Proceedings of IBIC2012, Tsukuba, Japan

MOPA18A PROTOTYPE CAVITY BEAM POSITION MONITORFOR THE CLIC MAIN BEAM∗
F. Cullinan† , S.T. Boogert, N. Joshi, A. Lyapin, JAI at Royal Holloway, Egham, UK
D. Bastard, E. Calvo, N. Chritin, F. Guillot-Vignot, T. Lefevre, L. Søby, M. Wendt, CERN, Geneva
A. Lunin, V.P. Yakovlev, Fermilab, Batavia, USA
S. Smith, SLAC, Menlo Park, California
Abstract

Figure 1: The brazed prototype pick-up assembly (left) andthe vacuum geometry (right) with fitted feedthroughs.

INTRODUCTION
The CLIC design for 3 TeV centre of mass energy includes a 40 km long main linac and a 6 km long beam
delivery system which require precise beam position monitoring for operation. This will be achieved using close to4800 cavity beam position monitors, one for each of the4196 quadrupole magnets in the main linac and a further
600 in the BDS. These must have a good spatial resolution of 50 nm and a time resolution of 50 ns in order toprovide multiple, accurate position measurements within asingle bunch train. They must also operate in an environment where large shifts in temperature are expected [1].
A prototype cavity BPM has been designed and built tobe tested on the CTF3 probe beam later this year. It consistsof two cavities, a position cavity and a reference cavity.
The position cavity is a cylindrical pillbox with rectangular
waveguides that strongly couple to the first resonant dipole
mode in two polarisations (TM110 ). The amplitude of eachpolarisation is proportional to the beam offset in one transverse dimension. The dipole mode frequency of 14.99 GHz
is close to 14 GHz which will be used for CLIC. It was chosen so that signals from consecutive bunches, separated by0.667 ns (0.5 ns for CLIC), add constructively and dominate signals from other modes excited by the beam. The
reference cavity is re-entrant and its first monopole mode
(TM010 ) is used for bunch charge normalisation and as areference beam arrival phase. It has the same frequency∗ The research leading to these results has received funding from theEuropean Commission under the FP7 Research Infrastructures project EuCARD, grant agreement no.227579, DITANET under EU contract PITNGA-2008-215080 and the STFC; Computing time with ACE3P provided
by US DOE at NERSC† Francis.Cullinan.2010@live.rhul.ac.uk

as the position cavity first dipole mode so that the same
signal processing can be applied to both for improved stability. Both cavities are designed to have low quality factors in order to achieve a good time resolution. The beam
pipe through the assembly has the same 4 mm radius as theCLIC main linac [2].
A photograph of the two cavity assembly, which consists of four stainless steel parts brazed together, is shownin Fig. 1. The signals are extracted via feedthrough antennas in the re-entrant part of the reference cavity and in thecase of the position cavity, at the end of the waveguides
where there are also tuning bolts. These push against thewall of each waveguide opposite the antenna to tune theantenna coupling to the dipole mode resonant frequency.
Each opposing pair of feedthrough antennas in the position
cavity measure the same dipole mode polarisation. Combination of the two signals is not intended but having twomeans a lower external quality factor and leaves the option
to suppress the TM210 quadrupole mode using a 180◦ hybrid or to compensate for cross coupling with adjustable
short-circuit terminations. The signal from one of the two
feedthrough antennas in the reference cavity could also be
diode rectified for timing measurements [3].
The prototype pick-up was brazed at the end of April andthe feedthroughs, which are aligned with dowel pins andsealed with silver-coated copper gaskets, were fitted afterwards by hand. The assembly then successfully passed avacuum test. This paper describes the latest RF measurements of the cavity, progress towards developing downconversion electronics and finally, beam test plans.
ISBN 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 ○The Compact Linear Collider (CLIC) places unprecedented demands on its diagnostics systems. A large numberof cavity beam position monitors (BPMs) throughout themain linac and beam delivery system (BDS) must routinely
perform with 50 nm spatial resolution. Multiple position
measurements within a single 156 ns bunch train are also
required. A prototype low-Q cavity beam position monitor
has been designed and built to be tested on the CLIC Test
Facility (CTF3) probe beam. This paper presents the latest
measurements of the prototype cavity BPM and the designand simulation of the radio frequency (RF) signal processing electronics with regards to the final performance. Installation of the BPM in the CTF3 probe beamline is also
discussed.

MOPA18

Proceedings of IBIC2012, Tsukuba, JapanRESONANT MODE MEASUREMENTS
The quality factors of the first dipole mode of the position cavity and the monopole mode of the reference cavity
were measured before [4] and after brazing. This was doneusing the transmission of a weakly coupled antenna positioned in the beampipe to each output port in turn with theremaining ports either short-circuited or terminated withmatched loads. The quality factors, both loaded QL andinternal Q0 could then be determined as outlined in [4].
Table 1: Quality factors and resonant frequency f0 of theposition cavity first dipole mode measured before and after
brazing.
Parameter

Predicted

f0 /GHz
QL
Q014.990274
450

Beforebrazing
14.993224306After
brazing
15.012198
292

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Table 2: Quality factors and resonant frequency f0 of thereference cavity first monopole mode measured before andafter brazing.
Parameter

Predictedf0 /GHz
QL
Q0

14.999150383

Beforebrazing
14.96074-

Afterbrazing
14.997130357The measured quality factors and resonant frequencies
are shown in Tables 1 and 2. In the case of the position
cavity, an increase of about 20 MHz in frequency and adecrease in the quality factor were observed after brazing. The value for the internal quality factor of the reference cavity before brazing is missing. This is because thereference cavity had to be modified [4] and the measurements were taken simply to confirm the correct resonant
frequency had been achieved. For the same reason, the predicted values were taken from simulations in Omega3p, theACE3P eigenmode solver [5], since no simulations of themodified cavity were performed at the design stage. Theresonant frequency increased by 37 MHz and the loaded
quality factor increased by a factor of almost two during
brazing. These changes are consistent with the correct positioning of the feedthrough antennas with the gaskets inplace as predicted in simulation [4].
After the measurements had been made, an attempt wasmade to tune the position cavity using the tuning bolts.
In this way, the coupling could be balanced between thewaveguides but the process was not completely reversible
and didn’t always result in an improvement. The maximum cross coupling between a pair of adjacent output ports
was -38 dB and this did not improve noticeably during tuning. The cross coupling between the other pairs of adjacentports were all below -40 dB.

Figure 2: Variation of the resonant frequencies of the position cavity dipole and reference cavity monopole modes
over time.
The stability of the resonant frequency with temperature
was also measured for both modes. The assembly was left
over one weekend and the resonant frequencies were measured from the transmission spectra between opposite ports
every ten minutes. The temperature was also recorded asshown in Fig. 2. The temperature stability of the positioncavity first dipole mode, as measured from fits of resonant
frequency against temperature, was -359 kHz/◦ C which
is worse than the previous measurement of -243 kHz/◦ C[4]. This is possibly due to the vacuum sealing of thefeedthrough antennas. The temperature stability of the reference cavity monopole mode, -308 kHz/◦ C, is significantly lower, giving a relative temperature stability of thetwo modes of 51 kHz/◦ C. Over the 20 ◦ C maximum temperature shift expected during CLIC operation, this corresponds to a relative frequency shift of 1 MHz, which willcertainly be significant when looking at measurements of
one or a few bunches and so may need to be improved. This
could possibly be achieved by using a reference cavity thatis a pillbox, similar to the position cavity.

SIGNAL PROCESSING
A schematic of the signal processing electronics to beused for the beam tests is shown in Fig. 3. There is a filter stage at RF to remove unwanted modes. However, thebandwidth must be kept larger than twice the bandwidth ofthe cavity so as not to have a significant effect on the risetime of the multiple bunch signal and degrade the temporalresolution. The electronics bandwidth also has a large effect on the maximum signal observed from a single bunch.
A digitally controlled attenuator will be used to control thedynamic range. A 7 GHz lowpass filter is used to remove
the high frequency outputs of the mixer and signals around
15 GHz while 800 MHz lowpass filters prevent aliasing in
the high speed digitiser which was chosen because of availability. In order to avoid any complications with timing, asingle independent voltage controlled oscillator (VCO) will

ISBN 978-3-95450-119-9
2Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

MOPA18Figure 3: Diagram of the signal processing electronics to be used for the beam tests.
ing pursued to achieve as many independent measurements
as possible within a single train as well as high precision
average position measurements. The former may employ
deconvolution of the single bunch response from the overlapping signals of multiple bunches [7].

Figure 4: Simulated output power against input power inthe electronics with the estimated corresponding offset.

be used for down-mixing to the 200 MHz intermediate frequency. The VCO can be driven to provide enough power
for the nine channels required for three BPMs in the future.
Before commissioning the electronics, schottky diode detectors will be used to obtain preliminary measurements of
the BPM coupling to beam.
A simple simulation using the PYTHON package, wfproc [6], was performed in order to see the effect of theelectronics in terms of gain and noise. The thermal noiseof the system was estimated to be 16 µV and the results
are shown in Fig. 4. The sensitivity of the pick-up was also
calculated from the measured quality factors and the R/Qdetermined using Omega3p [5]. This was used to estimate
a signal maximum of 0.6 V going into the electronics fora single bunch of 0.6 nC at 100 µm offset, taking into account the finite bandwidth of the electronics. Under theseassumptions and with no added attenuation, the right-handside vertical axis in Fig 4 shows the position offset corresponding to the calculated output power. The dynamic
range of the electronics is limited by the double-balanced
mixer and corresponds to ±240 µm and a resolution limitof 14 nm is imposed by the electronics noise.
The digital signal processing will be crucial for obtaining the desired performance. Several possibilities are be-A prototype cavity BPM for the CLIC main beam has
been designed and built. The RF characteristics of the assembly have been measured both before and after brazing
and a complete temperature stability measurement has beenmade. The signal processing electronics have been designed and simulated and the results will be directly comparable with measurements of a full channel.
The BPM is scheduled to be installed at the end of theprobe beamline at CTF3 [4] at the start of November for
running until the end of 2012 and at the start of 2013. It
will be mounted on a pair of stepper motor driven stages
for measurement of the sensitivity and for calibration inboth horizontal and vertical directions. A screen monitor
just downstream of the pick-up will help to commission
the new installation and ensure total beam transmission.

REFERENCES[1] Lefevre, T., 2011. “Overview of the CLIC Beam Instrumentation”, IPAC2011, San Sebastian, Spain.
[2] Schmickler, H. et al., 2010. “Submicron Multi-Bunch BPMfor CLIC”, IPAC2010, Kyoto, Japan.
[3] Lunin, A., 2012. “High Resolution RF Cavity BPM Designfor Linear Collider”, 8th DITANET Topical Workshop on
Beam Position Monitors, CERN, Switzerland.
[4] Cullinan, F. et al., 2012, “Development of a Cavity Beam
Position Monitor for CLIC”, IPAC12, New Orleans, USA.
[5] Ko, K. et al, “Advances in Parallel Electromagnetic Codes
for Accelerator Science and Development”, LINAC2010,Tsukuba, Japan.
[6] Lyapin, A., 2011, “Welcome to wfProcs documentation!”,http://wfproc.sourceforge.net/.
[7] Smith, S., 2012. “Beam Position Monitors for the CLIC DriveBeam”, 8th DITANET Topical Workshop on Beam Position
Monitors, CERN, Switzerland.

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 ○SUMMARY AND OUTLOOK