Proceedings of IBIC2012, Tsukuba, Japan

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BEAM TEST RESULTS OF UNDULATOR CAVITY BPM ELECTRONICS
FOR THE EUROPEAN XFEL
M. Stadler*, R. Baldinger, R. Ditter, B. Keil, R. Kramert, G. Marinkovic, M. Roggli,PSI, Villigen, Switzerland
D. Lipka, D. Nölle, M. Pelzer, S. Vilcins-Czvitkovits, DESY, Hamburg, Germany
Abstract

INTRODUCTION
The European XFEL (E-XFEL) [1] has asuperconducting 17.5GeV main linac that will provide
trains of up to 2700 bunches, with 0.1-1nC bunch charge
range, 600µs train length, 222ns bunch spacing, and 10Hz
train repetition rate. A kicker/septum scheme candistribute fractions of the bunch train to two main SASE
undulator lines followed by “secondary undulators” forspontaneous or FEL radiation. The E-XFEL will provide
SASE radiation down to below 0.1nm wavelength andsupports arbitrary bunch patterns within a bunch train,with bunch spacing of n*111ns, where n is an integer >1.
The E-XFEL is presently under construction inHamburg, with first injector beam scheduled for 2014 andfirst main linac beam and SASE for 2015.
The cavity BPM electronic system is being developed
at PSI [2,3]. For preliminary performance measurements
an array of 3 E-XFEL cavity BPMs have been installed at
the SwissFEL injector test facility [4].

BPM ELECTRONICS
The present BPM electronics prototype consists of:• The RF front-end electronics (RFFE): One I/Q
downconverter and LO synthesizer for
reference, x- and y-position signal channel,
and an ADC sampling clock synthesizer.
Active local temperature stabilizers areemployed for drift reduction.
• 6-channel, 16-bit 160MS/s analog-to-digitalconverters for all RFFE I and Q baseband
differential output signals.
• Digital signal processing hardware (“GPAC”board) for signal processing and interfacing tocontrol, feedback, timing and machine
protection systems.

RF Frontend
The simplified block diagram of the RFFE electronics
used is shown in Fig. 1. The basic principle of the BPM
electronic and cavity design is based upon ref. [6].

CAVITY PICKUPThe 3.3 GHz cavity pickups were designed at DESY[5]. They have the parameters given in Table 1.
Table 1: Undulator cavity BPM pickup parameters(10mm beam pipe aperture)

Resonant frequency
Sensitivity
Cavity loaded-Q

position cavity(TM110 mode)

reference cavity(TM010 mode)

3300 MHz

3300 MHz

2.8 mV/nC/ȝm

42 V/nC

70

70___________________________________________*markus.stadler@psi.ch

Figure 1: Simplified downconverter block diagram(one channel).
An input bandpass filter selects the cavity signal
components around 3.3GHz. The filter is followed by aswitchable gain section. The gain is selected dependingon the actual bunch charge (4 charge ranges).
The quadrature downconverter operates with an LO
frequency of approximately 3.3GHz. This frequency isequal for all three channels (reference-, x- and y-channel).
A reference signal (214 MHz at the SwissFEL injector
test facility, 216.66 MHz at EXFEL) is provided from themachine reference system. This signal is divided by 24with a divider common to all LO PLLs on the RFFE.
The ADC is clocked by a signal also generated on theRFFE. Its phase is controllable in 0.5º steps around theISBN 978-3-95450-119-9
Beam Position Monitor System1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The European X-ray Free Electron Laser (E-XFEL)
will use dual-resonator cavity BPMs (CBPMs) between
the SASE undulators and in the beam transfer lines tomeasure and stabilize the beam trajectory. The BPM
electronics is developed by PSI, while the pickup
mechanics is developed by DESY. BPM electronics beam
tests with three adjacent pickups have been performed at
the SwissFEL injector test facility (SITF) at PSI. Thesystem architecture and algorithms, achieved performanceand noise correlation measurements of the present
electronics prototypes will be presented.

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Proceedings of IBIC2012, Tsukuba, Japanfull circle by programming the phase tuning word of adirect digital synthesizer (DDS).
The RF front-end electronics was designed in a VME64x
compliant form factor (Fig. 2).

include Ethernet (with Linux running on the GPAC),
PCIe, or custom protocols e.g. for beam based feedbacks
using up to 8 multi-gigabit SFP+ transceiver links
provided by the MBU.

Figure 4: Lab test setup of a Modular BPM Unit(MBU), with one GPAC and two cavity BPMRFFE boards.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2: RF front-end card.

SIGNAL PROCESSING

ADC & Digital Backend

RF Cavity Pulse ShapeThe RFFE analog baseband signals are sampled by a 6channel 16-bit ADC board at 160 Ms/s sampling rate. The
sampling clock is provided by the RFFE. Programmabledelay units allow correction of individual path delay
variations for each ADC (Fig. 3).

The cavity signals as seen at the input of the RFFE
electronics have a shape as seen in Fig. 5. Because of thelow cavity loaded-Q factors the pulses durations is only
about 20ns. High loaded-Q factor cavities providing
longer pulse durations are unfavourable since bunch
spacing can be as short as 222ns in the EXFEL.

Figure 5: Cavity signals(timescale: 10ns/div).

atinputofRFFE

IF Pulse ShapeFigure 3: ADC block diagram (simplified).
The ADC mezzanine module resides on top of a VME64x
compliant FPGA carrier board (GPAC = Generic PSIADC Carrier). RFFEs and GPAC can be used both in
VME64x crates, or in the so-called modular BPM unit(MBU) that will be used for the final E-XFEL BPMsystems [3]. The MBU is a cost-efficient standalone crate
solution with a customized backplane. The housing of theMBU provides slots for one GPAC and either two EXFEL undulator or re-entrant cavity BPM RFFEs or four
E-XFEL button BPM RFFEs, or a combination of those.
By using MBU and GPAC for all BPMs in E-XFEL, astandardized interface to control, timing, interlock andfeedback systems is provided. Supported protocols

Down conversion and filtering in the RFFE producebaseband signals as seen in Fig. 6 top.
The IF pulses are relatively narrow compared to thesampling interval of 6.25 ns. The IF signal filter shape has
been chosen to maximize S/N ratio under the condition of
preventing bunch-to-bunch crosstalk. Figure 6 (top) shows
an example taken from real pick up signals.
Given the short signal pulse duration the sampled vector
magnitude (Fig. 6 bottom) is sensitive to the ADC
sampling clock phase. Therefore, an all-digital feedback
loop is used to compensate any ADC clock phasemisalignment and drift (Fig. 7).

ISBN 978-3-95450-119-92Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, JapanTUPA27

The values R and X are the peak vector magnitudes
calculated from the pulse peaks of I and Q baseband
waveforms of reference and x channel, respectively. For
example, in Fig. 6 (bottom) the vector magnitude isrepresented by the value of the highest sample. k is a gain
constant that we calibrate using a pickup mounted on a
motorized mover with a high-resolution position encoder.
In the above formula (1) |X| and |R| represent processedvalues, where downconverter I/Q imbalances and ADC
offsets have already been corrected for.

TEST SETUPADC Clock Phase Control LoopIn order to optimize signal to noise ratio and tocompensate drifts occurring in beam arrival time and/or
sampling clock phase the BPM electronics employs adigital phase feedback loop.
The clock phase offset is estimated from the threesampling values around the top of the reference IF vector
magnitude (Fig. 6, bottom). Then the DDS clock phase isadjusted to correct this error and keep on sample at thetop of the magnitude waveform (solid circles in Fig. 7).

Figure 7: Principle of sampling phase alignment
feedback loop.

Figure 8: BPM test array at SwissFEL 250MeV
test injector facility.

The phase estimator currently employs a polynomialinterpolation of the top section of the reference IF vector
magnitude waveform. This approach allows a relatively
precise estimation to be computed within a bunch spacingof 220 ns in an FPGA. Other approaches offering higher
accuracy were discarded due to their higher latency.

Beam Offset Calculation
The beam offset in x direction Dx is basicallycalculated using the formula

DX = k ⋅XR(1)Figure 9: BPM electronics test rack (RFFE, ADC& FPGA board) at SwissFEL injector facility.

ISBN 978-3-95450-119-9
Beam Position Monitor System

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 6: Baseband waveforms samples (top: I andQ signals, bottom: vector magnitude).

In order to test the BPM system with beam an array offour BPM pickups has been installed at the SwissFEL testinjector. A similar array is also installed at the FLASH
accelerator facility at DESY (Hamburg). The arrayconsists of three undulator BPM (10mm aperture) cavity
pickups followed by one beam transfer line BPM(40.5mm aperture). The test results reported here wereobtained using the three EXFEL undulator pickups. Thetest injector operates with a single-bunch repetition rate of10Hz. Using a VME64x crate instead of an MBU allowedeasy integration into the VME64x based control andtiming system of the test facility. Similar measurements
are planned using the cavity test array at DESY.

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Proceedings of IBIC2012, Tsukuba, Japan

Position Resolution
The three 10mm diameter cavities are arranged as seen inFig. 10.

We performed position resolution measurements basedon correlating the readings of three cavities (Fig. 12).
Using the position readings of BPM1 and BPM3 (d1 andd3) we calculate the difference between the linear
interpolation and the displacement measured by BPM2.
This difference is seen as ǻd in Fig. 12.

Figure 10: BPM (Ø10mm) pickup arrangement.

TEST RESULTS
Charge Resolution

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Since the reference cavity TM010 mode that is coupled
to the RFFE is sensitive to the beam charge only thereference channel information can be used as a sensitive
charge monitor after calibration of the scaling factor bymeans of a dedicated charge monitor. The charge
resolution RC is determined by taking the difference of thecharge readings of two BPMs and calculating

RC =std (Q1 − Q2 )2(2)std(x) is the standard deviation of the samples x.
A sample charge measurement at C§210 pC is shown inFig. 11. Applying (2) results in a charge resolution RC of§85 fC rms.
Those values are slightly higher than the theoretical value
obtained from ADC/RFFE noise calculations (Table 2).

Figure 12: BPM resolution measurement using 3 cavities.
If one assumes that all BPMs have equal position jitterthen the BPM resolution isδ BPM =2⋅ δ Δd3(3)Sample data for beam position jitter measurement is seenin Fig. 13. The bunch charge during this measurement
was 180pC. The RFFE gain settings were chosen toprovide a linear measurement range of +/-500ȝm.

Figure 13: Sample position measurement of BPMs
(180 pC bunch charge). Left: position data.
Right: Histogram of the error ǻd.
Figure 11: Charge measurement example at C§210 pC.
Table 2: Measured and calculated charge resolutions(bunch charge § 210pC)
Charge resolution at 210 pC
Resolution Calc. From Q1-Q2

82

fC rmsResolution Calc. From Q1-Q3

85

fC rmsResolution Calc. From Q2-Q3

80

fC rms

Theoretical Value

70

fC rmsCalculating the distribution of the displacement ǻdresults in the histogram seen in Fig. 13 (right). Themeasurement position resolution of the BPM is 183nm.
Table 3 summarizes sensitivity measurements at various
beam conditions.
The rightmost column in Table 3 contains predictedvalues based on system noise measurement. One possible
cause for the deviation from measured values is LO phasejitter that causes an increase of the position jitter of theI/Q imbalance is not fully corrected. However, thedeviation could also be caused by any uncorrelated

ISBN 978-3-95450-119-94

Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

Table 3: Measured and predicted measurement jitter fordifferent settings (x-direction)
Beam
Offset(mm)Beam
Charge(pC)

LinearMeas.
Range

Measured
Resolution(ȝm-rms)

Predicted
Resolution(ȝm-rms)

0.1

285±2mm

0.35

0.33

0.5

285±2mm

0.40

0.36

1285±2mm

0.56

0.4

0.05

183±500ȝm

0.18

0.165

0.2

2±6.4mm11.2

13§0.06350±250ȝm

0.12(0.06)

varying

It should be noted that any positively correlated noise or
drift is not included in the measured resolution. Such adrift factor could be temperature drift of the electronics.
Temperature drift measurements under realistic beamconditions will be conducted in near future.
Measurements of the vibration levels at the pickups arealso planned in order to determine the electronicresolution more precisely.

AKNOWLEDGEMENT
The authors wish to thank their colleague Daniel M.
Treyer for valuable discussions and review of the RF
circuit design.

REFERENCES[1] R. Brinkmann, “The European XFEL Project”, Proc.
FEL´06, Berlin, Germany, 2006.
[2] B. Keil, et al.: ”The European XFEL Beam Position
Monitor System”, Proc. IPAC, Kyoto, Japan, 2010[3] B. Keil et al., “A Generic BPM Electronics Platformfor European XFEL, SwissFEL and SLS”, theseproceedings[4] M. Pedrozzi, “250 MeV Injector Facility for theSwissFEL Project”, Proc. of FEL2009, Liverpool,UK, 2009[5] D. Lipka, et al.: “Cavity BPM Designs, relatedElectronics and Measured Performances”, Proc.
DIPAC’09, Basel, Switzerland, 2009.
[6] H. Maesaka, et al., “Development of the RF Cavity
BPM of XFEL/Spring-8”, Proc. of DIPAC09, Basel,Switzerland, 2009.
[7] B. Keil, et al.: “Design Status of the European X-FELTransverse Intra Bunch Train Feedback”, theseproceedings

CONCLUSION
We reported first tests of the EXFEL cavity BPMelectronics. The tests were performed at the SwissFEL
injector facility under single-bunch conditions. Firstresults show beam position resolution well below 1ȝm
rms even at a linear measurement range >±1mm. These
results fulfill the resolution requirements for the EXFEL
undulator BPMs.

ISBN 978-3-95450-119-9
Beam Position Monitor System

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○mechanical vibrations of the three pickups (also includedin the measured resolution above).

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