Proceedings of IBIC2012, Tsukuba, Japan

TUPA24

DESIGN OF CAVITY BPM PICKUPS FOR SWISSFEL
Fabio Marcellini, Boris Keil, Martin Rohrer, Markus Stadler, Jerome Stettler, Daniel Marco Treyer,
PSI, Villigen, Switzerland
Dirk Lipka, Dirk Noelle, Maike Pelzer, Silke Vilcins, DESY, Hamburg, Germany
Abstract
SwissFEL is a 0.1nm hard X-ray Free Electron Laser
being built at PSI. A photocathode gun, S-band injector
and C-band linac provide 2 bunches at 28ns spacing, 10200pC charge range, 100Hz repetition rate, and 5.8GeV
maximum energy. A fast distribution kicker will provide
one bunch each to one hard X-ray and one soft X-ray
undulator line. For linac and undulators, first prototypesof dual-resonator cavity BPM pickups have been designed
and one undulator prototype has been fabricated. The
pickups were optimized for low charge and short bunch
spacing in the linac. Design considerations, simulationand first test results will be reported.

As shown in Fig. 1, each pickup consists of two cavities
having a folded shape to limit their transverse dimensions
and get a more compact device. The working modes are
TM110 (dipole) for the so called position cavity and TM010(monopole) for the reference cavity. When the beamcrosses the two cavity gaps it induces signals proportional
to the product of charge and position offset in the position
cavity, and to the charge only in the reference cavity. The
beam position is then obtained by dividing the two signal
amplitudes, which are available at the outside by meansof properly designed couplers. In particular, the position
cavity has four rectangular waveguides that couple to thedipole mode while rejecting the monopole mode thatwould otherwise limit the resolution of the electronics.

SwissFEL [1] requires three different BPM pickup
types, due to the different beam pipe apertures along themachine: 38mm in the injector, 16mm in the linac, and
8mm in the undulators. The desired BPM RMS noise anddrift is <1 µm for the undulators, <5 µm for the linac, and<10 µm for the injector, for a charge range from 10200pC. For the SwissFEL undulators, we will use dualresonator cavity BPMs due to their good achievable drift
and noise level even at low charge.
At the SwissFEL injector test facility (SITF) that is
operational since 2010, resonant striplines are used as“working-horse” BPMs, achieving 7 µm RMS noise
down to charges of 5pC [2], while a test section at the endof the machine is used for cavity BPM tests.
For the final SwissFEL machine, we plan to use cavity
BPMs in the accelerator, including injector and linac.
This will allow to have a homogeneous BPM system andto be able to use the latest electronics generation for thewhole machine, based on our developments for the EXFEL BPM system [3].
Both the RF and mechanical designs of the present
SwissFEL cavity pickup prototypes were largely inspiredby devices developed in Japan for the XFEL/Spring-8 [4]and at DESY for the E-XFEL [5]. In particular, we plan to
use the 3.3GHz working frequency of the E-XFEL
undulator BPMs also for SwissFEL injector and linac
pickups. This choice allows us to adapt our E-XFEL
cavity BPM electronics to SwissFEL linac and injector
BPM requirements with relatively small effort, thus
maximizing the synergies of the E-XFEL BPM
collaboration between PSI and DESY on this subject andminimizing costs and man power.
For the present SwissFEL BPM pickup prototypes, we
adapted the loaded quality factor (Q) and sensitivity to thelower bunch charge range (10-200pC) and bunch spacing(28ns) of SwissFEL, since the E-XFEL BPM pickups
were designed for 100-1000pC and 222ns bunch spacing.

Beam Position Monitor System

Figure 1: Cavity BPM pickup schematic view (shown:vacuum).
The waveguides are connected to the cavity volume
through slots placed 90° apart from each other on one
cavity side wall. Each waveguide has a transition to a
coaxial line which ends with a standard type-N connector
output. In the reference cavity the signal is coupled out bymeans of a coaxial line where the inner conductor passes
through the cavity (predominantly magnetic coupling).
The position and the reference cavities must have asufficiently large distance to each other to avoid crosstalk.
This distance needs to be increased with increasing
diameter of the beam pipe.
In the following sections, the undulator and linac
pickup design as well as lab measurements of the RF
characteristics of the first undulator pickup prototypes arepresented. The design of the injector BPM has not yet
started.

UNDULATOR CAVITY BPM
Design and Simulation Results
The design of the SwissFEL BPM system is based on
the E-XFEL cavity BPMs where the pickup is developedby DESY and the electronics by PSI. Since SwissFEL
ISBN 978-3-95450-119-9

1

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

Proceedings of IBIC2012, Tsukuba, Japanand waveguides, some dimensional tolerances, especiallyslot misalignments in position and angle may causemonopole mode leakage, where the amplitude of theleaking signal is proportional to the mechanical error, as
shown in the simulation results [7] plotted in Fig. 2. The
frequency of the monopole mode in the dipole cavity isaround 2.5GHz, but the amplitude of the tail is still not
negligible at the 3.3GHz working frequency.
For the material of the inner cavity surface, two options
have been considered in the simulations: stainless steel,and copper (or copper plated steel). In the second case thecommon mode Q, which essentially depends on the losses
at the cavity walls, is higher, and the tail amplitude at
3.3GHz is lower. As a consequence also the TM010
leakage is lower, as shown by the blue curve in Fig.2.
The effect of slot position and angle misalignment has
been simulated also concerning the X-Y coupling. In
Fig. 3 the ratio between the signal voltage at orthogonal
outputs, VX/VY (VY/VX), due to a purely vertical(horizontal) beam position offset, is plotted as a functionof the misalignment error. The allowed mechanicaltolerances for a given upper limit of orthogonal coupling
can be estimated from these plots.

1

+

V /V

//

1.5

0.5
0

0

0.1

0.2
0.3
waveguide position error [mm]

0.4

0.5

1.5
V /V

4

+

Sensitivity [mV/nC]//

operates at much lower beam current than E-XFEL, oneof the main requirements is to maximize the monitor
sensitivity. For the present SwissFEL linac and undulator
BPM pickup prototypes this has been achieved byincreasing the cavity gap from 3 to 7 mm, extending the
waveguide length and connecting the waveguide to thebeam pipe along its whole length. After readjusting theother dimensions to obtain the desired TM110 mode
frequency at 3.3GHz with a Q of 70, HFSS simulations
showed an increase of the position sensitivity to
S=9.3V/nC/mm, compared to 2.89V/nC/mm for the EXFEL undulator BPM pickup.
The improved sensitivity also lead to an increase of thebeam angle sensitivity, i.e. the additional signal generated
when beam and cavity have a relative dX/dZ or dY/dZ
angle, where X, Y and Z are the horizontal, vertical andlongitudinal coordinates. According to simulations, apickup misalignment angle dX/dZ or dY/dZ of 1mrad
gives the same output amplitude (for a centered beam) asa trajectory with 9µm offset. The undesired impact of thisangle signal on the position resolution, linearity and drift
will be minimized by suppressing the angle signal
digitally (exploiting the fact that angle and position signal
have 90° phase difference), and by survey-based and thenbeam based angle measurement and correction. The angle
is dominated by the misalignment of the pickup, while theadditional angle signal caused by betatron oscillations of
the beam around its ideal trajectory is negligible.
Other properties of the pickup that may affect theperformance are the leakage of the TM010[6] (monopole)mode in the dipole cavity as well as the coupling between
horizontal and vertical plane, e.g. due to manufacturing
tolerances.

1

3
0.5

2
0

1
0

Sensitivity [mV/nC]

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○TUPA24

0

0.1
0.2
0.3
0.4
waveguide position error [mm]

0.5

0.6
0.4
0.2
0

0

1

2
3
4
waveguide angle error [deg]5

Figure 2: Simulated output signal, generated by 1nC,
centred beam, due to TM010 coupling as a function of
waveguide misalignments. Red and blue curves refer tostainless steel and copper (or copper plated) structures
respectively.
Although the monopole mode leakage is minimized by
the arrangement of the mode-suppressing coupling slots

0

0.5

1

1.5
2
2.5
3
waveguide angle error [deg]3.5

4Figure 3: X-Y coupling due to position and angle error of
the waveguides in the 3.3GHz undulator pickup. V+ andV// are the voltages detected respectively at the orthogonalplane and at the same plane of the beam offset plane.
A 4.8GHz cavity BPM has also been considered andsimulated as an alternative to the design described above.
The pickup sensitivity increases with the frequency, thus a
4.8GHz pickup can give better performance at low charge
operation, which would mainly be attractive for theundulator BPMs. The frequency has been chosen as the
maximum to still have a safe distance to the C-band
accelerating section frequency (5.712 GHz). Moreoverstaying below 6GHz simplifies the electronics design andminimizes drift effects that tend to increase with thefrequency. A quality factor of 100 was chosen to have thesame bandwidth and decay time constant as for the
3.3GHz version. For a pickup made of stainless steel thesensitivity exceeds 14V/nC/mm, while a 10% higher

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

Proceedings of IBIC2012, Tsukuba, Japan

Table 1: RF characteristics of the 3.3GHz and 4.8GHz
BPM position cavities, both made of stainless steel
Frequency

3.3 GHz

4.8 GHzQ loaded

70

100

Q external

160

232
2

2

R/Q

0.65 Ω/mm

Sensitivity

9.3 V/nC/mm 14.1 V/nC/mm

TM010 frequency

2.534 GHz3.492 GHz

Angle/position signal ratio 9 rad/mm

7 rad/mm

1.01 Ω/mm

Prototype Measurement Results
For the 3.3GHz undulator BPM prototype four pickups
have been fabricated, with the aim to both validate thesimulation results and to test different manufacturing
techniques.
The pickup body, made of LN316 stainless steel, iscomposed of three pieces, forming both the position andthe reference cavities. The vacuum feedthroughs, whichconstitute the end part of the coaxial line outputs, arewelded onto the pickup central body. Figure 4 shows
pictures of the three main parts and the final assembly of
the pickup.

then inserting the feedthroughs. This allowed to have an
early indication of the manufacturing quality and RF
tolerances. Afterwards, two prototypes have been electron
beam welded and the other two brazed. Finally thefrequency and Q were measured again to test the impactof the welding and brazing on the pickup RF parameters.
The measurement results are summarized in Table 2,
where the numbers in black (blue) refer to themeasurements before (after) the brazing or welding. For
each device, the table shows the frequencies and thequality factors of both the TM110 polarizations of theposition cavity and of the TM010 of the reference cavity.
The undulator prototypes named UP1 and UP2 have beenbrazed, while UP3 and UP4 were electron beam welded.
Moreover, the feedthroughs have been welded only in
prototypes UP1 and UP4, while they have just been
plugged into the unbrazed/unwelded and thenbrazed/welded main body parts of UP2 and UP3.
Table 2: Measured frequency and quality factor of theposition and reference cavity working modes of the four
prototypes, before (black) and after (blue) brazing (UP1and UP2) or electron beam welding (UP3 and UP4)
Position cavity
BPM # f

S31

Ref. cavity[GHz] QS31 fS42 [GHz] QS42

f
[GHz]

Q

3.300564.1 3.3010

64.7 3.3010 62.0

3.300863.9 3.3004

65.4 3.3009 68.7

3.300265.4 3.2995

66.0 3.2995 65.0

3.299071.7 3.2998

70.4 3.3010 73.0

3.300664.9 3.3021

63.7 3.2984 72.0

3.296968.2 3.2970

68.5 3.2935 72.0

3.299964.7 3.2999

63.6 3.2984 72.0

3.298566.0 3.2978

68.7 3.2925 74.0

UP1

UP2

UP3

UP4

Figure 4: SwissFEL undulator prototype: pictures of the 3parts composing the pickup body and final assembly after
brazing.
A first RF measurement of the pickup frequency and Q
has been done before welding/brazing the parts together,just by applying pressure to the stacked body parts andBefore the brazing/welding process, the measured
frequencies deviated -1.6 to +2.1MHz from the design
values, which is very well acceptable. This means that themechanical tolerances are well in the range of
specifications for all four prototypes. The position cavity
quality factors are 6 to 9% lower than expected, which
may be caused by imperfect electrical contacts of theloosely assembled body pieces before welding/brazing.
The reference cavity measured quality factors are in alarger range of values, being between 62 and 74. A reason
for that could be the larger sensitivity of Q to the coupler
insertion position and to the VSWR of the feedthrough, as
the analysis of dimensional tolerances has pointed out.
The brazing has not caused any significant change ofthe frequencies, while the electron beam welding caused astill negligible shift of -4 to -6MHz, with the exception of
the position cavity of the prototype UP4, where thefrequency change has been smaller. Due to these results,
and considering that the electron beam welding is a

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 ○value could by achieved by copper plating of the surfaces.
The common mode coupling, the orthogonal coupling andthe beam angle signal are lower in the case of the 4.8GHz
cavity. The main parameters characterizing the two
designs are listed in Table 1.

TUPA24

TUPA24

Proceedings of IBIC2012, Tsukuba, Japantechnique which only few companies can provide, wefavour brazing for the future series production.
The quality factors measured after brazing or welding
are generally higher than before, because of the improved
electric contacts, and are still within the desired range of±10%.

LINAC CAVITY BPM DESIGNFor the linac BPMs, we are aiming for <5µm RMS
noise and drift at 10-200pC, compared to <1µm for theundulators. Another difference is that the linac BPMs
must be able to measure independently the position of two
bunches with 28ns spacing. To reduce the bunch-to-bunch
crosstalk and to simplify the digital suppression of thiscrosstalk in the BPM electronics, we reduced the Q value
to ~40 while keeping the working frequency at 3.3GHz.
Table 3: RF characteristics of linac BPM position cavity
Frequency

3.300 GHz

Sensitivity

7.1 V/nC/mm

Q loaded

39

TM010 freq.

2.252 GHz

Q ext

83

Angle/position
signal ratio

4.3 rad/mm

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

0.19 Ω/mm

2

Comparing the geometries of this pickup with the
3.3GHz version for the undulators, the main differences,
for the position cavity, are the larger size of the beam pipe
aperture (16mm), and the shorter length of the
waveguides (reduced from 25 to 14mm, for a more cost
efficient machining of the body parts). Both modifications
cause a reduction of the R/Q and thus of the resolution at
low charge that mainly depends on this value. The shorter
waveguides have a positive impact on the beam angle
sensitivity, that is ~2 times lower, as well as on thecommon mode coupling, due to a larger frequency
separation between TM010 and TM110. The X-Y coupling
benefits from the larger pipe diameter. For the same error
in the waveguide position or angle, the ratio V+/V// isreduced by a factor 4 (position) and a factor 2.3(angle)
compared to the undulator pickups (see Fig. 3). Table 3
reports the main parameters obtained from thesimulations.
A further consequence of the larger beam tube
diameter, for a given cavity distance, is a larger coupling
between the working modes of position and reference
cavity. Therefore the distance between the cavities wasincreased by 10mm to 60mm for the linac pickup withrespect to the undulator 3.3GHz prototype. The residualcross coupling signal produces an equivalent position
offset of 150nm in the position cavity.

Measurement results for frequency and Q are in good
agreement with our expectations. To validate RF
properties and to measure the performance at low charge,
we plan to make also beam test of the SwissFEL pickup
prototypes. The tests will be performed at the SwissFEL
test injector facility, where a BPM pickup test area withspace for four cavity pickups (two of them on motorized
X-Y movers) is available. We intend to use the E-XFEL
cavity BPM electronics that already has been proved towork successfully [8].

REFERENCES[1] ftp://ftp.psi.ch/psi/SwissFEL_CDR/_V20_23.04.12.pdf[2] B. Keil et al., “Commissioning of the Resonant Stripline
BPM System of the SwissFEL Test Injector”, Proc.
FEL2010, Malmö, Sweden, p. 429.
[3] B. Keil et al., “The European XFEL Beam Position
Monitor System”, Proceedings IPAC’10, p. 1125 ff.
[4] H. Maesaka et al., “Development of RF cavity BPM of
XFEL/SPRING-8”, Proceedings of DIPAC ’09, Basel,
Switzerland, MOPD07, p. 56-58.
[5] D. Lipka, Cavity BPM Designs, related Electronics andMeasured Performances”, Proceedings of DIPAC ’09,
Basel, Switzerland, TUOC02, p. 280-284.
[6] T. Shintake, “Development of Nanometer Resolution RFBPMs”, Proc. of HEAC’99 (1999).
[7] http://www.ansys.com[8] M. Stadler et al., “Beam Test Results of Undulator Cavity
BPM Electronics For The European XFEL“, theseproceedings.

CONCLUSIONS AND OUTLOOKThe development of cavity BPMs for the SwissFEL
project is in progress at PSI. So far, a 3.3GHz and a
4.8GHz version for the undulator and a 3.3GHz version
for the linac have been designed. The 3.3GHz undulator
prototype has been fabricated, and the linac prototype iscurrently in production.

ISBN 978-3-95450-119-9
4Beam Position Monitor System