platform allows
a very quick way to make this measurement, whereas theprevious method required disabling a PSC in thecorrection algorithm and driving it with an analog
external generator, which would takes too many time.

Figure 4 shows the power spectral density and its
integration over the frequency band, for the mean orbit
over all BPMs, obtained from the FA data. These signals
are measured with a 5 mA beam stored, only one plane isshown for clarity. We can see that same performances arereached with the new platform. Sharp eyes can observe aslightly increase of the resonance crossover frequency.
This comes from a small latency reduction obtained withour new platform. Indeed, the computation on the Libera
starts after a waiting time of 60 µs after the loop iterationstart, whereas the new platform starts the correction assoon as all position are received, which is around 30 µs
after the loop iteration start.
The FOFB running on the new platform has been kept
on while the beam was accumulated up to 450 mA
without any issues.

Figure 5: FOFB efficiency for several PSC.

FUTURE STEPS
Integration to the Control System is still to be deployed
and tested. This will allow SOFB and FOFB interaction
which is mandatory for nominal operation. With this lastaddition, the new platform will be selected for operation,with the possibility to fall back to the previous system forthe first few weeks.

CONCLUSION
The new FOFB platform based on MTCA was installedat SOLEIL and has reached the actual performances. This
new platform unlocks new identification features that will
be used to improve daily operations, diagnostic andcorrection scheme.

ACKNOWLEDGEMENTS

Figure 4: Position PSD and integrated PSD, for x plane.
Green is FOFB off, blue and red is FOFB running on newand old platform respectively. Measured with a 450 mA
stored beam.

FOFB Efficiency
This measure uses the on-board sine generator to add asine wave on one selected PSC. The position orbit is thencaptured with and without the FOFB running. As for theTF identification, we compute the DTFT of the mean
position over all BPM at the PSC sine wave frequency,
only the module is kept. We consider efficiency as theratio of the mean orbit oscillation with and without theWe would like to thank S. Zhang, Y.M. Abiven andF. Bouvet from SOLEIL for their work or views on theproject. We equally thank C. Guemues, L. Butkowski,
B. Dursun, J. Georg and M. Killenberg from DESY fortheir help with FWK and ChimeraTK.

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at SOLEIL”, in Proc. IBIC'22, Kraków, Poland, Sep . 2022,
pp. 339-342. doi:10.18429/JACoW-IBIC2022-TUP42[2] N. Hubert, L. Cassinari, J. Denard, A. Nadji, andL. S. Nadolski, “Global Fast Orbit Feedback System Downto DC using Fast and Slow Correctors”, in Proc. DIPAC'09
Basel, Switzerland, May 2009, paper MOOC01, pp. 27-31.
[3] Z. Marti et al., “Fast Orbit Response Matrix Measurements
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doi:10.18429/JACoW-IPAC2017-MOPAB102

MO2C041408 Feedback Systems and Beam Stability