Chapter 9
Beam Instrumentation
9.1

Overview

This chapter discusses system design of beam instrumentation in SuperKEKB.
Table 9.1 summarizes main instrumentation to be installed in SuperKEKB
rings. Beam position monitors (BPMs) , 466 in HER and 444 in LER, will
be located next to every quadrupole magnets. Signal detecters of the BPM
system will be super-heterodyne detectors, turn-by-turn log-ratio detectors
with a fast gate which selects one bunch signal, and a detector for BPMs
in orbit feedback system which maintains stable collision. Extraction mirrors made of diamond bonded by gold will be prepared for visible photon
monitors to reduce the heat deformation by synchrotron light. New X-ray
beam profile monitors based on the coded aperture imaging method will be
installed for bunch by bunch measurement of the beam profile. A large angle beamstrahlung monitor near the interaction point (IP) will measure thepolarization of light generated by the beamstrahlung to obtain information
about the beam-beam geometry. The transverse bunch-by-bunch feedback
system will be upgraded using new low noise front-end electronics and new
12-bit iGp digital filters. The longitudinal bunch-by-bunch feedback system,
which was not equipped in KEKB, will be installed to damp the coherent
longitudinal beam instability due to higher order mode in RF cavities.

300

Table 9.1: Instrumentation in SuperKEKB rings.
SystemQuantity
HER
LER
Beam position monitor
466
444
Displacement sensor
110
108
Transverse bunch feedback system
22Longitudinal bunch feedback system
01Visible synchrotron radiation size monitor
1
1
X-ray size monitor
1
1
Beamstrahlung monitor
1
1
Betatron tune monitor
22Longitudinal phase monitor
1
1
Beam loss monitor
200
Direct current-current transformer (DCCT)
1
1
Current transformer (CT)11Bunch current monitor
1
1

9.2

Beam Position Monitor System

BPM system in SuperKEKB will maximally use that in KEKB to reduce
construction cost. Since BPM chambers in LER will be replaced by new
ante-chambers, BPM blocks and button electrodes will be newly designed
and fabricated. While the 1GHz narrowband detectors in KEKB will be reused in SuperKEKB HER, those with 509 MHz detection frequency will be
used in SuperKEKB LER to fit the ante-chambers whose cut-oﬀ frequency
is bellow 1 GHz. In addition to the narrowband detectors, 270 gated turnby-turn detectors will be installed to measure optics function during collision
operation. Button electrodes, a wideband detector, a processor unit and acontroller for power supplies of actuators will be developed for orbit feedback
system to keep the stable collision.

9.2.1

Button Electrode and BPM Chamber

While BPM chambers and button electrodes in KEKB HER will be mostly
re-used in SuperKEKB, those in SuperKEKB LER will be newly fabricated.
A button electrode has been developed to fit the ante-chamber in LER as
301

Figure 9.1: BPM electrode for SuperKEKB.

shown in Figure 9.1[2].
A diameter of the electrode is 6 mm to reduce the beam power at theelectrode. The electrode is a flange type for easy replacement on troubles
and for detachment from the chamber during the TiN coating process on
the chamber wall. A pin-type inner conductor ensures tight electrical connection. The estimated longitudinal loss factor of the beam chamber withfour electrodes is 0.16 mV/pC. The coupling impedance is 2Ω at the center
frequency of 14.8 GHz and the Q value is 38. The estimated growth time of
the longitudinal coupled-bunch instability is slower than 120 ms at the maximum beam current, which is much longer than the radiation damping timeof 43 ms. A prototype of the electrode was successfully tested at KEKB.
BPMs closest to IP will be located between IP and the final focusing
quadrupole magnets. These BPMs are important because they measure beam
position without influence of focussing by strong magnets around IP. Since
the output power from the beam at these BPMs are much larger than thatat the normal BPM due to small inner-diameter of the vacuum chamber of
20mm, a special feedthrough has been designed as shown in Fig. 9.2. The
302

Figure 9.2: Electrode for beam position monitors at closest to collision point.

button head is a simple rod with diameter of 1.8 mm to reduce the powerfrom the beam. The sealing and supporting ceramics is made of silicon
nitride with the dielectric constant of 7.9. It has smaller dielectric loss thanalumina ceramics. The body and the rod are made of titanium and blazed
to a flange made of CuNi. The button signal was simulated using HFSS and
GdfidL. The frequency response shows that the trapped modes around thehead and ceramics have well higher frequency than that of beam spectrum.
The impedance was also estimated from the wake simulation with GdfidL.
The shunt impedance of the coupling impedance is 0.3Ω and the Q value
is 200 at 14.3 GHz. With the maximum beam current of 3.8 A, estimatedpower level at 508 MHz is around 6 dBm and estimated total power frombeam is around 10W, which is similar level as that at the feedthrough of
BPMs near IP in KEKB where both beams in HER and LER pass throughthe same BPM chamber with large horizontal oﬀset.
BPM chambers made of aluminum for LER are designed by the KEKB
vacuum group. A schematic drawing of the BPM chamber is shown in Fig.
9.3[3].

303

Figure 9.3: BPM chamber in the arc sections in the LER.

304

9.2.2

BPM detectors

The main BPM detector system in SuperKEKB will follow that in KEKB.
The detector in KEKB was a 1 GHz narrowband super-heterodyne detector module accommodated in a VXI main frame[1] . One module covered
four BPMs by multiplexing the signals with a switch module. While thenarrowband detectors in KEKB will be re-used in SuperKEKB HER, a new
narrowband detector with detection frequency of 509 MHz will be used in SuperKEKB LER, since the cutoﬀ frequency of the new ante-chamber is below
1 GHz, i.e. 990MHz. In addition to the narrowband detectors, turn-by-turndetectors with a fast gate will be installed at selected BPMs at the rate ofthree per betatron wavelength so as to measure the optics during collision.
A wideband detector will be introduced at four BPMs closest to IP for orbit
feedback to maintain the stable collision. Eight commercial signal processors
will be installed for the measurement of orbit oscillation around IP and fast
orbit interlock in order to protect extraction windows for aborted beam.Two
longitudinal phase monitors, which are re-use of KEKB, will be installed for
injection tuning.
Table 9.2 is a list of the detectors in SuperKEKB.
Table 9.2: Detectors for BPM System in SuperKEKB.
Type
Narrowband in KEKB
New narrowband
Turn-by-turn
IR feedback
Orbit interlock
Fast oscillation monitor
Longitudinal phase monitor

Resolution( µm) Repetition(Hz)
2
0.25
2
0.25
50 - 100
100k
1
5k
100
100k
5
10k to 100k
10
100k

Number of units
117
127
270
4
4
4
2

Narrowband Detector
A 509 MHz narrowband detector which meets the VXI standards is developed based on the 1GHz narrowband detector in KEKB[4]. A schematic
diagram of the detector is shown in Fig. 9.4. A selector and a variable305

Figure 9.4: 509 MHz narrowband detector[4].

attenuator use PIN-diodes which are directly connected each other without
decoupling capacitors. The isolation between channels in the selector is better than 100dB. Its insertion loss is about 1.5dB. An isolator is put between
the PIN switch selector and the variable attenuator to reduce the apparent
shift of beam position caused by impedance mismatch between the selector
and the attenuator. Local signal for a mixer is generated with a high precision synthesizer and a high precision crystal oscillator to reduce phase noise.
Polyethylene capacitors are used in the IF(intermediate frequency) circuit toreduce temperature drift.
Achieved signal to noise ratio of B/A, C/A and D/A, where A to D arevoltages detected at four channels, is larger than 90dB in a laboratory testwith CW as shown in Fig. 9.5.
Gated turn-by-turn detector
The newly developed gated turn-by-turn detectors[5] will be introduced inSuperKEKB. The detector measures the orbit oscillation of a pilot bunch306

Figure 9.5: Signal to noise ratio normalized by a signal level of the electrodeA at the temperature of 20 degrees. The data of five detectors are shown.

at selected BPMs using fast gate, where the pilot bunch is ”a non-colliding
bunch” located at the end of a bunch train. It also measures the beam orbit
of injected beam with small bunch current. About 110 detectors will be
installed in the early stage of commissioning. In the future, they will be
added depending on the budget.
Since the detector will be inserted between the BPM heads and the narrowband detectors, it should fulfill the following characteristics:• It should not disturb the measurement of narrowband detectors verymuch. Noise from the detector, especially switching noise from the fastgate should be suppressed as much as possible.
• The rise and fall time of the gate need to be short enough in order todistinguish the bunch signals with the minimum bunch separation of
4 ns. Also isolation to the turn-by-turn line should be large enough not
to be aﬀected by the large power (70dB larger) coming from bunches.
• The system should be compact, all-in-one package to save the mounting
space and the fabrication cost.
307

Figure 9.6: Block diagram of the gated turn-by-turn detector.

The block diagram of the gated turn-by-turn detector is shown in Fig.
9.6. It has four independent channels corresponding to four BPM electrodes.
Each channel has a fast gate switch, 508 MHz band-pass filters (BPF), low
noise amplifiers (LNA, HMC616) with total gain of 40 dB, a logarithmic
amplifier (ADL5513), a peak-holding circuit with input to reset timing, anda 14 bit ADC (ADS850). All the timing generation and the data processing
are done by a Spartan-6 FPGA (XC6SLX100T-3FGG484) with a 128 MB
DDR3 SDRAM. The data and the command is transferred through Gigabit
Ethernet interface. The system is installed in a box with 1U size.
A Hittite HMC232LP4 is selected as a SPDT IC. As the measured switching noise is large, typically 70 mV peak to peak, and the isolation less than 67
dB is not so good, we employ a switching-noise cancelling method[6] which
combines four SPDT ICs and three 180-deg hybrids as shown in Fig. 9.7.
The switching noise is subtracted using two pairs of a 180- deg hybrid, while
BPM signal is summed with the same polarity to reduce the insertion loss.
For the turn-by-turn measurement, two stages of the SPDT switch are used
to enhance the isolation. According to the measurement by a developed module the measured noise is well suppressed down to 2 mVp-p in both channels.
The rise and fall response is about 0.6 ns and is short enough for bunch separation of 4 ns. The measured insertion loss and the isolation to narrowband
channel around 508 MHz is 2.7 dB and 63 dB, respectively, and those to theturn by turn channel is 4 dB and 80 dB, respectively.
308

Figure 9.7: Block diagram of the noise-cancelling switch.

The timing jitter was measured using histogram function of a sampling
scope with a bandwidth of 20 GHz where the timing of the RF signal andthe gate signal was compared. The jitter was around 3.2 ps and the mean
timing oﬀset stayed within 20 ps, which is small enough for our purpose.
Giga-bit Ethernet connection is used to control the system. On the Spartan6 FPGA, MicroBlase is used to handle the command from the server. As
the large data transfer with the MicroBlase is unacceptably slow, i.e. muchslower than 1 MB/s, we implement SiTCP[7] method on the FPGA. Throughthe SiTCP under GbE connection, data transfer rate amounts to larger than
400 MB/s including the data copy from a DDR3 memory to a FGPA stack.
IP Feedback Detector and Processor Unit
Vertical position at IP of both beams should be controlled by a collision
feedback system to maintain stable collision. The four BPMs closest to IP,
two in each ring, are used for the IP orbit feedback. The vertical orbit shift
at the BPMs by beam-beam deflection is estimated to be several microns.
A simulation study shows that the main frequency components of the orbit
oscillation due to the vibration of the quadrupole magnets closest to the IP
are 25 and 69 Hz.
A special wideband detector for the IP orbit feedback is being developed
to measure the orbit at the BPMs[4, 8]. Target performance of the detector
is resolution less than 1 µm and repetition rate higher than 5 kHz. The

309

Figure 9.8: IP feedback detector.

schematic diagram of the detector is shown in Fig. 9.8.
The detector converts the 508.8MHz component in BPM signal to anintermediate frequency (IF) of 16.9 MHz with an analog mixer to reduce thedegradation of the signal to noise ratio by the clock jitter of an ADC. The
IF signal is digitized by the 16 bit ADC with sampling rate of 99.4 MHz,
then processed by a chain of digital filters (two CICs and a FIR). The cutoﬀ
frequency of the filter is 1 kHz. The digital part is implemented in a microTCA board developed for the SuperKEKB LLRF system[9]. A prototype
circuit is being tested with CW signal. The jitter of the sampling clock of
1.16ps and the position resolution less than 0.1µm is achieved. The groupdelay is 1.1 ms for the FIR with 73 taps.
The calculation of the kicks by the correctors will be done by a processor
unit based on PID control using the detected beam position[4]. The unit
has a FPGA Vertex-5 PowerPC440 on a micro-TCA board as shown in Fig.
9.9. A feedback algorithm is implemented with Matlab/Simulink with systemgenerator from Xilinx for easy coding.

310

Figure 9.9: Digital Signal Processor for IR feedback[4].

Special purpose detectors
If the beam is aborted while it is oscillating horizontally with large amplitude, the aborted beam may hit outside of the abort window then destroy
it. In order to protect the abort window the beam should be aborted if thehorizontal orbit exceeds a predetermined level. This interlock system is also
useful to protect vacuum components from hit of the beam with large oscillation amplitude. The Libera brilliance+ by Instrumentation Technologies[10] modified for our purpose will be used for this orbit interlock [11]. It continuously monitors the turn-by-turn orbit, then the abort request is asserted
in ten turns if the orbit exceeds a predefined interlock level. Fig. 9.10 showsa measurement of interlock latency of a modified Libera brilliance+. Two
Libera brilliance+ in each ring will be installed at positions where diﬀerenceof betatron phase advance is 90 degrees.
Measurement of the beam oscillation is important to get the informationof its aﬀect to the luminosity. Also measurement of the beam oscillation
at the sextupole magnets, where the vertical beta function is very large inorder to correct the chromaticity caused by quadrupole magnets around IP,
is important because the vertical orbit deviation at the sextupole magnets
311

Figure 9.10: A measurement of interlock-latency of a modified Libera brilliance+ processor for orbit interlock. Upper trace is sinusoidal signal supplied
to one of input channels of the processor. Lower trace is an interlock signal
from the processor. The data shows that the latency is about 40 µs(i.e. 4
turns).

312

may cause the enlargement of the vertical emitttance or the vertical beam
size. For these purposes two Libera brilliance+’s in each ring will be installedat the BPMs near the sextupole magnets (i.e. MQLB1RE and MQLC7RE
in HER, MQLB1RP and MQLC3RP in LER) around IP [11].
Two longitudinal phase monitors which measure the longitudinal beamphase with respect to the RF signal will be installed for injection tuning.
They are re-use of KEKB.

9.2.3

Displacement sensor

Displacement sensors were introduced in KEKB to measure the mechanical displacement between a BPM head and a neighboring sextupole magnet
which was supposed to be caused by deformation of vacuum chambers heatedby synchrotron radiation. They will be installed also in SuperKEKB at all
BPMs next to the sextupole magnets.
While most displacement sensors will be re-use of KEKB[12], new displacement sensors will be introduced at BPMs next to twenty-four rotatablesextupole magnets installed in the region for the local chromaticity correction around IP at LER, because tolerance for the displacement of the beamat these magnets is tight, 20µm vertically and 40µm horizontally. A sensor
support made of MMC (Metal Matrix Composite) which is made of SiC (Silicon Carbide) ceramics and silicon metal and has small coeﬃcient of thermal
expansion of 3 10−6 /K, is being developed for the new sensor in order toreduce the systematic measurement error due to the thermal expansion of
the sensor supports as shown in Fig. 9.11.

313

Figure 9.11: A prototype of a sensor support of a BPM displacement sensor
made of Metal Matrix Composite. Parts in gray are made of MMC. A sensor
target made of MMC is mounted on a silver aluminum rod. Actually thetarget will be mounted on a BPM block. A horizontal sensor is seen at theside of the sensor target.

314

Bibliography[1] M. Arinaga et al., ”KEKB beam instrumentation systems”, Nuclear
Instruments and Methods in Physics Research A 499 (2003) 100???137.
[2] M. Tobiyama et al., ”DEVELOPMENT OF BUTTON ELECTRODESFOR SUPERKEKB RINGS”, Proceedings of BIW10, Santa Fe, New
Mexico, US, 2010.
[3] ”Vacuum System”, in this design report.
[4] M. Arinaga et al., ”BEAM INSTRUMENTATION FOR THE SUPERKEKB RINGS”, in proceedings of IBIC2012, Tsukuba, Japan,
2013, p.6-10.
[5] M. Tobiyama, H.Fukuma, H.Ishii and K.Mori, in proceedings of
IBIC2013, Oxford, UK. 2013 MOPF23.
[6] T. Naito, et al., in proceedings of IPAC2013, Shanghai, China, p.506p.508, (2013)[7] T. Uchida, IEEE Trans. Nucl. Sci. Vol. 55, No.3, 1631-1637, 2008.
[8] H. Ishii et al., ”DEVELOPMENT OF A BEAM POSITION DETECTOR FOR AN ORBIT FEEDBACK SYSTEM IN SuperKEKB”, inProceedings of the 8th Annual meeting of Particle Accelerator Societyof Japan, 2011.
[9] T. Kobayashi et al., ” PROTOTYPE PERFORMANCE OF DIGITALLLRF CONTROL SYSTEM FOR SUPERKEKB”, in Proceedings of
IPAC’12, 3470(2012).
[10] http://www.i-tech.si/315

[11] S. Kanaeda et al., ” PERFORMANCE EVALUATION OF THE LIBERA BRILLIANCE+ AS A BEAM ORBIT OSCILLATION MONITOR AND A BEAM POSITION INTERLOCK MONITOR IN SUPERKEKB”, in Proceedings of the 10th Annual meeting of Particle
Accelerator Society of Japan, 2013.
[12] Mitsuhiro Arinaga, John W. Flanagan, Hitoshi Fukuma, Takaaki Furuya, Shigenori Hiramatsu, Hitomi Ikeda, Hitoshi Ishii, Eiji Kikutani,
Toshiyuki Mitsuhashi, Kenji Mori, Masaki Tejima, and Makoto Tobiyama, Prog. Theor. Exp. Phys. (2013) 03A007.

9.3

Bunch by Bunch Feedback System

9.3.1

Design Considerations

The machine development on the bunch feedback system during the KEKB
operation has shown the growth rates of the rings were around the orderof few 100 µs or faster[1]. The main source of the impedance has been
estimated to be the electron cloud instability for LER[1, 2, 3], and the fastion instability for HER. They has been anticipated to play major role also
in the SuperKEKB rings. The use of the transverse bunch feedback systems
will be essential for the smooth commissioning of the rings.
The eﬀect of the noise in the bunch feedback system during collision has
also been measured during KEKB operation[4, 5]. Due to the beam-beam
eﬀect, the colliding beam showed very wide frequency response besides thebetatron frequency. Reduction of the noise components in the bunch feedback
system, especially in the detector part is essential to keep the noise eﬀect
lower than the level that aﬀects the beam size at the collision point. The
signal coupling between the neighboring bunches should also be suppressed.
Though the betatron tunes at the design stage are not very near to thehalf integer in both horizontal and vertical planes, it is still near to the half
integer which limits the stable maximum gain of the feedback system. We willsolve the diﬃculty by employing the dual feedback loops with the betatronphase response roughly 90◦ apart on both horizontal and vertical planes.
In KEKB operation, the longitudinal bunch feedback system was notneeded thanks to the slower growth rate of the instability in nominal operation mode. However, it is expected that the trapped modes in the ARES
cavities might cause longitudinal coupled-bunch instabilities at high beam316

Figure 9.12: Location of the bunch feedback components in SuperKEKB. All
the components will be installed at Fuji straight section same as the case of
KEKB.

current in LER. We will prepare longitudinal feedback system on LER. For
HER, the system is optional because of estimated low growth rate and will
not be installed at the start of the beam commissioning.
All the bunch feedback components will be installed in the Fuji straightsection as shown in Fig. 9.12. The number of feedback components installedis shown in Table 9.3.
Figure 9.13 shows the block diagram of the transverse and longitudinal
feedback systems for SuperKEKB.
Table 9.3: The number of bunch feedback vacuum components installed in
Fuji straight section. Parenthesized numbers show future plans.
Component
HER LER
Bunch position monitor
22Transverse feedback kicker
2
2
Longitudinal feedback kicker 0 (6) 2 (4)9.3.2

Signal Pickup

We have developed a button electrode with improved impulse response by
employing a glass-type sealing with a low relative dielectric constant ϵr = 4,
as shown in Fig. 9.14[6].
317Figure 9.13: Block diagram of the transverse feedback systems (upper) andlongitudinal feedback systems (lower) for SuperKEKB.

318

Figure 9.14: Glass-type feedthrough for bunch feedback systems.

The diameter of the button electrode is 6 mm, same as ones used at
KEKB. A reverse-type SMA connector with gold-plating is used for the RF
output connector. We will attach a 3-dB attenuator directly to the SMA-R
connector to reduce the impedance mismatch.
The simulated S-parameter using ANSYS-HFSS[7] is shown in Fig. 9.15.
Twenty four button electrodes (8 horizontal, 8 vertical and 8 between
horizontal and vertical planes) will be welded to a circular monitor chamberwith inner diameter of 64 mm as shown in Fig. 9.16.
The cutoﬀ frequency of the monitor chamber can be calculated analytically as shown in Table 9.4.
Table 9.4: Cutoﬀ frequency of the monitor chamber.
Mode Frequency (GHz)
TE11
2.70
TM01
3.53
TE12
4.48

Though the lowest cutoﬀ frequency is higher than 2.5 GHz, we will use
the 2 GHz(= 4 × fRF ) components of the beam, at least in the early stageof the commissioning, to keep the compatibility with the previous detection
system[8, 9]. The GdfidL[10] model of an electrode is shown in Fig. 9.19.
319

Figure 9.15: Simulated S21 of the feedthrough using HFSS.

The simulated output and the coupling impedance for a bunch of σz = 6 mmlong is shown in Figs. 9.17, 9.18. The longitudinal loss factor for the bunch
length σz = 6mm is 0.9 mV/pC per four electrodes. The beam responseof the electrodes using a short beam at test line of KEK-PF BT from linac
has been measured and confirmed to be better than that of ones used at
KEKB[6].

9.3.3

Bunch position detection

Figure detect shows the block diagram of the bunch position and bunchcharge detectors.
The 2 GHz component (= 4 × fRF ) of the signals from the electrodeis filtered by the 3-cycles-comb-coupled filter with the center frequency of
2 GHz. Though the insertion loss of the filter is huge, i.e. about 20 dB, therejection around lower frequency is much larger and the bunch isolation is alsomuch better than that made of a cable-combiner type comb filter. The outputof the comb filter is amplified by the wideband low noise amplifier with thenoise figure less than 1 dB. For the fine adjustment of the timing between twoelectrodes, a trombone-delay with maximum stroke of 10 cm is inserted. To
make a diﬀerential signal and a sum signal from the two inputs, a wideband

320Figure 9.16: Drawing of the HER monitor chamber.

321

Figure 9.17: Simulated BPM output for 1 mA with σz = 6mm bunch signal.

Figure 9.18: Simulated BPM impedance (real part and imaginary part).

322

Figure 9.19: GdfidL model of the BPM for the bunch by bunch feedback
system.

Figure 9.20: Block diagram of the transverse bunch position detector (top)and the longitudinal bunch phase and bunch charge detector (bottom).

323

Figure 9.21: Picture of the transverse detector.

180-degree hybrid (H-184-3) is used. To cancel the DC components from thediﬀerence signal, an amplitude and phase (+ or -) controlled signal-sum is
injected with the same timing as the diﬀerence signal. The timing between
two paths has been tuned using a SRD test pulsar. After the homodyne
detection using a double balanced mixer (DBM), the baseband component isfiltered by a Bessel type low pass filter with the cutoﬀ frequency of 600 MHz.
The output is split into two ways and amplified with DC amplifiers. For thelongitudinal detection, we replace the hybrid with a simple wideband power
combiner and make a homodyne detection, similar to the transverse detector.
Table 9.5 shows the total number of detectors installed in the local control
rack for the bunch by bunch feedback system.
Table 9.5: The number of bunch position detectors.
HER LER
Transverse detector
44Longitudinal detector
11Bunch current detector
11Figure 9.21 shows the inside of the transverse detector. The size fits EIA1U as shown in Fig. 9.22 and has a slide mechanism which enables us to tune
the internal components, especially the trombone-delay to adjust the timing
diﬀerence between two electrodes without dismounting the system.

324

Figure 9.22: Picture of the transverse and longitudinal detectors installed ina rack.

9.3.4

Signal Processing

The iGp12 digital filters[11] will be used for the signal processing part ofthe systems. It has 12-bit ADC, 12-bit DAC and 12 Msamples of external
recording memory (SRAM). We have received 10 iGp12’s in total, 8 of them
are with a large Virtex-5 FPGA (VSX95T) and the others are with a normal
FPGA (VSX50T). With the normal FPGA, a 10-taps FIR filter is realized,
while with the larger FPGA, a 20-taps FIR configuration is available. On thetransverse feedback systems, as the time lag coming from the longer FIR filter
reduces maximum stable feedback gain especially when the betatron tune isnear half or integer tune[12], a 8-taps FIR or a 6-taps FIR filter configuration
will be used in real operation. On the other hand for the longitudinal system,a FIR filter with more taps enables us to configure the system without or withsmall number of down-sampling factor which also limits the maximum stable
feedback gain. Though we still need downsampling for fs ∼ 0.02 even with amaximum 20-taps FIR filter, we expect the reduction of the downsampling
factor enables us larger feedback gain and easier operation.
The following special functions will be implemented in the iGp12s.
• Beam transfer function measurement for a selected bunch.
• Single bunch excitation function using PLL.
This function enables us to measure the optics function, especially the
betatron phase advance between the gated turn-by-turn monitors during the collision.
325

Figure 9.23: iGp12 digital filters installed in the LER-feedback control room.

We have installed the received iGp12’s in the feedback control room asshown in Fig. 9.23 and examined the long-term stability. Some diﬃculties
have already fixed. The current 8-bit iGps will be used as the feedback signal
processors for the damping ring and also for the longitudinal phase monitor
for HER.
Since we will use the horizontal-vertical combined stripline kickers sameas ones used at KEKB, it is necessary to combine two outputs (horizontal
and vertical) of the iGp12s. We have developed the active hybrid circuits
made with wideband diﬀerential amplifiers which pass from DC to nearly
500 MHz signals. The active hybrid circuits will be replaced with the passive
RF hybrid circuits used in KEKB.
For the longitudinal modulation, we will continue to use QPSK modulation with the carrier frequency of 1.15GHz(= 2.25 × fRF ). A digital QPSK
modulator has already been developed and has shown excellent performanceon the KEK-PF longitudinal feedback system[13].

9.3.5Power Amplifiers

Transverse amplifiers
To make the dual feedback loop on the transverse plane, we will add fourhigh power feedback amplifiers for each rings. Tables 9.6 and 9.7 show main
parameters of the high power amplifiers, including old amplifiers used at
KEKB bunch feedback systems.
The amplifiers will be installed on B4 floor under the Fuji crossing bridge
326

Table 9.6: Main parameters of 500 W transverse feedback amplifiers.
Name
R&K CA010K251-5757R
No. of available Amps.
12
units
Output polarity
Negative
Bandwidth0.01 ∼ 254
MHz
Gain
> 57
dB
Gain flatness
±2
dB
P1 dB
> 350
W
Psat dB> 500
W
Input VSWR
< 1.5
Output VSWR
< 3.0
Max. Consumption Curr.(200 V)
at Pout = 500 W
20
A
Size
480 × 650 × 310
mm
Weight70kg

where almost no radiation is expected. They will be monitored and controlled remotely through GP-IB (R&K amplifiers) or parallel interface (ARamplifiers). As both the pulse response and the maximum power of theCA010K251-5757R amplifiers are much better than the others, we will replace the old amplifiers with CA010K251-5757R after the commissioning.
Old 250A250 amplifiers will be used at the bunch feedback systems for thedamping ring.
The CA010K251-5757R amplifiers have been tested at the J-PARC MR
intra-bunch feedback system with the required bandwidth of 10 kHz ∼ 100 MHz
and has shown excellent performance. In addition, several minor diﬃculties
have been fixed during the operation.
To protect the transverse amplifiers from beam induced power, we willinsert high-power low pass filters (FSY LD 430-7NN-HP) with the cutoﬀ
frequency of 300 MHz and the maximum power rating of 1 kW.
Figure 9.24 shows the transverse feedback amplifiers installed at Fuji-B4.
Longitudinal amplifiers
As the carrier frequency of the longitudinal kickers is around 1.14GHz = (fRF × 2.25),
we have prepared wideband UHF amplifiers with maximum power of 500 W,
327

Figure 9.24: Transverse feedback amplifiers 250A250s (left) and CA010K2515757Rs (right).

328

Table 9.7: Main parameters of 250 W transverse feedback amplifiers.
Name
AR 250A250
No. of available Amps.
11
units
Output polarity
Negative
Bandwidth0.01 ∼ 250
MHz
Gain
> 54
dB
Gain flatness
± 1.5
dB
P1 dB
> 150
W
Psat dB> 250
W
Input VSWR
< 1.5
Output VSWR
< 2.5
Max. Consumption Curr.(200 V)
at Pout = 250 W
7ASize
503 × 427 × 551 mm
Weight68kg

as shown in Table 9.8.
In addition to the CA901M182-5757R, we have two sets of MILIMEGA
AS0920-500T amplifiers. The parameters of the AS0920-500T amplifiers are
similar to those of the CA901M182K-5757R amplifiers, with slightly poor
pulse response. Again, all the amplifiers will be installed on the B4 floor
under the Fuji crossing bridge as shown in Fig. 9.8 where almost no radiation
is expected.
As the longitudinal kicker has no directionality in power, we will installa high power wideband circulator (Pmax = 5 kW, BW 900 MHz ∼ 1.8 GHz)and a high power low pass filter of power-consumption type between theamplifier and the kicker.

9.3.6

Transverse feedback kickers

Very similar stripline kickers to those used at the KEKB bunch feedback
systems will be used, at the least at the early stage of the commissioning.
Table 9.9 shows main parameters of the transverse kickers.
Examples of the drawing and the photo of the transverse stripline kicker
are shown in Figs. 9.26, 9.27. The stripline electrode is 40 cm long andmade of stainless steel coated with Cu thicker than 10 µm. The downstream329

Figure 9.25: Longitudinal feedback amplifiers.

330

Table 9.8: Main parameters of longitudinal feedback amplifiers.
Name
R&K CA901M182K-5757R
No. of available Amps.
8units
Output Polarity
Negative
Bandwidth
900 ∼ 1800
MHz
Gain
> 57
dB
Gain flatness
± 1.5
dB
P1 dB
> 398
W
Psat dB> 500
W
Input VSWR
< 2.0
Output VSWR
< 2.0
Max. Consumption Curr.(200 V)
at Pout = 500 W
25
A
Size
480 × 650 × 265.9mm
Weight68kg

feedthroughs have bending structure made of BeCu to release thermal expansion due to the heat cycle of the stripline electrode[9]. We have changed
the RF connector interface of the kicker from 20D-type to EIA-7/8 type[14],though all the dimensions are completely same except for the fixing hole on
the flange.
The longitudinal and transverse impedance and the output of the stripline
kicker are simulated using GdfidL. Figures 9.28, 9.29, 9.30, 9.31 show the
GdfidL model (1/4), longitudinal impedance, simulated output for σz = 6mm
bunches at the upstream and downstream ports, respectively.
The longitudinal loss factor for σz = 6 mm bunch is 0.185 V/pC.
Figure 9.32 shows the transverse impedance with horizontal oﬀset of 1 mm
for the LER stripline kicker.
For higher beam power after phase 3 operation will has started, where
maximum beam current of LER will exceed 2 A, we might need better structure tolerant toward much larger power. We will prepare new structure before
phase 3 operation with the collaboration of SLAC.

331

Table 9.9: Main parameters of the transverse feedback kicker.
HER LER
Number of kickers
22
Number of electrodes
44
Inner diameter
104
94
mm
Stripline length
4141cm
Stripline angle
60
60
degreeStripline position from center
39.2 35.2 mm
Stripline thickness
22mm
Stripline characteristic impedance
50
50Ω
Shunt impedance at lowest freq.
∼ 10 ∼ 10 kΩ
Loss factor
0.176 0.185 V/pC
Kick factor
4.15 5.28 V/pC/m

9.3.7

Longitudinal feedback kickers

We have designed a new longitudinal feedback kicker based on the overcoupled cavity developed at DAFNE[15] to fit our requirement. The basic
parameters of the kicker are shown in Table 9.10.
Table 9.10: Basic parameters of the longitudinal feedback kicker.
Input port2
Output port
2
Center Frequency
1.145
GHz
3dB Bandwidth
240
MHz
Shunt Impedance
1.68
kΩLongitudinal loss factor
0.406
V/pC /Cavity
Transverse kick factor
2.06 V/pC/m /Cavity
Length (flange-to-flange) 0.44m

Figures 9.33, 9.34 show the HFSS model and the simulated S-parameter,
respectively.
Using GdfidL, we have also estimated the time-domain behavior of the
kicker. Figures 9.35, 9.36 and 9.37 show the GdfidL model, estimated longitudinal impedance and port output for σz = 6 mm bunch, respectively.
332

Figure 9.26: Drawing of the LER transverse stripline kicker.

9.3.8

Bunch feedback related systems

We will use the (part of the) bunch feedback systems for the measurementof betatron tunes, bunch current, bunch oscillation and other related measurements such as the beam-beam kick due to collision.
Betatron tune measurement
We will keep the current betatron tune measurement system used at KEKB[8,9]. There are two systems:• Global tune measurement system.
We detect the BPM signal directly using a spectrum analyzer. The
output of the tracking generator of the spectrum analyzer is downconverted to baseband signal, and is injected to the feedback loop after
the digital filter. We will use this scheme to measure the tunes during the optics correction at low beam current (∼50 mA) where manybunches with fairly small bunch current will exist. The waveform of
the spectrum is transferred to EPICS waveform and will be fitted on
the optics panel.
• Gated tune measurement system.
We can excite the non-colliding bunch (“Pilot bunch”) not being ap333

Figure 9.27: Picture of the HER transverse stripline kicker.

Figure 9.28: GdfidL model of the LER transverse stripline kicker(1/4).

334

Figure 9.29: Calculated longitudinal impedance of the LER transverse kicker.

Figure 9.30: Simulated output at upstream port from 1mA/bunch beam.

335

Figure 9.31: Simulated output at downstream port from 1mA/bunch beam.

Figure 9.32: Calculated transverse impedance of the LER transverse kicker
for the horizontal beam oﬀset of 1 mm.

336

Figure 9.33: HFSS model and simulated E-field magnitude of the longitudinal
kicker.

Figure 9.34: Simulated S-parameter using HFSS of the longitudinal kicker.

337

Figure 9.35: GdfidL model of the longitudinal kicker (1/4).

Figure 9.36: Calculated longitudinal impedance of the longitudinal kicker.

338

Figure 9.37: Calculated output at the port from σz = 6 mm bunch for thelongitudinal kicker.

plied transverse feedback. The system consists of a gated pulsar, single
bunch oscillation detection system (BOD) and a spectrum analyzer
with a tracking generator. We have used this system to monitor the
betatron tune during collision in KEKB. Though we plan to keep thesame system, we will omit the analog fast gate to turn oﬀ the feedback signal to the pilot bunch– such fast switching will be made in theiGp12 digital filter, if necessary. Moreover, as we will prepare singlebunch betatron oscillation excitation system using PLL-function of theiGp12, this old-fashioned system will be expected to be obsolete soon.
Bunch current monitor
In KEKB, we have measured the bunch current using the bunch feedback
techniques (bunch phase detector, hardware two-tap FIR filter, etc.)[16]. We
will upgrade the ADC system from old two-taps based system to new systemby employing modern technique, such as fast wideband ADC, very fast FPGA
and so on. The MAX108 ADC with the bandwidth larger than 1.5 GHz andresolution of 8-bit will be used. The output of the ADC is accumulated using
Spartan-6 FPGA. For the bunch current monitor, a block-RAM in the FPGA
should be enough even for the KEKB rings. We will also use a huge external339

DDR3 memory to measure the longer-time bunch behavior which enable us
to replace the old bunch oscillation recorder based on also two-taps FIR filter
system to the same ADC system.
Beam-beam kick measurements
We have installed special beam measurement systems using the bunch feedback signal in KEKB to measure special beam behavior such as beam-beamkick or transient beam phase during collision[17, 18]. Very old electronics has
been used in this system and it is not easy to keep the function using same
modules. We hope to keep this system on SuperKEKB but the resource isfairly limited so the priority for the system is fairly low.

340

Bibliography[1] M. Tobiyama, J. W. Flanagan, H. Fukuma, S. Kurokawa, K. Ohmi and
S. S. Win, Phys. Rev. ST Accl. Beams 9, 012801 (2006).
[2] J. W. Flanagan, K. Ohmi, H. Fukuma, S. Hiramatsu, M. Tobiyama, and
E. Perevedentsev Phys. Rev. Lett. 94, 054801 (2005).
[3] S. S. Win, K. Ohmi, H. Fukuma, M. Tobiyama, J. W. Flanagan and
S. Kurokawa, Phys. Rev. ST Accl. Beams 8, 094401 (2005)[4] M. Tobiyama and K. Ohmi, in proceedings of DIPAC2011, Hamburg,
Germany, MOPD73.
[5] K. Ohmi, R. Tomas, Y. Funakoshi, R. Calaga, T. Ieiri, Y. Morita,
K. Nakanishi, K. Oide, Y. Ohnishi, Y. Sun, M. Tobiyama and F. Zimmerman PRST-AB, 14, 111003 (2011).
[6] M. Tobiyama, H. Fukuma, K. Shibata, M. Tejima, S. Hiramatsu,
K. Mori, H. Ishii, T. Obina, in proceedings of BIW2010, Santa Fe, 2010.
[7] ANSYS HFSS, http://ansys.jp/products/electromagnetics/hfss/[8] M. Arinaga, J. Flanagan, S. Hiramatsu, T. Ieiri, H. Ikeda, H. Ishii,
E. Kikutani , T. Mimashi, T. Mitsuhashi, H. Mizuno, K. Mori,
M. Tejima and M. Tobiyama Nucl. Instrum. Meth. Phys. Res. A499,
p100-137, (2003).
[9] Mitsuhiro Arinaga, John W. Flanagan, Hitoshi Fukuma, Takaaki Furuya, Shigenori Hiramatsu, Hitomi Ikeda, Hitoshi Ishii, Eiji Kikutani,
Toshiyuki Mitsuhashi, Kenji Mori, Masaki Tejima, and Makoto Tobiyama, Prog. Theor. Exp. Phys. (2013) 03A007.
[10] http://www.gdfidl.de/341

[11] http://www.dimtel.com/[12] S. Hiramatsu, private communication.
[13] R. Takai, M. Tobiyama, T. Obina, J. W. Flanagan, M. Tadano, T. Mitsuhahi, in proceedings of DIPAC09, Basel, Switzerland, p.59-61, 2009[14] EIA Standard RS-225.
[15] R. Boni et al., Particle Accelerators 52 (1996) 95-113.
[16] Makoto Tobiyama and Eiji Kikutani Phys. Rev. ST Accl. Beams
3,012801(2000).
[17] T. Ieiri, K. Akai, H. Fukuma, T. Kawamoto, E. Kikutani, and M. Tobiyama Phys. Rev. ST Accl. Beams 5,094402(2002).
[18] T. Ieiri, K. Akai, H. Fukuma and M. Tobiyama Nuclear Instruments andMethods in Physics Research A 606 (2009) 248-256.

9.4

Photon Monitors

We will use two diﬀerent types of synchrotron radiation (SR) monitors forbeam size diagnostics: x-ray monitors and visible-light monitors. The x-ray
monitors will be used primarily for vertical bunch profile measurements, andthe visible light monitors will be used primarily for horizontal and longitudinal bunch profile measurements, with the possibility of vertical measurements
at larger beam sizes for comparison with the x-ray monitor measurements.
The collision quality will be monitored using a large-angle beamstrahlung
monitor. An energy monitor based on inverse Compton scattering is underconsideration for future development, though will not be required at start-up,
and is not discussed here.

9.4.1

Visible-light SR monitor

Overview
As at KEKB[1], we will use SR interferometers (using either the horizontal
or vertical polarization component) to measure beam sizes from the visible342

Table 9.11: Visible SR monitor source parameters.
Parameter
Horizontal emittance ϵxEmittance coupling κ
Vertical emittance ϵyHorizontal beta function βyVertical beta function βyHorizontal beam size σyVertical beam size σyBeam Energy
Bend eﬀective length
Bend angle
Bend radius ρ
Observation wavelength λSR opening angle θc (λ)
Hor. slits opening angle αVer. slits opening angle α
Required max. measurable hor. visibility γxmax
Required max. measurable ver. visibility γymax
Min. measurable hor. beam size σxmin
Min. measurable ver. beam size σymin

343

LER
3.20 × 10−90.27%
8.64 × 10−12
8.86
30.71
168
16.3
4
0.89
5.04
179
400
1.0
0.2
0.8
90%
98%
164
15.7

HER
4.54 × 10−90.25%
1.14 × 10−11
10.17
26.92
215
17.5
7
2.90
5.00
580
400
0.7
0.2
0.9
90%
98%
157
15.0

Units
m
m
m
m
µm
µm
GeV
m
mrad
m
nm
mrad
mrad
mrad

µm
µm

component of synchrotron radiation from source bends in the LER and HER.
The parameters of the source points are shown in Table 9.11.
When the LER and HER beams have achieved their targeted low emittances, the vertical beam sizes at the monitor source points are below 18 µm
in both the LER and the HER. The resolution of the SR interferometer isfundamentally limited by the measurement wavelength λ and the opening
angle α between slits as seen from the beam source point, as well as the
maximum visibility (fringe modulation) that can be reliably
measured; for a√λ1Gaussian beam, the beam size σ is given by σ = π(α)
ln γ1 . The opening
2
angle is in principle limited by the SR opening angle and by mechanical considerations, with the latter being the dominant restriction at SuperKEKB;
the mirrors are located in antechambers to minimize HOM losses, with theheight of the antechambers limited by the pole gap of upstream quadrupole
magnets to 24 mm in height. The mirrors are located at a distance F of
24.5 (23.5) m downstream of the source point in the LER (HER), so witha maximum eﬀective slit separation D of 20 mm at the mirror location, theslits opening angle α = D/F is 0.8 (0.9) mrad in the LER (HER). For a 400
nm measurement wavelength, it would be necessary to be able to measure
visibilities of around 98% to measure the vertical beam sizes at the source
points. This would be extremely challenging.
In the horizontal direction, the beam sizes are 10 times or more larger
than the vertical ones, so the required measurable maximum visibility is amuch more reasonable 90%, assuming 9 mm-wide mirrors.
Accordingly, the visible SR interferometers will primarily be used forhorizontal beam size measurements. In the vertical direction, it can still
be used at larger beam sizes for cross-calibration with the x-ray beam size
monitor.
The SR interferometer will not be capable of single-bunch, single-shotmeasurements, only integrated ones over many turns of the whole beam, due
to limited photon statistics at shorter integration times.
The longitudinal bunch profiles will be measured by streak-cameras.
Extraction mirrors
Beryllium mirrors used at KEKB suﬀered from heat distortion due to absorbed SR power, which caused the apparent beam size to change as a function of beam current. The synchrotron radiation monitors at SuperKEKB

344

Table 9.12: Visible SR monitor source point power.
KEKB
SuperKEKBParameter
LER HER LER HER Units
Beam energy
3.5
8
4
7
GeV
Beam current
2
1.4
3.6
2.6 A
Bending radius 85.7 580 177.4 580 m
SR power
48
136
72
149 W/mrad

will use source bends in the same locations as at KEKB, which are the 5 mrad???weak bends??? heading into the Fuji (LER) and Oho (HER) straight sections. At SuperKEKB, the beam currents will be higher, as shown in Table
9.12. The source bend for the LER will be replaced with one with a longer
core and larger bending radius (a re-used LER regular arc bend magnet) inorder to reduce the SR power intensity. Despite the reduced bending radius,
the increase in beam energy and beam current means that the incident angular power density will be higher (72 W/mrad) than that of the KEKB LER(48 W/mrad). The source bend will remain the same at the HER, and theeﬀect of the increased current of the beam is almost cancelled out by the
reduced beam energy, so that the angular power density is only a little bit
higher than at KEKB.
It is planned to move the chambers a bit further downstream from thesource bends than they were at KEKB, which will help reduce the SR power
intensity hitting the primary extraction mirrors. However, the heat deformation of the KEKB mirrors was already a very significant problem, requiring
complicated measures to measure and compensate for the distortion in realtime in order to correct the beam-current dependence on the measured beam
size. For this reason, we are planning to use mirrors made of diamond[2].
The superior thermal conductivity and low thermal expansion coeﬃcient of
diamond is expected to make the the apparent change in magnification atfull current much lower than was seen with the beryllium mirrors at KEKB.
The design of the mirror is based on CVD diamond, made by Cornes
Technology and EDP Corporation. For best thermal conductivity, the ideal
mirror material would be a pure monocrystalline diamond, rather than apolycrystalline one. The mirrors that have been fabricated for initial use
at SuperKEKB consist of 3 monocrystalline segments, each approximately
9 mm x 9 mm, joined together to make a 9 mm x 28 mm rectangle, 0.8345Figure 9.38: Gold-coated diamond mirror for SuperKEKB visible SR monitor.

mm thick. For optical reflectivity, a metallic surface is needed. A 1 µm Au
reflective surface is bonded to the diamond substrate with a Cr bonding layer
between the gold and diamond. An example of one of the produced mirrors isshown in Figure 9.38. The monocrystalline diamond also gives good surface
smoothness after polishing; production samples show Ra values between 1.1
and 7 nm.
ANSYS simulations showed that for any holder design which touches themirror on all edges or along the back side, the deformation of the holder
would be large enough to cause unacceptable deformations in the surface ofthe diamond mirror, much larger than the deformations caused by the heat
distribution pattern in the diamond mirror itself. The holder design thathas been adopted is a split cylinder of soft copper, which grips the mirror
near one edge only. The soft copper permits good heat-sinking contact withthe portion of the mirror surface gripped within the split (about 5 mm),without placing extraneous strain on the surface of the mirror due to heat
deformation of the copper. The holder is shown in Figure 9.39.
The mirror is set inside a 24 mm-high antechamber, as shown in Figure
9.40, to minimize HOM losses. The loss factor for the mounted mirror assembly is calculated using GdfidL to be 0.8 × 10−6 V/pC, which correspondto an expected loss of 41 mW at full current in the LER.
The visible-light SR monitor will start commissioning from Phase I.

346

Figure 9.39: Heat-sinking holder for diamond mirror. A metallic dummy
plate (with hole in it) is inserted in place of the mirror. Actual mirror has
no hole in it.

Figure 9.40: SR extraction chamber with metallic dummy plate (with hole init) inserted in place of the mirror. Actual mirror has no hole in it. (Extraction
chamber design by Y. Suetsugu, vacuum group.)347

9.4.2

X-ray beam profile monitor

Overview
X-ray beam profile monitors are being installed in each ring to provide highresolution bunch-by-bunch, turn-by-turn measurement capability for lowemittance tuning, collision tuning, and instability measurements. Two sets of
optics will be installed, one a simple pinhole (single-slit) type, and the other acoded aperture. Coded aperture imaging is a technique originally developedby X-ray astronomers[3], using a pattern mask to modulate the incoming
light. The projected image on the detector is then decoded using the known
mask pattern to reconstruct the original image. The large open aperture provides much higher photon throughput than a single pinhole, allowing better
photon statistics-limited resolution in single-shot measurements. Researchand development of the monitors has been carried out in collaboration with
Cornell University, University of Hawaii and SLAC. Beam size measurements
using coded apertures down to ≈ 10 µm have been demonstrated at the CesrTA accelerator at Cornell[4].
Beam lines and heat load
The X-ray sources are the last arc-bends located immediately upstream ofthe straight sections in Fuji (LER) and Oho (HER). The final LER arc bend
for SuperKEKB has been split into two re-used KEKB arc bends, with the xray source point being located in the upstream half of the pair. This provides
more x-ray flux for the LER monitor than would have been received fromone of the new SuperKEKB LER arc bends.
The parameters for the beamlines are shown in Table 9.13. The beamlines are about 40 meters long from the source points to the detectors. The
optical elements (pinhole, coded aperture) are located in optics boxes ≈ 9−10
meters from the source point, for geometrical magnification factors of ≈ 3×for both lines.
The beam lines are connected directly to the ring vacuums. Gate valves
will be located upstream of the optics boxes for isolation when opening theoptics boxes or doing other vacuum work downstream on the beam lines, orin the event of a vacuum leak downstream. Insertable beam stoppers will belocated directly upstream of the gate valves to protect them from the highincident SR power when needed.
The incident power density on the optics boxes with no filters are 23 and348

Figure 9.41: Location of x-ray monitor beam line for the LER in the Fuji
straight section.

Figure 9.42: Location of x-ray monitor beam line for the HER in the Fuji
straight section.

349

Table 9.13: X-ray monitor beam line parameters.
Parameter
LER
Vertical beam size σy at source point
20
Beam energy
4
Source bend radius ρ
31.74
Critical energy
4.4
Zero-degree source SR power density
0.56
Distance from source to optic
9.39
Beam current
3.6
Incident power density on optic (w/no filter)
23
Distance from source to detector
40.7
Be extraction window thickness
200
Power density on detector (w/no filter, optics out) 0.7

HER
11
7
106
7.1
2.8
10.27
2.6
69
42.55
200
2.8

Units
µm
GeV
m
keV
kW/mrad2 /A
m
A
W/mm2
m
µm
W/mm2

69 W/mm2 for the LER and HER, respectively. The optics elements consistof ≈ 20 µm thick gold masking material on silicon or diamond substrates.
High-power tests carried out at CesrTA[5] indicate that silicon-substrate xray optics elements can just tolerate the LER power density, but with very
little margin. The diamond-substrate mask is more robust (due to the betterhear-conduction of diamond over silicon), so has more margin available. A
Be filter may be used upstream in the LER to bring incident power levels
down a bit further, though this would come at the expense of photon flux. In
the HER, upstream Be filters are required to bring the incident power level
down to that of the LER line. The Be filters will be placed upstream of the
stoppers. Further upstream of the Be filters will be 4 mm-wide collimators,
to block unnecessary oﬀ-axis x-ray power from hitting the beam pipe walls.
Screen monitors will be installed on the downstream sides of the beamlines, midway between the optics boxes and detectors, to aid in final, x-ray
beam-based alignment.
Detector and read-out
A one-dimensional pixel array will be used, primarily for vertical beam size
measurements. An InGaAs array with 50 µm pitch has been used at CesrTA,and it is very eﬃcient for low-energy x-rays (up to a few keV). A similar array350

has been prepared for SuperKEKB, but its eﬃciency for x-ray detection drops
oﬀ at higher energies, due to the thinness of the sensing element (about
3.5 µm deep). In order to detect more eﬃciently x-rays in the 10-20 keV
range, where the SuperKEKB spectrum peaks after passage through beamline elements, a novel type of detector is under development at SLAC for use
at SuperKEKB. This detector is made of a silicon wafer mounted edge-on to
the beam, providing a sensing depth of 2 mm, which is enough to absorb 90%of the incident spectrum expected. The high collection eﬃciency will improve
the statistical resolution for single-shot measurements. Readout traces run
parallel to the beam direction, so that electron-hole pairs created anywhere
along the sensing depth can be read out rapidly. The thickness of the wafer
in the direction perpendicular to the beam is 75 µm. Simulations show thatthe tails of the current time distribution fall oﬀ within 2 ns at a 200 V bias
voltage, suﬃcient for good bunch-to-bunch separation. (The rise time of thecharge pulse is a few tens of ps, which may potentially open up possibilities
for crude longitudinal diagnostics for head-tail instability studies, dependingon the coupling characteristics of the readout.)
A layout diagram of the detector strip design in fabrication at SLAC isshown on the left in Figure 9.43. The collection eﬃciency can be improved
further by stacking detector strips perpendicular to the beam axis and ganging corresponding pixels of parallel strips together; the strip on the right in
the figure is designed for easy stacking with the strip on the left. Both strips
comprise 128 traces at 50 µm spacing. This spacing was chosen based on simulations of the statistical resolution achievable with various candidate optics
patterns as a function of detector pixel size. Charge blooming within thedetector is expected to be about 2-4 µm, based on CASINO[6] simulations.
A high detection eﬃciency implies a high absorbed heat load. It isplanned to mount the detector strips on monocrystalline diamond heatsinks(similar to the ones used for the mask substrates). Calculations of the detectable spectra using this detector are shown in Figure 9.44.
The digitizing and read-out electronics are being developed at the University of Hawaii, based on high-speed read-out systems developed there forhigh-energy physics detectors. A 64-channel development motherboard isshown in Figure 9.45. It features interchangeable daughter cards for use
with diﬀerent digitizer ASICs. Two such custom ASICs have been developed
so far, one optimized for reading out a few bunches (or bunch slices) overmany turns, and one for reading out all bunches over a few turns. Further
ASIC development is expected as familiarity with the current ones is devel351

Figure 9.43: High-energy x-ray detector layout. Left: Primary detector strip
layout. Right: Secondary detector strip for stacking in parallel with primary
detector strip.

oped, and as needs evolve. Full 128-channel readout is also planned for the
future.
To analyze the data, a template-fitting procedure has been developed foranalysis of CesrTA data. A 64-CPU analysis computer will be dedicated toeach beam line for performing rapid fitting of the x-ray monitor data, withresults to be written to the EPICS database for machine tuning purposes.
Optics and resolution
The single-shot resolution of the x-ray monitor at low intensities is dominatedby photon statistics. The resolutions given by diﬀerent mask patterns havebeen evaluated by calculating the distinguishability between the images seenby the detector for beams of diﬀerent sizes. An approach similar to that
described in Ref. [7] has been used to calculate the suitability of diﬀerent
x-ray optic (mask) patterns at diﬀerent beam sizes.
Figure 9.46 shows, on the left, the simulated detector images for a 32 µm
pinhole (slit) pattern, assuming gold masking on a diamond substrate andthe high-energy detector under development, at the SuperKEKB LER. On
the right are the resolution contours for a range of beam sizes, where diﬀerent
colors denote diﬀerent bunch currents. The inner, light-blue contours correspond to full beam current, and the outer, red ones correspond to 1/128th
beam current (equivalent to the typical beam currents used for machine optics correction). While the single pinhole is in some ways the easiest one toanalyze, it is not expected to be useful for single-shot measurements during352

Figure 9.44: Calculations of x-ray spectra detectable with high-energy detector for raw beam and various types of optics. Left: LER, assuming 200 µm
Be extraction window. Right: HER, assuming 200 µm Be extraction window
and 1.4 cm Be filter upstream.

353

Figure 9.45: 64-channel motherboard for development, with digitizer daughter cards.

standard optics correction modes, unless the beam current is concentrated
in a small number of bunches.
For better resolution, it is planned to use a coded aperture pattern. The
coded aperture pattern used at CesrTA is a Uniformly Redundant Array(URA)[8], which features an open aperture of 50% with an even sampling of
spatial frequencies in the non-diﬀractive limit. An example of a candidate
URA mask for use at SuperKEKB is shown in Figure 9.47. It is a 59-element(where each element can be filled or not) URA mask pattern with minimum
feature size of 10 µm, and is thus 590 µm in height.
The simulated detector images and resolution contours for the URA pattern are shown in Figure 9.48. The resolutions at low beam currents are
significantly better than those for the single pinhole. As mentioned above,
this is due to the greater open aperture, leading to greater detected photon
flux, as well as the presence of beam size-dependent modulation depths.
Other patterns are currently under investigation as well. Figure 9.49
shows the results for one recently developed pattern being considered thatuses 32 µm pinholes in a logarithmically distributed range of spacings, designed to provide good resolution over a range of beam sizes. This patternclearly improves on the resolution of the URA pattern.

354

Figure 9.46: Left: Simulated detector images for diﬀerent beam sizes using
32-micron pinhole and 50 micron pitch deep silicon detector at LER. Right:
Single-shot statistical resolution contour plots for the same combination.

Figure 9.47: 59-element URA mask pattern for SuperKEKB.

355

Figure 9.48: Left: Simulated detector images for diﬀerent beam sizes using
URA mask pattern and 50 micron pitch deep silicon detector at LER. Right:
Single-shot statistical resolution contour plots for the same combination.

356

Figure 9.49: Left: Simulated detector images for diﬀerent beam sizes using
multiple 32-micron pinhole pattern and 50 micron pitch deep silicon detector
at LER. Right: Single-shot statistical resolution contour plots for the samecombination.

357

The x-ray monitor will start commissioning from Phase I, for use as soon
as possible for low-emittance tuning.

9.4.3

Large Angle Beamstrahlung Monitor

We have a plan to install a Large Angle Beamstrahlung Monitor (LABM),
originally developed by G. Bonvicini of Wayne State U. for use at CESR.
Beamstrahlung is light generated by the beam-beam collision, as the particles in each beam are deflected by the opposing beam, similar in principle
to synchrotron radiation. Like synchrotron radiation, the resulting beamstrahlung light is polarized in diﬀerent axes depending on the direction ofdeflection of each particle. The monitor does not measure beam sizes directly, but uses the relative strengths of horizontal and vertical polarization
components of large-angle (≈ 10 mrad oﬀ-axis) beamstrahlung to diagnose
the quality of collisions, allowing the evaluation of parameters such as beam
separation and relative beam sizes at the collision point, as shown schematically in Figure 9.50. Estimation of signal and background level shows that amuch stronger signal than that seen at CESR is expected at SuperKEKB.
To extract the light, two pairs of mirrors are installed, one on either sideof the IR. One mirror on each side is mounted on the upper wall of the IR
chamber, and reflects the light out of an extraction port on the lower wall.
The other mirror on each side is mounted on the lower wall, reflecting upwards. The four sets of extracted beamstrahlung light are then transferred
by a series of mirrors to optics boxes mounted at the Tsukuba B4 level below
the accelerator beam line, and directed into optics boxes (see Figure 9.51).
The light from each beam is split into vertical and horizontal polarization
components via a Wollaston prism, then further divided into 4 wavelength
bands, with the intensity of light in each polarization and wavelength combination being measured by PMTs.
The Be extraction mirror and extraction chamber design design are shownin Figure 9.52. The front face of the mirror, at a 45-degree angle, is 2
mm wide, and extends 2 mm in height and in longitudinal extent, with ashallower-sloped trailing edge. The extraction mirror is set within a slotted
ramp structure, which shields the mirror from electric fields generated by
the passing beam. The ramp is 6 mm in height around the mirror, with 30degree sloped leading and trailing edges. The mirror sits inside a 3 mm-wideslot. GdfidL calculations give a loss factor of 12.3 × 10−3 V/pC for eachchamber, which will generate about 645 W at full LER current, with the358

Figure 9.50: Example of collision mismatch diagnosis using x- and ypolarization components. Upper plots represent, from left to right: oﬀset, tilt
and size mismatches between the two colliding beams. Lower plots show thecorresponding shifts in measured polarization vectors (x-axis = normalized
x-polarization; y-axis = normalized y-polarization component).

Figure 9.51: Design of optics box that splits beamstrahlung light into diﬀerent polarization components and wavelengths for measurement by PMTs.

359

Figure 9.52: Left: Be extraction mirror for LABM. Right: Extraction mirror
chamber design (by K. Kanazawa, vacuum group).

majority of that due to the shield ramps, and only a few Watts due to the
8 mm-wide extraction window holes. Very little field leakage into the mirror
area is found.
The LABM will begin initial commissioning and background studies inPhase I, when there will be no background from the QCS quadrupoles magnets. If beams can be collided at some point in Phase I, even at extremely
low luminosity, that would provide valuable data for systematic studies.

360

Bibliography[1] M. Arinaga, J. Flanagan, S. Hiramatsu, T. Ieiri, H. Ikeda, H. Ishii,
E. Kikutani , T. Mimashi, T. Mitsuhashi, H. Mizuno, K. Mori,
M. Tejima and M. Tobiyama Nucl. Instrum. Meth. Phys. Res. A499,
p100-137, (2003).
[2] J.W. Flanagan, M. Arinaga, H. Fukuma, H. Ikeda, “Diamond mirrors
for the SuperKEKB synchrotron radiation monitor,” Proc. IBIC12 ,Tsukuba (2012).
[3] R.H. Dicke, Astrophys. Journ., 153, L101, (1968).
[4] J.P Alexander, A. Chatterjee, C. Connolly, M.P. Ehrlichman, E. Fontes,
B.K. Heltsley, W. Hopkins, A. Lyndaker, D.P. Peterson, N.T. Rider,
D.L. Rubin, J. Savino, R. Seeley, J. Shanks, J.W. Flanagan Nucl. Instrum. Meth. Phys. Res. A748, p96-125, (2014).
[5] J.W. Flanagan, M. Arinaga, H. Fukuma, H. Ikeda, A. Lyndaker, D.P.
Peterson and N. Rider, “High-power tests at CesrTA of x-ray optics
elements for SuperKEKB,”Proc. IBIC13, Oxford (2013).
[6] Drouin, D., Couture, A. R., Joly, D., Tastet, X., Aimez, V. and Gauvin, R. “CASINO V2.42???A Fast and Easy-to-use Modeling Tool for
Scanning Electron Microscopy and Microanalysis Users,” Scanning, 29:
92???101 (2007).
[7] J.W. Flanagan, M. Arinaga, H. Fukuma, H. Ikeda, T. Mitsuhashi, J.S.
Alexander, M.A. Palmer, D.P. Peterson, N. Rider, G. Varner “SINGLESHOT RESOLUTION OF X-RAY MONITOR USING CODED APERTURE IMAGING,” Proc. DIPAC11, Hamburg (2011).

361

[8] E.E. Fenimore and T.M. Cannon, Appl. Optics, V17, No. 3, p. 337(1978).

9.5

Other instrumentations

9.5.1

Beam loss monitors

Beam loss monitor (BLM) system is used for the protection of accelerator
components against unexpected beam loss caused by, for example, beam
instabilities. An interlock signal from a BLM triggers a beam abort. We will
use the same BLM system as that in KEKB [1] that consists of ion chambers
and PIN photo-diodes. The number of channels of BLM is approximately
200. We use data loggers to diagnose whether the abort is correctly requested
by BLM and other sensors or not.
Ion Chamber
Ion chambers which are shown in Fig. 9.53 are re-used sensors of KEKB.
The 5m long chambers are mounted on cable racks located on the outer wallalong the tunnel. The ion chamber is a Fujikura FC-20D co-axial cable.
The inner and outer conductors are separated by an air gap. The applied
voltage at outer conductor is 200V. The typical drift time where the positive
ions are reached at inner conductor is 1ms. The resulting current is sent toreadout electronics which includes an integrator and an amplifier as shownin Fig. 9.54. Each module handles eight channels. The amplifier gain canbe selected to 1, 10 or 100. The RC time constant also can be selected to10, 100, 300 or 1000ms. The beam abort is requested when the beam loss
level exceeds a threshold that is set for individual channel. The loss signal isbuﬀered and sent to a 16-bit, 64 channel ADC (DGX 18K14B) and loggedwith 1Hz. The same signal is sent to the data logger with high-sampling-rate
immediately after the beam abort.
PIN photo-diode
Collimators are installed in beam line in order to protect the Belle-II detectors
from the beam. The number of the collimators is 13 and 16 in LER and HER
respectively. High current beam may damage the collimator itself when the
beam orbit changes rapidly. PIN photo-diodes (PDs) are used as sensors of362

Figure 9.53: Ion chamber for the beam loss monitor[1].

the BLM at the collimators since the interlock signal from the ion chamber
is not fast enough to protect the collimators. The rise time of PD is 60µs which is determined by the capacitance of the signal cable (30nF for a
400m cable) and an input resistance of 2kΩ of the readout electronics. The
readout electronics is a modified ion chamber readout that has a track-andhold circuit as shown in Fig. 9.55. The interlock signal is asserted within a
few turns after beam loss happened. The PIN signal can protect the hardware
from the localized and rapid beam loss, and be used to distinguish the ringwhere the beam loss occurred. Therefore we install PINs to several places
where we expect the localized beam loss besides the collimators as shown in
Fig. 9.56. The signal flow after the readout module is same as that of theion chamber signal.
Data logger system
The diagnostic system of the beam abort is based on a high-sampling-rate
data logger that records BLM and other signals at the moment of the beamabort [9]. BLM signals of all over the rings are collected to four local control
rooms (LCRs). The data loggers are located in five LCRs where the BLM
signals and signals from RF cavities are corrected. The logged time period
is 600 ms or 300 ms. Sampling intervals are 5 µs or 1 µs depending on the363

Figure 9.54:
chambers[1].

Block diagram of the read-out electronics for the ion

364Figure 9.55: Block diagram of the read-out electronics for the PIN photodiodes[1].

365Figure 9.56: An example of PIN photo-diodes which are put near bellows.

366

Figure 9.57: A signal flow of data loggers[2].

logged time periods of 600 ms or 300 ms respectively. The signal flow is shownin Fig. 9.57. Beam intensity, signals from the RF cavities and beam phaseshowing deviation of synchronous phase from the timing signal of injection
trigger are logged with a part of BLM signals. These fast signals are useful
to diagnose the cause of the beam abort. The recorded data are sent to thecentral control room (CCR) via the KEK internal network. The informationis ready for inspection within a few minutes after the abort. Analyzing thedata, the reason of beam loss and the beam abort is identified and the best
condition of the beam abort is determined.

9.5.2

DCCT and CT

In SuperKEKB we will use the same DCCT and CT systems as those used in
KEKB[1, 2] except for the DCCT circuit in LER whose maximum range of367

∼ 3 A exceeds the design beam current of SuperKEKB LER. We will modify
it to accept the beam current up to 5 A. Location of the DCCT and CT in
LER will be slightly moved due to change in positron injection system.
The measurement system will be slightly modified to realize faster measurement of the beam current and beam lifetime. Employing the Agilent
34410A fast digital volt meter, we plan to measure the beam current at the
sampling rate of 5k samples per second with the resolution of about 16 bit.
The average beam current, average injection rate and fitted lifetime will becalculated on a Linux IOC to update the data once per second. R&D of sucha system is in progress.

368

Bibliography[1] M. Arinaga, J. Flanagan, S. Hiramatsu, T. Ieiri, H. Ikeda, H. Ishii,
E. Kikutani , T. Mimashi, T. Mitsuhashi, H. Mizuno, K. Mori, M. Tejima
and M. Tobiyama Nucl. Instrum. Meth. Phys. Res. A499, p100-137,(2003).
[2] Mitsuhiro Arinaga, John W. Flanagan, Hitoshi Fukuma, Takaaki Furuya, Shigenori Hiramatsu, Hitomi Ikeda, Hitoshi Ishii, Eiji Kikutani,
Toshiyuki Mitsuhashi, Kenji Mori, Masaki Tejima, and Makoto Tobiyama, Prog. Theor. Exp. Phys. (2013) 03A007.

369