Proceedings of IBIC2012, Tsukuba, Japan

WECC04ANALYSIS OF THE ELECTRO-OPTICAL FRONT END FOR THE NEW
40 GHz BUNCH ARRIVAL TIME MONITOR∗
A. Kuhl† , J. Rönsch-Schulenburg, J. Roßbach, Universität Hamburg, Germany
M.K. Czwalinna (née Bock), M. Bousonville, H. Schlarb, C. Sydlo, DESY, Hamburg, Germany
S.M. Schnepp, Laboratory for Electromagnetic Fields and Microwave Electronics (IFH),ETH Zurich, Switzerland
T. Weiland, Institut für Theorie Elektromagnetischer Felder,
Technische Universität Darmstadt, Germany
A. Angelovski, R. Jakoby, A. Penirschke, Institut für Mikrowellentechnik und Photonik,
Technische Universität Darmstadt, Germany
Abstract
The Free-electron LASer in Hamburg (FLASH) is currently equipped with four Bunch Arrival time Monitors(BAMs) which achieve a measurement accuracy below
10 fs for bunch charges higher than 500 pC. In order toachieve single spike FEL pulses at FLASH, electron bunchcharges down to 20 pC are required. To achieve a measurement accuracy of 10 fs also at such a small bunch charge a
new BAM containing new pickups with a bandwidth of up
to 40 GHz has been designed and manufactured. The signal of the pickups will be evaluated using a time-stabilizedreference laser pulse which is modulated with an ElectroOptical intensity Modulator (EOM). The theoretical measurement accuracy depends on several parameters and theirfluctuations. The impact of these fluctuations on the measurement accuracy will be discussed.

INTRODUCTION
The Free-electron LASer in Hamburg (FLASH) is asource of short photon pulses tunable within a wavelength range from 4.12 to 45 nm [1]. It is equipped withfour Bunch Arrival time Monitors (BAMs), which provide
a measurement accuracy below 10 fs for bunch charges
above 0.5 nC [2]. In order to reach FEL pulses with a duration of a single mode only low bunch charges are required.
For FLASH a bunch charge down to 20 pC is necessary [3].
Thus a BAM is required which allows the determination of
the arrival time with a precision of 10 fs for bunch charges
down to 20 pC. Therefore, a new pickup with a bandwidthof 40 GHz has been developed [4, 5, 6] and installed at
FLASH. Besides the new pickup, a new electro-optical
front-end is required for such a new BAM. The new frontend will use an electro-optical modulator (EOM) with abandwidth of 40 GHz which corresponds to the bandwidthof the new pickup. Also a new readout electronic basedon μTCA 4 [7] will be used. In order to preserve the large
operating range of the bunch charges up to 3 nC a special
wiring scheme is needed.
∗ The work is supported by Federal Ministry of Education and Research of Germany (BMBF) within FSP 301 under the contract numbers
05K10GU2 and 05K10RDA.
† alexander.kuhl@desy.de

The arrival times of the electron bunches are detectedwith a pickup and compared with the timing of a laser
pulse which is synchronized to a reference master oscillator. The new 40 GHz pickup contains four cone-shaped
pick-up electrodes [4]. The electro-magnetic field of anelectron bunch passing the BAM induces a short bipolar
RF signal in each of the four pick-up electrodes. The RF
signals of opposite pickup electrodes are combined to increase the amplitude and to reduce the influence on theorbit position of the electron bunches of the measured arrival time. One of these RF signals is directed to a 40 GHz
EOM. This branch will be used as a fine channel with ahigh accuracy for the low bunch charge operation mode,but for high charges the amplitude of the pickup signal rises
above the usable range of the EOM. The other branch iscarried to a 10 GHz EOM (see Fig. 1). This branch will be
used as coarse channel in low bunch charge mode and as
standard channel for the high charge operation at FLASH.
The laser pulse from the synchronisation system is approximately 100 times shorter than the RF signal pulse and theratio of the laser amplitude of the output signal of the EOM(Iout ) and the input signal (Iin ) is given by the following
equation [8]:Msignal =

Iout
Iin



2πUbias
2πURF (t)= cos2 2δ0 ++Uπ,biasUπ,RFπUbiasπURF (t)1 1+= + cos δ0 +2 2
Uπ,biasUπ,RF(1)

The parameters δ0 , Uπ,bias , and Uπ,RF are device specificconstants of the EOM. The intrinsic operation point is presented by δ0 . Uπ,bias and Uπ,RF are the voltages to change
the modulation M between 0 and 1 at the bias port respectively at the RF port. By setting the modulation to M = 0.5
with a DC bias voltage Ubias an optimized determination of
the timing difference between the RF and the laser pulse at
the EOM are feasible. With a correct timing of the electron bunch, the zero-crossing of the RF signal reaches theEOM at the same time as the reference laser pulse and theoutput of the EOM will be M = 0.5. When the electronISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○PRINCIPLE OF MEASUREMENTWECC04

Proceedings of IBIC2012, Tsukuba, Japanbunch reaches the EOM with a timing offset the pickup signal shows a non-zero voltage at the RF input and therefore
the amplitude of the laser pulse is modulated (see Fig. 2).
The amplitude of the laser pulse is detected with a photodiode and digitized by a fast ADC. The performance of thissystem depends on the slope at the zero-crossing of the RF
signal which directly depends on the electron bunch charge.
Furthermore, a small jitter in the other components such as
the reference laser pulse is necessary for a measurement
accuracy in the order of 10 fs.

Figure 2: Principle of the arrival time measurement. The
laser pulses are modulated by the EOM which is driven by
the RF signal from the pickup. Arrival time changes of theelectron bunch cause different modulation voltage at thelaser pulse arrival time. [9]calculated signal at EOM
analytical function

amplitude in V

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

1

0

−1
−0.05

0

0.05

0.1

0.15

time in ns

Figure 1: Schematic design of the BAM system.

ANALYSIS MODEL
For a detailed analysis of the measurement accuracy ofthe new BAM the knowledge of the shape of the RF signal at the entrance of the EOM is required. The performance of the pickup has been simulated using CST PARR [4, 6]. The simulation results of the
TICLE STUDIO 
new pickup were calculated directly behind the vacuum
feedthroughs of the pickup electrodes. Additionally the RF
cabling between the feedthroughs and the EOM changes
the shape of the RF signal. The S-parameters have beenmeasured for a 37 cm long RF cable. These results havebeen extrapolated to a 2.87 m long RF cable as it has been
installed at the new BAM at FLASH. Furthermore, the RF
combiner was considered with +2 dB (+3 dB by combination and -1 dB insertion loss). By using these extrapolated
S-parameters the RF signal was calculated at the end of thecable. The resulting RF signal slope is 286 mV/ps and theamplitude at the entrance of the EOM is 1.463 V at a bunchcharge of 20 pC. The RF signal and its slope and amplitude

Figure 3: Comparison between the calculated RF signal
with an electron bunch charge of 20 pC (blue) and the analytical function (red). The interesting area is around thefirst zero-crossing.
scales linearly with the bunch charge. For simplification
it is advisable to use a deterministic function (see Fig. 3).
Therefore, for further investigations the following analytical function, which corresponds to a derivative of the normally function, has been used to describe the RF signal
2

URF (t) = tSe− e ( A )
1

tS

(2)

with the slope at the zero-crossing S and the amplitude of
the pickup signal A. The ringing of the RF signal is notof interest for this analysis. To avoid errors of the bunch
arrival time measurement due to long-term drifts of δ0 or
Uπ,bias , a baseline modulation will be detected during operation. This is possible because the repetition rate of thelaser pulses (216.67 MHz which is one sixth of the 1.3 GHz
reference source) is much higher than the repetition rateof the electron bunches (max. 1 MHz). Therefore a laser
pulse before the one which is modulated by the RF signal of
the pickup serves as a non-RF-modulated reference. These
non-RF-modulated reference laser pulses will be detectedISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan
15000

10000

10000

Count

Count

15000

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5000

5000

0
−40

−20

0

20

0
−40

40

Time difference in fs

−20

0

20

40

20

40

Time difference in fs

10000

10000

Count

15000

Count

15000

5000

5000

−20

0

20

40

Time difference in fs

0
−40

−20

0

Time difference in fs

Figure 4: Count of the difference between tbunch and tsim,meas for different settings. The bunch charge is set to 20 pC in the
top row and to 200 pC in the bottom row. The timing jitter of the bunch is set to 25 fs in the left column and to 100 fs in
the right column.
as baseline modulation Mbaseline . Compared with formulaEq. (1) we obtained:


1
1 1
πUbias(3)
Mbaseline = + cos δ0 +≈2 2
Uπ,bias set to 2
The real amplitude modulation can be calculated by:
M = Msignal − Mbaseline(4)

Furthermore a conversion from the real amplitude modulation of the laser pulse to the timing shift of the RF pulse andaccordingly of the electron bunch is required. A linearization around the operating point of the RF signal leads to thecalibration constant [8]S(5)K=
2Uπ,signal

arrival time jitter of the electron bunch and therefore themeasurement jitter has to be determined. The analysis was
performed for different bunch charges. For one certain
bunch charge the values of the real amplitude modulation
M (Eq. 4) will be calculated. In the next step the simulated
arrival time measurement tsim,meas will be determined. This
process will be repeated 105 times. For each of these calculations a different random set of the jitter parameters in
Table 1 are generated . The simulated arrival time measurement value tsim,meas differs from the set value of the arrival
time tbunch . Note, tbunch is the arrival time which is also effected by jitter (see Table 1) . The differences between t bunch
and tsim,meas of each iteration were calculated (see Fig. 4).
From all of these 105 samples the RMS of these differenceswere determined. This will be done for a set of different
bunch charges.

and therefore the simulated arrival time measurement is
tsim,meas =M 2M 3 6M 5
arcsin(2M )≈+++... (6)Taylor K
2K
3K
5K

Monte Carlo Simulation
The performance of the bunch arrival time measurement
depends on the stability of the laser pulse. Furthermore
the noise of the bunch charge, the noise of bias voltage of
the EOM, and the amplitude noise of the RF signal also
influence the bunch arrival time measurements. The discrepancies of these values have been considered as normally distributed with an rms width given in Table 1. The

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Copyright ○

0
−40

80

Proceedings of IBIC2012, Tsukuba, Japan
1. order3. order5. orderarccos

60

40

20

0
0

200

400

600

800

1000

measurement accuracy in fs (RMS)

measurement accuracy in fs (RMS)

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1. order3. order5. orderarccos

60

40

20

0
0

200

bunch charge in pC

Figure 5: Calculated measurement accuracy of the BAM
system at the 40 GHz EOM for different bunch charges
with a timing jitter of 25 fs.

400

600

800

1000

bunch charge in pC

Figure 6: Calculated measurement accuracy of the BAM
system at the 40 GHz EOM for different bunch charges
with a timing jitter of 100 fs.

Table 1: List of Assumed Normally Distributed Jitter
Values for the Monte Carlo Simulation

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

assumed RMS values

bunch charge
bias voltage of the EOM
RF voltage at the EOM
laser amplitude1
laser timing
ADC channel
bunch arrival time2
1 Only for the calculation of the M

1%
0.5 mV
0.5 mV
0.35 %
2.5 fs20
25 fs and 100 fs
signal .

2 Stored and compared to the simulated measurements for each

iteration.

Figure 4 exemplary shows the results for two different
charges. For a bunch charge of 20 pC the calculation of
the performance of the new BAM achieved a measurement
accuracy of about 11 fs (RMS). The measurement jitter
improves slightly if the arrival time jitter of the electron
bunches decreases. A reduction of the arrival time jitter
down to 25 fs is possible by using the intra bunch train feedback at FLASH [10]. The measurement jitter for different
bunch charges is shown in Fig. 5 and 6. The results are calculated applying different orders of Equation (6). Increasing the bunch charge starting from 5 pC the measurement
jitter will be reduced down to 4 fs. For a bunch arrival time
jitter of 100 fs the measurement jitter is starting to degrade
above 250 pC. This is because of the unambiguousness ofthe modulation in the EOM is lost when URF > 12 Uπ,RFarcording to Eq. (3). As result the Tsim,meas can not be calculated correctly with Eq. (6). This failure will also occur
in the real BAM system. Figure 7 shows the relative incidence of URF which is greater than 12 Uπ,RF . For bunchcharges above approximately 200 pC the 10 GHz EOM has
to be used as the fine channel for the arrival time measurement.

relative count in %

40

30

20

10

0
0

200

400

600

800

1000

bunch charge in pC

Figure 7: The working range of the EOM for detecting thecorrect timing is limited by 12 Uπ,RF . The graph shows therelative count with URF higher than 12 Uπ,RF for different
bunch charges by using a timing jitter of 100 fs. For bunchcharges higher than 200 pC the 10 GHz EOM has to be usedas the fine channel for measurement of the timing shift.

CONCLUSON AND OUTLOOKA new 40 GHz BAM was designed and simulated [4].
The performance of the BAM system was calculated withsimplified mathematical models including various jittersources. The calculation reaches a measurement accuracy of approximately 11 fs for a bunch charge of 20 pC
and better than 10 fs for bunch charges higher than 25 pC.
Compared with the currently running 10 GHz BAM system which achieves a measurement accuracy below 10 fs
for bunch charges above 500 pC, the new system will bea significant improvement. It also has been observed thatthe jitter of the laser amplitude has the strongest influenceon the measurement accuracy for low bunch charges. An
improvement of this jitter to 0.2 % would result in a measurement accuracy below 10 fs for 15 pC bunch charge.
The new BAM pickup has been installed at FLASH. The
next steps are the manufacturing and installation of the newelectro-optical BAM front end.

ISBN 978-3-95450-119-9
4Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

WECC04

REFERENCES[1] K. Honkavaara et al., “Status of FLASH”, TUPPP052, Proceedings of IPAC 2012, New Orleans, USA.
[2] M.K. Bock et al., “Recent Developments of the Beam
Arrival Time Monitor with Femtosecond Resolution at
FLASH”, WEOCMH02, Proceedings of IPAC 2010, Kyoto,
Japan.
[3] J. Rönsch-Schulenburg et al., “Generation of Ultra-ShortElectron Bunches at FLASH”, THPD33, will be publishedat FEL 2012, Nara, Japan.
[4] A. Angelovski et al., “High Bandwidth Pickup Design for
Bunch Arrival-time Monitors for Free-Electron Laser”, will
be published at Physical Review Special Topics - Accelerators and Beams (PRST-AB), 2012.
[5] A. Angelovski et al., “Realization of a High Bandwidth
Bunch Arrival-time Monitor with Cone-shaped Pickup
Electrodes for FLASH and XFEL”, TUPC076, Proceedingsof IPAC 2011, San Sebastián, Spain.
[6] A. Kuhl et al., “Sensitivity and Tolerance Analysis of a new
Bunch Arrival Time Monitor Pickup Design for FLASHand XFEL”, TUPC079, Proceedings of IPAC 2011, San Sebastián, Spain.
[8] M.K. Bock, “Measuring the Electron Bunch Timing with
Femtosecond Resolution at FLASH”, dissertation, Hamburg
2012, Germany.
[9] F. Loehl et al., “Electron Bunch Timing with Femtosecond
Precision in Superconducting Free-Electron Laser”, published at Physical Review Lett. 104, 144801, 2010.
[10] C. Schmidt et al., “Feedback Strategies for Bunch Arrival
Time Stabilization at FLASH Towards 10 fs”, THPA26, Proceedings of FEL 2011, Shanghai, China.

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