Proceedings of IBIC2012, Tsukuba, Japan

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HOM CHOICE STUDY WITH TEST ELECTRONICS
FOR USE AS BEAM POSITION DIAGNOSTICS
IN 3.9 GHZ ACCELERATING CAVITIES IN FLASH§
Nicoleta Baboi#, Bastian Lorbeer, DESY, Hamburg, Germany
Pei Zhang, DESY, Hamburg, Germany; UMAN, Manchester, UK
Nathan Eddy, Brian Fellenz, Manfred Wendt, Fermilab, Batavia, USA
Higher Order Modes (HOM) excited by the beam in the
3.9 GHz accelerating cavities in FLASH can be used forbeam position diagnostics, as in a cavity beam position
monitor. Previous studies of the modal choices within thecomplicated spectrum have revealed several options:cavity modes with strong coupling to the beam, andtherefore with the potential for better position resolution,
but which are propagating within all 4 cavities, andmodes localized in the cavities or the beam pipes, which
can give localized position information, but which
provide worse resolution. For a better characterization ofthese options, a set of test electronics has been built,
which can down-convert various frequencies between
about 4 and 9 GHz to 70 MHz. The performance ofvarious 20 MHz bands has been estimated. The best
resolution of 20 m was found for some propagating
modes. Based on this study one band at ca. 5 GHz waschosen for high resolution position monitoring and a bandat ca. 9 GHz for localized monitoring.

HOM spectrum suitable for beam position monitoring[6,7]. The studies culminated with the examination of thepotential of each region with a set of test electronics [8],which makes the subject of this paper. As a result, thespecifications for the final electronics have been defined.

FLASH
FLASH [4] is a Free Electron Laser facility generating
short laser-like pulses with a wavelength between about 4
and 45 nm. It is also a test facility for the European X-ray
FEL and the International Linear Collider. The first partof the linac is shown in Fig. 1. The photo-electric gun
generates electron bunch trains with an energy of 5 MeV.
These are accelerated by the first cryo-module, ACC1,
containing 8 TESLA cavities. The subsequent cryomodule ACC39 is used to linearize the energy spreadalong the bunch generated by the non-linear accelerating
field in ACC1 [9].

INTRODUCTION
Higher Order Mode Beam Position Monitors (HOMBPM) are devices that can be used to center the beam in
accelerating cavities [1,2]. Since their principle relies onmonitoring beam excited dipole modes, which are themain transverse component of the potentially damaging
wakefields [3], they can help improve the quality of thecharged particle beam. Moreover, they can be calibrated
in terms of beam offsets, resembling a classical BPM.
HOM-BPMs have been built for the TESLA 1.3 GHz
cavities at FLASH [1,4] and are routinely used forcentering the beam. We are planning to build similar
monitors for the 3.9 GHz cavities [5] in the same facility,
often referred to as third harmonic cavities. The
implementation is however much more complicated thanin TESLA cavities for various reasons, briefly reviewed in
the next section. Extensive studies have been made withthe aim of defining the specifications of the HOMBPMs.
The studies started with transmission measurements ineach of the 4 cavities before and after installation in the
cryo-module [6]. They continued with the dependencies
of dipole modes on the transverse beam position, which
had as result the identification of several regions in the___________________________________________§Work supported in part by the European Commission under the FP7
Research Infrastructures grant agreement No.227579#nicoleta.baboi@desy.de

Figure 1: Schematics of the FLASH injector section.

The Third Harmonic Cavities
Four 3rd harmonics cavities are installed in the ACC39
cryo-module (Fig. 2). They are denoted with C1 to C4, inthe beam direction. There are 9-cells per cavity. Eachcavity is equipped with an input coupler and 2 HOM
couplers, placed in the connecting beam-pipes, to extractenergy from the beam excited HOMs and thus reduce
their effect on the beam quality.

Figure 2: The four cavities in the ACC39 cryo-module.
C2H1 means cavity 2, HOM-coupler 1.
The cavity design is inherited from the TESLA cavity.
Figure 3 shows a picture of a 3.9 GHz cavity compared to a
TESLA cavity. The main difference is the beam pipe
diameter, which is larger than one third of the beam pipein the TESLA cavity. This enables all HOMs to propagate
along the module and therefore be damped by the
couplers of all cavities.

ISBN 978-3-95450-119-9
Beam Position Monitor System1

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Copyright ○Abstract

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Proceedings of IBIC2012, Tsukuba, Japan
former requires careful electronics design and the latter
will impact the position resolution of the HOM-BPM.

Figure 3: Picture of a 3.9 GHz cavity (down) and a
1.3 GHz cavity (up).

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The Principle of HOM-BPMs
Dipole modes are the main component of transverse
HOMs and therefore have the highest damaging potential.
Their strength depends linearly on the beam offset fromthe cavity center and therefore can be used for beamalignment and the monitoring of transverse beam
positions, like classical cavity BPMs.
HOM-BPMs have been previously built for TESLA
cavities at FLASH [1]. In that case, a dipole mode at
ca. 1.7 GHz, one of the modes in the first dipole band,
was used (see Fig. 4(a)). This mode has a high R/Q [10], aparameter which indicates the interaction strength of themode with the beam. Thus a high R/Q has the potential
for better position resolution.
Dipole modes in 3rd harmonic cavities are however
overlapping with each other and are not easy to separate(see Fig. 4(b)). This is mainly due to the larger beam pipe,
which makes the modes couple among cavities. This fact
makes the beam position unable to be determined in eachcavity, but only for the whole module. This is also true forthe high R/Q modes.
Some modes in the fifth dipole band are found to be
trapped inside each cavity [11,12] and could be used for
local position diagnostics. However, these modes arelocated in an upper frequency range (ca. 9 GHz) and have
only weak coupling to the beam (small R/Q values). The

Figure 4: Beam-excited dipole modes in the 1st dipole
band of a TESLA cavity and a 3rd harmonic cavity.
Since we cannot separate single modes, for the 3rd
harmonic cavities multiple modes are used.. Initially thisseemed to be a large disadvantage, but it may have somebenefit. Using more modes means more information andhas the potential for more resolution.

MEASUREMENTS WITH
THE TEST ELECTRONICS
Results from Previous Studies
Along with extensive simulations [11-14], spectra
measurements have been made with network andspectrum analyzers [6]. A beam-excited spectrum
measured with a real-time spectrum analyzer (RSA) at
coupler C1H1 is shown in Fig. 5 together with simulation
results. The first two cavity dipole bands are between
about 4.2 and 5.5 GHz, while the trapped cavity modes in
the fifth dipole band are between 9.03 and 9.08 GHz.

Figure 5: Beam-excited spectrum measured at coupler C1H1. Every 50 MHz from 4 GHz to 9.5 GHz was excited by asingle electron bunch. The vertical lines indicate simulation results of an ideal cavity [11].

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

Proceedings of IBIC2012, Tsukuba, Japan

Mode
type

Freq.
(GHz)

Advantages

Disadvantages

Beampipe~ 4.1

Localized

Low R/Q;
Not cavity-based

Cavity/1st & 2ndband

4.2-5.5

High R/Q

Not localized

Cavity/5th band9.059.08

Localized
Cavity-based

Low R/Q

Note that in our previous studies we could not study the
resolution achievable with the various options, due to thelimitations of the devices we used: oscilloscope and RSA.
Therefore, in order to be able to make the final choice for
the HOM-BPM electronics, flexible test electronics havebeen built.

Setup
The test electronics was designed to have the flexibility
to study the various modal options of interest as well as
accommodate the large mode bandwidths (BW) of these
options. Its simplified block diagram is shown in Fig. 6.
One of four different analog bandpass filters (BPF) can beconnected into the chain to study the localized dipole
beam-pipe modes at ca. 4.1 GHz, strong multi-cavity
modes in the first dipole band at ca. 4.9 GHz and in thesecond dipole band at ca. 5.4 GHz, and the trapped cavity
modes in the fifth dipole band at ca. 9 GHz. Afterfiltering, the signal is mixed with a selectable local
oscillator (LO) to an intermediate frequency (IF) of
ca. 70 MHz. Then the IF signal is further filtered with a
20 MHz analog BPF to select the specific band of modes.
In order to ensure that the possible remaining highfrequency components of the IF signal generated during
the mixing step are well suppressed, a lowpass filter(LPF) is applied to preserve only the frequencies below
105 MHz with a dominant component from 60 to
80 MHz. In Fig. 6, the “INPUT” is connected to the HOM
signal through a multiplexer for easy switching between
couplers. The 70 MHz IF out of the analog box is further
split into two signals. One is digitized by a VME digitizer

operating at 216 MS/s with 14 bit resolution along with aprogrammable FPGA for signal processing. The other IF
signal is processed by a µTCA digitizer SIS8300 [15,16].
The VME digitizer is triggered by a 10 Hz FLASH beam
trigger. Both the selectable LO and the digitizer clock arelocked to the FLASH accelerator by using RF signalsdelivered from the master oscillator as a reference. This
locking allows correct phase information of the digitizedsignal.

Figure 6: Schematics of the test electronics. One of thefour BPFs was connected in front of the mixer during
each measurement.
All the devices were set up outside the FLASH tunnel.
Digitized data was collected from the VME digitizer withan EPICS software tool, while the beam charge, steerer
current and BPM readouts were recorded synchronously
from FLASH control system DOOCS. The dataprocessing for position diagnostics is conducted offline
using MATLAB.
This paper cites primarily the results from the VME
digitizer. Results obtained with the TCA digitizer can befound in Ref. [15].
An example of a signal measured with the VME
digitizer for the input filter with a center frequency of
5.437 GHz and its Fourier transform are shown in Fig. 7.

Figure 7: An example output signal (a) and its Fouriertransform (b) measured from coupler C3H2 for a center
frequency of 5.437 GHz.

ISBN 978-3-95450-119-9
Beam Position Monitor System3

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Studies on the behavior of the dipole modes on the
beam position enabled finding of three regions of interestin the HOM spectrum promising for diagnostics, assummarized in Table 1 [2]. Beam-pipe dipole modes are
localized between cavities and could deliver localized
beam position estimations. However they have low R/Q’s
and are not cavity-based, therefore not relevant for beamalignment and the reduction of HOM effects throughbeam alignment. The first two cavity dipole bands containmodes with high R/Q’s and thus have the potential forhigh resolutions, but they can only offer a measurement
for the whole module. In the fifth dipole band there aretrapped modes due to the geometry of end-cells and canprovide a localized cavity-based beam measurement, butwith low resolutions.
Table 1: Modal Options for HOM-BPM

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Proceedings of IBIC2012, Tsukuba, Japan

Measurements
The HOM frequency regions to be studied have been
selected based on the previous measurements andsimulation results as summarized in Table 2. The
bandwidth was 20 MHz. A couple of tests with abandwidth of 100 MHz have been made as well, with lesspromising results.
Table 2: Modal Options Selected from Previous Studies
Mode type

Centre frequency (MHz)Beam pipe

4082, 4118

st

1 dipole band

4859, 4904, 4940

2nd dipole band

5437, 5464, 5482

thc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○5 dipole band

9048, 9066

The schematic of the measurement setup is shown in
Fig. 8. An electron bunch of approximately 0.5 nC was
accelerated on-crest to ca. 150 MeV by ACC1 before
entering the ACC39 module. Two steering magnets
located upstream of ACC1 were used to produce
horizontal and vertical offsets of the electron bunch in
ACC39. Two beam position monitors (BPM-A and BPMB) were used to record transverse beam positions before
and after ACC39. By switching off the accelerating fieldin ACC39 and all quadrupoles nearby, a straight line
trajectory of the electron bunch was produced between
those two BPMs. Therefore, the transverse offset of theelectron bunch in the module can be determined byinterpolating the readouts of the two BPMs.

Figure 8: Schematic of measurement (not to scale; ACC1
is larger than ACC39).
In order to study the position dependencies of HOMs,
we moved the beam in a 2D grid manner. Figure 9(a) shows
the steerer current during the scan, while the readouts ofthe two BPMs are shown in Fig. 9(b) and Fig. 9(c). The
samples in blue are used for calibration, and thenvalidation samples were taken as shown in red to estimate
the performance. The integrated HOM power over thefrequency range shown in Fig. 7(b) is calculated for eachbeam position. Figure 9(d) shows the integrated power
distribution measured from coupler C3H2. The position,
which has minimum integrated power, is marked withwhite pentagon.

Figure 9: Steerer current (a) and readouts of BPM-A (b)and BPM-B (c) during a 2D grid-like beam scan(Calibration: blue; Validation: red). (d) Integrated power
at different beam position. The center frequency was set
to 5437 MHz with a BW of 20MHz.
The main SVD components have then be used toconstruct a “cleaner” waveform matrix, which has thenbeen correlated to the matrix of beam positions where
HOM signals have been measured. This means the centerof the cavity for the localized modes, and the center of thecryo-module for the case of propagating modes. In thisway the HOM signals are calibrated with beam offsets.
By applying this calibration to newly measured HOM
waveforms, a HOM-BPM is obtained. By comparing thereading of the HOM-BPM for the validation samples (reddots in Fig. 9) to the measurement interpolated from
BPMs, an estimation of the measurement accuracy is
made.
Figure 10(a) shows an example of the HOM-BPM
readings (red) and the interpolated positions from BPMs
(blue) when using the HOM signals shown in Fig. 7 forvalidation samples. The center frequency was 5.437 GHz,
with a BW of 20 MHz. The differences between these two
positions are shown in Fig. 10(b) as prediction errors:,
20 µm rms for x and 22 µm rms for y.

Data Analysis

Figure 10: (a) Measurements (blue) and predictions (red)of the transverse beam position for a center frequency of
5.437 GHz. (b) Prediction errors.

Singular Value Decomposition (SVD) has been applied
to the matrix consisting of HOM waveforms in order tolower the system dimension by reducing the noise, andthus retain only the relevant information related to thebeam position [7].

It is worthwhile mentioning that this RMS error does
not represent the usual resolution. We obtained smaller
values when estimating this error for a smaller scan rangeof beam offsets. We expect to get even better results whenmeasuring on beam jitter.

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4Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

Localized Modes
Beam pipe modes
For the beam pipe modes, the first filter in Fig. 6 wasused. Only the TCA-based digitizer was available at thetime. The minimum number of SVD modes was chosenthat leads to a smaller RMS error and it differs for thevarious couplers between 5 and 13. The RMS error
obtained for the second frequency range, centered at
4.118 GHz, varies from coupler to coupler and is between
40 and 100 m for the horizontal plane and between 80
and 180 m for the vertical plane. The large error values,
particularly for the y plane led to the elimination of thisoption early in the study.
Trapped cavity modes
The fourth filter, centered at 9.085 GHz, was used totest the trapped modes. An ideal mathematical filter cut
the propagating modes above 9.08 GHz. Similar results
were obtained for the two frequency regions in the 5th
dipole band tested. RMS errors between 40 and 100 mwere obtained for all couplers and both transverse planes.
7 to 15 SVD modes were used. In spite of the large
relatively large errors, we kept this option, since it candeliver localized, cavity based measurements. Also, we
expect the resolution with the final electronics to bebetter.

Propagating Cavity Modes
Surprisingly, the tested frequencies in the 1st dipole
band performed slightly worse than the 2nd band. The best
results obtained were for a center frequency of
5.437 GHz. This may be due to the presence of high R/Q
modes in this range, as predicted by simulations. The
RMS errors are between 20 and 30 m for the x plane andbetween 20 and 50 m for the y plane (Fig. 11). Between
5 and 15 SVD modes were used. 10 - 20 m rms is about
the resolution of the BPMs used and therefore may be alimiting factor of our measurement. Due to the higher
resolution, we chose to build an electronics version basedon the second dipole band.

Figure 11: Position prediction accuracy for each coupler.
The center frequency is 5437 MHz with a 20 MHz BW.

Specifications for HOMBPM Electronics
Based on the test electronics results, the design of the
final HOM-BPM electronics for the 3.9 GHz module has

been finalized. Two couplers will be equipped withelectronics centered at ca. 5440 MHz and a bandwidth of
100 MHz to provide high resolution positions in themodule, and six couplers with electronics centered at
ca. 9060 MHz, same bandwidth, to deliver local position
inside each cavity. Since not all modes in the fifth dipole
band are trapped [11,17], the frequency band has been
carefully chosen in order to mitigate the contaminationsof travelling modes.

CONCLUSION AND OUTLOOKDefining the frequency regions to be used for beam
monitoring was no easy task, since all frequency regionsin the spectra previously considered had advantages anddisadvantages. Only with the flexible test electronics,
which overcame the resolution limitations of the test
devices previously used, oscilloscope and real-time
spectrum analyzer, could these regions be further
differentiated, and thus the specification of the final
HOM-BPM electronics could be defined.
The HOM-BPM electronics is currently being designed
by FNAL in collaboration with DESY. First tests shouldtake place till spring 2013. The achieved resolution is
expected to be at least as good as the one achieved withthe test electronics.
Installing the electronics at FLASH is not the end of the
project, since we experienced difficulties with thecalibration stability. This may be due to phase drifts oreven to drifts of the HOM frequencies. Further studies are
required to understand this issue. Nevertheless as soon as
the electronics is installed, it will already benefit beamalignment in ACC39.

ACKNOWLEDGMENT
We are indebted to the accelerator operators and the
FLASH team, as well as to the MCS group, for theirsupport in preparing and conducting our measurements.

REFERENCES[1] S. Molloy et al., Phys. Rev. ST-AB 9, 112802 (2006).
[2] N. Baboi et al., SRF2011, MOPO060.
[3] K.L.F. Bane, SLAC-PUB-4169, 1986.
[4] S. Schreiber et al., IPAC2012, TUPPP052.
[5] J. Sekutowicz et al., TESLA-FEL 2002-05, 2002.
[6] P. Zhang et al., DESY 12-109, 2012.
[7] P. Zhang et al., Rev. Sci. Instrum. 83, 085117(2012).
[8] P. Zhang et al., DESY 12-143, 2012.
[9] K. Flöttmann et al., TESLA-FEL 2001-06, 2001.
[10] R. Wanzenberg, TESLA 2001-33, 2001.
[11] P. Zhang et al., DESY 12-101, 2012.
[12] I.R.R. Shinton et al., DESY 12-053, 2012.
[13] I.R.R. Shinton et al., IPAC2011, WEPC125.
[14] T. Flisgen et al., IPAC2011, WEPC097.
[15] T. Wamsat et al., this conference, TUPA23.
[16] B. Lorbeer et al., this conference, TUPA19.
[17] P. Zhang et al., IPAC2011, THPPA00.

ISBN 978-3-95450-119-9
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