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

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FIRST MEASUREMENTS OF AN ELECTRO-OPTICAL BUNCHARRIVAL-TIME MONITOR PROTOTYPE WITH
PCB-BASED PICKUPS FOR ELBE
B. E. J. Scheible1∗ , A. Penirschke, Technische Hochschule Mittelhessen, Germany
M. K. Czwalinna, N. T. A. Nazer, H. Schlarb, S. Vilcins-Czvitkovits
Deutsches Elektronen-Synchrotron DESY, Germany
M. Freitag, M. Kuntzsch, Helmholtz-Zentrum Dresden Rossendorf HZDR, Germany
W. Ackermann, H. De Gersem, Technische Universität Darmstadt, Germany
1
also at Technische Universität Darmstadt, Germany
Abstract
A vacuum-sealed prototype of an electro-optical buncharrival-time monitor has been commissioned in 2023. It
consists of a pickup structure and a low-𝜋-voltage ultrawideband traveling-wave electro-optical modulator. The
stainless-steel body of the pickup structure is partially produced by additive manufacturing and includes four pickups
as well as an integrated combination network on a printed
circuit board. This novel design aims to enable single-shotbunch-arrival-time measurements for electron beams in freeelectron lasers with single-digit fs precision for low bunchcharges down to 1 pC. The theoretical jitter-charge-producthas been estimated by simulation and modeling to be in theorder of 9 fs pC. The new prototype is tailored for validation experiments at the ELBE accelerator beamline. In thiscontribution first measurement results are presented.

INTRODUCTION
For synchronizing many subsystems and for stabilizing
the arrival time of electron bunches an all-optical approach isused in several large free-electron-laser (FEL) facilities [1],where the arrival time is particularly critical for applications
like pump-probe experiments [2]. The system requires amaster laser oscillator phase-locked to the main RF oscillator, which emits picosecond laser pulses as timing reference [1, 3]. Furthermore, an actively stabilized distribution
system must be established to transfer these pulses along
the facility with minimal drift [1]. The electro-optical (EO)
bunch arrival-time monitor (BAM) [1] is one of various
end-stations in the all-optical synchronization system.

EO-BAM Operating Principle
In contrast to cavity-based BAM in radio-frequency (RF)synchronization [4], where the electron bunch excites a rather
slowly decaying resonance, the EO-BAM detects an evanescent bipolar voltage signal [1]. The voltage is induced by the
co-moving electric field of a bunch as the surface currents
are disturbed by the pickup. The transient voltage leads to a
current, which is transmitted via coaxial cables to a MachZehnder-type electro-optical modulator (EOM) [1]. Usually
the signals of two or more angular distributed pickups are∗ bernhard.scheible@iem.thm.decombined to compensate for orbit variations [5]. The voltage
signal serves as input for the EOM, where the intensity of
the reference laser pulse is modulated accordingly [1]. The
EOM bias voltage is set to a value such that the laser pulse
amplitude is reduced to 50 % of the maximum value in orderto make use of the bipolar pickup signal. By adjustable delaylines the system is tuned to perfect timing, so that the reference pulse and the zero crossing (ZC) of the voltage signal
coincide at nominal bunch arrival time without additional
modulation [6]. Early or late bunches result in a positive or
negative signal, which causes an intensity modulation [1].
The laser-intensity modulation is processed by dedicatedreceiver electronics [3, 6]. A calibration curve is used todeduce the arrival time for every electron bunch [3, 6].
For future applications it is desirable to reach lower bunchcharges, while at the same time the demands for synchronization accuracy rise as well. The current BAMs areequipped with 40 GHz cone-shaped pickups designed forbunch charges down to 20 pC [5], which achieve an overall
resolution of 3.5 fs with 250 pC nominal bunch charge inthe synchronization system of the European XFEL [7]. For
low charges the signal strength and therefore also the BAM
sensitivity decreases rapidly.

EO-BAM Upgrade
To pave the way for 1 pC operation of the European XFEL,an upgrade of the arrival-time diagnostics is mandatory. To
reach the goal of maintaining a resolution below 10 fs evenat this low-charge operation, an upgrade of the main BAM
components is necessary. First the RF part, comprisingof the pickup structure and the signal transmission, andsecond, the EOM must be redesigned. The bandwidth ofboth components should be increased up to 100 GHz andthe length of the transmission line has to be reduced [8].
The EOM’s half-wave voltage 𝑉𝜋 needs to be loweredin order to reach a high modulation with the low expectedsignal strength. To reduce cable losses especially at high frequencies, the EOM will be placed closer to the beamline andthus the radiation hardness has been examined. In a future
upgrade it might even be integrated in the RF structure.
For the pickup system, it was proposed to have pickups andcombination network integrated on a printed circuit board(PCB) [8, 9]. This idea was first combined with rod-shaped

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pickups mounted on top of this PCB (rPCB) to maximize
the effective pickup surface and reduce the wake loss while
minimizing the distance between pickup and beam [10]. The
reported slew rate at ZC was 171.6 mV ps−1 pC−1 , exceeding
the project goal by 14 % [10]. For the construction of a firstvacuum-sealed prototype, it was decided to use a planar version of the rod-shaped pickups, for simplified manufacturingwith a slight improvement of the signal [11].
The potential resolution of the upgraded BAM was estimated using the simulation results of rPCB pickups, themeasured response of a new EOM prototype and typicalnoise contribution. With the assumptions made in [12] ajitter-charge product of about 9 fs pC is expected, whichreaches the project goal.
Figure 1: Rendering of the PCB-based BAM prototype.

PROTOTYPE DESIGNThe first vacuum-sealed prototype of an EO-BAM basedon planar pickups with integrated combination network,
shown in Fig. 1, has been tailored for tests at the ELBE
accelerator in the Helmholtz-Zentrum Dresden Rossendorf(HZDR). Vacuum compatibility of the prototype was realized in close cooperation with DESY and HZDR. It isconnected with a DN40 ConFlat (CF) flange to an inner
beamline diameter of 36 mm, while the minimal clearing
between the pickups is 10 mm. The feedthrough module canbe attached to the side with a DN25 CF flange. A sliding
contact is used for the connection between the feedthroughs
inner conductor and the end of the two stage combination
network, which is integrated on a substrate. The V-type
RF connector is limited to a bandwidth from DC to 65 GHz
according to the manufacturer’s specification.
The layout of the PCB was chosen similar to the PCB
presented in [11] and shown in Fig. 1, but adapted for themechanical and electrical material properties. As substratea Rogers TMM® 10i laminate with dielectric constant Dkof 9.8 ± 0.245, a thickness of 380 µm and 35 µm of copper
cladding was chosen [13]. The short-term vacuum compatibility has been tested at HZDR in a single batch of four
different 20×40 mm substrate samples at once. The baselinepressure in the test chamber was 3 × 10−9 mbar. Due to theraw samples the pressure did not drop below 2 × 10−7 mbar.
After the samples were baked at up to 105 °C for a total ofalmost three days, the pressure dropped to 7 × 10−9 mbar.
The baseline could presumably be reached after some time,so that the test is considered successful.
The position of the PCB relative to the feedthrough is a
critical dimension. The PCB rests on a ledge in the body andfits into a notch cut into a bulge at the feedthrough-module’s
surface to ensure that a good electrical contact is provided
between inner conductor and trace, as well as between the
ground plate and the outer conductor. On the opposite side
two dowel pins jam the PCB in position, because a tilt outof the transverse axis was also identified as a critical defectin the tolerance analysis [11].

Figure 2: Close-up of the prototype installed between two
bellows in the THz beamline of ELBE (top) and measurement setup in the beamline cave, before installing additionalfans and 𝛼-radiation absorber (bottom).

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ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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

IBIC2023, Saskatoon, Canada
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In the course of a dedicated machine development shift at
ELBE, two 2-day shifts were allocated for RF measurementswith the novel EO-PCB-BAM prototype. A third shift isscheduled for December 2023, to measure the performanceof the system with EOM.

Measurement Setup

Measurement Results
Figure 3 shows an exemplary result of the first measurement shift. The full bandwidth of the Keysight UXR1102A
oscilloscope was utilized in this case, with a brick-wall filter with roll-off at 113 GHz. The charge measurement was
unreliable in this charge regime, but with a cathode currentof (1.4 ± 0.2) µA and a repetition rate of 0.41 MHz in CWmode, the mean bunch charge at the source is approximately
3.45 pC. Since a strong fluctuation was observed during themeasurement, the segmented mode was used to save a batchof 1024 pulses for averaging.
The decisive slew rate is (99.27 ± 10.62) mV ps−1 in thismeasurement. The error is expected to be mainly a consequence of charge fluctuations, thus the average slew rateshould match the average charge with significantly lower
uncertainty. The peak-to-peak voltage and rise time are(1.52 ± 0.11) V respectively (24.52 ± 1.09) ps.

Discussion
The measurements were carried out using a combinedtransmission line length of more than 1.4 m with high attenuation above 11 dB at 67 GHz. For the final EOM-integrated
BAM it is planned to omit the external cable completely. Furthermore, the simulated signal was also taken directly at the
feedthrough port. The scattering parameters of the assemblyof the two cables and the v-female-female connector, shownin Fig. 4, have been measured for de-embedding.
The transmission line is modeled with a two-port network
as shown in Fig. 5. The outgoing signal at port 2 in a twoport network is defined as
𝑉2− = |𝑆21| 𝑉1+ + |𝑆22| 𝑉2+ ,

(1)

1.0

0.5
U (V)

Figure 2 (top) shows a close-up to the prototype between
two bellows in the ELBE THz beamline. Off measurement
time, the output was terminated with 50 Ω as shown in the
picture. For measuring the RF signal, a two-port real-time oscilloscope, the Keysight UXR1102A, with maximum bandwidth of 110 GHz was used. To protect the oscilloscopeagainst radiation, it has been encased in the custom lead crateshown in Fig. 2 (bottom), which was afterwards covered in
an absorber to moderate the 𝛼-radiation stemming from thebeam dump. The oscilloscope was remotely operated fromthe control room during measurements. Due to the distance
between the oscilloscope and the BAM, two coaxial v-cable
had to be used. A 121.9 mm Huber+Suhner SUCOFLEX®
570S and a 20.3 mm Rosenberger 70W-08S1-08S1-00203
extended by a v-female-female connector.

0.0

−0.5

−1.0
−20

0
t (ps)

−10

10

20

Figure 3: Measurement #34 (2023-07-27) with a total bunchcharge of approximately 3.45 pC. A set of 1024 pulses isplotted in gray and the average signal in red. The top figure
includes about 2 ns of ringing and the bottom is zoomed tothe main pulse.
0−2−4|S21 | (dB)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

RF MEASUREMENT AT ELBE

−6
−8
−10
−12

0

10

20

30
40
f (GHz)50

60

Figure 4: Scattering parameter of the 20.3 mm Rosenberger
and 121.9 mm Huber+Suhner cables with the connector as
used in the measurements.
where + denotes the incident and − the reflected wave [14].
In these measurements it will be assumed that no wave will
be reflected in the oscilloscope, leading to 𝑉2+ ≈ 0. With
𝑆21 ≠ 0 in the whole signal spectrum, it is possible to get
the simplified input signal by
𝑉1+ =

𝑉2−
|𝑆21|

(2)

in frequency domain. The measured time-domain signal
must be transformed by fast Fourier transform before this

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S11 V1121.9 mm

Female-Female

20.3 mm

V-cableV-connector

V-cable-Pickup structure+

V2 S22-

S12

Keysight UXR1102A

Transmission line

Oscilloscope

Figure 5: Block diagram of the electrical setup with scattering parameter for de-embedding.
step. With inverse fast Fourier transform, the de-embedded
time-domain signal is retrieved afterwards.
After de-embedding the whole RF transmission line, theslew rate at main zero-crossing is (275.71 ± 34.64) mV ps−1with a peak-to-peak voltage and rise time of (4.16 ± 0.31) V
respectively (24.08 ± 1.34) ps. Supposing a linear dependency on the bunch charge [15, 16], the measured slew rate
is normalized to 79.9 mV ps−1 pC−1 and just above half ofthe project goal. This could potentially allow for the desired
BAM resolution down to 2 pC.
Figure 6 shows the simulated signal compared to the meanof 1024 measured pulses after de-embedding. For the simulation the bunch-charge was set to 3.5 pC for comparable
results. The measured and the simulated signals coincidevery well in the vicinity of the bunch and deviate otherwise.
The slew rate is even lower, at 235.50 mV ps−1 , which is significantly worse than in the previously proposed design witha thin glass substrate. This indicates that the TMM® 10i
substrate is not optimal.
mean measurement
simulated

2

U (V)

1

use due to its hydrocarbon content. After disassembling,
the BAM must also be analyzed for defects caused by beamincidence and radiation.
Although the first prototype could not reach the project
goal in accordance with simulations using the same materials, it already exceeds the performance reported for thecurrent pickups [15, 17]. Since not all relevant data wasreported with the results, the comparison must be treatedwith caution and ideally a comparative study should follow
for a robust validation.
In summary, the preliminary review suggests that thenovel design approach is operational and provides a significant improvement. By simulation it is likely that a different
substrate will considerably improve the signal. Furthermore,
the principle is expected to be transferable to other applications. If the in-depth analysis confirms the positive result
and the measurements with an EOM, planned for December
2023, are successful, an improved prototype will be built.
If possible, a W-type connector should be used, to extend
the bandwidth limit to more than 100 GHz. In addition, it
is reasonable to explore different PCB topologies in a nonvacuum structure, in a test chamber or with a free-spacebeam. Finally, it is foreseen to build a new BAM for FLASHand for the European XFEL.

ACKNOWLEDGEMENTS

0
−1
−2
−300

−200

−100

0
t (ps)

100

200

300

Figure 6: Comparison of the simulated signal and the average measured signal given in Fig. 3 after simplified deembedding.

CONCLUSION AND OUTLOOKThe first prototype led to promising results and showedthat a PCB-based BAM with planar implementation of rodshaped pickups is viable. It must be emphasized that the
prototype was not built to exploit its full potential. In particular, the use of a thinner fused-silica substrate should provide
for further improvements. This is also advisable, since thecurrent substrate might suffer radiation damage in long-term

Work supported by the German Federal Ministry of Education and Research (BMBF) under contracts No. 05K19RO1
and No. 05K22RO2.
We would like to thank Keysight for their generous offer,to provide the UXR1102A oscilloscope for these measurements, as well as Dr. Thomas Kirchner from Keysight, for his
technical support. We would also like to thank the HelmholtzZentrum Dresden Rossendorf (HZDR) for granting us beamtime and all the staff involved in installing the BAM or working in the control room during measurements. Finally, we
would also like to thank Neue Technologien GmbH & Co.
KG (NTG) for the close cooperation during constructionof the prototype and the opportunity to carry out the final
assembly on their premises.

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S21

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