Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB5STATUS OF THE LASER STRIPPING OF H− BEAM AT J-PARC RCS
P.K. Saha∗ , A , H. HaradaA , H. YonedaB , Y. MichineB , A. SatoC , T. ShibataD , M. KinshoA
A
J-PARC Center, KEK and JAEA, Tokai-mura, Ibaraki, JapanB
Univ. of Electro-Communications, Tokyo 182-1112, JapanC
NAT, Tokai-mura, Ibaraki 319-1112, Japan
DKEK, Tsukuba, Ibaraki 305-0801, JapanAbstract
To overcome the realistic issues and limitations associatedwith a stripper foil used for H− charge-exchange injection(CEI) at proton accelerators for high-intensity beam, we are
studying a foil-less H− CEI by using only lasers. To establish
our principle, a POP (proof-of-principle) demonstration of
400 MeV H− stripping to proton by using lasers is underpreparation at the Rapid Cycling Synchrotron of Japan Proton Accelerator Research Complex. The H− is neutralized
to H0 first by stripping its loosely bound electron. The H0is excited by a deep UV laser before stripping it to protonby the same YAG laser. A prototype YAG laser system has
been developed through experimental studies of 3 MeV H−neutralization. A multi-reflection YAG laser cavity system
has also been developed and tested at 3 MeV. The installationof the YAG laser system at 400 MeV beam line of J-PARC
Linac including R&D of the UV laser are in progress to start
the POP experimental studies in 2024.

INTRODUCTION
The multi-turn charge-exchange injection of H− (negativehydrogen) by using a thin solid stripper foil is an effective
way to achieve high-intensity proton beam [1–4]. However,a short and unexpected lifetime of the foil as well as theuncontrolled beam losses caused by foil scattering of thecirculating beam during injection period and the corresponding high residual radiation are two serious issues, especiallyat high-intensity operation [5–7]. Although, remarkableprogress has been made for producing stronger foils [8], but
it is still hard to maintain reliable and longer lifetime due tooverheating of the foil at high-intensity [9], and it is one of
the big concern to realize next generation multi-MW proton
accelerators.
Laser manipulations of the H− beam by single or double neutralization is a very promising technique and highly
essential to utilize in accelerator process, such as beam diagnostics, collimation, extraction and pulse chopping as well
as stripping for the present and future high-intensity proton
accelerators. To overcome the realistic issues and practical limitations associated with a stripper foil, a foil-less
H− charge-exchange injection by using only lasers is under studied at J-PARC [10]. To establish the method, a POP(proof-of-principle) demonstration of 400 MeV H− stripping
to proton by using only lasers is under preparation [11–13].
Figure 1 shows a schematic view of our present concept.
The H− is first neutralized to H0 by removing its outer most∗ saha.pranab@j-parc.jp

electron by an YAG laser of 1064 nm. The ground state(1s) electron in the H0 is excited to 3rd excited state (3p)named H0∗ by using a deep UV laser of 213 nm, and finallythe H0∗ is stripped to proton (p) by removing the excitedelectron from the H0∗ by using the YAG laser. A prototype YAG laser system and also a completely new type ofmulti-reflection cavity have been developed and step by stepfurther developments are continued for higher energy, robust
uses, reliability and long term stability through experimental studies of 3 MeV H− neutralization at J-PARC RFQ test
facility (RFQ-TF) [14].

Figure 1: A schematic view of the principle of H− stripping
to proton by using only lasers. Noted parameters are for an
H− stripping at 400 MeV.

THE YAG LASER AND
MULTI-REFLECTION CAVITY SYSTEMS
Figure 2 shows a layout the prototype YAG laser system [13]. To start with, a combination of Arbitrary Wave
Generator (AWG) and Electro Optic Modulator is used togenerate programmable short pulse at high quality and highrepetition, which is then fed into multi stage fiber amplifiersystems. The design repetition rate is same as the H− micro pulse frequency of 324 MHz, but at present it is set to162 MHz mainly for clearly and uniquely identifying theinteraction signal at a different frequency than that of themain beam and with a less background. The laser pulses are
finally amplified by Laser diode for about several mJ/micropulse (design). The laser output pulses are then transfer
to the multi-reflection cavity system. The laser energy at
the latest experiment was 150 mJ for a duration of about
40𝜇s. The micro pulse length was typically 100 psec (𝜎)with 0.023 mJ/pulse at 6.172 ns interval, corresponding to afrequency of 162 MHz.
A requirement of considerably higher laser energy/pulse
is one of main difficulties of the laser stripping, while it ishard to handle a bigger laser system. To minimize the seed- 59 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB5Figure 5 shows an expanded view of the time domain
signal of the FCT at the center of a 50 𝜇s H− macro pulse.
A reduction of the pulse height at every alternate H− pulses
occurred due to a neutralization by a laser interaction. Figure 6 shows FFT spectra of the time domain signals of theFCT time domain data with laser ON and OFF depictedby the red and black lines, respectively. The FFT peak at
162 MHz appears only when the laser is ON, which wasused for obtaining the neutralization fraction.
Figure 2: A schematic view of the YAG laser system developed for the POP demonstration, which was successfully
tested for 3 MeV H− neutralization.

Figure 3: A schematic view of the 32 passes multi-reflection
laser cavity system also tested for 3 MeV H− neutralization.

Figure 4: Setup of the laser system for 3 MeV H− beam
neutralization study at J-PARC RFQ-TF. The neutralization
fraction is obtained by measuring the H− signals with laser
ON and OFF by the FCT.

-0.11
-0.12
-0.13

-0.1529.80

29.85

29.90 29.95
Time (µs)

30.00

Figure 5: Expanded view of the H− signal taken by the FCT.
The neutralization occurs for every alternate H− micro pulse.

EXPERIMENTAL RESULTS OF THE 3 MeV
H− NEUTRALIZATION
Figure 4 shows the experimental setup for 3 MeV H−
neutralization at the RFQ-TF. The IP is set at the upstreamof a bending magnet (BM). The H− beam neutralized bythe laser (H− + 𝛾 = H0 + e) becomes neutral (H0 ), which
is separated from the primary H− by the BM. The H− isdeflected by the BM and goes to the 11-degree beam line
and measured by a fast current transformer (FCT). The peakcurrent of the H− beam was 50 mA, same as for J-PARC
Linac, where a macro pulse of 50 𝜇s was used.

-0.1

-0.14

FFT Amplitude (Arb. units)laser energy, we have developed a multi-reflection YAG laser
cavity system to overlap many laser pulses at the interaction
point (IP). Figure 3 shows a schematic view of the final partof the cavity developed for 32 passes. This can essentiallyreduce the seeder power to ∼1/32 by considering negligible light losses at the optical devices and vacuum windows.
A roof-top transverse tiny light image produced at the upstream of the cavity is transferred to the IP for maximizing
the photon flux, while maximizing the spots at the mirrors tominimize their damage. A micro pulse energy of 0.023 mJ
after 32 overlaps at the IP was obtained to be 0.38 mJ, nearly
half than the number of passes, which was due to photon
losses mainly at the vacuum windows (∼1%/pass) as the cavity was placed outside the beam chamber. The laser systemand the cavity were tested for 3 MeV H− neutralization atthe latest in 2022. We have also developed high efficientvacuum windows to achieve the loss rate less than 0.1%/pass,which will be tested in a near future experiment.

FCT signal (V)

-0.09

106

162 MHz105

Laser ON
Laser OFF

104
103
102
101
100

0100 200 300 400 500 600 700
Frequency (MHz)Figure 6: FFT spectra of the FCT time domain signals withlaser ON (red) and OFF (black). The peak at 162 MHz withlaser ON corresponds to a neutralization signal.
The neutralization fraction for every 2 𝜇s was calculatedas shown in Fig. 7. The AWG waveform was carefully tuned- 60 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

30Longer flatPeaky laser

252015105001020304050

Time (µs)
Figure 7: Time dependent neutralization over the entire H−
macro pulse. A uniform neutralization throughout the H−
pulse has been obtained. A further higher efficiency at thecentral part has also been obtain by using a peaky laser pulse.
with a precision better than 10−3 GHz for a precise micropulse frequency of the light pulse to obtain a flat neutralization of 18% over the entire H− macro pulse (red), while byusing a further peaky laser pulse, a higher neutralization fraction of 25% around the middle of the H− macro pulse wasobtained as depicted by the blue color. We also calculated theneutralization of individual micro pulses (Fig. 5) by integrating and comparing with neighboring un-neutralized pulse,which was also consistent with the FFT analysis results. Figure 8 shows the pass number dependence laser energy gain
and the corresponding neutralization. The present result
demonstrates the merit of the cavity to increase the laser
pulse energy at the IP by reducing the seeder energy. We obtained a 1/16 reduction of the seeder energy at present, which
was due to significant photon losses at vacuum windows, but
it can be reached to ∼1/32 in the next trial by replacing with
newly developed windows with higher efficiency coating aspresented in the next section.

Gain factor

20160
14012010080604020

Window loss 1%
Window loss 0.1%
05101520253035

Pass number

Laser energy gainNeutralization gain

15

pared to the present windows used so far. Figure 9 showsthe estimated pass dependent laser energy drop with 1% and0.1% photon losses at vacuum windows at present (red) and
newly developed higher efficient windows (blue), respectively. A phone loss of 75% can be reduced to at least 10%
by using the new windows. As a result, a micro pulse energyof only 0.023 mJ from the present seed laser would give morethan 0.67 mJ at IP with 32 reflections at the IP as compared
to 0.38 mJ at present. Figure 10 shows an estimated neutralization as a function of the seeded pulse energy [15, 16]. At
present with 1% photon loss/pass at the vacuum windows
gives 18% neutralization (red) for 0.023 mJ/pulse from theseeder (0.38 mJ at the IP), but it can be increased to 30%(blue) by minimizing the window losses to 0.1%. Then, aseeder micro pulse energy of only 0.15 mJ will give us a
more than 4 mJ energy at the IP to obtain a more than 90%
neutralization. It is worth mentioning that we also plan tofurther double the number of reflections to 64. As a result,the seeder pulse energy can be further reduced as well as ahigher neutralization can be achieved. Such a reduction of
the seeder energy is more useful for the UV laser as a higher
energy is required for the H0 excitation as well as difficulties
to handling such a deep UV laser with a higher pulse energy.
A reduction of the seeder pulse energy is thus very effective
to reduce average power of the seed laser to cover the whole
injection period.

Laser energy

Neutralization fraction

PASJ2023 WEOB5

Figure 9: Estimated pass dependent laser energy drop due
to photon loss at vacuum windows at present (red) and with
newly developed higher efficiency ones.

105PREPARATION STATUS OF THE POPTEST AT 400 MeV

008162432

Pass number
Figure 8: Comparison of the pass dependent laser energy
gain and the corresponding neutralization gain.

DEVELOPMENT OF HIGH-REFLECTIVE
VACUUM WINDOW
To realize perfect benefit of the cavity system, the photon losses, especially at the vacuum windows have to beminimized. We have developed vacuum windows with highefficient coating for the YAG laser by which the photon loss
can be reduced to more than one order of magnitude as com-Figure 11 shows a schematic view of laser transport line
setup for the 400 MeV POP test at the L-3BT. The main laser
system will be installed at the power supply (PS) room of theL-3BT magnets, which is outside the accelerator tunnel andlocated at the 2nd floor. The laser will be guided from there
to the POP chamber (already installed) at the downstream of
the L-3BT by using sepatate vacuum pipes for the YAG andUV lasers. The total length from the laser main station to thePOP chamber is about 70 m. This year the laser transportsystem installation at the upstream part (shown by the redline), which is from the laser main station (L-3BT PS room)
to the sub station set at the at the L-3BT utility tunnel will be- 61 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

Neutralization fraction

PASJ2023 WEOB5
10.8

SUMMARY

Window loss 1%
Window loss 0.1%

0.6
0.4
0.201101001000

Seed laser pulse energy (µJ)

Figure 10: Estimated seeder pulse energy versus neutralization for two cases of photon losses at vacuum windows. A
seeder pulse energy of only 0.015 mJ gives more than 4 mJ
at the IP after 32 reflections with using newly developed
vacuum windows to obtain a more than 90% neutralization
fraction as shown by the red line.

completed. The laser transport vacuum pipes from from theutility tunnel to near the POP chamber, which is about 40 m(shown by the yellow arrow) have already been completed
and tested for the laser alignment and stability by using agreen laser. The R&D of the UV laser as well as opticaldevices to handle such a deep UV laser have also been started
through close collaboration with the manufacturer. Similar
to the YAG laser, we will also develop a UV laser cavity
system to minimize the seeder pulse energy. The 1st stageof POP demonstration of 400 MeV H− laser stripping has
been scheduled to stat in 2024. It is also worth mentioningthat we have demonstrated non-destructive measurement ofboth longitudinal and transverse beam profiles of the H−
beam at 3 MeV by laser manipulation, which can be easilyimplemented at 400 MeV and as an online monitoring during
beam operation.

To replace the carbon foil used for the conventional H−
charge-exchange injection (CEI), a foil-less H− CEI by using only lasers is under study at J-PARC. To establish themethod a POP demonstration of 400 MeV H− stripping toproton by using only lasers will be carried out. A prototype
YAG laser system including a multi-reflection laser cavity
system to significantly reduce the seed laser power havebeen developed and tested for 3 MeV H− neutralization atJ-PARC RFQ-TF. A maximum neutralization of 25% has
been obtained by 0.38 mJ energy at the IP with 32 passes of
only 0.023 mJ micro pulse energy from the seeder. A reduction of the seeder was 1/16 at present due to photon losses
at the vacuum windows, but it can be reached to ∼1/32 by
replacing the vacuum windows with newly developed higher
efficient ones. As a result, the present seeder micro pulse
energy of only 0.015 mJ will give more than 4 mJ at the IPto obtain a more than 90% neutralization. In addition, wewill also upgrade the cavity system to double the reflections.
Such a reduction of the seeder energy is more useful for theUV laser. The R&D of the UV laser produced by higher
harmonic generation from the YAG laser is also in progress.
The POP experimental studies for 400 MeV H− stripping
to proton will be started in 2024. We will first study theneutralization and non-destructive beam diagnostics of the400 MeV H− beam by a laser manipulation.

ACKNOWLEDGMENTS
The authors would like to acknowledge our J-PARC colleagues for continuous support and cooperation in the presentresearch. This work was partly supported by the JSPS KAKENHI Grant Number JP20H04456, JAEA HOUGA researchfunds (2017-2018, 2019-2020) including US-Japan collaboration for high-power neutrino beams.

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Figure 11: Schematic view of the installation of laser vacuumtransport lines at the L-3BT through the utility tunnel. Theupstream part (red) will be completed in 2023, where theyellow part has already been completed in 2022. The POPchamber has also been installed.

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Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

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