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 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 1 9

FIRST TEST WITH MICROTCA BASED CAVITY BPM ELECTRONICSFOR THE EUROPEAN XFEL AND FLASH
B. Lorbeer∗ , H. T. Duhme, I. Krouptchenkov, T. Lensch, D. Lipka, M. Werner
Deutsches Elektronen-Synchrotron DESY, Germany
Abstract
The European X-ray free-electron laser (E-XFEL) and the
FLASH2020+ project for the free electron laser Hamburg(FLASH) at DESY in Hamburg, Germany foresee several
machine upgrades in the years to come. At FLASH a whole
undulator section in a shutdown starting in summer 2024
and finishing in autumn 2025 is going to be rebuild. Existing button beam position monitors installed in this sectionof the machine do not deliver sufficient signal strength for
future required resolution specification and orbit feedback
optimization for machine operation. The resolution limitations will be overcome by replacing the button-based beam
position monitors with in-house developed cavity beam position monitors and compact microTCA based radio frequency
receiver read-out electronics. The measurement system has
been tested and evaluated in a test setup at FLASH.

INTRODUCTION
The two international user facilities European X-ray freeelectron (E-XFEL) and the free-electron laser of Hamburg (FLASH) are operated by the Deutsches ElektronenSynchrotron (DESY) in Hamburg, Germany. Both machines are based on the effect of self-amplified spontaneous emission (SASE) which generates ultra-short photon pulses by cascading several undulators. The photon
pulses are based on accelerated electron-bunches which
originate from a photo injector driven radio frequency (RF)electron gun. The electron bunches are accelerated in apulsed superconducting linac at a repetition rate of up to4.5 MHz. More specifications on the different machine parameters can be found here [1]. The high-resolution transversal beam position measurement of the electron beam along
the machines is realized with cavity beam position monitors(CAVBPM). The E-XFEL is already equipped with well
performing monitors including read-out electronics [2] fromin-kind contribution partners. However, future planned upgrades of the machine [3] with additional CAVBPM will bebased on in-house developed read-out electronics. Like theE-XFEL the FLASH facility will be upgraded with approximately 20 monitors within the FLASH2020+ project [4]. ACAVBPM electronics read-out system has been developedin the past few years and is already in operation [5]. Theseelectronics have been modified and enhanced to meet thespecifications of the E-XFEL and FLASH where the mostdemanding requirement is a single bunch resolution of better
than 1 um in a charge range from 20 pC up to 1 nC. First
tests with these CAVBPMs have been made with a test cavity BPM at FLASH to evaluate the adapted and extended∗ bastian.lorbeer@desy.deread-out electronics. The charge has been varied over thespecified range and the measurement results demonstrate tofulfill the specified resolution.

CAVITY BEAM POSITION MONITORS AT
FLASH AND XFEL
Cavity beam position monitors are composed of a monopole and a di-pole resonator from which the electron beam
excited modes deliver RF signals from which the charge
and the transversal position of the electron beam can beevaluated [6]. The monitors in use at the E-XFEL and in thedesign procedure for FLASH are also developed in-house [7]and have the characteristics listed in Table 1.
Table 1: Cavity Beam Position Monitor Characteristics
E-XFEL and FLASH
QualityRange

resonance frequency
beam pipe diametermono-pole sensitivity
dipole sensitivity
loaded quality factor

3.3 GHz
10 – 40.5 mm
42.5 – 42.9 Vpk/nC
2.46 – 2.87 Vpk/nC/mm
70

ANALOG SIGNAL DYNAMICS
The original developed CAVBPM monitor [8] and corresponding RF front-end electronics [5] have been designed tooperate in a charge range from 500 fC up to 100 pC, while thespecified charge range in the E-XFEL and at FLASH varies
from 20 pC - 1 nC. Due to these different charge ranges andequivalent signal amplitudes modifications in the RF frontend were necessary. For this purpose a typical signal from amono-pole resonator has been measured at a test cavity at
FLASH. The signal is shown in Fig. 1.
Since the signals from the cavity are band-pass filtered in
the first place in the RF front-end the signal strength fromthe band-pass filtered signal has been chosen to determine
the necessary front-end dynamics. The derived quantities
are summarized in Table 2.
Table 2: Estimation of Expected Signal Powers for theCharge Range From 20 pC–1 nC
Mode/StatesCharge

voltage/Vpppower/dBm

min. charge
max. charge

20 pC
1 nC0.044
22.25

−2331

MOP0197002 Beam Position 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 - M O P 0 1 9

X1

Figure 1: Analog signal from reference resonator and 20 dB
additional attenuation measured at 400 pC. White trace:
without band-pass filter. Yellow trace: with band-pass filter.
Therefore the mono-pole signal covers a dynamic rangeof approximately 60 dB with a headroom of 6 dB. A similar estimation for the signal level dynamics of the dipole
signal has been made. The front-end receiver dipole channel can accept a minimum signal power of −76 dBm anda maximum of 37.75 dBm exceeding a dynamic range of
100 dB. Special care needs to be taken to avoid an operationof the front-end receiver components beyond the P1dB in
amplifiers and demodulator. This is done empirically duringcommissioning of the electronics. For these first tests the RF
front-end receiver introduced in [5] has been modified in thecharge channel. The remaining component chain consists ofa band-pass filter and a step-attenuator before the RF signal
enters the demodulator.

X2

Y1 Y2 Q1XYQ

Q2108MHz108MHz

Figure 2: Micro TCA based read-out electronics including
CPU, MCH, X2timer, and three digitizer cards. Three power
splitters for x,y and q signals from the cavity and an additional splitter for the Master Oscillator reference frequencyof 108 MHz.
to digital converters with a 16 bit resolution and a sample
rate of 108 MHz. The digitized data is collected in a field
programmable gate array (FPGA) and the sampled data isaccessed by a hardware (HW) server process in which thefinal calculation of beam charge and position takes place.
To synchronize the local oscillator (LO) phase and obtainphase stable measurements it is necessary to program thephase locked loop (PLL) on the RF front-end receiver fromthe server panel shown in Fig. 3.

MEASUREMENT SYSTEM SETUP
The measurement principle to estimate the resolution performance of the electronics is based on the idea to separate
correlated beam-jitter and un-correlated electronics noise
contributions in the measurement. This is done by splitting
the signal from one cavity and measure the signal fluctuations in two identical electronics. To obtain a prediction of
the position and charge measurement a singular value decomposition (SVD) of two independent measurement channels(either horizontal, vertical, or charge) is applied [9]. The
remaining standard deviation of position and charge fluctuations reveals the un-correlated electronics noise from thetwo systems 1 . Figure 2 shows the connection sheet fromthe splitters to the modified RF front-end which is housed
in a Micro TCA crate.

Data Acquisition
The signals received by the modified RF front-end receiverare digitized on a separate electronics card with six analog
1 Excluding the losses in the splitter, which promises an improved resolution

by at least a factor of √2.

Figure 3: Hardware server with RF front-end interface, raw
data read-out and calculated horizontal and vertical positionand charge data.
Additional important features of the server are:• data timing and phasing of intermediate (IF) signals• a global gain control to adjust signal level depending
on charge and x,y-offset
MOP01902 Beam Position Monitors

71

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 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 1 9Calibration
The test cavity BPM is mounted on a mover stage withan internal accuracy of 1 µm which can be moved during
electron beam operation. Finding the correct scaling factor
has been done in an iterative manner by physical movementof the cavity with the help from the mover stage and thencomparing the measured value from the electronics. Aftercomparison the scaling factor has been corrected so themeasured value matches the physical movement. This has
been done over a measurement range of +/- 500 µm. The
scaling of the charge values has been done by comparing thereadings from a nearby charge measurement monitor andadjusted accordingly to deliver similar results.

MEASUREMENT RESULTS
A synchronous read-out of position and charge data for
220 subsequent bunches has been realized to enable theseparation of correlated beam jitter and the un-correlated
electronics noise. The result can be shown as a function of
the predicted value for the measured charge and position
data sets. During the measurement the minimum measured
charge has been 19 pC and the maximum was around 307 pC.
Figure 4 shows the measured charge versus the predictedcharge at 19 pC and Fig. 5 shows the measured x-position
versus the predicted x position at a charge of 91 pC.
2019.51918.50.08
0.07

horiz. measured position / mm

• LO reference phase control to stabilize the reference
cavity with respect to machine phase and optimize stability and reproducability• phase offset which need to be adjusted once to findthe correct quadrant and sign of di-pole signals phasechange• scaling factors for x, y, and q to adjust calculated values
to reasonable physical quantities.

charge measured / pC

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

0.06
0.05
0.04
0.03
0.02
0.01
00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

horiz. position prediction / mmFigure 5: Measured x-position versus the predicted x position. The average charge here is 𝜎q = 91 pC and thestandard deviation of the uncorrelated horizontal jitter is𝜎x = 0.542 µm.
Table 3: Calculated resolutions based on measurements atFLASH at different charges. The calculation do not include
the factor of √2 to compensate for the losses in the splitter.
Charge [pC]
18.78
91
153
307

𝜎𝑞 [fC]𝜎𝑥 [µm]𝜎𝑦 [µm]

74
177
279
437

2.637
0.542
0.795
0.412

2.737
1.09
1.421
0.895

SUMMARY AND CONCLUSION
The measurements show that required electron beam position resolution of 1 µm could almost be met for a large charge
range neglecting the fact of a reduced signal power of −3 dBin each RF front-end due to the power splitters in the measurement setup. The RF cables from tunnel to rack in thissetup is about 50 m long. RF cables in existing cavity BPMs
at E-XFEL and for future installations at FLASH2020+ will
be more on the order of 30 m which will improve the signal
to noise ratio to finally meet the specification goal of 1 µm
also at the small charges of 20 pC.

18REFERENCES

17.5[1] W. Decking et al., “A MHz-repetition-rate hard X-ray freeelectron laser driven by a superconducting linear accelerator”,
Nat. Photonics, vol. 14, pp. 391–397, July 2020.

171717.5

1818.5

1919.520

20.5

charge prediction / pC

Figure 4: Correlated electronics noise from charge channel
front-end receiver. The average charge is avgq =18.78 pC
and the standard deviation of the uncorrelated measurement
noise is 𝜎q = 74 fC.
The following Table 3 shows additional charges wheremeasurement data has been analyzed.

[2] M. Stadler et al., “Low-Q Cavity BPM Electronics for E-XFEL,FLASH-II and SwissFEL”, in Proc. IBIC’14, Monterey, CA,USA, Sep. 2014, paper WEPD12, pp. 670–674.
[3] S. Casalbuoni et al., “European XFEL Undulators - Statusand Plans”, in Proc. IPAC’22, Bangkok, Thailand, Jun. 2022,
pp. 2737–2740.
doi:10.18429/JACoW-IPAC2022-THPOPT061[4] J. Zemella, J. Roensch-Schulenburg, S. Schreiber, and M. Vogt,“Recommissioning of the FLASH injector and linac”, in Proc.

MOP01972

02 Beam Position 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 - M O P 0 1 9

IPAC’23, Venice, Italy, May 2023.
doi:10.18429/JACoW-IPAC2023-TUPL022[5] B. Lorbeer et al., “Cavity BPM Electronics for SINBAD
at DESY”, in Proc. IBIC’22, Kraków, Poland, Sep. 2022,
pp. 413–415. doi:10.18429/JACoW-IBIC2022-WEP14[6] R. Lorenz, “Cavity beam position monitors”, in AIP Conf.
Proc., vol. 451, pp. 53–73, 1998. doi:10.1063/1.57039[7] D. Lipka et al., “Production of Cavity BPMs for the European
XFEL”, presented at IBIC’13, Oxford, UK, 2013, FRWMJ5.

[8] D. Lipka, M. Dohlus, M. Marx, S. Vilcins, and M. Werner, “Design of a Cavity Beam Position Monitor for the ARES Accelerator at DESY”, in Proc. IBIC’18, Shanghai, China, Sep. 2018,
pp. 269–272. doi:10.18429/JACoW-IBIC2018-TUPB05[9] N. Baboi et al., “Resolution Studies at Beam Position Monitors
at the FLASH Facility at DESY”, in AIP Conf. Proc., vol. 868,
pp. 227–237, 2006. doi:10.1063/1.2401409

MOP01902 Beam Position Monitors

73

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