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

PASJ2022 TUOPA03

ACHIEVEMENT OF LOW BEAM LOSS AT HIGH-INTENSITYOPERATION OF J-PARC 3 GeV RCS
P.K. Saha∗ , K. Okabe, T. Nakanoya, M. Yoshimoto, Y. Shobuda, H. Harada, F. Tamura, H. Okita,
S. Hatakeyama, K. Moriya, K. Yamamoto, J-PARC Center, KEK and JAEA, Ibaraki, Japan
H. Hotchi, KEK/J-PARC, Ibaraki, Japan
Abstract
The beam power 3-GeV RCS of J-PARC for operation
to the MLF has been increased to 800 kW recently. The
total beam loss even at the designed 1 MW beam powerhas been well controlled. However, the residual radiation
at the injection area caused by the uncontrolled beam loss
occurred by foil scattering of the circulating beam is a serious issue for regular maintenance works. To minimize thebeam loss further but for further, especially the uncontrolled
beam losses caused by the foil scattering, recently we have
implemented a new approach by minimizing the injection
beam size and using a smaller size stripper foil. A foil hittingof the circulating beam is reduced so as the foil scattering
uncontrolled beam losses mainly at the injection area. In addition, an optimized transverse painting area matching witha smaller injection beam gives a smaller circulating beamemittance to reduce the beam loss at the collimator section
and its downstream. The corresponding residual radiationfor operation at 700 kW beam power was measured to besignificantly reduced as compared to that with an original
bigger injection beam size and a bigger foil. A smaller injection beam size including other potential optimizations
were also extensively tested at 1 MW beam power and obtaina residual beam loss of nearly 0.05%, which is nearly 3/4
reduction as compared to the previous 1 MW test operation
done in 2020.

INTRODUCTION
The 3-GeV RCS (Rapid Cycling Synchrotron) at J-PARC(Japan Proton Accelerator Research Complex) delivers highintensity proton beam to both MLF (Materials and Life Science Experimental Facility) and the MR (30-GeV Main Ring
Synchrotron) [1]. The injection beam energy is 400 MeV,
which is accelerated to 3 GeV at repetition rate of 25 Hz andsimultaneously delivered to the MLF and MR. The designed
beam power is 1 MW (8.33×1013 protons/pulse), while atthe latest it is 800 kW to the MLF and also nearly 800 kW
equivalent beam power to the MR. Figure 1 shows the historyof the beam power to the MLF. The beam power to the MLF
is gradually approaching to the designed 1 MW by carefully
investigating the neutron production target. Meanwhile wehave conducted 1 MW test operation several times to checkthe feasibility of continuous operation.
Usually, a more than 90% of the beam from the RCS isdelivered to the MLF. A beam loss reduction in the RCS
for operation to the MLF is highly important. Based on the∗ saha.pranab@j-parc.jp

Figure 1: History of RCS beam power to the MLF.

detailed studies and numerical simulations, The beam loss at
the RCS has been well controlled even at the designed 1 MW
beam power [2]. Recently, we have achieved further beamloss mitigation by optimizing the betatron tune as well as
adopting a momentum offset (Δ𝑝/𝑝) of the injection beamitself instead of RF bucket offset as a process of longitudinalpainting. However, the uncontrolled beam loss caused by the
foil scattering of the circulating proton beams during multiturn charge-exchange injection of 𝐻 − and the corresponding
residual radiation at the injection area is still considerably
high. It is thus a serious concern at 1 MW regular operation
in a near future [3, 4]. A large transverse painting (TP) at
injection is adopted for the MLF to reduce average foil hitsof the circulating beam to only 7 [5], but it is not sufficient
to keep the residual radiation at the injection area to a permissible level. A further technique for foil hit reduction isthus highly essential.
To further reducing the foil hits of the circulating beamand the corresponding foil scattering beam losses, we have
implemented a smaller size foil by minimizing the injection
beam size at the 1st stripper foil. This was done by manipulating vertical twiss parameter (𝛽𝑦 ) of the injection beam.
As a result, a significant reduction of the uncontrolled beamlosses at the injection area and the corresponding foil hit reduction were obtained. In addition, an optimized transverse
painting area matching with a smaller injection beam gives asmaller circulating beam emittance for further reducing thebeam loss also at the collimator section and its downstream.

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

PASJ2022 TUOPA03

PRESENT APPROACH
Figure 2 shows a schematic view of the present approachto reduce foil hits of the circulating beam by minimizing
vertical size of the injection beam and using a smaller size
foil. This allows us to reduce both sides of the vertical foil
instead of a single side if it is done for the horizontal size, as
the foil is horizontally mounted. The vertical twiss parameter(𝛽𝑦 ) of the injection beam is minimized to 2.4 m from its
typical value of 8 m by using 4 quadrupole magnets locatedat the upstream of the stripper foil.

Figure 4: Schematic view of vertical TP done by varying
𝑦′ of the injection beam. The 𝑦′ for a smaller 𝛽𝑦 can be
minimized while keeping a same painting area to reducelarge amplitude particles in the circulating beam.
painting area of 200𝜋 mm for the MLF. As emittance of
the injection beam is unchanged, then the beam ellipse fora smaller 𝛽𝑦 is changed as shown in the right figure. The
angle of the injection beam can be thus minimized to keep
the painting area unchanged. Such an optimization of thevertical painting improves vertical beam distribution by minimizing the number of large amplitude particles as comparedto that with a bigger 𝛽𝑦 of the injection beam. While due
to space charge effect the maximum beam emittance goesbeyond 200𝜋 mm at high intensity, the beam loss caused by
the large amplitude particles at the collimator and its downstream sections can also be further mitigated by minimizing
𝛽𝑦 of the injection beam for a smaller vertical beam size.

Signal (Arb. units)5

εun-normalized 99% (π mm mrad)

Figure 2: Schematic view of the present approach to reduce
circulating beam hits on the foil and the corresponding foil
scattering beam losses by minimizing vertical size of theinjection beam to place a smaller size foil.

βy = 8 m
βy = 2.4 m

4
3

150

βy = 8 m
βy = 2.4 m
Horizontal 150

100

100

50

50

0
1

2

3

4

5

6

0

1

Time (ms)

-5

0

5

2

3

4

5

6

Time (ms)Figure 5: Reduction of circulating beam emittances obtainedin the simulation by using a smaller injection 𝛽𝑦 and optimizing the painting area.

1
0
-10

0
0

2

βy = 8 m
βy = 2.4 m
Vertical

10

Vertical position (mm)Figure 3: Measured vertical profiles of the injection beam at
a location of the foil. A profile of 2 mm (in 𝜎) was minimized
to nearly half of 1.1 mm.

Beam survival (%)

Figure 3 shows the measured vertical profiles of the injection beam at the 1st foil location for an original 𝛽𝑦 of 8 m
and minimized one of 2.4 m depicted by the black and redlines, respectively. The profile width (𝜎) of 2 mm has been
reduced to nearly half of 1.1 mm.
Figure 4 shows a schematic demonstration of verticaltransverse painting (TP) done by changing vertical angle(𝑦′ ) of the injection beam by using two vertical painting
magnets placed the beam transport, upstream of the injection point. The edge of the injection beam determines the
maximum painting area, where a 𝑦′ of -3.4 mrad gives a100βy = 8 m
βy = 2.4 m

99.98
99.96
99.94
99.92
0

5

10

15

20

Time (ms)Figure 6: Simulation results of beam survival improvement
at 700 kW by minimizing injection 𝛽𝑦 8 m to 2.4 m andreducing the foil size 20 mm to 14 mm, respectively.

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

PASJ2022 TUOPA03

Figure 5 shows 99% unnormalized emittances of the circulating beam obtained in the numerical simulations for 𝛽𝑦of 8 m (black) and 2.4 m (red), where left plots are for thehorizontal and right plots are for vertical emittances, respectively. In this simulation, is it found that a smaller 𝛽𝑦 with an
optimized painting area (shown in Fig. 4) gives a reductionof the circulating beam emittances.
Figure 6 shows numerical simulation results of beam survival for 𝛽𝑦 of 8 m (black) and 2.4 m (red). The simulation
was done for a beam power of 700 kW by taking into account measured twiss parameters of the injection beam andrealistic machine parameters including foil scattering. The
vertical foil size for a bigger and smaller 𝛽𝑦 was 20 mm and
14 mm, respectively. A smaller 𝛽𝑦 gives a significant improvement of the beam survival by reducing the total beamloss as much as 45%. The corresponding average foil hits
was also estimated to be reduced 30%. The simulation results thus indicate that the present approach can reduce theuncontrolled foil scattering beam losses at the injection area
while reducing the overall beam loss at the collimator section
and its downstream.

EXPERIMENTAL RESULTS AT 700 KW
BEAM POWER
Figure 7 shows the beam losses at 700 kW equivalentbeam power measured by the beam loss monitors (BLM) at
the collimator and the 1st arc sections. Each BLM signal isintegrated over the whole cycle of 20 ms (injection to extraction). A smaller 𝛽𝑦 (red) gives 42% beam loss reduction in
average as compared to a bigger 𝛽𝑦 (black), where more than
50% reductions have also been obtained at several points. It
is worth mentioning that the absolute beam losses at the 1st
arc section is comparatively much lower than the collimator
section. The high voltage of the BLM devices are set higher
to measure even a lower beam loss signal. The measurement
result is quite consistent with simulation result as shown abeam survival improvement (Fig. 6). The rms emittances ofthe extracted beam for both horizontal and vertical planes
were measured to be more than 10% reduced by applying asmaller 𝛽𝑦 of the injection beam.

BLM’s signal (Arb. units)

Collimator section

1st arc section

0.3

As for the beam loss reduction at the injection section, it
was separately measured by using a small plastic scintillator
counter type BLM placed 90𝑜 above the foil in the horizontal
direction. In this measurement, secondary charged particles
such as, 𝛾 rays generated by large angle foil scattering primary protons lost at the nearby beam pipe. A 30% reductionof such a beam loss was measured corresponding to a same
reduction of foil hits obtained with a smaller 𝛽𝑦 as comparedto that with a bigger one [6], and was consistent with 27%
reduction estimated from the numerical simulation.
A smaller injection 𝛽𝑦 and a smaller vertical size foil of
14 mm were tested for RCS operation at 700 kW beam power.
Figure 8 shows a comparison of the measured residual radiation (on contact and after 4 hours cooling from beam stop)
between two 𝛽𝑦 s used each for 1 month operation. Similar
to the measured beam loss, the residual radiation was also
successfully reduced significantly by implementing a minimized injection 𝛽𝑦 , where reductions at the injection and1st arc sections have significant importance due frequent
access on these areas for regular maintenance works. As aresult, a smaller injection 𝛽𝑦 and a smaller foil were implemented for RCS routine operation, and there is no issues so
far even at the present beam power of 800 kW. They werealso been successfully tested for several hours operation at 1
MW beam power in June 2022.

Figure 8: Comparison of residual radiation (on contact) at
the injection, collimator and 1st arc sections with a bigger(black) and smaller (red) 𝛽𝑦 measured after 1 month operation at 700 KW beam power. The residual radiation was
significantly reduced by implementing a smaller 𝛽𝑦 and asmaller size foil.

βy = 8 m
βy = 2.4 m

0.2

BEAM LOSS MITIGATION AT 1 MW
BEAM POWER

0.1

We have also carried out detail studies at 1 MW beam
power by using a smaller 𝛽𝑦 and a smaller size foil. In addition, we have applied following several other parameters
step by step to for beam loss at 1 MW.
Parameter ID #1: Original with 𝛽𝑦 = 8 m.
Parameter ID #2: Optimization of betatron tune and Δ𝑝/𝑝offset by the injection beam instead of RF bucket offset.
Parameter ID #3: 𝛽𝑦 = 2.4 m and optimization of horizontal

0
0

5

10

15

20

25

30

BLM’s ID

Figure 7: Measured beam loss as a function of BLM IDs at
a beam power of 700 kW. A beam loss reduction of 42% (inaverage) is obtained by minimizing the injection 𝛽𝑦 .

-3-Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUOPA03

transverse painting to produced more uniform distribution.
Details can be found in Ref. [7].
Parameter ID #4: Reduction of chicane bump height (20%)
mainly to suppress their non vanished sextupole field component that excites the 3𝜈𝑥 = 19 resonance causing a horizontal
emittance growth.
Parameter ID #5: Application of 1 ms longer 2nd harmonic
RF voltage to 7 ms for further mitigation of the space charge
effect producing more uniform longitudinal beam distribution and achieving a higher bunching factor [8].
Collimator

1st Arc

BLM signals (arb. unit)

0.8

Extraction

2nd ArcRF

3rd Arc

Injection 3N H0-D

βy = 8 m originalβy = 2.4 m including parameters up to ID #5

0.6

obtained by mitigating nearly 3/4 from an original beam loss
of 0.2% at the previous 1 MW test operation in 2020.
It is thus can be noted that the remaining beam loss isdominated by the foil scattering ones and a complete mitigation is practically impossible as long a stripper foil is used.
The additional beam losses even at 1 MW beam power has
been well mitigated. In the next beam study, we will test alittle further smaller size foil of 12 mm for another at least
10% reduction of the foil scattering beam losses. To eliminate the foil scattering beam losses as well as to avoid issues
associated with a stripper foil short lifetime and failures, weare developing a foil-less 𝐻 − charge-exchange injection byusing only lasers [9]. One can thus expect an achievement of
MW level beam power essentially without almost any beamlosses.

SUMMARY

0.4

0.2

0
0

10

20

30

40

50

60

70

80

90

BLM IDs

Figure 9: Measurement of beam loss mitigation at 1 MW
beam power by using a smaller injection 𝛽𝑦 of 2.4 m andapplying measures taken by parameter IDs #2 through #5 ascompared to the original one with a larger 𝛽𝑦 of 8 m.

Beam loss (%)

0.2

ID #1
ID #2
ID #3
ID #4
ID #5

0.15

0.1

0.05

0
0

1

2

3

4

5

Parameter ID

Figure 10: Reduction of the residual beam loss at 1 MW in
the collimator through 1st arc sections as a function of theapplied parameter ID.
Figure 9 shows the measured beam losses 1 MW beam
power throughout the RCS. The beam loss has been significantly mitigated by using a smaller injection 𝛽𝑦 of 2.4 m
and applying all parameters mentioned in IDs #2 through #5
as compared to that of original one with using a 𝛽𝑦 of 8 m.
An average beam loss reduction of more than 70% has beenobtained at the collimator through 1st arc section. Figure 10
shows a step by step reduction of the residual beam loss estimated from measured BLM signals at the collimator through1st arc section as a function of the parameters applied. The
residual beam loss at the latest remains only around 0.05%,

To reduce the residual radiation at the injection area of JPARC RCS caused by the uncontrolled foil scattering beamlosses, we have implemented a smaller size foil for RCS
operation by minimizing the injection beam size. The vertical beam size (𝜎) at the foil from 2 mm was minimized
to 1.1 mm by reducing a 𝛽𝑦 of 8 m to 2 m and replace afoil size of 20 mm to 14 mm, respectively. The beam loss
at the injection area and the corresponding average foil hitsof the circulating beam was measured to be 30% reduced,
consistent with an expectation of 27% reduction. In addition, an optimized vertical transverse painting area matchingfor a smaller injection beam reduces the circulating beamemittance to mitigate the beam losses at the collimator andits downstream sections. The residual radiation at 700 kW
beam power operation with a smaller 𝛽𝑦 was measured to besignificantly reduced as compared to that with a bigger one
used so far. A smaller injection beam with other potential
optimizations have also been extensively tested at 1 MW
beam power. The residual beam loss at the latest beam test
is minimized to nearly 0.05%, which is nearly 3/4 reduction
from that of 0.2% beam loss at the previous 1 MW test operation done in 2020. The remaining beam losses are mainly
dominated by the foil scattering, which can be further mitigated by reducing the foil size to 12 mm in the next beam
test. A foil-less charge-exchange injection of 𝐻 − by using
lasers, which we are developing at J-PARC can eliminate
realistic issues and limitations associated with a stripper foil
including foil scattering uncontrolled beam losses.

ACKNOWLEDGMENTS
The authors would like to acknowledge all members of
the J-PARC accelerator division for continuous support andcooperation throughout the present work.

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REFERENCES[1] High-intensity Proton Accelerator Project Team, “Accelerator
Technical Design Report for J-PARC”, JAERI-Tech 2003-044
and KEK Report 2002-13.

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

PASJ2022 TUOPA03[2] H. Hotchi et al., “Achievement of a low-loss 1-MW beam
operation in the 3-GeV rapid cycling synchrotron of the Japan
Proton Accelerator Research Complex”, Phys. Rev. ST Accel.
Beams, vol 20, p. 060402, 2017.
[3] Shinichi. Kato, Kazami Yamamoto, Masahiro Yoshimoto,
Hiroyuki Harada and Michikazu Kinsho, “Localization of
the large-angle foil-scattering beam loss caused by multiturncharge-exchange injection”, Phys. Rev. ST Accel. Beams, vol.
16, p. 071003, 2013.
[4] M. Yoshimoto et al., “Radio-activation caused by secondary
particles due to nuclear reactions at the stripper foil in the
J-PARC RCS”, in proceedings of international particle accelerator conference (IPAC’17), Copenhagen, Denmark, 2017,
paper TUPVA093, pp. 2300-2303.
[5] P.K. Saha et al., “First measurement and online monitoring of
the stripper foil thinning and pinhole formation to achieve a
longer foil lifetime in high-intensity accelerators”, Phys. Rev.
ST Accel. Beams, vol 23, p. 082801, 2020.

-5-[6] P.K Saha et al., “Recent results of beam loss mitigation andextremely low beam loss operation of J-PARC RCS”, Proc.
of IPAC’22, Bangkok, Thailand, 2022, paper WEOYGD1, p.
1616.
[7] H. Hotchi, “Effects of the Montigue resonance on the formation of the beam distribution during multiturn injection
painting in a high-intensity proton ring”, Phys. Rev. ST Accel.
Beams, vol 23, p. 050401, 2020.
[8] F. Tamura et al., “Longitudinal painting with large amplitudesecond harmonic rf voltages in the rapid cycling synchrotronof the Japan Proton Accelerator Research Complex”, Phys.
Rev. ST Accel. Beams, vol 12, p. 040001, 2009.
[9] P.K. Saha et al., “RECENET PROGRESS OF LASER STRIPPING POP DEMONSTRATION STUDY AT J-PARC RCS”,
Proc. of PASJ2021, p. 656, 2021.