Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEOB09

INSTALLATION AND TEST OF THE PHASE DRIFT MONITOR AT
J-PARC LINAC
E. Cicek∗ , Z. Fang, Y. Fukui, K. Futatsukawa, S. Mizobata, KEK, Ibaraki, Japan
Y. Sato, Nippon Advanced Technology Co., Ltd., Tokai, Japan
Abstract
Achieving a stable RF field for all cavities in the J-PARC
linac is one of the significant tasks for the successful operation of J-PARC. In particular, a long-term stable RF field isa crucial challenge in suppressing the drift of the injection
beam momentum from the linac to the rapid cycling synchrotron (RCS). In addition, we found that environmental
factors such as humidity and temperature cause RF phasedrift in the cavities, resulting in the drift in the beam injection momentum, making the compensation of phase drifta priority job. To this end, a phase drift correction algorithm is carried out within the low-level radio frequency(LLRF) system at linac. Although this system has been operated successfully, we aim for an updated design to directly
measure and correct the phase drift of the cavities operating at different frequencies. Therefore, a real-time phasedrift compensation system based on a direct sampling technique deploying RF system-on-chip (RFSoC) technology isadopted to measure two different RF frequencies without
local oscillator (LO) signals. This study presents the installation stage of the device in the linac and long-term cavity
RF phase results.

INTRODUCTION
The Japan Proton Accelerator Research Complex (JPARC) linac consists of a negative hydrogen 𝐻 − ion source(IS), 3 MeV radio frequency quadrupole (RFQ), 50 MeVdrift-tube linac (DTL), 190.8 MeV separated-type DTL(SDTL), and 400 MeV annular ring coupled structure ACS,
as shown in Fig. 1. The linac also has a medium-energy
beam transport line 2 (MEBT2), which is equipped withtwo buncher cavities (B1 and B2), and L3BT (linac-to-3GeV RCS beam transport). A 50 mA 𝐻 − beam is injected
into the rapid cycling synchrotron (RCS) through the L3BTline [1, 2].
The drift in the momentum of the injection beam must bewithin ±0.05% due to the RCS acceptance to avoid beamlosses [3]. Environmental factors that can cause drift inbeam injection momentum from the linac to the RCS are
predominantly the humidity and temperature at the klystrongallery. Thus, stabilizing the accelerating RF field in the
short-term and long-term is necessary during beam operation. A low-level radio frequency (LLRF) system withdigital feedback (DFB) and feedforward (DFF) is utilized tofulfill the required specifications of Δ𝐴/𝐴 < ±1% peak-topeak (pp) in amplitude and Δ𝜃 < ±1 deg (pp) in phase [4].
Meanwhile, a compensation system using cavity phase monitors (henceforth referred to as “CPM”) located at each RF∗ ecicek@post.kek.jp

station compensates for phase and amplitude drifts in RF
frequencies of 324 MHz and 972 MHz individually [5].
Table 1: The Comparison of the Extant and Updated Phase
Drift Compensation SystemParameters
DeviceTechnique
LOFrequency
Key point
𝑎ExtantUpdated

Cavity phase
monitor
Downconverter +IF sampling
Used
324 MHz𝑎 or
972 MHz𝑎
Conventional

Phase drift
monitorDirect samplingNot used
324 MHz and972 MHzRFSoC

Two frequencies are individually measured at CPMs.

Although the extant drift compensation system has provenreliable, measuring and correcting drift in RF cavities operating at different frequencies necessitates an updated technique. Therefore, we have developed a new system usinga phase drift monitor (henceforth called “PDM”) based on
the Xilinx Zynq UltraScale+ RF system-on-chip (RFSoC)
ZCU111 evaluation platform [6]. The comparison of extant
and updated systems is listed in Table 1. Local oscillator(LO) signals, which may be a source of drift, are eliminatedwith the direct sampling technique where pick-up RF signals from accelerating cavities operating at 324 MHz and
972 MHz are directly digitized by analog-to-digital converters (ADC). The PDM calculates phase differences between
cavities and compensates for drift in RF signals within dedicated DFB systems. This study presents the performance
analysis completion of the PDM, its installation procedure,
and long-term cavity RF phase measurement results.

HARDWARE
The PDM architecture consists of a ZCU111 board, andthe analog front end was implemented by the Mitsubishi
Electric TOKKI Systems [7]. Figure 2 shows the hardware inside the PDM chassis and its front panel. Theboard was populated with Zynq UltraScale+ XCZU28DR2FFVG1517 RFSoC combining a processing system andprogrammable logic in the same device. The processor runs
Linux OS and integrates the experimental physics and industrial control system (EPICS) input/output controller (IOC)for data communication. The ZCU111 board contains eight
12-bit 4 GSPS RF-ADC channels (ADC00∼07) and eight
14-bit 6.554 GSPS digital-to-analog (RF-DAC) channels(DAC00∼07). An ethernet interface allows integration withthe linac control system. An RFMC connector on the board- 97 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEOB09

Figure 1: J-PARC linac configuration.
provides direct access to the RFSoC analog inputs. The
analog front-end comprises an RF differential breakout card
(AES-LPA-502-G) [8] and ADC wideband panel mountbalun boards (ADC-WB-BB) [9]. The baluns are employed
to convert RF signals into differential outputs and provide
access between eight analog input channels from the front
panel to the breakout card. The breakout card is directly
connected to the RFMC mezzanine interface.

Figure 4 shows the temperature and humidity characteristics of the PDM. Input channels from ADC00 to ADC03and ADC04 to ADC07 measured phase of 324 MHz (from𝜙0 to 𝜙3 ) and 972 MHz (from 𝜙4 to 𝜙7 ) RF signals, respectively. 𝜙3 and 𝜙7 were assigned as reference phases for two
different frequencies, and phase differences with respect tothe references were computed. The temperature inside thechamber (𝑇𝑠𝑒𝑡 ) was changed from 23∘ 𝐶 to 31∘ 𝐶 (round trip)
with a 2∘ 𝐶/3 h ramping rate at a fixed humidity of 65%RH.
Afterward, humidity (𝑅𝐻𝑠𝑒𝑡 ) was changed from 15%RH to65%RH (round trip) with a ramping rate of 25%RH/24 hsteps at a fixed temperature of 27∘ 𝐶.

Figure 2: (Color online) The top view (top) and front view(bottom) of the PDM chassis.

PERFORMANCE ANALYSISCOMPLETION
The first results regarding performance analysis of thePDM were reported in [10]. Afterward, to evaluate the driftin phase differences measured on ADC channels, PDM wasplaced in a commercial environmental test chamber (ESPECPDR-3J) [11], as illustrated in Fig. 3.

Figure 3: Test bench of the temperature and humidity characteristics measurement for the PDM.

Figure 4: Temperature (top) and humidity (bottom) dependence of RF phase differences. Δ𝜙32 , Δ𝜙31 , and Δ𝜙30 denote phase differences between ADC03 and ADC02, ADC03
and ADC01, and ADC03 and ADC00, respectively. Δ𝜙76 ,Δ𝜙75 , and Δ𝜙74 are phase differences between ADC07
and ADC06, ADC07 and ADC05, and ADC07 and ADC04,respectively.Δ𝜙73 is phase difference between references.

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October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEOB09

The temperature and humidity characteristics results ofthe PDM using an infinite impulse response (IIR) filter, thesimplest LPF, implementation are listed in Table 2. Drift inphase differences (pp) of 324 MHz (Δ𝜑324 ) and 972 MHz(Δ𝜑972 ) ADC channels per 1∘ 𝐶 and 1%RH steps were verysmall. Though the relative phase difference (Δ𝜙73 ) hada similar trend with rising and decreasing temperature andhumidity, the drift denoted as Δ𝜑𝑟𝑒𝑓 was also measured assmall. In addition, the PDM was less humidity-dependent.

To connect cavity pick-up loop and reference clock (REFCLK) signals to MEBT2B1 control rack, 3/8-inch foam
dielectric, phase-stabilized Andrew Heliax coaxial cables
were used. Front panel mount SMA cables were used toconnect the eight analog input channels to an RF panel inthe control rack. RF power for 324 MHz REFCLK generated from an RF&CLK board in the SDTL16 RF station was+3.8 dBm. RF trigger (RFTRG) signal was provided fromthe timing system at the SDTL16 RF station.

Table 2: Summary of Temperature and Humidity Characteristics for the PDM

LONG-TERM PHASE DIFFERENCES

Ramping rate

StepPhase drift [deg]Δ𝜑324 Δ𝜑972
Δ𝜑𝑟𝑒𝑓2∘ 𝐶/3 h (fixed65%RH)
25%RH/24 h(fixed 27∘ 𝐶)

1∘ 𝐶

1.23E-3

6.68E-3

3.75E-2

1%RH 1.20E-4

6.63E-4

4.20E-4

Meanwhile, long-term temperature and humidity in theMEBT2B1 LLRF rack, where the PDM was installed, canbe kept within 1∘ 𝐶 (pp) and 1%RH (pp), respectively. Accordingly, the PDM is ideal to be implemented in the LLRF
system in terms of temperature and humidity characteristics.

The PDM was tested online to evaluate long-term cavity
RF phase differences because we aimed to utilize it within
the LLRF system. Figure 6 shows the RF phase measurement scheme. Cavity phases of 𝜙23_𝑣𝑠 , the vector sum (VS)phase of SDTL16A and SDTL16B tanks, and 𝜙4_𝑝𝑑𝑚 wereassigned as reference phases because the MEBT2B1 andSDTL16 19-inch LLRF control racks were placed in a constant local temperature&humidity environment. 𝜙01_𝑣𝑠 isthe VS RF phase of SDTL15A and SDTL15B tanks.

INSTALLATION
Figure 5 shows the PDM installed at the MEBT2B1 LLRF
control rack, where a constant temperature and humidity
environment is provided. The cavity pick-up RF signals
from SDTL15A and SDTL15B, SDTL16A, and SDTL16B
tanks operating at 324 MHz were connected to the channels of ADC00, ADC01, ADC02, and ADC03 channels,respectively. In addition, the pick-up signals from 972 MHz
MEBT2B1, MEBT2B2, and ACS1 cavities were connected
to ADC04, ADC05, and ADC06, channels, respectively.

Figure 5: Photograph of PDM installed at MEBT2B1 station.

Figure 6: Long-term cavity RF phase measurement schemeon the PDM and CPMs. RF1 and RF2 inputs on the CPMs
represent ADC inputs where two neighbor cavity tanks areconnected. The phases from 𝜙0_𝑝𝑑𝑚 to 𝜙6_𝑝𝑑𝑚 denote theRF phases of each cavity tank measured by the PDM.

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Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEOB09

Figure 7: Long-term cavity RF phase differences, measured and computed in the PDM when there is no change in theMEBT2B1 rack temperature and humidity. Δ𝜙2301_𝑝𝑑𝑚 represents VS phase difference between SDTL16 and SDTL15;
Δ𝜙54_𝑝𝑑𝑚 denotes phase difference between MEBT2B2 and MEBT2B1; and Δ𝜙64_𝑝𝑑𝑚 represents phase difference between
ACS01 and MEBT2B2 cavities.

Figure 8: Comparison of long-term cavity RF phase differences measured and computed on the PDM and CPMs: (top)Δ𝜙21_𝑝𝑑𝑚 and Δ𝜙21_𝑐𝑝𝑚 represent the phase differences between SDTL16A and SDTL15B tanks, (bottom) Δ𝜙54_𝑝𝑑𝑚
and Δ𝜙54_𝑐𝑝𝑚 denote the phase differences between MEBT2B2 and MEBT2B1 cavities. The black plots depict the phasedifference measured on the PDM with a MAF. MAF represents a moving average filter.
Figure 7 presents the long-term RF phase differences withrespect to reference phases that can be measured only on thePDM. Figure 8 shows phase differences compared with permanently installed and successfully operated CPMs. Hereby,the EPICS archiver appliance stored real-time cavity RFphase data as well as temperature and humidity data inside
the MET2B1 rack. The results measured on the PDM wereconsistent with those on the CPMs; no change in the centerof phase differences for two weeks where stable temperatureand humidity in the rack were provided, see Fig. 8.
As the last and primary step, we evaluated Δ𝜙𝑟𝑒𝑙 relative
phase difference derived from the phase difference between
two different frequencies of 324 MHz and 972 MHz on
the PDM. Write down an expression of the relative phasedifference,Δ𝜙𝑟𝑒𝑙 = Δ𝜙23_𝑣𝑠 − (Δ𝜙4_𝑝𝑑𝑚 /3),- 100 -(1)Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEOB09

Figure 9: Humidity dependency of the relative phase difference.
where Δ𝜙23_𝑣𝑠 and Δ𝜙4_𝑝𝑑𝑚 denote phase changes (pp) in
the reference channels of ADC03 (SDTL16 VS) and ADC04(MEBT2B1), respectively. Figure 9 presents the humidity
dependence of relative phase difference evaluated for about
one week. The relative phase difference tends to be similar
to the humidity change in the klystron gallery and the opticalduct, where RF reference signals are distributed [12].

CONCLUSIONS
Phase differences between 324 MHz cavities and their respective reference phase, as well as that of 972 MHz cavities,
are measured to be stable for the long-term on the CPMs andPDM. However, we have found that environmental factors,
particularly humidity changes, cause long-term phase drift
in the relative phase difference, critical information for stabilizing accelerating RF fields through linac cavities operating
at two different frequencies. The source of variation in therelative phase difference is thought to be humidity change
in the optical duct or klystron gallery. To this end, the PDM
will be employed within the LLRF system to compensate
for possible phase and amplitude drifts in cavity RF signals
by eliminating environmental effects, which is crucial for astable long-term operation.

ACKNOWLEDGMENTS
Special thanks to the Mitsubishi Electric Tokki Systems
Corporation team for implementing a phase drift monitor.
The authors would also like to thank the Mitsubishi SC members for their excellent work during the device installation.

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