Proceedings of IBIC2012, Tsukuba, Japan

MOPA16DESIGN AND CHARACTERIZATION OF A PROTOTYPESTRIPLINE BEAM POSITION MONITOR FOR THE CLIC DRIVE BEAM*
A. Benot-Morell, CERN, Geneva, Switzerland and IFIC (CSIC-UV), Valencia, Spain
L. Søby, M. Wendt, CERN, Geneva, Switzerland
J.M. Nappa, J. Tassan-Viol, S. Vilalte, IN2P3-LAPP, Annecy-le-Vieux, France
S. Smith, SLAC National Accelerator Lab, Menlo Park CA, USA
The prototype of a stripline Beam Position Monitor(BPM) with its associated readout electronics is underdevelopment at CERN, in collaboration with SLAC,LAPP and IFIC. The anticipated position resolution andaccuracy are expected to be below 2μm and 20μm
respectively for operation of the BPM in the CLIC drive
beam (DB) linac. This paper describes the particular
CLIC DB conditions with respect to the beam position
monitoring, presents the measurement concept, andsummarizes electromagnetic simulations and RF
measurements performed on the prototype.

INTRODUCTION

n=2, where c is the speed of light in vacuum) to be at
12GHz, which is also the bunch frequency. Therefore, inthe time domain, the idealized response to a multi-bunchtrain will only show the first and last pair of bunches, allother bunches in-between will be cancelled (Fig. 1). Otherrelevant design parameters are listed in Table 1.

Figure 1: Idealized BPM multi-bunch train response.
Table 1: Drive Beam Stripline BPM Parameters

CLIC, a Compact electron-positron LInear Collider
proposed to probe high energy physics (HEP) in the TeV
energy scale, is based on a two-beam scheme. RF power,
required to accelerate a high energy luminosity beam isextracted from a high current Drive Beam (DB), whosedecelerator requires more than 40000 quadrupoles, eachholding a BPM. These BPMs face several challenges, as
they will be operated in close proximity to the Power
Extraction and Transfer Structures (PETS), while theaccuracy requirements are demanding (20μm). They haveto be compact, inexpensive and operate below thewaveguide (WG) cut-off frequency of the beam pipe toensure locality of the position signals, which rules out thesignal processing at the 12GHz bunch frequency. Also
wakefields, and hence the longitudinal impedance, must
be kept low. This first proposed solution is a compact,
conventional stripline BPM utilizing a signal processing
scheme operating below 40MHz. Before installation into
the CLIC Test Facility (CTF3), the manufactured
prototype has been characterized in detail on an RF bench
setup. In parallel, the design of a readout system isprogressing.

The signal processing will be performed at baseband
frequencies ranging 4 to 40MHz, to avoid non-local
confounding signals, mainly coming from the PETS,
starting at 7.6GHz, the cut-off frequency of the TE11
mode for a circular waveguide of 23mm pipe aperture.
The position signal sensitivity of a stripline BPM
detector is based on the image charge model, and isapproximately pos=(r/2)¨/Ȉ, being r the beam pipe
radius, ¨ and Ȉ the difference and sum of the opposite
electrode signals, and pos the horizontal (x) or vertical (y)beam position. A thorough description of the readout
electronics and further details of the design can be foundin [1] and [2].

CONCEPTUAL BPM PICKUP DESIGNPROTOTYPE CHARACTERIZATIONThe CLIC DB stripline BPM pickup is compact and fits
into the quadrupole vacuum chamber. Each of the fourelectrodes spans an angular coverage of 45º, having acharacteristic impedance of 50 and a physical electrodelength of L=25mm. As of the proximity of 12GHz highpower accelerating structures (PETS), L was chosen toutilize one of the notches in the transfer function (nc/2L,___________________________________________*Work supported by MINECO contract FPA2010-21456-C02-01,
SEIC-2010-00028 and U.S. Department of Energy contract DE-AC0276SF00515

Parameter
Diameter
Stripline length
WidthCh. Impedance
Duct apertureResolution
Accuracy
Temporal resolution

Value
24 mm
25 mm
12.5 %
50 ȍ23 mm
2 μm
20 μm
10 ns

Comment
stripline IDof circumference (450)
In decelerator
Full train
BW > 20 MHz

Position Characteristics and Linearity
The full system was tested using the proceduredescribed in [2] to check its performance. The position
characteristic was analysed using a stretched wire fed by
an RF excitation signal, while being moved in 1mm steps
in the range ±6mm. Table 2 shows the results for theposition sensitivity at the origin, the electrical offset andthe RMS linearity error for both planes. Although theperformance of the electronics was satisfactory, showing
the expected signal shape and levels, [2], the obtainedISBN 978-3-95450-119-9
Beam Position Monitor System

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractProceedings of IBIC2012, Tsukuba, Japansensitivity was lower than computed using the simplistic
formula above: 166m-1.
In order to rule out inadequacies of the setup due to theadjacent vacuum chambers to the pick-up being muchsmaller than the wavelength of the excitation signal (440MHz), a brief measurement was performed at 3GHz,
without electronics. This result (value raw in Table 2) isalmost equivalent to a numerical simulation (EM PICsolver), which gives 137m-1 as pickup sensitivity.
Table 2: Sensitivity and Linearity Parameters
xH,V = (SH,V-1)¨/Ȉ + EOSH,VParameter
Value (Full System)
Value (Raw)
V sensitivity SV(115.19 ± 2.32) m-1(130.44 ± 2.59) m-1
H sensitivity SH(115.17 ± 1.98) m-1(130.28 ± 2.37) m-1
V offset EOSV
(0.03 ± 0.08) mm(-0.51 ± 0.08) mmH offset EOSH
(0.02 ± 0.07) mm(-0.30 ± 0.07) mm
V RMS linearity error 251.07 μm
247.71 μm
H RMS linearity error 214.07 μm
225.83 μm

The transfer function of the BPM was analysed with acoaxial transmission-line setup, [3]. An inner rod,traversing the BPM, serves as center conductor of thecoaxial structure, which has conical transitions towards
the SMA connectors at both ends, providing a constantcharacteristic impedance of 50. A sine wave stimulus
signal was swept in the range 0.003-20GHz to measure
the magnitude of the transfer function (Fig. 2). We
observed major differences between measured and ideal(theoretical) transfer functions, e.g. the first notch
frequency was found at 4.5GHz, while expecting 6GHz,
which hints to an electrical length of 33mm.

Effects of a SiC HOM Damping Ring
A strong resonance peak of the transverse wake
impedance of the striplines was observed around 12GHz
in simulations [1]. A ring of SiC RF damping material
was placed at the downstream end of the electrodes toabsorb this, and other higher-order modes (HOM).
However, while successfully damping HOMs, thedielectric ring also shifts the transfer function to lower
frequencies, i.e. causing an extension of the effective
electrical length of the striplines, as Fig. 2 indicates.
Therefore, the pick-up can no longer behave as a notch
filter for 12GHz, which unfortunately prevents the “bunchcancellation” scheme (see also Fig. 6).

Geometric Issues
Despite the extended electrical length, the measured
magnitude response (Fig. 2) hints for other design issues,
which we tried to understand by empirical EM
simulations.
−90−100−120−130−140−150−1700−20

phi = 45°phi = 30°
phi = 10°2468 10 12 14
Frequency (GHz)161820Figure 3: Transfer function for different electrode angularcoverages, without SiC damping ring.

−40−50
Theoretical−60

Measured
510Frequency (GHz)

1520

Figure 2: Transfer function of the stripline BPM
prototype.

Time Domain Measurements
The frequency domain result was verified by a S21time domain measurement, using a 50ps full width at halfmaximum (FWHM) Gaussian stimulus pulse passed on
the wire conductor, emulating a single bunch. A 210ps
time difference was measured between the two oppositepolarity pulses out of one pickup electrode, confirming aneffective electrode length of ~32mm.

The width of the 450 stripline electrodes is large withrespect to its length, causing non-TEM fields. Simulationsof the transfer function towards more narrow, TEM-likeelectrodes reduce some of the spurious resonances (Fig.
3). Particularly, the 100 electrodes approximate thetheoretical magnitude response quite well, except for one
resonance around 10GHz.
−90
−100

Transfer Function (dB)−30−700−110−160−10Transfer Function (dB)

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

EM SIMULATIONS AND DISCUSSION

Transfer Function (dB)

MOPA16−110−120−130−140−150

D = 23mm
D = 11mm−160−1700246810 12 14
Frequency (GHz)161820Figure 4: Transfer functions for different pipe apertures.

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

Proceedings of IBIC2012, Tsukuba, Japan−10−30−40Voltage (mV)

Transfer Function (dB)

−20However, passing this pickup signal through an ideal
40MHz low-pass network, similar to that of the read-outelectronics, returns the expected baseband waveform (Fig.
7). This is not surprising, as the ideal low-pass filter in thesimulation is perfectly attenuating all high frequencies,including 12GHz. In practice, the prototype BPM can be
still made operational, a dedicated external 12GHz notch
filter will be mandatory however.

−50L = 25mm
L = 20mm
L = 18mm
L = 15mm−60−70−8002468
10 12 14Frequency (GHz)161820Figure 5: Transfer function for different electrode lengths
with SiC ring.
Figure 5 shows the simulation efforts to compensate thelength extension effects caused by the SiC damping ring.
Evidently, also the notch effect is damped, so relocating
the notch to 12GHz will only be of minor benefit.
A 2D EM analysis of the prototype geometry showedfurthermore, that the characteristic impedance of thestripline electrodes is ~40, not 50 as anticipated. Somecorrections on electrode thickness (3.1 Æ 1.37mm) and agap (2.4 Æ 0.63mm) would be required.

Expected Beam Signals
As the transfer response of the prototype BPM (Fig. 1)
has some drawbacks with respect to the design intentions,
we analysed the expected signal response to a CLIC-likebeam with an EM simulation, based on the CST StudioParticle-in-Cell (PIC) solver. As stimulus signal, arelativistic beam of 20 Gaussian bunches spaced by
83.33ps, each one with 8.4 nC charge and 1 mm length(RMS), was supplied. As expected, the time domain
output signal does not show any bunch cancellation (seeFig. 6). The raw waveform recorded by a voltage monitor
at the output feedthrough port shows strong highfrequency signal contents, particularly at 12GHz.
4000

Theoretical
PIC Solver

3000

Voltage (V)PIC Solver
Theoretical

20406080100 120 140 160 180 200

Time (ns)Figure 7: Filtered signals at the output of the readout
electronics for simulated (blue) and ideal (red) cases.

CONCLUSIONS AND FUTURE WORK
Due to the described geometry constraints and thefrequency shift caused by the damping material, the tested
prototype is conceptually not behaving close to an ideal
stripline BPM at high frequencies. We do not expect asufficient filtering of 12GHz CLIC DB beam harmonics
by this first DB stripline BPM prototype. However, thepickup may still operate very well as BPM, assuming wewill be able to provide a sufficient external 12GHz notch
filter solution.
The presented results have to be confirmed by beam
tests in CTF3, foreseen for late 2012. More studies,including other BPM pickup solutions will be performedin the future.

ACKNOWLEDGEMENT
F. Guillot-Vignot, J.Tan (CERN), J.J. García-Garrigósand A. Faus-Golfe (IFIC) are warmly acknowledged fortheir valuable and continuous support.

REFERENCES[1] S. Smith, et al., "CLIC Drive Beam Position Monitor",
DIPAC'11, Hamburg, May 2011, TUPD13.
[2] A. Benot-Morell, et al., “Characterization Tests of a
Stripline Beam Position Monitor for the CLIC DriveBeam”, IPAC’12, New Orleans, May 2012, MOPPR042.
[3] J.J. García-Garrigós, et al., "High Frequency Measurementsof the Beam Position Monitors for the TBL Line of theCTF3 at CERN", DIPAC'11, Hamburg, May 2011,
TUPD16.

2000
1000
0−1000−2000−3000−400001009080706050403020100−10−20−30−4000.5

11.5Time (ns)22.5

3Figure 6: Stripline BPM raw output.

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 ○This unavoidable resonance seems to be caused by theTM01 waveguide mode of the vacuum chamber. A
simulation with an 11mm aperture shifts the effect well
above 15GHz (Fig. 4).

MOPA16