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

PASJ2022 TUP003

STATUS OF POP DEMONSTRATION OF 400 MeV H− LASER STRIPPINGAT J-PARC RCS
P.K. Saha∗ , H. Harada, M. Kinsho, J-PARC Center, KEK and JAEA, Tokai-mura, Ibaraki, Japan
H. Yoneda, Y. Michine, University of Electro-Communications, Chofu, Tokyo, Japan
A. Sato, NAT, Tokai-mura, Ibaraki, Japan
T. Shibata, High Energy Accelerator Research Organization, Tsukuba, Ibaraki, Japan
Abstract
To overcome the realistic issues and practical limitations
associated with H− charge-exchange injection (CEI) doneby using a solid stripper foil, a Proof-of-principle (POP)demonstration studies of 400 MeV H− stripping by using
only lasers is under preparation at J-PARC. In this method,the H− is first neutralized to H0 by using an YAG laser, theH0 is then excited (called H0∗ ) by using a UV laser andfinally stripped it to proton by the YAG laser. A prototype
YAG laser system has been developed including a multireflection laser cavity system to significantly reduce the seedlaser energy. The R&D of the laser and the cavity systems arebeing upgraded through experimental studies for 3 MeV H−
neutralization. The R&D of the UV laser produced by higher
harmonic generation from the YAG laser is also in progress.
The experimental studies for the POP demonstration will bestarted in 2023. In this report, progress of YAG laser andthe cavity systems including latest experimental results of 3
MeV H− neutralization are mainly presented.

only lasers. The noted laser parameters are considered for
H− of 400 MeV. In this method. the H− is first neutralized to
H0 by stripping its loosely bound electron by an YAG laser of1064 nm. The ground state (1s) electron in the H0 is excited
to 3rd excited state (3p) denoted as H0∗ by using a deep UV
laser of around 200 nm, and finally the H0∗ is stripped to p byremoving its excited electron by the YAG laser. To establish
the method, a proof-of-principle (POP) demonstration study
for 400 MeV H− stripping to proton is under preparation
at the 3 GeV Rapid Cycling Synchrotron (RCS) of Japan
Proton Accelerator Research Complex (J-PARC) [10–14].

INTRODUCTION
The multi-turn charge-exchange injection (CEI) of H−(negative hydrogen) conventionally done by using a thinsolid stripper foil is an effective way to achieve high-intensity
proton beam [1, 2]. The CEI allows stacking of many turns
because of ideally no beam emittance growth due to injecting
in a different charge state and it provides the opportunity ofunlimited multi-turn injection until stacking particles exceed
the aperture. The CEI with a stripper foil has been successfully utilized to achieve high-intensity beam of around 1 MW
in existing accelerators [3, 4]. However, a short and unexpected lifetime of the foil as well as uncontrolled beam losses
and the corresponding high residual radiation are two serious
issues, especially at high-intensity operation [5–7]. The continuous efforts on durable foil production made remarkableprogress on the foil lifetime [8], but it is still unclear how
to deal with multi-MW beam power. It is hard to maintainreliable and longer lifetime due to overheating of the foil
at high-intensity operation and might be the most serious
concern and a practical limitation to realize a multi-MW
beam power.
To overcome the issues and limitations associated withthe stripper foil, we proposed a foil-less alternate method of
H− CEI by using only lasers [9]. Figure 1 shows a schematic
view of the principle for H− stripping to protons (p) by using∗ saha.pranab@j-parc.jp

Figure 1: A schematic view of our proposed principle ofH− stripping to proton by using only lasers. Typical laserparameters for an H− stripping at 400 MeV are also noted.

A prototype YAG laser system has been developed andfurther upgrades are continued for higher energy, robust
uses and reliability. To significantly reduce the seed laser
power, we have also developed a multi-reflection YAG laser
cavity system to overlap many laser pulses at the interaction
point (IP) of the laser and ion beam [15]. This year we havedoubled the number of reflections from 16 to 32. The YAG
laser and the cavity system are being developed throughexperimental studies of 3 MeV H− neutralization J-PARC
RFQ-TF (Radio Frequency Quadrupole test facility) [16].
The UV laser of 213 nm, which is a 5th harmonic of YAG
laser, is also under developed by higher harmonic generation
techniques from the YAG laser. In addition, to handle sucha deep UV laser, R&D for the coating of UV laser opticaldevices have also been started. A detail strategies for a POP
demonstration of 400 MeV H− laser stripping was presented
in the PASJ meeting in 2020 [13]. In this paper, progress on
the YAG laser and the laser cavity system including latest
experimental results of 3 MeV H− neutralization are presented.

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

PASJ2022 TUP003

THE YAG LASER AND
MULTI-REFLECTION CAVITY SYSTEMSThe structure of the YAG laser system is essentially sameas used in 2021 except additional upgrades were done toincrease the pulse energy with a precise control for a longer
duration, robust uses with a good stability. More details
about the laser system can be found in [14]. In this experiment, a maximum photon energy of every single micropulse was about 0.023 mJ with 6.17 s micro pulse interval
for about 40 𝜇s total duration.
As a requirement of huge laser energy is one big issue torealize a laser stripping with higher efficiency, a reductionof the seeder energy is highly essential. For that purpose,
we have developed a multi-reflection (multi-pass) cavity
system to overlap many laser pulses at the interaction point(IP) [15]. This year we have doubled reflections to 32 fromthat of 16 in 2021. Figure 2 shows a schematic view ofthe final overlaps of 32 passes at the IP, which would allow
nearly a 1/32 reduction of the seeder energy. The laser beam
are focused at the IP for maximizing the photon flux, while
minimizing the mirror damage. The micro pulse durationof the laser was about 100 psec with a transverse spot sizes
for both horizontal and vertical planes with only 0.1 mm at
the IP. The output energy of each micro pulse was 0.023 mJ
at maximum, which after 32 overlaps gave about 0.38 mJ.
This number is nearly half than a linear consideration of 32
times increase, but photon losses due to the mirror efficiency
as well as at the vacuum windows are considerably higher,
which can be almost compensated with higher reflective
materials and placing the cavity system in vacuum. The
performance of the updated laser cavity system was tested
for 3 MeV H− neutralization in June, 2022.

which is then measured by a fast current transformer (FCT)with laser ON and OFF. Similar to the previous study, thelaser pulse interval was 6.17 s instead of 3.08 s of the H−
pulse as shown in Fig. 4. As a result, the interaction occurredfor every alternate H− pulses. The neutralization yield was
obtained by the FFT (Fast Fourier Transformation) of the
FCT data for a signal at 162 MHz. The present geometry inthis sense is very useful to uniquely detecting any signal at
162 MHz, differing from the main beam signal frequency of
324 MHz and with sufficiently less background level. The
neutralization efficiency for every individual micro pulse
can also be determined as the FCT can measure micro pulse
structure of the H− beam. The peak current of the H− beam
was 50 mA, same as used for J-PARC operation at present,
where a macro of 50 𝜇s was typically used.
A precise calibration of the laser to synchronize with theion beam is very essential to ensure a perfect overlapping fora higher stripping efficiency. At first the calibration of thelaser was performed by looking a neutralization peak at 162
MHz by an oscilloscope as shown in Fig. 5. The upper plot
is a time domain data of the FCT for un-neutralized H− beamof 50 𝜇s duration with laser ON. The middle plot shows an
expanded view, where a comb structure of the pulses due toneutralization for every alternate pulses can be seen, whichis confirmed by an FFT analysis and finding a maximum
peak height at 162 MHz as shown in the lower plot. Next,
a tuning of the AWG waveform for a precise micro pulse
frequency of the laser pulses is done for a perfect matchingwith every alternate H− micro pulse. This gives a uniform
neutralization over the entire H− pulse as shown in Fig. 6
the amplitude of 162 MHz obtained for every 2 𝜇s intervalof the time domain data. The AWG was tuned better than
10−3 precision to obtain a flat neutralization over the entire
macro pulse of the H− beam.

EXPERIMENTAL RESULTS OF 3 MEV H−
NEUTRALIZATION IN 2022

Figure 2: A schematic view of the latest multi-reflection
laser cavity system with 32 reflections overlapped at the IP.

EXPERIMENTAL SETUP
The setup for 3 MeV H− neutralization at the RFQ-TF
was essentially same as done in 2021 as shown in Fig. 3.
The laser output pulse is sent to the multi-reflection cavity
and interact to the H− beam. The IP is at the upstream of abending magnet (BM). The H− beam neutralized by the laserinteraction (H− + 𝛾 = H0 + e) becomes neutral in charge
called H0 , which is separated from the un-neutralized H−
beam by using the BM. The H0 thus goes straight, while theH− is bent by the BM and goes to the 11-degree beam line,

Figure 7 shows an expanded view of the time domain
signal at the central region from a 50 𝜇s H− pulse obtainedby applying a maximum laser energy. A reduction of every
alternate pulses can be seen caused by the neutralization
occurred due to laser interaction. The neutralized H0 beam
went straight to the 0-deg beam dump. Figure 8 shows FFT
spectra of the time domain signals with laser ON and OFF
depicted by the red and black lines, respectively. The peak at
162 MHz appears only when the laser is ON. In order to calculate the neutralization efficiency, the time domain micropulses are separated first with and without laser interaction.
The interval of the time domain pulses then becomes 6.17s
to have a frequency of 162 MHz. A ratio of the amplitude of
162 MHz peak for the pulses with and without an interaction
with the laser gives a neutralization efficiency. On the other
hand, we can also calculate neutralization of individual micro pulses (Fig. 7) by integrating and comparing with nearest
pulse with no interaction with the laser. By using the former
method, the neutralization for several 𝜇s at the middle of- 273 -

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

PASJ2022 TUP003

Figure 3: Setup of the laser system for 3 MeV H− beam neutralization study at J-PARC RFQ-TF. The neutralized H0 isseparated from the un-neutralized H− by a BM placed just downstream of the IP. The neutralization efficiency is obtainedby measuring the H− signals without and with laser taken by a current monitor (FCT) at the downstream.

FFT amplitude (Arb. units)

150

AWG freq. best
AWG ∆f: -8E-4 GHz
AWG ∆f: +8E-4 GHz

100

50

0
0

Figure 4: Synchronization of the laser pulse (purple) to every
alternate H− micro pulses (sky blue). The timing system
including the logic were updated to improve a timing jitter.

Figure 5: Online confirmation interaction by FFT analysisof the FCT time domain data for a signal at 162 MHz.
the H− pulse was calculated to be 24.8%, which is 1.5 times
higher than the previous result obtained in 2021 [15].
It is worth mentioning that the vertical laser beam size
should be higher than the ion beam for a complete overlapping as the laser crosses horizontally. In our case the laser
spot sizes were much smaller than the ion beam. The laser
spot at the IP was moved according to permissible limit of

10

20

30

40

50

Time (µs)

Figure 6: Tuning of the AWG waveform for a perfect matching with all alternate H− micro pulse to obtain a flat neutralization over the entire H− pulse.
the cavity geometry to have a higher beam size and obtained
nearly a half overlapping with the ion beam. As a result,the neutralization efficiency by taking into account of theoverlapping factor is estimated to be nearly 50%.
Figure 9 shows a comparison of the time dependent neutralization between obtained at the present and in 2021. The
vertical axis the FFT amplitude for every 2 𝜇s, meaninga neutralization efficiency. This time the laser pulse wastuning to be flat for a longer enough duration to obtain auniform neutralization over the entire H− macro pulse ascompared to no such a flatness in the previous year.

Laser Energy and Pass Dependent Neutralization
Efficiency
Finally we have also studied a neutralization efficiency
depending on the laser energy and also the number of reflections (passes) in the cavity. The energy dependence wasstudied for a maximum of 32 passes, where the input energy was varied. Differing from the above setup, the laser
spot size was kept original of only 0.11 mm at the IP for aninteraction occurring at a tiny region with a higher photon- 274 -

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

PASJ2022 TUP003

Neutralization efficiency (%)-0.09

FCT signal (V)

-0.1
-0.11
-0.12
-0.13
-0.14
-0.15
29.8

29.9

100
75
50
25
0

30.0

0

Time (µs)

Figure 7: Expanded view of the FCT domain signal of H−
at the central region. A reduction of every alternate H−
occurred due to the neutralization with laser interaction.
162 MHz

Laser ON
Laser OFF

105

20

104
103
2

10

1

10

100

0

100

200

300

400

500

600

100

Figure 10: Neutralization efficiency of the overlap region
covered by the tiny laser spot of 0.11 mm as a function oflaser energy. The neutralization efficiency is almost saturated by applying a maximum laser energy.

Gain factor

FFT Amplitude (Arb. units)

106

25
50
75
Laser energy (%)

Laser energy gain
Neutralization gain

15
10
5

700

Frequency (MHz)Figure 8: FFT spectra of the time domain signals with laser
ON (red) and OFF (black). The peak at 162 MHz appears
only with a laser ON.

FFT amplitude (Neutralization)flux. As as result, we estimated the spot neutralization as afunction of the laser energy as shown in Fig. 10. The neutralization efficiency is almost saturated at a higher energy
and was obtained to be 88%. This results indicates that aphoton flux which has been obtained by applying a maximum energy for a spot size of 0.11 mm gives almost fullneutralization of the spot. As a results, a bigger laser beam
size well covering the ion beam beam by keeping the present
photon flux can give a full neutralization of the H− beam.
2000

2022
2021

1500

1000

500

0
0

10

20

30

40

50

Time (µs)

Figure 9: Comparison of the time dependent neutralization
between 2022 and 2021 results. A uniform neutralization
over the entire H− macro pulse is obtained in 2022.

0
0

8

16

24

32

Pass number

Figure 11: Comparison of the laser energy gain and the corresponding neutralization gain as a function of the numberof the passes in the cavity.
Figure 11 shows a gain on the neutralization efficiency as afunction of the number of reflections overlapped in the cavity.
The laser energy gain, which was estimated by measuring
the photon losses at the mirrors and vacuum windows duringmulti passes due to their reflection efficiency of each device.
The laser energy gain was estimated to the about 16 for
32 passes and was quite consistent with neutralization gain
shown by the black and red symbols, respectively. The
present results thus demonstrate the merit of the present
multi-reflection cavity system to increase the laser pulse
energy at the IP by reducing the seed lase pulse energy. At
present a 1/16 reduction of the seed laser energy is obtained,
which can be reached to almost 1/32 with better coating of
the optical devices and placing them in vacuum.
It is worth mentioning that by reducing the laser peak
power, the pulse duration was extended to more than 300
𝜇s and measured a corresponding almost flat neutralization
over 300 𝜇s of the H− beam by scanning the laser with 20 𝜇s
step. We have also successfully measured both longitudinal
and transverse H− beam profiles by a laser interaction. Being completely non-destructive the method would be soon
applicable to J-PARC Linac for beam diagnostics, online
beam monitoring and control as well.

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

PASJ2022 TUP003

SUMMARY
To overcome the issues and limitation associated with astripper foil used for H− charge-exchange injection, a proofof-principle (POP) demonstration of 400 MeV H− stripping to proton by using only lasers is under preparation at
J-PARC RCS. A prototype YAG laser system including a
multi-reflection laser cavity system to significantly reduce
the seed laser power have been developed and further upgrades are continued through experimental studies for 3 MeV
H− neutralization at J-PARC RFQ-TF. The latest experimental studies were conducted in June, 2022. We have obtained
nearly a doubled pulse energy from the laser, which was
also precisely tuned to achieve a uniform neutralization overthe entire H− macro pulse of 50 𝜇s. The cavity system wasalso upgraded to doubled the reflections to 32 from that of
16 last year. The laser energy at the IP for each pulse wasobtained to be 0.38 mJ to achieve an effective neutralizationof 50% for each alternative micro pulses of the H− beam.
However, due a tiny laser spot size, a spot neutralization was
also estimated to be almost 100% with 32 passes. A passdependent neutralization gain was also obtained to be 16
at 32 passes, which was consistent with the estimated laser
energy gain. The laser energy gain such as the neutralization can be increased proportional to the number of passes
by minimizing the photon losses in the optical devices andplacing the cavity system in vacuum. For an near futureapplication to the accelerators, we have also successfullymeasured both longitudinal and transverse H− profiles by alaser interaction.
The R&D of the UV laser produced by higher harmonic
generation from the YAG laser is progress. The experimental
studies for the POP demonstration of 400 MeV H− stripping
to proton will be started in 2023.

ACKNOWLEDGMENTS
The authors would like to acknowledge many of our JPARC colleagues for continuous support and cooperationin the present research. This work was partly supportedby the JSPS KAKENHI Grant Number JP20H04456. The
present research was also partly supported by JAEA Chief
director fund under “Innovative research and development
system” for the fiscal years 2017-2018, 2019-2020 including
the US-Japan collaboration for high-power neutrino beams.

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