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12th Int. Beam Instrum. Conf.
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THE CONCEPTUAL DESIGN STUDY FOR NEW BPM SIGNAL
PROCESSING SYSTEM OF J-PARC MAIN RING (MR)*
K. Satou†, T. Toyama, S. Yamada, J-PARC/KEK, Tokai, Japan
Abstract
The beam position monitor signal processing system,
which is a 19-year-old system, has been suffering fromgain fluctuation due to contact resistance of the mechanicalgain selector and communication disruption caused by an
unstable contact of a card edge connector. It also has difficult repairments because some onboard parts have alreadyreached the end of a product-life cycle, and some units
have been in an unusable situation. currently, we are on the
beam power upgrade campaign to 1.3 MW by increasing
the beam bunch current and shortening the main ring operation cycle, and precise beam tunings would require massive waveform data processing and transfer to a storage
system than the present system. For this, we have been developing a system based on the 10 GbE optical link. The
ADC board, which is under development, performs direct
sampling using the third harmonics of accelerating radio
frequency. The digital IQ demodulation technique is used
to extract the baseband oscillation from the raw data. The
obtained raw waveform and closed orbit data are stored in
the data storage system. In the presentation, we will reporton the progress of development aimed at operation in 2025
and the conceptual design of the new system.

INTRODUCTION
In the J-PARC main ring (MR), 186 beam position monitors (BPMs) have been used to measure beam orbit andbeam optic parameters [1, 2]. Its signal processor, BPM circuit (BPMC), designed 19 years ago, has two modes of position measurement:
1) Closed orbit distortion (COD) mode,2) bunch-by-bunch (BxB) mode.
The first mode is a narrowband measurement that performs a fast Fourier transform on the waveforms digitizedwith 80 MHz to extract the second harmonic component of
the accelerating radio frequency (RF) to obtain the COD.
Meanwhile, the second mode is a broadband measurement
for measuring bunch waveforms to measure the beam center of each bunch. The position measurement error is approximately 30 and 300 μm for the COD and the BxB
modes, respectively [3].
At some BPMC, communication disruption was causedby an unstable contact of a card edge connector. However,this BPMC is too old that some circuit parts have alreadybeen discontinued. This makes maintenance work difficultand some BPMCs unusable. If this situation progresses,
there is a risk that sooner or later stable MR operation will
be hindered.
___________________________________________* Work supported by Accelerator and Beamline Research and Technology
Development for High-Power Neutrino Beams in the U.S.-Japan Scienceand Technology Cooperation Program in High Energy Physics.
† email address: kenichirou.satou@j-parc.jp

The BPMC shows the beam position shift due to thechange in the contact resistance of a relay. To optimize theinput signal level, it has a 20 dB fixed attenuator placed
just before the input terminal and a variable attenuator in
the BPMC. Depending on the beam condition, the attenuation factor is selected by switching a relay element on thecircuit, and its switching frequency is approximately several times per year. The contact surfaces are then exposed
to atmospheric gas for a long time because the relay is notstrictly shielded. The inspection of the relay surface confirmed that the silicon-based compound was layered. This
results in surface contamination and consequently resistance changes. A 0.5 Ω change in contact resistance results in an error of 1.3 mm. Moreover, an error of several
millimeters– may occur because of individual differencesin relays. This resistance can be reduced by applying a voltage [4]. However, there are large individual differences inthis effect, and no clear systematicity has been confirmed
so far, therefore, calibration is difficult. To examine the impact of this effect, some consistency check methods between the BPM sensors are now proposed [4, 5].
Moreover, beam tuning above 1 MW is supposed to require high-precision measurement of optics parameters [3].
Thus, it is necessary to process a large amount of highly
accurate position data. Improvement of the system is also
necessary.

NEW BPM SIGNAL PROCESSOR
We are developing a new signal processor as part of the
MR beam power upgrade campaign to 1.3 MW. In the new
system, we aim to reduce the position error to 1/3 or less inboth modes. To achieve this, we will use the following
items:
1. Reduction of the reflection coefficient of the attenuator input terminal,
2. reduction of signal distortion and gain temperature
coefficient in the attenuator and ADC,
3. improvement of the vertical resolution of the ADC,
4. improved digital signal processing section.
Figure 1 shows a block diagram of the new system installed in four different buildings. It uses the existing BPM
sensors and its signal cables, which are 100-300 m depending on the sensor location. An attenuator unit, ADC card,
network interface controller unit (NIC unit), and data storage system have been developed.
The attenuator unit is powered by a nuclear instrumentation module (NIM) standard bin power supply and theADC card is housed in the same NIM case. The digitizedwaveform data are delivered over a 10 GbE optical private
network and forwarded through the NIC unit to the data
storage system. This network uses the user datagram protocol (UDP).

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02 Beam Position Monitors

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ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 2 3The configuration of the attenuator units and the ADC
cards will be made through the experimental physics andindustrial control system (EPICS), which is a distributed
control system on the accelerator control network. The
COD and alarm data will also be sent to the control network through the NIC unit. The development of the attenuator unit has already been completed, and we are currently
developing the ADC card, NIC unit, and data storage system.
The development of each component will be completed
by the end of this fiscal year, and part of the system will bebuilt and tested in the next fiscal year, and mass production
will be completed. We plan to start operating the new BPM
system by FY2025. This paper provides an overview of the
new system and the functions and outlines of each component.

Attenuator Unit
The new attenuator device has already been developedand is in mass production [6]. Figure 3 shows a block diagram of the new attenuator device.
As the beam bunch shape becomes sharper with acceleration, the signal reaches a maximum voltage of 200 V peak
to peak (Vpp) when the beam energy of 30 GeV is reached.
To match the input level of an ADC of 2 Vpp, the input
signal is attenuated through an RF2033 power divider(−12 dB), a switchable attenuator (−10 dB or 0 dB), a variable gain (G = 1 or 1/2), and an RF transformer (1/2).
The new attenuator uses a reed relay, HFS-1C-05W(manufactured by OKITA Works Co., Ltd.), enclosed in a
glass tube filled with nitrogen. It is expected that the contact resistance will be stable for a long time because it is
not affected by dust or atmospheric gas because of its sealing. From the Ref. [7], it can be seen that if the reed relay
was used within 2E6 times, the contact resistance would beapproximately 10 mΩ. This impedance change induces
beam position variation of approximately 4 μm. It was implemented using a special socket so that it can be replacedin case the relay exceeds its usage limit or is damaged.

Figure 1: Block diagram of the new BPM system.

COMPONENT DESIGNIn MR, 3 GeV protons are accelerated to 30 GeV in
about 1 second. The accelerating frequency of the RF accelerating devices changes from 1.672 to 1.720 MHz, witha harmonic number of 9.
The BPM sensor is a diagonal cut type, and the four signals R, L, U, and D from the right, left, top, and bottom
electrodes are transmitted to the BPM signal processing
unit via 5D-FB coaxial cables. The sensor capacitance and50 Ω input impedance form a high-pass filter with a cutoff
frequency of approximately 16 MHz, resulting in an output
pulse shape of the derivative of the charge bunch as shownin Fig. 2.

Figure 3: Block diagram of the new attenuator unit.
In the BxB measurements, impedance matching between
the signal cable and the attenuator device is important. To
achieve fine matching, a circuit to adjust the input impedance was implemented. For adjustment, the power divider
RF2033 produced by HIRAKU Ltd. is used (Fig. 3). If a
17 Ω resistor is connected to terminal 4, the input impedance (Zin) and attenuation (Att) seen from the input becomes 50 Ω and 1/4 (−12 dB), respectively.
When 17−δ Ω is connected to this terminal, the impedance seen from the input is given by
𝛿
3
(1)
𝑍

50

4

800

𝛿 .

Therefore, Zin can be finely adjusted by the δ with a variable resistor. At the same time, the δ modulates the attenuation to be
3 400 3𝛿(2)
𝐴𝑡𝑡–
Figure 2: Beam pulse shape of 3 GeV beam.

8 600

3𝛿

.

A high intensity beam can produce up to 200 Vpp at the
input terminal of the attenuator unit, so the signal must be
attenuated to match the input level of the ADC circuit.
By driving the Zin with any test signal, the gain balanceof each channel, cable attenuation, and matching between
MOP02302 Beam Position Monitors

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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 2 3

the cable and Zin can be measured. As shown in Fig. 1, theZin tuning stab circuit is connected by turning on RelayCAL. Then, the Zin can be excited with a waveform input
from CAL_SIG. The relay is used here because the voltage
at Zin is expected to be at a maximum of 200 Vpp for thedesigned beam power operations.
A burst wave of 5 MHz, its frequency in the main signal
band to be processed at the ADC card, is used as a test signal. The wave height of the input wave (first wave) determines the total gain of the attenuator unit and ADC card.
The first wave is reflected by the BPM sensor’s capacitance (130 pF), and the returned wave is observed as thesecond wave. The cable attenuation can be estimated fromthe signal ratio of the second wave to the first wave considering the reflection at the BPM sensor. Before the second wave arrives, it turns off the MOSFET switch
ADG5401 by the CAL CNTL GATE to disconnect the impedance of the drive amp from Zin. If the Zin does not matchthe cable impedance, a part of the second wave will be reflected to the BPM sensor and then observed as the third
wave. The reflection coefficient at Zin can be calculated
from the third wave, where the third wave height depends
on the reflection at Zin, cable attenuation, and reflection at
the BPM sensor. Here, at a frequency of approximately
5 MHz, the electrical length is approximately 40 m. Thus,
the reflection inside the N-type connector used can be ignored.
Figure 4 shows the test results using a 200 m 3D-FB cable with a Zin of 50 Ω. From the wave heights of the firstand second waves, the cable attenuation was estimated to
be 32 dB/km at 5 MHz. The actual BPM cable is NH-5DFB, and its cable attenuation is approximately 1/5. From
the third wave, the reflection efficiency at Zin was estimated to be 1.02. In the actual operation, matching isachieved by adjusting Zin so that the wave height of thethird wave becomes the minimum value.

Figure 4: Calibration test signal input at the Zin and multireflected waves. The third wave shows impedance mismatching at the input terminal.
In the actual operation, the Relay-CAL is turned off.
Therefore, it is necessary that Zin does not change due tothe relay operation. The upper part of Fig. 5 shows the difference in S11 with and without the Zin tuning stub circuit.
Here, Zin was set to 50 Ω (δ=17). The voltage standing
wave ratio (VSWR) at the input terminal was measured in

the range of 1 to 10 MHz. In the range of 1 to 5 MHz, they
are almost the same within the error. However, in the rangeof 5 to 10 MHz, the compensation circuit is slightly overcompensated, and a difference of approximately VSWR =0.002 appears. Currently, the VSWR is within 1.005 in therange of 1 to 10 MHz. The lower part of Fig. 5 shows theSmith chart of Zin at 5 MHz where Zin was shifted bychanging δ. As shown in the figure, the impedance moves
in the range of 50 ± 1 Ω along the line Im(Zin) = 0. The
impedance step is 0.037 Ω, which allows impedance
matching down to VSWR < 1.001.

Figure 5: Measurement of impedance matching at the input terminal with and without the Zin tuning stub circuit.
The upper part shows VSWR when Zin = 50 Ω. The lower
part shows the footprint in the Smith chart when changing
the input impedance.

ADC Card
Figure 6 shows a block diagram of the ADC card underdevelopment. The ADC card uses two 16-bit ADCs,
LTC2194 from Analog Devices, which perform direct sampling using a 48-multiplied RF clock of approximately
82 MHz. The sampling clock is directly generated using avector signal generator (SG) N5172B from Keysight basedon the RF pattern IQ data and input to the ADC card.
The raw waveform data from the LTC2194 is transmitted
and processed using an Arria 10 FPGA from Intel. The timing of each waveform data from a different BPM sensor issynchronized with a delay circuit in the FPGA. To decrease
beam signal fluctuations and random noises at each measurement, we average over 1 to N cycles (1 to N MR beam
shots) at 1.3 to 5 s MR trigger intervals. For the waveforms
processed, filtering with narrowband BPF and IQ demodulation with the third harmonics of the RF is performed to
extract a betatron oscillation. Then the I and Q data are processed with LPF, and the amplitude R and a phase θ arecalculated. The R data are further processed using LPF,
then a sum Σ and a difference δ between the R (U) and L(D) are calculated to obtain horizontal and vertical COD
waveform data of approximately 2kSps. When the COD
exceeds its preset threshold value, an alarm signal is generated to dump the beams.

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02 Beam Position Monitors

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d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 2 3Part of the raw waveform data, all waveform data of R,θ, δ, Σ, and the alarm status are stored in DDR4 memory
and transferred to the NIC unit by using UDP via a 10 GbE
SFP+ optical port.
The processing functions, COD alarm threshold level,
and raw data to be saved and transferred are dynamicallyset by reading the configuration register in the FPGA. The
configuration register can be edited through the USB port
or from the NIC unit via the SFP+ port.

Figure 6: Block diagram of the ADC card.

Network Interface Controller Unit
The NIC unit under development is responsible for communicating with the ADC card though the private optical
network, acquiring a set of waveform data from different
ADC cards, and sending it to the data storage system. Furthermore, it communicates with an EPICS IOC (Input output controller) via a dedicated API (Application Interface)and delivers the COD data and alarm status onto the control
network. The user can write the set value to the configuration register of the ADC card via EPICS records and thenread the set value back to the EPICS records.

Data Storage System
The data storage system under development stores alarge amount of waveform data sent via UDP from the NIC
unit. This system has 16 SATA 16 TB HDD storage andbuilds RAID5 or 6 with a dedicated hardware RAID card.
In UDP communication, the order in which the data
flames transferred from the NIC unit are received is different, and there is no protection against flame data loss. It has
the advantage of having less overhead during transfer andcan be processed at high speed. The input communication
card of the data storage system rearranges the data flames
based on the time information in the header information ofeach flame data and organizes them into a file at each MR
cycle. When a flame loss occurs, the user is notified, but noretransmission request is made.

A large amount of waveform data is written in this file,
however, the user usually needs only a part of the data, suchas the beam oscillation data immediately after beam injection and beam motions when the machine status ischanged. It is inefficient to read all data in the file andsearch for analysis points. Therefore, we consider adopting
the HDF5 format as a file system that can extract and readsome of the data stored.

CONCLUSION
We are developing a next-generation BPM data processing system to replace the outdated BPMC system, a 19year-old system since its design. In the new system components, an attenuator unit has been developed and is in
mass production. We are currently fine tuning the circuit.
We are now conducting a design study for the ADC card,
NIC unit, and data storage system. We plan to complete
these developments by JFY2023. Additionally, we plan tobuild a part of the full system with limited modules in
JFY2024, and launch a new system in JFY2025. However,
the availability of the FPGA chip, Arria10 270, used in the
ADC card is now poor, and there are no prospects for fullmass production. Although it appears to be improved in the
future, it is necessary to closely examine the FPGA production status.

REFERENCES[1] K. Satou et al., “Beam Diagnostic System of the Main Ring
Synchrotron of J-PARC”, in Proc. HB'08, Nashville, TN,
USA, Aug. 2008, paper WGF11, pp. 472-474.
[2] T. Toyama et al., “Performance and Upgrade of BPMs at the
J-PARC MR”, in Proc. IBIC2012, Tsukuba, Japan, Oct. 2012,
paper MOPA26, pp. 107-111.
[3] S. Igarashi et al., “Accelerator design for 1.3-MW beam
power operation of the J-PARC Main Ring”, Prog. Theor.
Exp. Phys. vol. 2021, p. 033G01, 2021.
doi.org/10.1093/ptep/ptab011[4] T. Toyama et al., “Recovery and check of the switching relay
in the BPMs in the J-PARC MR”, presented at the IPAC'23,
Venice, Italy, May 2023, paper THPL104, to be published.
[5] T. Asami et al., “High accuracy optics measurement in
JPARC MR for 1.3 MW upgrade plan”, in Proceedings ofthe 19th Annual Meeting of Particle Accelerator Society ofJapan, August 29 – September 1, 2023, Funabashi, Japan.
[6] K. Satou et al., “Development of Signal Attenuator with Low
Reflection and Low Distortion for New BPM Signal Acquisition System of J-PARC MR”, in Proceedings of the 19th
Annual Meeting of Particle Accelerator Society of Japan,
October 18-21, 2022, Online (Kyushu University), in Japanese.
[7] “OKITA RELAYS CATALOG”, OKITA Works Co., Ltd.

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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3