Proceedings of IBIC2012, Tsukuba, Japan

MOPA35

DESIGN STATUS OF THE EUROPEAN X-FEL TRANVERSE INTRABUNCH TRAIN FEEDBACK
Boris Keil, Raphael Baldinger, Carl David Beard, Micha Markus Dehler, Waldemar Koprek, GoranMarinkovic, Markus Roggli, Martin Rohrer, Markus Stadler, Daniel Marco Treyer,
PSI, Villigen, Switzerland
Vladimir Balandin, Winfried Decking, Nina Golubeva, DESY, Hamburg, Germany

The European X-Ray Free Electron Laser (E-XFEL)[1] will have a fast transverse intra-bunch train feedback(IBFB) system [2] to stabilize the beam position in the
SASE undulators. E-XFEL bunch trains consist of up to2700 bunches with a minimum bunch spacing of 222ns
and typ. 10Hz train repetition rate. The IBFB willmeasure the positions of each bunch in the bunch train,
and apply intra-train feedback corrections with fastkickers, in addition to a feed-forward correction forreproducible trajectory perturbations. By achieving a
feedback loop latency in the order of one microsecond,the IBFB will allow the beam position to converge
quickly to the nominal orbit as required for stable SASE
operation. The latest conceptual design of the IBFB andthe status of IBFB components will be presented.

INTRODUCTION
The E-XFEL has a superconducting 17.5GeV main
linac, with 0.1-1nC nominal bunch charge, and N·111ns
bunch spacing, where N is an integer > 1. One distinct
feature of the accelerator is its ability to generate bunchtrains of up to 600μs length with arbitrary bunch patterns
for the SASE undulators, where different parts of thesame bunch train can be distributed to different undulatorlines by means of a beam distribution system [3].

Transverse Beam Stability
In order to achieve sufficient and reproducible intensityand pointing stability of the X-ray photon pulses
generated in the E-XFEL SASE undulators, the electron
beam should deviate less than ~σ/10 from its nominal(ideally straight) trajectory in the undulators, with typicalbeam sizes of σ=30μm or less depending on beam charge

and resulting emittance. However, due to a number oftransverse perturbations sources, deviations of more than~σ/10 from this trajectory are expected to occur without
operational IBFB. Perturbations that are random, i.e. not
reproducible, will be corrected by a fast intra bunch train
feedback (IBFB) system can measure and correct the
trajectory individually for each bunch. In addition, for
perturbations that are reproducible from bunch train tobunch train (or change sufficiently slow) the IBFB will
apply a static (or adaptive) feed-forward correction.

Perturbation Sources, Frequencies, andFeedback Loop Latency
Table 1 shows the presently expected main horizontal(X) and vertical (Y) perturbation sources, their estimatedworst-case peak amplitudes and necessary correction
kicks [4], normalized to 30m beta function both at thelocation of position measurement and of the kicker. Sinceno significant random perturbations with very highfrequencies are expected, we aim for a feedback loop
latency of <1.5μs, allowing to correct non-reproducible
perturbations up to a maximum (0dB) frequency of~70kHz. Although a lower latency is possible, we favour
a latency that is somewhat larger that the technicallyfeasible minimum value, because this allows to use e.g.
ADCs with higher resolution (having higher latency) for
the BPMs, or more advanced FPGA algorithms to correct
BPM RF front-end IQ imbalance and X/Y-coupling, thus
reducing BPM-noise dominated perturbations that the
IBFB adds to the beam. Since the IBFB kickers can apply
arbitrary individual kicks for each bunch, the additional
feed-forward corrections applied by the IBFB allow tocorrect reproducible perturbations of any frequency fromseveral MHz down to DC within the available kick range.

Table 1: E-XFEL beam trajectory perturbation sources, estimated worst-case peak amplitudes, and frequencies
X
[μm]

Y
[μm]

Frequency
[kHz]

Plane

PerturbationType

Kick(X)[μrad]

Kick(Y)[μrad]

Magnet Vibrations
Power Supply Noise
Vibration-Induced Dispersion Jitter
Beam Distribution Kicker Drift
Beam Distribution Kicker Noise
Spurious Dispersion (3% Energy Chirp)
Nonlinear Dispersion (3% Energy Chirp)
Spurious Dispersion (1E-4 Energy Jitter)
Nonlinear Dispersion (1E-4 Energy Jitter)
Wakefields

±28
±12.6
±2.5
±0
±0
±15
±15
±0.5
±0.15
±25

±28
±12.6
±2.5
±1
±1
±15
±0
±0.5
±0
±25

<1
<1
<1
<1
<5000
<1
<1
<5000
<5000
<5000

X/Y
X/Y
X/Y
Y
Y
X/Y
X
X/Y
X
X/Y

RandomRandomRandomRepetitive
RandomRepetitive
Repetitive
RandomRandomRepetitive

±1.0
±0.5
±0.1
±0
±0
±0.5
±0.5
±0.02
±0.005
±0.9

±1.0
±0.5
±0.1
±0.04
±0.04
±0.5
±0
±0.02
±0
±0.9

Sum Of Peak Values

±98.8

±85.6

±3.5

±3.1

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPA35

Proceedings of IBIC2012, Tsukuba, Japan

Quadrupole Magnet Vibrations
One source of random perturbations where the expectedbeam movement is significant is the vibration ofquadrupole magnets in the cryostats of the linac, where
the comparatively complex mechanical support structure
is naturally not as stable as for normally conducting linacs
where quadrupoles sit on simpler massive girders. Since
the typical oscillation period of magnet vibrations isrelatively high (~10-100ms) compared to a bunch train
length of max. 0.6ms, the vibrations cause an overall
trajectory perturbation that varies very slowly along thebunch train, with nearly the same perturbation amplitude
and angle for adjacent bunches. Vibrations are therefore –despite their low perturbation frequency – one reason why
a low-latency IBFB is required: When the IBFB detects
the perturbation at the beginning of the bunch train andthen starts applying corrections so that the following
bunches converge towards their nominal orbit, theconvergence time (in the order of 10μs or less, dependingon perturbation amplitude) scales with the IBFB latency.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Dumping of Not Corrected Bunches
The IBFB as well as other beam based feedback
systems in E-XFEL like the longitudinal/LLRF feedback
may need a number of bunches at the head of the bunchtrain before their corrections become fully effective andreach the desired stability level. The unstable bunches at
the head of the train can be dumped by the beam
distribution system upstream of the SASE undulators, sothat only stabilized bunches reach the undulators. Due toa maximum bunch train length of 600μs, dumping
bunches at the head of the train for a duration of ~10μs
has little impact on the achievable average X-ray flux.

IBFB System Layout
Figure 1 shows the location and topology of the IBFB.
The main IBFB components are located just after thecollimator, upstream of the beam distribution system andits dump kicker.

for each transverse plane. In the standard operation modeof the IBFB, two adjacent BPMs downstream of the
kickers (“downstream BPMs”) send their position data via
multi-gigabit fiber optic links to an FPGA signal
processing board (originally named “PDC” = PSI DSPCarrier [2]) that performs the feedback and feed-forward
algorithms. The PDC calculates the kick angles, using amezzanine with fast (>500MSPS) DACs to directly
generate the stripline kicker signal waveforms via FPGAbased direct digital synthesis (DDS). The DAC signals areamplified via kW-range solid state low-latency RF power
amplifiers that drive the stripline kickers.
Two BPMs upstream of the kickers (“upstream BPMs”)
are used to monitor if the obtained kick angle has thedesired value, by comparing the predicted beam positionat the downstream BPMs with the measured one for eachbunch. This allows e.g. detection of IBFB system failures,or in-situ calibration and correction of nonlinearities andgain drift of the RF power amplifiers.
In addition to the above described standard operationmode, the IBFB can be operated in an alternative mode
where the upstream BPMs instead of the downstream
BPMs are used for the fast feedback loop. This mode
employs a model-based prediction of the necessary
correction kicks, where a comparison of expected andmeasured beam position at the downstream BPMs allows
to correct and adapt the model in real-time.
Compared to the standard mode, the alternative mode
reduces the feedback loop latency, and also the noise that
the IBFB adds to the beam, since this noise is not seen
and coupled back by the BPMs into the feedback loop
because they are upstream of the kickers.
However, for reasons of simplicity and robustness, anddue to moderate latency requirements and good BPM
noise levels of our present BPM prototypes, we favour the
standard mode for commissioning and first operation of
the E-XFEL and IBFB, where changes of energy andoptics that would affect the model and beam response
matrix may occur more frequently during tuning and testsof the accelerator and its subsystems.

Integration of Undulator BPMs

Figure 1: IBFB System Topology.

IBFB Operation Modes
The IBFB allows to correct the beam position and angle
individually for each bunch, using two stripline kickers

In order to be able to correct perturbations that occur
between the IBFB and the undulators, the IBFB also
receives the data of all undulator BPMs via multi-gigabit
fiber optic links, where the BPMs of each undulator areconnected in a bidirectional daisy chain.
For correction of non-reproducible perturbations
between IBFB and undulators, at least two undulator
BPMs will be equipped with the low-latency electronics,
since the latency requirement for the standard BPM
electronics is only <10ms. Due to a distance of several
100m between IBFB and undulators, the latency of theundulator BPM data received by the IBFB is several μs.
The E-XFEL beam distribution system [3] has a fast
dump kicker that can generate arbitrary bunch patterns forthe undulators. This kicker also dumps the beam while aslower but stronger kicker (based on a Lambertson
septum and a flat top pulser kicker) changes its field toISBN 978-3-95450-119-9
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Feedbacks and Beam Stability

Proceedings of IBIC2012, Tsukuba, Japan

Beam Optics, Magnet Lattice and BPM
LocationsFigure 2 shows the magnet lattice and beam optics inthe area of the IBFB. The end of the collimator is nears=0. The two vertical IBFB kickers are located at s=27
and s=57m, the horizontal ones at s=38m and s=68m.
IBFB Kickers and BPMs were placed at locations withlarge beta functions, and the betatron phases between two
kickers and two upstream and downstream BPMs were
each optimized to be as close as possible to an odd integer
multiple of 90° within the given optics and lattice design
constraints. This maximizes the resolution of the BPMs
and thus minimizes the BPM-dominated noise that the
IBFB adds to the beam.

Figure 2: Beta Functions and Betatron Phase in IBFB
Area.
Moreover, the optimized kicker locations minimize theRF amplifier power necessary to correct a given
perturbation, thus reducing amplifier costs andmaximizing the mean time between failures (MTBF) of
the amplifiers that improves with lower power.

IBFB SUBSYSTEMSIn the following, the concept and design status of thedifferent IBFB subsystems will be reported.

BPMs
The E-XFEL uses button as well as some re-entrant
single resonator cavity BPMs in the cold linac, and buttonand dual-resonator cavity BPMs in the warm part of theaccelerator [5]. The cost-efficient buttons are used where
their resolution is sufficient, while cavities are used where
higher resolution is required, e.g. in the E-XFEL
undulators.
IBFB upstream and downstream BPMs have the same
40.5mm aperture 3.3GHz cavity BPM pickups [6] as thenon-IBFB BPMs elsewhere in the accelerator. However,
the requirements for the BPMs used by the fast IBFB
feedback loop are more demanding than for the other
BPMs. On one hand, the IBFB should have a BPM
system latency of a few 100ns, while a latency of <10ms
is sufficient for non-IBFB BPMs that are used e.g. forslow global feedbacks or beam based alignment.
Moreover, noise of the single-bunch position
measurements of the IBFB BPMs will be added to thebeam by the feedback loop, with a gain depending on
IBFB algorithm, feedback loop parameter settings etc.
Thus, IBFB BPMs should ideally have sub-micron noisefor single-bunch position measurements over the wholebunch charge range. In contrast, non-IBFB applications,e.g. beam-based alignment of undulator quadrupoles and
BPMs where sub-micron resolution is also needed, canuse the average position of all bunches in a train, where
the alignment can be performed at high bunch charge
where the BPMs have their best resolution.
The present prototype version of the standard cavity
BPM electronics provides sub-micron resolution, where
beam tests with three cavity BPMs showed e.g. 120nm
RMS at 350pC and 180nm at 183pC bunch charge [7],with an electronics latency of ~400ns (including
calculation of beam position in an FPGA).

Figure 3: Left: E-XFEL cavity BPM electronics prototype(standalone unit for two cavity BPMs). Right: Cavity
BPM pickup test area at SwissFEL Injector Test Facility,with three E-XFEL undulator cavity BPMs and one IBFB
cavity BPM.
Although these results were obtained by correlating
beam measurements of three adjacent 10mm aperture
undulator BPM cavity pickups, the performance for the
40.5mm aperture IBFB upstream and downstream BPM
cavity pickup (to be tested in the near future) should be
similar, since its sensitivity (in units of V/mm/nC) is onlyISBN 978-3-95450-119-9
Feedbacks and Beam Stability

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○redirect part of a bunch train to another undulator [3]. Dueto this concept, the high-frequent trajectory perturbations
generated by the distribution system should be negligible.
Its low-frequent perturbations e.g. due to capacitor bankdroop, are minimized by suitable design techniques andare expected to be mainly reproducible.
Therefore, the only expected source of significant nonreproducible perturbations between IBFB and undulators
are mechanical magnet vibrations that have low
frequencies (typ. 10-100Hz), with a perturbation
amplitude that should be low compared to main linac
quadrupole vibrations. Thus, the higher latency of theundulator BPM data is uncritical: For the first bunches ina train, the IBFB will only use the downstream (and/or
upstream) BPM data to correct the trajectory, in addition
to the previously mentioned feed-forward corrections. As
soon as the IBFB receives the first readings from theundulator BPMs, it performs additional fine tuning of thebeam trajectory so that the following bunches shouldreach the desired position in the undulators.

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c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○MOPA35

Proceedings of IBIC2012, Tsukuba, Japan~20% lower and since 10mm and 40.5mm version havethe same loaded Q (~70) and frequency (3.3GHz).
Due to the different requirements for standard and
IBFB BPM electronics, we had originally intended todevelop a dedicated electronics version for the IBFB
BPMs. However, the low latency and good resolution that
we achieved with the standard BPM electronics motivatesto have a common design for IBFB and non-IBFB BPMs.
Therefore the next undulator BPM electronics prototype
version will have improvements beneficial for the IBFB,
e.g. programmable attenuators for the pickup signals at
the RFFE inputs, with >60dB range and 0.5dB steps,
compared to four attenuator/charge ranges with ~6dB
steps for the present RFFE version. The new RFFE willthus have a significantly reduced dependence of theresolution on the bunch charge compared to the present
electronics, where the ADC noise at the lower end of eachcharge range deteriorates the resolution since less than
50% of the ADC full scale range is used at these charges.
However, lab and beam tests will have to show if we
can use the new version for all cavity BPMs in E-XFEL,
including IBFB, or if we still need to develop a dedicated
IBFB version, e.g. due to the fact that the largest possible
signal level of the IBFB pickup is 50V, while a desired
resolution and drift of 1μm at a desired lowest bunchcharge of 20pC corresponds to a signal level of 50μV,
which makes the design of the input stage of the RFFE
rather challenging.

Signal Processing Electronics and Algorithms
During the preparatory phase of the E-XFEL, starting
2005, we had already developed a prototype for an IBFB
signal processing board, consisting of an FPGA Carrier
Board (PDC = PSI DSP Carrier) with two mezzaninesthat each have four 12-bit 500MSPS ADCs and two 16bit 1GSPS DACs. While we intend to keep the general
concept of the PDC board for the final IBFB version, weare currently developing a new carrier board [8] for which
we use Xilinx Artix-7 and Kintex-7 (28nm silicon
structure size) instead of Virtex-4 (90nm) FPGAs, with a
new DSP that is available with one or multiple cores and
20 GFLOPS per core, compared to 3+3 GFLOPS for the
DSPs on the previous PDC board version.
SFPSFP

D D R 3 4.5x 16x2 56 M b

D D R 3 4 .5x1 6 x2 56 M b

BPM FPG A 2
X ilin x A rtix
XC7A

BPM FPGA 1
X ilin x A rtix
XC7A

M e z za n in e 2

M e zz a n in e 1

ETH

Figure 4 shows the simplified block schematics of the
new board. For the data transfer between FPGAs, DSPs
and high-speed IO connectors (new VME-P0 with
>6Gbps/pin, SFP, mezzanines) we use mainly serial
multi-gigabit links (5-10Gbps). The board also has has
multiple parallel LVDS high-speed (~1Gbps) lines to two
mezzanines, for interfacing up to six parallel 16-bit ADCs
or DACs per mezzanine to so-called “BPM” FPGAs on
the mainboard. The “COM” FPGA has single-ended IOs
to the VME-P2 connector for slower (<100Mbps) control
and status signals from RFFEs or kicker amplifiers. 10G
Ethernet is supported via SFP transceivers and VME-P0.

Figure 5: IBFB Electronics / FPGA Board Topology.
By replacing the parallel DSP address/data bus of theprevious carrier board version with a few serial multigigabit links (using PCIe and SerialRapid IO), thecomplexity of the layout and thus the board price ismassively reduced. Therefore the new board may not only
be used as signal processing board for the IBFB
algorithms, but also as successor of our present “GPAC”(generic PSI ADC Carrier) board that serves as digitalback-end for the present E-XFEL BPM prototypes.

Debug

T I D S P F a m ily
T M S 32 0 C 6 6 x x
D D R 3 2 .5 x 16x 2 56 M b

CO M FPG A
X ilin x K in te x
XC7K

S y s te m F P G A
X ilin x K in te x
XC7K

C o n fig
SD CARDCLOCK

VME P2V M E -P 0

V M E 6 4x B u s

Figure 4: Block Schematics of new FPGA Carrier Board.

Figure 6: IBFB Feedback/Feedforward FPGA Firmware
Block Schematics.
For the IBFB BPMs, we plan to use the same 6-channel
16-bit ADC mezzanine as for the standard cavity BPMs.
For control of the kickers, a new DAC mezzanine with 4channels, 16 bit resolution and >500MSPS sample rate for
FPGA-based direct digital synthesis (DDC) of the kicker

ISBN 978-3-95450-119-9
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Feedbacks and Beam Stability

Proceedings of IBIC2012, Tsukuba, Japan

Kicker Magnets
During the E-XFEL preparatory phase [2], 1m long
stripline kicker prototypes have been developed [9]. For
the final IBFB system, we are currently designing 2mlong versions. The main differences of the new version to
the previous one – in addition to the length - are the morerobust mechanical construction that avoids sensitive
ceramic spacers to hold the strips, as well as tapered RF
ports to minimize VSWR. In order to get an effective
kicker length of 4m for each of the two IBFB kickers perplane, we intend to chain two 2m kickers via RF cables,with a quadrupole in between them. The resulting overall
kicker length of 4m allows to reduce the amplifier RF
power, thus further improving MTBF and minimizing
costs. An RF design and a conceptual mechanical designfor the new kicker have already been made (see Figure 7).
The detailed construction and production of a prototype isscheduled for the next months.

Figure 7: Latest IBFB Kicker Magnet Design (L=2m).

RF Power Amplifiers
The IBFB kickers will be driven by commercial pulsed
solid-state RF power amplifiers, where two amplifiers
drive opposite striplines of each kicker in push-pull mode.
We recently ordered prototypes with a saturated power of
3kW and a linear power (1dB compression) of 2kW.
More than ±8μrad kick angle at 3kW provides ample
safety margin with respect to the estimated perturbations(see Table 1), allowing to operate the amplifier well
below the saturation point in its linear regime. Anyremaining nonlinearities will be calibrated and correcteddigitally by the IBFB FPGA board.
The RF power amplifier is based on an off-the-shelf
version (shown in Figure 8) but is currently being
improved for PSI, e.g. by adding redundant primary(AC/DC) power supplies and redundant RF amplifier
modules in order to improve MTBF, and by using newer
as well as more robust RF components that tolerate 100%
reflection at the amplifier output at full power. The
maximum pulse length is 1ms, with up to 3% duty cycleand <50ns latency (input to output). A bandwidth of atleast 10-100MHz will allow to use DC-free sinusoidal orrectangular amplitude-modulated waveforms up to some
10MHz frequency to drive the kickers. The signal
frequency (integer multiple of the bunch rep. rate), phaseand chain cable length for the 2m elements is chosen sothat the beam sees just the positive (or negative) halfwaves of the stripline signals when passing the kickers. It
should be noted that, despite a lower amplifier bandwidthof ~10MHz, the kickers can correct orbit perturbations
down to DC: The kicker waveform is DC-free, but thebunches just see a fraction of the waveform that is not
DC-free and thus can be corrected down to DC.

Figure 8: Left: Prototype RF power amplifier, similar tothe dedicated IBFB version that is in production (4kW, 5175MHz BW, <30ns latency). Right: Amplifier latency
measurement (courtesy TOMCO Technologies, horizontal
scale 20-80MHz, vertical scale 5-55ns, latency <30ns).

STATUS AND OUTLOOKThe development of IBFB concept and subsystems at
PSI is progressing from prototypes towards pre-series
versions and the final design. We are currently merging
initially separate designs for IBFB and general BPM
system where possible, aiming at electronics modules thatmeet the more demanding latency and performancerequirements of the IBFB while being cost-effective
enough for large-scale use in the E-XFEL BPM system.
The IBFB will use cavity BPMs where beam tests havealready demonstrated sub-micron resolution. The next
electronics revision that is currently being developed willfurther minimize BPM-dominated noise added to thebeam by the IBFB, using the latest FPGA and DSP
technology. For the IBFB kicker system, prototype
production and test are planned for 2013, while IBFB
beam commissioning is scheduled for autumn 2015.

REFERENCES[1] R. Brinkmann, “The European XFEL Project”,
Proceedings FEL´06, Berlin, Germany, 2006.
[2] B. Keil et al., “Design of an Intra Bunch Train Feedback
System For The European XFEL”, Proceedings DIPAC’07,
p. 232.
[3] W. Decking et al., “Layout of the Beam Switchyard at theEuropean XFEL”, Proceedings EPAC’08, p. 2163.
[4] W. Decking, private communications.
[5] B. Keil et al., “The European XFEL Beam Position
Monitor System”, Proceedings IPAC’10, p. 1125.
[6] D. Lipka et al., “Development of Cavity BPM for theEuropean XFEL”, Proceedings Linac’10, p. 631.
[7] M. Stadler et al., “Beam Test Results of Undulator Cavity
BPM Electronics For European XFEL”, these proceedings.
[8] B. Keil et al., “A Generic BPM Electronics Platform for
European XFEL, SwissFEL and SLS”, these proceedings.
[9] M. Dehler et al., “Stripline Devices for FLASH andEuropean XFEL”, Proceedings BIW’08, p. 114.

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○waveforms will be developed. Figure 5 shows the
hardware layout and interconnections of the IBFB
electronics. Figure 6 contains the block schematics of the
feedback/feed-forward algorithm to be implemented on
the IBFB FPGA carrier board.

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