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 - M O 3 C 0 3

B. Keil, R. Ditter, F. Marcellini, G. Marinkovic, J. Purtschert, M. Rizzi,
M. Roggli, D. Stephan, X. Wang, Paul Scherrer Institut, Villigen, Switzerland
Abstract
After more than 20 years of operation, the storage ringof the Swiss Light Source (SLS) will be replaced. The newring called SLS 2.0 will have 40 times higher brilliance,
thanks to an innovative low-emittance magnet lattice and abeam pipe with smaller aperture. For SLS 2.0, the ageing
SLS RF beam position monitor (BPM) electronics will be
incrementally replaced for the whole accelerator, including
linac, booster, transfer lines and storage ring. This contribution presents the development status and latest prototype
test results of the SLS 2.0 BPM system, including pickups,
mechanics, and electronics.

INTRODUCTION
Machine and Beam Parameters
Table 1 shows the parameters of the SLS 2.0 [1] compared to the present SLS (“1.0”) ring. The nominal beamenergy for user operation will be slightly increased from
2.4 GeV to 2.7 GeV, which is already supported by the
SLS 1.0 full energy booster synchrotron that is re-used for
SLS 2.0, as well as the linac and linac-to-booster transfer
line. The booster-to-ring transfer lines will be modified, including a new magnet lattice and additional BPMs for improved control of the more critical injection process.
Table 1: SLS Storage Ring Beam Parameters
Parameter
Circumference
Beam Current
Injection Charge
Beam Energy
Main RF
Harmonic No.
Hor. EmittanceVert. Emittance
Ring BPMs

Units
m
mA
nC
GeV
MHz
#
pm
pm
#

SLS 1.0

SLS 2.0

288
400
~0.15
2.4
499.6372.7
499.654
480

5030
5-10
75

131-158
10
136

BPM REQUIREMENTS AND TYPES
Table 2 shows the BPM requirements for the SLS 2.0
storage ring, with an expected minimal beam size of σ ≥ 5µm at the BPMs, where <0.05 µm desired electronics RMS
noise from 0.1 Hz to 1 kHz translates to 1% of this beam
size. Like SLS 1.0, SLS 2.0 will operate in top-up injection
mode at 400 mA with typ. 2-3 mA (max. 4 mA) periodic
current variation within a few minutes. 430-460 of 480 successive RF buckets are typically filled with approximately
the same charge, with ~10% charge variation between
buckets, excluding an optional so-called “cam-shaft”
bunch in the bunch gap with ~3-5 times higher charge. For
the linac, booster and transfer lines operated with single

bunches of ~0.15 nC at 3 Hz injection rate, the position resolution requirement is also <50 µm, aiming at<10 µm/week long-term drift of the averaged position
readings in the booster-to-ring transfer line for negligible
variations of bunch charge transfer efficiency to the ring.
Table 2: SLS 2.0 Storage Ring BPM Position Measurement
Requirements
Parameter
RMS Noise, 0.1 Hz-1 kHz, 400 mA
RMS Noise, 0.1 Hz-0.5 MHz, 400 mA
RMS Noise, 1 Bunch, 0.15 nC
Electronics Drift (400 mA Top-Up)

Overall Drift (incl. Cables/Mechan.)Beam Current Dependence

Goal
0.05 µm
1 µm
50 µm
0.1 µm/h
0.4 µm/week
1 µm/year
0.25 µm/h
1 µm/week
2.5 µm/year
0.02 µm/mA

Table 3: SLS 1.0 / 2.0 BPM Types
Location
Linac &
Transfer Lines
Booster
SLS 1.0 Ring
SLS 2.0 Ring

Resonant Striplinegeom. factors
kx/ky [mm]various

ButtonButtonButton8.3/7.7
16.7/14.3
7.1/7.2

Type

Table 3 shows a list of BPM types used in SLS. Boosterand storage ring have button BPMs with four diagonalelectrodes. Linac and transfer lines have so-called resonant
stripline BPMs, generating a decaying 500 MHz sine signal which can be processed by the normal storage ring
BPM electronics also working at 500 MHz. The beam position is calculated by the BPM electronics from the button/stripline signal voltage amplitudes A, B, C and D (upper outer, upper inner, lower inner and lower outer button/stripline) for the horizontal (X) and vertical (Y) planewith the common approximation formulas
X = kx * (A-B-C+D)/(A+B+C+D)
Y = ky * (A+B-C-D)/(A+B+C+D)

Storage Ring Beam Pipe
Figure 1 shows the cross section of the SLS 1.0 storage
ring beam pipe in blue and the smaller SLS 2.0 pipe in orange. The latter has an octagonal shape, usually with
18 mm aperture. While the SLS 1.0 pipe is made of stainless steel, SLS 2.0 uses a NEG-coated copper pipe. However, at the BPMs and their adjacent horizontal and verticalorbit corrector dipole magnets, the pipe is made of stainless
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DEVELOPMENT OF THE SLS 2.0 BPM SYSTEM

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 - M O 3 C 0 3

steel, with, with a ~5 µm copper coating (to reduce impedance) and ~0.5 µm NEG coating like in the surrounding
solid copper pipe, where the button electrodes are only
NEG coated but not copper coated.

Figure 1: SLS 2.0 beam pipe (yellow) compared to the previous SLS pipe (blue), with dimensions in [mm].

BPM MECHANICS
The inner beam pipe aperture at the SLS 2.0 BPMs andcorrectors is tapered from the nominal 18 mm to 21 mm,
thus shielding BPMs and correctors from synchrotron radiation, reducing beam-induced temperature variations andmechanical position drift (see Fig. 2).

Button Electrode Design
Figure 3 shows the cross section of the SLS 2.0 storage
ring button BPM at one electrode (left) and the button electrode before insertion into the steel BPM block (right). The
electrode (red) with 6.5 mm diameter and 0.25 mm gap to
the BPM block is round and symmetric towards the beam
pipe surface, with a radial asymmetry of the part going
through the borosilicate glass dielectric (dark blue), thus
suppressing higher-order modes (HOMS) in the electrode.
The TIG welding of the buttons into the BPM block is doneat the upper side between the steel (grey) and outer button(light blue) body that is also made from steel. The electrodehas an SMA male connector (red/orange/yellow), where afemale-to-female connector is screwed from the outside,using RF cables with SMA male connectors.

Figure 3: SLS 2.0 storage ring button BPM electrode.

Figure 2: SLS2 BPM storage ring block with corrector
magnet beam pipe.
The combined BPM/corrector beam pipe block hasflanges to the surrounding copper beam pipe, and a dedicated rigid mechanical support for the BPM block, aiming
at keeping the BPM position stable, while the softer surrounding copper pipe can float. The SLS 2.0 storage ring
will have 136 BPMs overall. 115 of them have an adjacenthorizontal and vertical corrector magnet to be used by the
global fast orbit feedback (FOFB). The beam pipe at thecorrector magnets is only 0.5 mm thick, compared to 2 mm
for SLS 1.0, reducing eddy currents and improving the
FOFB bandwidth.
The mechanical supports of the BPM blocks are double
steel plates with a sealed compound of balsawood and viscoelastic glue in between that damps vibrations. The upper
part of the BPM support is a water-cooled copper alloy
block (CuCrZr), aiming at reducing the beam-induced temperature variation and thus thermal drift of the BPM block
when the ring is filled from 0 mA to 400 mA. The expected
remaining beam-induced power dissipation for the combined BPM/corrector beam pipe is ~2.6 W due to stray synchrotron radiation, and ~1 W losses due to the beam pipe
impedance, assuming 400 mA and 9 ps worst-case bunch
length (that is several times larger during normal user operation). Additional temperature sensors will enable feedforward correction of any remaining temperature-induced
position drift of the BPM block.

While the SLS 1.0 storage ring BPMs use Molybdenum
button electrodes with brazed Al2O3 dielectric, the SLS 2.0
ring will have Inconel® electrodes with a borosilicate glassdielectric. Although Molybdenum has better thermal andelectrical conductivity, we chose Inconel for several reasons. Firstly, the Swiss company that produces the buttons(designed by PSI) uses Inconel for nearly all their other borosilicate glass feedthroughs, mainly for non-RF industry
applications in harsh environments, with proven mechanical robustness. Secondly, simulations showed that the expected Inconel button temperature for worst-case SLS 2.0
beam conditions (400 mA, 9 ps bunch length, 3rd harmonic
cavity inactive) is 51.8 °C (for ~0.4 W power dissipation
per button), which is only 11 °C higher than Molybdenum.
The resulting simulated mechanical stress of the borosilicate glass is only 15% higher and well below any criticalthreshold. Last but not least, we exposed buttons in the labto thermal stress (much higher than during production,
beam operation and bake out procedures) without causing
any vacuum leaks.

Production Status and Offset Measurements
About 25% of the BPM blocks have been produced sofar, and one of twelve sectors of the SLS 2.0 beam pipe has
been assembled, including 12 BPMs, where all BPMs
passed all vacuum tests. The button electrodes were sortedby their transfer impedance which was measured before
welding, thus reducing the resulting position offset fromtens of microns without sorting to typically <1 µm withsorting. In addition, we estimated the beam position offset
after welding using the Lambertson method, where the RF
asymmetry of the BPM electrodes and general geometry ismeasured with a 4-port RF network analyzer. Combinedwith future pre-beam calibration of RF cables and BPM
electronics, we aim for an overall position offset uncertainty of < 100 µm for SLS 2.0 beam commissioning.

MO3C0316

02 Beam Position Monitors

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BPM ELECTRONICS
The SLS 2.0 BPM electronics is based on the so-called
DBPM3 platform that was developed at PSI and is alreadyused for cavity BPMs at SwissFEL [2]. The platform has ageneric back-end with a Xilinx/AMD Zynq UltraScale+(ZynqU+) MultiProcessing System-on-Chip (MPSoC),
combined with accelerator-specific RF front-end modules(RFFEs) with integrated ADCs. The RFFEs are inserted at
the rear side of the 19’’ DBPM3 unit (see Fig. 4). The
MPSoC has a 2-core 32-bit CPU (“RPU”) for real-time signal processing in addition to the programmable logic (PL)of the chip, and a 4-core 64-bit CPU (“APU”) runningLinux and an EPICS7 IOC for easy control and readout of
the BPM system.

caused by cables and electronics. The DBPM3 RFFE performs low pass and bandpass filtering and amplification/attenuation with 63 dB overall dynamic range for the incoming beam/pilot signals. In addition to the support of an external beam-pilot signal combiner, the DBPM3 RFFE also
has an additional internal beam-pilot signal combiner on
the RFFE PCB, close to the SMA inputs. The user can select for each of the two generated pilot tones if it goes tothe internal combiner (used e.g. for pre-beam tests and calibration of the electronics after production), or to the external combiner, or both. The DBPM3 RFFE also has two 2x2
crossbar switches close to the SMA beam signal inputs,
where the switch swaps the RFFE channels used for the
input signals of opposite BPM button electrodes. This enables suppression of gain asymmetries and drift of the RFFE
channels by sufficiently fast switching (and digital uncrossing of ADC output signals in the ZynqU+).

ADCs and Digital Signal Processing

Figure 4: DBPM3 BPM electronics unit for SLS 2.0 withfront-to-rear ventilation, redundant fans, and redundant
power supply.

RF Front-End and ADC
For SLS 2.0, each DBPM3 unit has three RFFEs, one per
BPM. Figure 5 shows a simplified block schematics of theRFFE. It has four SMA beam signal inputs, as well as an
SMA output providing two mono-frequent “pilot tone” signals with adjustable frequencies and amplitudes.

The present DBPM3 RFFE prototype for SLS 2.0 uses
two dual-channel 16-bit 500 MSPS JESD204B ADCs.
Digital down-converters (DDCs) in the ZynqU+ MPSoC
process the beam and the two pilot signals simultaneously
in separate DDC channels. The DDC, implemented at PSI,
decimates and lowpass filters the ADC data in three successive stages with decreasing data rates and bandwidth.
For the tests presented in this paper, we used an ADC sample rate of ~433 MSPS, thus enabling an integer fraction of
ADC samples both for the storage ring and booster thathave slightly different revolution frequencies. The ADC
data was decimated to ~1 MSPS (beam revolution frequency) with ~0.5 MHz bandwidth, to 20 kSPS with
3.3 kHz bandwidth, and to 20 SPS with 11 Hz bandwidth.
The DDC data rates, filters and filter bandwidths can be
reprogrammed during operation using a Python GUI.

Drift Suppression

Figure 5: DBPM3 RF Front-End electronics with integrated 2-channel 16-bit 500 MS/s JESD204B ADC.
The pilot signals will be recombined with the four SLS
2.0 BPM beam signals close to the beam pipe, using a socalled “pilot combiner box”, with four ~2 m long RF cablesfrom the pickup to the pilot combiner box, and five 2036 m long RF cables from the pilot combiner box to theelectronics racks: One cable delivers the pilot tone from the
DBPM3 unit to the pilot combiner box, and four cables
with the combined beam/pilot signals go back to the
DBPM3 unit in the electronics racks located in the SLS
technical gallery. This setup enables pilot-tone-based compensation of beam position drift and low-frequent noise

The ZynqU+ provides beam position readings calculated
only from the beam signal (representing the real beam position in the accelerator), from the pilot signals (result is
ideally zero, with a small offset mainly due to electronics
and cable drift), and from the difference of both. This difference represents the beam position, but suppresses measurement errors caused by drift of the insertion loss / gain ofeach BPM channel, including all components downstreamof the beam/pilot signal combiner, including the ADCs.
The 2x2 crossbar switches serve the same purpose, thus
supporting two different methods of drift suppression. The
pilot tones provide the additional benefit of including RF
cable induced drifts. It also enables easy pre-beam tests andcalibration of the cables before 1st SLS beam, as well as
monitoring cable attenuation and general health during
beam operation and after maintenance shutdowns.
In addition to these active drift suppression approaches,
we plan to sort all SLS 2.0 BPM RF cables from beam pipe
to electronics by measured time of flight (TOF) and attenuation, and then install cables with similar TOF and attenuation for each BPM. Combined with a common thermal
isolation of these cables, we thus aim at reducing cableMO3C03

02 Beam Position Monitors

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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 - 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 - M O 3 C 0 3

induced beam position drift by equalizing cable properties
and related drift for each BPM.

BEAM TEST RESULTS
Booster and Transfer Lines
We tested the DBPM3 electronics in the SLS l.0 boosterthat has similar BPM signal level and geometry factor likethe SLS 2.0 storage ring. The measured turn-by-turn single
bunch turn-by-turn position resolution was <40 µm RMS
at 0.15 nC, thus meeting our goal of <50 µm for booster
and SLS 2.0 ring. For the SLS 1.0 transfer line resonant
striplines, we measured <10 µm single bunch resolution at
0.15 nC, thanks to the larger signal levels and matched signal spectrum. We improved the single-bunch noise performance by feeding only ADC data near the beam signal
peaks into the DDCs, zeroing all other ADC samples near
the noise level (not using pilot tones here). This optional
function is implemented in programmable logic and canrun continuously at the full ADC sample rate, with adjustable signal level and pre-/post peak sample counts, thus improving position resolution for all DDC decimation stagesof the booster BPMs.

Storage Ring
For tests of the new DBPM3 electronics with the SLS
1.0 storage ring beam, we installed a DBPM3 unit with two
RFFEs in a prototype SLS 2.0 rack that is water-cooled andhas an active temperature stabilization. The RFFEs received signals from two unused SLS 1.0 storage ringBPMs, where their four button signals were going to a 4x
RF combiner. The sum signal was then combined with thepilot output signal of a DBPM3 RFFE. The combined
beam/pilot signal was split 4 times to the four signal inputsof the RFFE, using only short (<20 cm) cables for our firsttests to characterize the electronics alone. This setup simulates a centered beam with a signal spectrum similar to thereal SLS beam (at ~400 mA beam current and the usual
bunch filling pattern). We used the SLS 2.0 storage ringgeometry factors to calculate the beam positions. So far, weonly used one pilot tone, with a frequency offset of
0.531 MHz (0.5 times the beam revolution frequency of
1.04 MHz plus 10 kHz) above the beam signal frequencyof ~499.637 MHz. The ADC sample rate was ~433 MS/s,
the crossbar switching frequency ~130 kHz, both synchronized to the accelerator main RF. As shown in Figs. 6 and7, the measured noise and 24 hour drift of the present
DBPM3 prototype meets SLS 2.0 requirements.

Figure 7: Horizontal and vertical beam position drift of two
DBPM3 RFFEs (11 Hz bandwidth) over ~24 hours, using
the pure beam signal (left side, no pilot-based drift suppression), and the pilot subtracted from the beam signal position (right side), with one data point every ~30 seconds.

SUMMARY AND OUTLOOKThe SLS 2.0 BPM system design and first electronics
prototype test results at SLS 1.0 were presented. The present basic ZynqU+ firmware and software will be extended, supporting presently unused hardware features likethe integrated temperature regulation of the RFFEs using
14 heating zones. The signal processing will also be improved, including the digital suppression of crossbar
switching noise and balancing of signal levels and phasesof the different RFFE channels. Long-term measurementswith larger quantities are needed to investigate if the performance of present prototypes is reached reliably forevery single BPM and large quantities over longer timescales, where only two prototypes of the latest RFFE version were available during two weeks for the results presented in this paper.
The “dark time” during which SLS 1.0 is replaced by
SLS 2.0 starts 1.10.2023. For 1st SLS 2.0 beam in January2025, we will equip only the SLS 2.0 ring mainly with thepresent 1st generation of DBPM3 electronics, keeping theold SLS 1.0 DBPM1 electronics for linac, booster, transfer
lines and some uncritical ring BPMs. We intend to install a
2nd DBPM3 generation in the ring during a 2nd shorter darktime in Q1/2026, then moving the 1st generation to thelinac, booster and transfer lines, thus finally replacing all
DBPM1 systems. This incremental upgrade is aiming to reduce risks and get experience with production and operation of DBPM3 gen. 1, allowing to improve and optimize
DBPM3 gen. 2 for highest ring performance.

ACKNOWLEDGEMENTS
The authors would like to thank all contributing colleagues from the SLS 2.0 project team for their support
with the design, production, procurement, installation andtest of the mechanical and electrical BPM system components and related infrastructure.

REFERENCES[1] A. Streun et al., “SLS 2.0, the Upgrade of the Swiss Light
Source”, in Proc. IPAC’22, Bangkok, Thailand, June 2022,
pp. 925-928.

Figure 6: 20 nm horizontal (X) and 15 nm vertical (Y)RMS beam position noise of DBPM3 (0.001-3.3 kHz
bandwidth, 50% ADC full scale, one pilot tone active,
same signal level as beam signal).

doi:10.18429/JACoW-IPAC2022-TUPOST032[2] B. Keil et al., “First Application of a Multiprocessing
System-on-Chip BPM Electronics Platform at SwissFEL”, in
Proc. IBIC’22, Krakow, Poland, September 2022, pp. 245248. doi:10.18429/JACoW-IBIC2022-TUP12

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02 Beam Position Monitors