Proceedings of IBIC2012, Tsukuba, Japan

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BEAM INSTRUMENTATION FOR THE SUPERKEKB RINGS
M. Arinaga, J. W. Flanagan, H. Fukuma*, H. Ikeda, H. Ishii, S. Kanaeda,
K. Mori, M. Tejima, M. Tobiyama, KEK, Tsukuba, Japan
G. Bonvicini, H. Farhat, R. Gillard, Wayne State U., Detroit, MI 48202, USA
G.S. Varner, U. Hawaii, Honolulu, HI 96822, USA
The electron-positron collider KEKB B-factory iscurrently being upgraded to SuperKEKB. The design
luminosity of 8 x 1035 /cm2/s will be achieved using
beams with low emittance, of several nm and doubling
beam currents to 2.6 A in the electron ring (HER) and 3.6
A in the positron ring (LER). A beam position monitor(BPM) system for the HER and LER will be equippedwith super-heterodyne detectors, turn-by-turn log-ratio
detectors with a fast gate to measure optics parameters
during collision operation and detectors of BPMs near thecollision point (IP) for orbit feedback to maintain stable
collision. New X-ray beam profile monitors based on thecoded aperture imaging method will be installed aiming
at bunch by bunch measurement of the beam profile. A
large angle beamstrahlung monitor detecting polarizationof the synchrotron radiation generated by beam-beaminteraction will be installed near IP to obtain information
about the beam-beam geometry. The bunch-by-bunch
feedback system will be upgraded using low noise frontend electronics and new 12-bit iGp digital filters. An
overview of beam instrumentation for the SuperKEKB
rings will be given in this paper.

INTRODUCTION
The electron-positron collider KEKB B-factory iscurrently being upgraded to SuperKEKB[1]. The design
luminosity of 8 x 1035 /cm2/s will be achieved using theso called nano-beam scheme[2]. Machine upgrades
include the replacement of the current, cylindrical LER
beam pipes to one with ante-chambers so as to withstandlarge beam currents and mitigate the electron cloud effect,a new final focus in the interaction region (IR) in order toadopt the nano-beam scheme and the construction of apositron damping ring for positron injection. The firstbeam is expected in the Japanese FY 2014. Machineparameters of SuperKEKB are shown in Table 1.

BEAM POSITION MONITOR SYSTEM
The number of beam position monitors (BPMs) inSuperKEKB is 445 in the LER and 466 in the HER. The
closed orbit measurement system of KEKB utilized a
VXI system[3]. The KEKB detector was a 1 GHz
narrowband superheterodyne detector module. One
module covered four beam position monitors (BPMs),
two in the LER and two in the HER, by multiplexing thesignals with switch modules. The main detector system ofSuperKEKB follows that of KEKB. The narrowband____________________________________________*hitoshi.fukuma@kek.jp

Table 1: Machine Parameters of SuperKEKB
Energy (GeV)

HER

LER

7

4

Circumference (m)3016

Beam current (A)2.6

Number of bunches

3.6
2500

Single bunch current (mA)

1.04

Bunch separation (ns)1.44
4

Bunch length (mm)5625/0.30

32/0.27

Emittance (nm)

4.6

3.2

X-Y coupling (%)

0.28

0.27

Vertical beam size at IP (nm)59

48

Damping time: trans./long. (ms)

58/29

43/22

Beta function @IP hor./ver.
(mm)detectors of KEKB are reused in the SuperKEKB HER.
A new narrowband detector with a detection frequency of
509 MHz is being developed, since the cutoff frequencyof the new LER ante-chamber is below 1 GHz.
Additionally, turn by turn detectors will be installed atselected BPMs at the rate of three per betatron wave
length to measure the optics during collision. Also, aspecial wideband detector is being installed for the four
BPMs closest to the collision point (IP) for orbit feedback
to maintain stable collision. Table 2 shows a list of
detectors in SuperKEKB.
Displacement sensors, which measure the mechanicaldisplacement between a BPM head and a sextupole
magnet, are installed at all BPMs neighboring the
sextupole magnets, same as at KEKB.

Button Electrode and BPM Chamber
The BPM chambers and button electrodes in the HER
are reused from KEKB. A button electrode with adiameter of 6 mm has been developed for the LER to
reduce the beam power at the electrode[4]. The electrodeis a flange type for easy replacement and for removal
during the TiN coating process of the chamber to reduce
the electron cloud. A pin-type inner conductor is used fortight electrical connection. The estimated longitudinalloss factor of a beam chamber with four electrodes is 0.16
mV/pC. The coupling impedance is 2 ohm at the center
frequency of 14.8 GHz and the Q value is 38. The
estimated growth time of the longitudinal coupled-bunchISBN 978-3-95450-119-9
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Copyright ○Abstract

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Table 2: Detectors for BPM System in SuperKEKB
Resolution

Repetition

Numberof units

2µm

0.25Hz117

2µm

0.25Hz127

Turn-by-turn

50 - 100µm

100kHz270

IR feedback

1µm

5kHz

4

Type
Narrowband
KEKB
New
narrowband

instability is slower than 120 ms at the maximum beam
current, which is much longer than the radiation damping
time of 43 ms. A prototype was tested successfully at
KEKB.
BPM chambers are being fabricated by the KEKB
vacuum group. TiN coating and baking of the chambers
started this past April.

Figure 1: 509 MHz narrowband detector.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Narrowband Detector
Development of the 509 MHz narrowband detector isin the final stage. A schematic diagram of the detector isshown in Fig. 1. An isolator is put between a selector with
PIN-switches and a variable attenuator to reduce theapparent shift of beam position caused by impedancemismatch between the selector and the attenuator. A
signal to noise ratio larger than 90 dB was achieved in alaboratory test with CW.

Figure 2: Turn-by-turn detector with fast gate.

Turn-by-Turn Detector with Fast Gate
The schematic diagram of the turn by turn detector isshown in Fig. 2. The detector has GaAs fast gates toselect the signal of a non-colliding pilot bunch locatedtypically 6 ns behind a bunch train. Signals of remaining
bunches are sent to the output port of the detector, whichis connected to the narrowband detector in order to enablea simultaneous measurement of the narrowband detector
and the turn-by-turn detector. A glitch cancellation circuit
by T. Naito is adopted in the gate circuit to remove switch
noise[5]. A 0-π splitter and a combiner placed before andafter the switches, respectively, remove a common mode
glitch of the switches. The turn-by-turn detection is done
by a log amp to reduce the cost of the detector. A
prototype of the switch was tested at laboratory. The
result shows the isolation larger than 85 dB. Also, thedetection by the narrowband detector connected to theturn-by-turn detector was not affected by the switches.

Figure 3: IR feedback detector.

IR Feedback Detector and Processor Unit
The wideband detector for IR orbit feedback is underdevelopment. Vertical positions of both beams are used
for orbit feedback to maintain stable beam collision. The
orbit shift at the BPMs by a beam-beam deflection isestimated to be several microns. A simulation study
shows that the main frequency components due tovibration of the quadrupole closest to the IP are 3, 36 and
53 Hz. Target performance for the development of the

Figure 4: Digital Signal Processor for IR feedback.
detector was tentatively set to a resolution of 1 µm andrepetition rate of 5 kHz.
The schematic diagram of the detector is shown in
Fig. 3. The detector converts the 508.8MHz component toan intermediate frequency (IF) of 16.9 MHz with an

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Proceedings of IBIC2012, Tsukuba, JapanSYNCHROTRON RADIATION MONITOR
We will install three kinds of synchrotron radiation
monitor: a visible light monitor using an interferometerand a streak camera to measure horizontal beam size andlongitudinal beam profile respectively, an X-ray monitor
for vertical beam size measurement, and a large angle
beamstrahlung monitor to get information on collision
status at the IP.

Visible Light Monitor
The large power incident on the extraction mirror is asource of heat-load induced deformations. A source bendin the SuperKEKB LER is a recycled bend in the arc
section of SuperKEKB with bending radius twice that of
the KEKB source bend. Incident power is less than that of
the KEKB HER but more than that of the KEKB LER.
The HER bend does not change, and the incident powerof 283 W is slightly higher than that of the KEKB HER.
Since the heat deformation was already a great problem
at KEKB, it is desirable to have a mirror that does notdeform as much under the same heat load. For this reason
we are developing diamond mirrors[7]. The mirrors are
made of quasi-monocrystalline diamond. A prototype 10
mm x 20 mm x 0.5 mm was completed, and as was one
20 mm x 20 mm x 1 mm. The surface of the mirror iscoated by 3 µm Au with a thin Cr layer between gold anddiamond. The diamond surface is nearly a single crystal,so that good surface smoothness (Ra ~ 2 nm, <~λ/50) is
expected. The very good heat conductance and low
thermal expansion coefficient reduce the apparent change
in magnification, compared to the Be mirrors used at
KEKB. A simulation by ANSYS at HER full current
shows the magnification error to be 43 % for berylliumand 3 % for diamond. The deformation of the diamondmirror is being measured in a test bench.
The resolution of the interferometer is fundamentally
limited by the wavelength for measurement and by the
opening angle between slits which is limited by the antechamber height. A calculation shows that a vertical beam
size measurement would be possible with theinterferometer, though it is near the limit of theinterferometer resolution.

X-ray Monitor
An X-ray beam size monitor will be installed because
the interferometer will have marginal vertical resolution
and is not capable of single-shot (single bunch, singleturn) measurements, which will likely be needed for lowemittance tuning, based on experience at CesrTA. The
typical vertical beam sizes at the source points in the LER
and the HER are 20 µm and 10 µm, respectively.
We are developing a coded aperture imaging
method[8]. This technique was developed by X-ray
astronomers using a mask to modulate incoming light.
The resulting image must be deconvolved through themask response, including diffraction and spectral width,
to reconstruct the object. An open aperture of 50% gives
high flux throughput for bunch-by-bunch measurements.
A heat-sensitive and flux-limiting monochromator is notneeded. The method has been successfully tested at
CesrTA[9].
Tests at CesrTA showed that a URA (Uniformly
Redundant Array) mask gives the predicted single-shot
resolution. Two types of URA mask, made of Si anddiamond, are considered for use at SuperKEKB, as shownin Fig. 5. A thick Si mask at SuperKEKB full current
power load was successfully tested at CesrTA. The
diamond substrate mask was also fabricated and installed
at CesrTA and is ready to be tested.
A measurement at the Photon Factory revealed that the
Fermionics photon detector had low detection efficiency
at high energies due to a small active pixel depth of only
3.5 microns. A simulation estimates single-shot resolutionof ± 2.5microns at a beam size of 10 microns with thisdetector. Though this resolution satisfies minimum
requirement for SuperKEKB, we are pursuing new
detectors which have higher efficiency in the hard X-ray
spectrum region and faster response. A trench diodedesign is being considered as a candidate. The diode has

Figure 5: Uniformly redundant array (URA) masks forcoded aperture imaging. Left: Gold on a silicon substrateinstalled at CesrTA. Minimum slit width is 5 microns.
Right: Gold on a diamond substrate to be tested at
CesrTA. Minimum slit width is 10 microns.

ISBN 978-3-95450-119-9
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Copyright ○analog mixer to reduce the degradation of the signal tonoise ratio by the clock jitter of ADC. The IF signal isdigitized by a 16 bit ADC with a sampling rate of 99.4
MHz, then processed by a chain of digital filters (i.e. two
CICs and a FIR). The cutoff frequency of the filter is 1
kHz. The digital part is implemented in a micro-TCA
board developed for the SuperKEKB LLRF system[6].
A prototype circuit is being tested. Achieved
performances of the analogue section are clock jitter of
1.30 ps, noise figure of 14.3dB at 16.9MHz andtemperature coefficient of IF-out of -0.03dB/deg. The
frequency characteristics of the digital filter was as
expected. The position resolution obtained in the test
bench is about 50 nm for CW signal. The group delay of
the first prototype of the digital filter was 3.3 ms. A
simulation shows that the group delay should be less than
1 ms to get enough rejection gain against the disturbance.
Taps, i.e. the number of filter coefficients, of the FIR will
be reduced to get a shorter group delay.
We are also developing a processor unit for the
calculation of the orbit feedback as shown in Fig. 4. The
unit has FPGA Vertex 5 on a micro-TCA board. A
feedback algorithm is implemented with Matlab/Simulink
with system generator (HDL coder) for easy coding.

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Proceedings of IBIC2012, Tsukuba, Japan

deep but narrow pixels. The first prototype was fabricatedat SLAC and U. Hawaii. The digitizer is the STURM(Sampler of Transients for Uniformly Redundant Mask)
ASIC for high-speed readout developed at U. Hawaii.
Ver. 1 of the STURM digitizer was tested at the KEK PF
in March 2009. A 64-channel system with a Fermionics
detector, a preamp board and a STURM digitizer board
integrated on a motherboard is being developed fortesting at the ATF2[10].

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Large Angle Beamstrahlung Monitor
The LABM is being built in the US, mostly at Wayne
State University, with Japanese funding. The device
measures the beamstrahlung light emitted at large angle(large compared to the typical 1/γ angle of radiation
produced by ultra relativistic beams), and relates it tobeam parameters so that the beam-beam collision can be
optimized for luminosity. Figure 6 shows how polarized
light yields measured at specific azimuthal points (in thiscase, at the top and bottom of the beam pipe, or φ=±90
degrees) relate to parameters of the beam-beamcollision[11].
The deviation of the measured polarization vectors in
Fig. 6, with respect to the nominal point for optimal
collisions (by construction, the point (1,1)), represents aquantitative measurement of the type of optics present.
From the diagram alone, and assuming only one beamneeds correction, it is possible to decide which beamneeds correction, which type of corrector (dipole,quadrupole or sextupole) to use, and how much correction
is needed.
The monitor consists of four viewports, located at the
top and bottom of the beam pipe, and respectively at 7
and 8 mrad in the LER and HER. Each viewport is a 2 X
2.8 mm2 primary mirror, reflecting light out of the beam
pipe. Light is transported through an optical channel to an
optical box (one box serves each side, or two viewports
per box), where it is separated into two transverse
polarizations and four different wavelength bands,
totaling 32 optical and electronic channels. The signal isthen counted, time-stamped, and recorded. At fullFigure 6: Relation between beam-beam imperfectcollisions (top row) and the corresponding large angle
beamstrahlung yield, normalized to a calculable quantity
U0. The beams move in and out of the page.

Figure 7: Layout of one half of an optical box. Shown area polarization splitter, ruled gratings, light collectors and
PMTs.
SuperKEKB luminosity, the photon flux at the primary
mirror is of the order 1012 Hz, and a factor of 50-500
lower for each individual electronic channel.
The design of the device is dominated by control ofsystematics, based on extensive experience at CESR[12].
Each beam is observed by two viewports located opposite
each other, at the azimuth where machine backgrounds
are expected to be smallest. Errors due to a misalignment
between beam velocity and detector will cancel out when
the two signals are summed. Inside the optical box,
shown schematically in Fig. 7, the photon counters(typically PMTs) are placed on a small conveyor belt andcan be rotated between different optical channels, so that
the PMT relative efficiency, which dominates thesystematic spectral and polarization error, can bemeasured online. The replacement of a single ruled
grating allows the device to be sensitive to thewavelength ranges 250-500, 300-600, or 450-900 nm.

BUNCH-BY-BUNCH FEEDBACK
SYSTEM
Transverse Feedback System
Since the horizontal tune is close to a half integer,
several turns are needed to observe the orbit change at themonitor if the phase relation between the source of the
kick and the monitor is not good. Using two feedback
loops with betatron phase advance around 90 degrees
might solve this difficulty. For this reason two sets ofshort stripline kickers which cover two feedback loops
with 90 degrees phase difference will be installed in eachring. The phase between the kicker and the monitor isadjusted by the digital filter. The block diagram of thetransverse feedback system is shown in Fig. 8.
The monitor chamber has a diameter of 64 mm andthe cutoff frequency is 2.7 GHz. The button electrode has
a glass feedthrough sealing with low relative dielectric
constant of about 4 which has good time and frequency
response.
A low-noise front-end for the transverse feedback
system is under development. The detection frequency is
2 GHz. A 2 GHz comb filter is used to cut low frequency
noise and to improve isolation between bunch signals. A

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low noise amp with high IP3 is put immediately after the
comb filter. A difference signal is mixed with LO with afrequency of 4 x RF frequency, amplified to increase
gain, fed to a 600 MHz Bessel LPF, then amplified by a
DC amp to improve transient response. Configuration of
the amps is being optimized for better gain and noise
figure.
Baseline system of the digital filter is the iGp processor
developed with SLAC and INFN-LNF supported by USJapan collaboration on High Energy Physics. It consistsof a fast 8-bit ADC, a Virtex-II FPGA with 8 Mbytes ofexternal SRAM, a fast 12-bit DAC and controlled bybuilt-in EPICS-IOC. Since the FPGA is completely
programmable, it supports almost any harmonic number,including odd numbers. The iGp was installed at KEKB
and tested. It showed excellent performance both in single
beam run and in collision runs. The ADC is upgraded to a
12 bit system to extend dynamic range in order to capture
the large offset signal due to the beam-beam kick.
The design of the kicker is similar to that of the present
kicker. A shunt impedance of 10k ohm per kicker is
expected. A feedthrough is changed from 20D type toEIA-7/8 with low loss ceramics. The high-power cables
are also changed from 10D to 20D. Power amps are oldfour 250W-amps and new four 500W-amps in each ring.

Longitudinal Feedback System
A growth rate of the longitudinal coupled bunch
instability due to the impedance of ARES cavities in LER
is estimated to be 15 ms which is shorter than thelongitudinal radiation damping time. Thus a longitudinal
feedback system is indispensable in the LER. The frontend electronics and the digital filters are same as those oftransverse system. A digital QPSK modulator is used.
Four DAFNE type kickers with two inputs and two
outputs will be installed to get larger capture range. The
design of the cavity has started. So far the shunt
impedance of 1k ohm at 1.15GHz and the bandwidth of
225 MHz are obtained by a HFSS calculation. Two
500W- amps per kicker are used.

OTHERS
The tune monitor is reused from KEKB. It is a tracking
sweeper with a gate to extract the signal of the pilot

bunch. The oscillation of the pilot bunch is excited by thebunch feedback system with phase locked loop.
The bunch current monitor is revised using a MAX108
8bit ADC and a Spartan6 FGPA in a form of a VME
module[13]. Bunch current information will be sent
through reflective memory in real time to the bucket
selection system during injection.
A DCCT for beam current measurement is reused from
KEKB. A loss monitor system consisting of ion chambers
and PIN photo-diodes is also reused from KEKB, with anincreased number of detectors.
The instrumentations of the damping ring are describedbriefly. The number of BPMs is eighty-four. Two button
electrodes are attached in one flange due to narrow space
for their installation. The detector for the BPM is a logratio detector accommodated in a VME module. A bunch
feedback system is installed to damp the residual bunch
oscillation at injection and extraction. The DCCT isreused from KEKB. A measurement room for asynchrotron radiation monitor is located at the tunnel
floor level. A streak camera and a gated camera areplanned to observe transverse and longitudinal profile.

ACKNOWLEDGMENTS
The authors would like to thank the KEKB vacuum
group for manufacturing BPM chambers. They also thankProf. J. Fox for valuable advices for the instrumentation
system of SuperKEKB. The authors thank staffs at
Cornell University for their assistance and efforts fordevelopment of the X-ray monitor at CesrTA and Prof. T.
Mitsuhashi for cooperation to test the X-ray monitor at
ATF2.

REFERENCES[1] H. Koiso, Proceedings of IPAC'11, 1931(2011).
[2]“SuperB Conceptual Design Report”, INFN/AE-07/2,
SLAC-R-856, LAL 07-15, March, 2007.
[3] M.Arinaga et al., Nucl. Instrum. Meth. A499, 100(2003).
[4] M. Tobiyama et al., Proceedings of BIW10, 223(2010).
[5] T. Naito et al., Proceedings of 6th Particle
Accelerator Society Meeting, 88 (2009).
[6] T. Kobayashi et al., Proceedings of IPAC'12,
3470(2012).
[7] J. W. Flanagan et al., in these proceedings, TUPB74.
[8] J. W. Flanagan et al., Proceedings of EPAC'08,
1029(2008).
[9] J. W. Flanagan et al., Proceedings of IPAC'10,
966(2010).
[10] J. W. Flanagan et al., in these proceedings, MOPB72.
[11] E. Luckwald, G. Bonvicini and D. Cinabro, Phys
.Rev. E59, 4584(1999).
[12] G. Bonvicini, ALCPG 2009, Albuquerque, NM,
September 2009.
[13] M. Tobiyama and J. W. Flanagan, in theseproceedings, MOPA36.

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Copyright ○Figure 8: Transverse bunch-by-bunch feedback system.

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