Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, Japan

PASJ2018 FROL12

MEASUREMENT OF PROTON BEAM PROFILE AT 8 GEVACCELERATION COMMISSIONING FOR THE J-PARC COMET
EXPERIMENT
Y. Fukao∗ , K. Agari, H. Akiyama, K. Aoki, Y. FujiiA) , E. Hirose, M. Ieiri, Y. Igarashi,Y. Kato, R. Kurasaki, S. Mihara, M. Minakawa, Y. Morino, M. Moritsu, R. Muto,
H. Nishiguchi, Y. Sato, S. Sawada, H. Takahashi, K. Tanaka, M. Tomizawa, A. Toyoda,
K. Ueno, M. Ukai, H. Watanabe, T. Yamamoto, Y. Yamanoi
KEK, Oho, Tsukuba, 305-0801, JapanA) Monash University, Clayton, VIC 3800, Australia
Abstract
The first commissioning of a bunched slow extractionof a 8 GeV proton beam was performed at J-PARC for theCOMET experiment in January and February, 2018. Thebeam profile of the extracted beam was measured at theprimary proton beamline in Hadron Facility and the resulting
2σ emittance was 0.78 π mm mrad and 4.6 π mm mrad inhorizontal and vertical direction, respectively. The beam
extinction factor of the secondary beam was also measured
for the first time at the K1.8 beamline.

INTRODUCTION
The COMET experiment aims at searching the phenomena of the muon-to-electron (µ-e) conversion without neutrino emission [1]. Since such process violates lepton flavor
conservation, discovery of the µ-e conversion indicates existence of new physics. The COMET experiment will searchfor electrons with energy of the muon mass from the muon
decay. To avoid background related to the beam, it’s preferable for COMET to utilize a bunched beam with the bunchinterval of about muon life time. In terms of the beam intensity, the number of beam particles per bunch is limitedby the rate capability of the detectors. As the best choiceof a source of the muon production, we decided to utilize
a proton beam with the bunched-slow extraction methodfrom J-PARC Main Ring (MR) to Hadron Facility (Fig. 1).
The beam energy is 8 GeV, which is lower than the normal
beam energy of 30 GeV, to suppress background caused byanti-proton production. The extracted beam will be transported via Switch Yard and Hadron Hall to the COMET
experimental hall as shown in Fig. 2.
The time structure of the bunched-slow extracted beam for
COMET is displayed in Fig. 1. The cycle of the accelerationand extraction of the proton beam at MR is 2.48 s. The
beam is extracted for about 500 ms during the flat top. The
bunch interval of 1.17 µs or 1.75 µs is achieved by filling thebeam to four out of nine MR buckets for MR operation at aharmonic number of nine. The bunch length is estimated to
be about 100 ns. In the commissioning in 2018, idle timeof about three seconds was added to each cycle to smoothly
change accelerator setting between the commissioning andthe normal beam operation. Number of protons per bunchin the commissioning was 1.6×107 and was same as thatplanned in the actual operation at COMET Phase-I.
Two important informations were obtained in the commissioning. One was the measurement of the extracted beamemittance and the other was the measurement of an extinction factor with the secondary beam. The extinction factor
is a ratio of the residual beam particles between bunches tothose of the main bunch. The extinction factor directly contributes to a prompt beam-related background. The COMET
experiment requires the extinction factor of smaller than
10−10 . Although the test of the acceleration and extraction
for the COMET experiment were partially performed in thepast years, the commissioning of the bunched slow extraction of the 8 GeV proton beam was carried out for the firsttime in this January and February. Most of the beam time
was used for the accelerator commissioning and the detector calibration while the last day in February was used forthe measurement of the beam emittance and the extinctionfactor.

���� � ������������������������������Figure 1: Cycle of the proton beam acceleration at MR andtime structure of the bunched-slow extraction beam.

∗ fukao@post.kek.jp- 231 -Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, Japan

PASJ2018 FROL12

Hadron Hall

K1.8
Area

Main Ring
A-line0B-lineSwitch Yard

20m

Beam Monitor

COMET
HallFigure 2: Proton beamline of J-PARC Hadron Facility.

2σ width of dX [mrad]
2σ width of Y [cm]

2.5
21.5
10.5
3.540
32.5

6080100
Measurement120140160

180200220

2σ width of dY [mrad]

2σ vertical beam size [m]2σ horizontal beam size [m]Because the new beamline for the COMET experiment iscurrently under construction, the extracted proton beam wastransported via A-line to an existing primary proton target.
To obtain the emittance of the extracted beam, existing beam
monitors installed in A-line were used. The beam monitors
were Residual Gas Ionization Profile Monitors [2]. Eachmonitor can measure one-dimensional beam profile and aset of horizontal and vertical monitors was installed at eachcircle indicated in Fig. 2. Using the PSI TRANSPORT
simulation [3], the beam envelope was fitted to the measured
beam size to obtain the beam emittance at the point of theextraction from MR to Hadron Facility.

2σ width of X [cm]

BEAM EMITTANCE

Fit

1.22
1.21
1.2
1.19
1.18
1.17
1.16
1.15
1.14
0.0900:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00
Time
0.08
0.07
0.06
0.05
0.04
0.400:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00
Time
0.39
0.38
0.37
0.36
0.35
0.34
1.3400:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00
Time
1.32
1.3
1.28
1.26
1.24
1.22
1.2
1.1800:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00
TimeFigure 4: Shot-by-shot beam extent and divergence as afunction of time.

2
1.5
10.5
040

60

80

100

120

140

160

180

200

220

Monitor position [m]Figure 3: 2σ beam size at the position of the beam monitors.
Horizontal axis indicates distance from the beam extraction
point.

A comparison of the beam size and the simulated beamenvelope for a typical shot (one cycle of the beam extraction)is displayed in Fig. 3. The shot-by-shot width of the beam
extent and divergence was calculated for about 24 hours toevaluate stability of the beam and the results are shown inFig. 4. A shift around 11:00 was due to power recycle of abeamline magnet. Frequent deviation with interval of 20 –30 minutes was caused by instability of a debuncher of theaccelerator. Mean values of 2σ width of horizontal extent,
its divergence, vertical extent and its divergence were 11.7(0.09) mm, 0.067 (0.0054) mrad, 3.7 (0.07) mm and 1.25(0.019) mrad, respectively. Numbers in parenthesis are rootmean-square of each fluctuation. Resulting 2σ horizontal
and vertical emittance were 0.78 (0.063) π mm mrad and4.6 (0.14) π mm mrad, respectively.
Based on the measured beam emittance, the beam opticsof the new proton beamline for the COMET experiment was- 232 -Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, JapanPASJ2018 FROL12� ��� �������������� ����������������������������������������������+� �����

�����������������!"#$�%"&�'��$�()*Figure 5: Schematic view of the detector system at K1.8 area.

calculated using the PSI TRANSPORT and TURTLE [3]simulation. The optics was optimized to minimize the beam
size at the target of the COMET experiment. The resulting
horizontal and vertical beam size at the target was 1.4 mm
and 1.0 mm, respectively, in 1σ of Gaussian fit.

������� ����EXTINCTION FACTOR MEASUREMENTAT K1.8 AREA IN HADRON HALL
Several measurements of the extinction factor were performed at J-PARC in the past years. However those earlytests were done with the beam energy of 30 GeV and/or at
the MR abort line without extraction. The measurementwith 8 GeV and bunched slow extraction was performed forthe first time in 2018. The measurement was carried out at
the K1.8 experimental area in Hadron Hall (Fig. 2). The
extracted proton beam was injected to the primary target andproduced secondary particles were transported to the K1.8area. The secondary beamline was adjusted to transport
1.32 GeV/c negative pions with typical beam intensity of
1.5×106 per shot. The pion beam was used for the extinction
factor measurement. The bunch structure of the secondarypions is inherited from the primary proton beam and its
intensity is proportional to the proton intensity.
The schematic view of the detectors for the measurement
is displayed in Fig. 5. Coincidence of five layers of the scintillation counters or hodoscopes was employed to identify
the beam pions. The most upstream beam hodoscope (BH1)
was installed in the middle of the secondary beamline. The
second hodoscope (BH2) was located near the focal point.
Signal OR of slabs of each hodoscope of BH1 and BH2 wasused in the measurement. Another hodoscope (SH) and twocounters (SC1 and SC2) were installed at the most downstream region. Their photo was shown in Fig. 6. SH was themain detector to count beam pions. SH consists of 16 scintillation counters read out by photomultiplier tubes. The size ofeach scintillator was 40 mm wide, 250 mm high and 20 mm
thick. The signal of each slab was recorded separately toavoid signal pileup because about four pions per bunch wereinjected on average in the measurement. Two ion chambers������Figure 6: Photo of the downstream part of the detectors.

(IC1 and IC2) were used to supplement shot-by-shot beam
intensity.
The signals from counters were recorded by three different
TDCs (Time-to-Digital Converter) for redundancy. One is acommercial TDC (Acqiris TC890) with high time resolutionof 50 ps but the number of input channels is limited to six.
Signals from SH were additionally merged only for this TDC.
Another is TDC based on Xilinx Kintex-7 FPGA evaluation
board with 16 input channels. The third is FPGA-based
TDC with 64 input channels and was developed for general
purpose used at Hadron Facility. Signals from SH wererecorded without coincidence with BH1, BH2, SC1 and SC2
for this TDC and the coincidence was performed in offline
analysis. We finally obtained consistent results between
these TDCs.
In addition to the signals from the detectors, three timing
signals from the accelerator were used in the measurement.
One is a synchrotron RF which is synchronized with beam
bunches. Another is MR_sync which is a timing of the firstbucket out of nine in MR. MR_sync is used as a time zero of- 233 -Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, Japan

PASJ2018 FROL12TDC. The third is P3 which is a timing of the beam extraction
as displayed in Fig. 1.
Performance of the counters was evaluated during thecommissioning period as well as beam time with normal
slow extraction. Overall efficiency of the detectors was 92%
for single pion injection. The inefficiency mainly came fromacceptance of BH1 and BH2 because pion beam size waslarger than usual experiment at K1.8 area. Count loss ofSH due to signal pileup was evaluated by hit multiplicity ofSH as well as comparison between SH and ion chambers
and was estimated to be 10%. A fake signal due to accidental coincidence of the detectors was evaluated based on
single count rate of each detector and was obtained to be
4×10−13 Hz. It corresponds to 10−8 hits or less during themeasurement time.

The obtained hit time distribution is displayed in Fig. 7.
Four large peaks correspond to filled bunches. The numberof the accumulated pions was 1.7×1010 . No particles between bunches were observed before the 4th bunch while
202 counts of residual beam pions were detected after the4th bunch. The resulting extinction factor was the orderof 10−8 and is unacceptable for the COMET experiment.
By masking the 4th bunch, we achieve acceptable level of
6×10−11 but signal yield decreases to 75%. Correlation withP3 timing is displayed in Fig. 8. The undesirable hits concentrate near the beginning of the beam extraction. Notethat the extinction factor was also measured at MR abort line
directly using proton beam to be better than 10−11 . However the region after the 4th bunch was invisible due to itsmeasurement method.

9�����

10810710SUMMARY AND PROSPECTS

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Figure 7: Hit time distribution. Time zero was determined byMR_sync with arbitrary offset. Four large peaks correspondto the filled bunches.

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Commissioning of the bunched slow extraction of 8 GeV
proton beam was performed at J-PARC for the first time. The
emittance of the extracted beam and the extinction factor
were measured during the commissioning. Based on themeasurements, it becomes possible to evaluate a design ofthe new beamline for the COMET experiment. More realistic simulation of the COMET experiment is also ongoingto evaluate signal yield and sensitivity. We achieved theextinction factor of 6×10−11 or less while the significant
beam leakage was observed after the 4th bunch. The leakageshows characteristic time structure and it would help to solve
the source of the leakage.

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REFERENCES

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[1] http://comet.kek.jp/������

[2] Y. Sato et al., “Residual gas ionization profile monitors in JPARC slow-extraction beam line”, Proceedings of IBIC2012,
Tsukuba, Japan, pp. 267-270, 2013.

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Figure 8: Time evolution of the hit time distribution.

[3] PSI Graphic Transport and Turtle Framework by U. Rohrer
based on a CERN-SLAC-FERMILAB version by K.L. Brown
et al.;
http://aea.web.psi.ch/Urs_Rohrer/MyWeb/- 234 -