US-Japan
collaboration

Status of the laser stripping of H- beamat J-PARC RCS[J-PARC 3GeVにおける負水素イオンのレーザー荷電変換の状況]

Pranab K. Saha, H. Harada, M. Kinsho (JAEA/J-PARC)
H. Yoneda, Y. Michine (ILS, UEC), A. Sato (NAT), T. Shibata (J-PARC/KEK)
PASJ annual meeting 日本大学理工学部船橋キャンパス, August 30, 2023

Outline:・Introduction
・Laser stripping scheme and experimental setup
・Overview of the laser system and the laser cavity
・Results of 3 MeV H- neutralization study
・Laser power reduction scheme・Summary and outlook

P.K. Saha

PASJ 20232J-PARC accelerators
J-PARC
KEK & JAEA)

Introduction:

400 MeV H− Linac

A stripper foil is need for multi-turn H- chargeexchange injection (CEI) at RCS to accumulate~1014 ppp for 1 MW beam power.

Foil
3 GeV Rapid Cycling
Synchrotron (RCS)◆ However, foil scattering uncontrolled beam losses

Neutrino Beam Line
to Kamiokande (NU)

and the corresponding high residual radiation is aserious issue at high-intensity operation.
◆ A shorter foil lifetime is also another big issue.

30 GeV Main
Ring synchrotron(MR)◆ These issues become further complicated atmulti-MW beam power.

Material and Life
Science Experimental
Facility (MLF)

Hadron Expt.
PASJ 2023
Hall (HD)To overcome these issues, we aredeveloping a foil-less H- CEI
by using lasers.
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Stripper foil issues:
I. Foil scattering uncontrolled beam losses:
 Extremely high residual radiation at theinjection area.

II. Short lifetime and foil failure: Serious issues at high-intensity operation.
J-PARC foil degradation@ 800 kW (1 Month)

Foil failures at the SNS,Oak Ridge.

~1 MW
2 daysonly

A foil-less H- charge-exchange injection
has to be realized!
P.K. Saha

PASJ 20234

RCS beam loss mitigation at 1 MW
Until laser stripping is established, we continue our effort to minimize the beam losses including
foil scattering ones by using a smaller size foil, optimizing transverse and longitudinal paintings....
Measured circulating
beam intensity

Fast extraction

Beam loss throughout the ring

- 1 MW- 0.87 MW- 0.77 MW- 0.5 MW- 0.3 MW- 0,1 MWTime structure of the beam loss

Beam loss occurs only atinjection energy◆ The beam loss at 1 MW until 2020 was only 0.2%, well mitigated and controlled.
◆ We have further reduced (~80%) to remain now only ~ 0.05%.

 Dominated by the foil scattering beam loss.
 A laser stripping can give almost no beam loss at 1 MW in the RCS!
P.K. Saha

PASJ 2023

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High magnetic fields for stripping
Laser for H0 excitation

J-PARC Scheme: Only lasers
Step 3:
photo-ionization
H0* + γ → p + e-p

Step 1:
Photo-detection
H- + γ → H0 + e-

Step 2:
Excitation of H0
H0 + γ → H0* (n=3)
e-H0*H0H-ee-

SNS achieved 10µs stripping in 20 years.
(Requirement: 1000 µs)

YAG Laser
λ=1064 nm

n=3−1.51

n=2−3.40

UV laser
λ= 213 nm

YAG Laser
λ=1064 nm

α=28o
α=40o
α=50o
Groundstaten=1−13.60J-PARC strategies:● Avoid difficulties and issues with high magnetic field.
● Sufficient reduction of the laser power.

Laser power is the main limitation!

P.K. Saha

0.00-H-

10 µs

-n=∞

ePRL 118, 074801 (2017)

pIonization (Electron is free)Electron binding
energy (eV)Laser assisted

Laser stripping schemes

Energy state

SNS (Oak Ridge) scheme:

We are preparing a POP demonstration of 400 MeV
H- stripping by using only lasers.
PASJ 2023

External funds:◆ US-Japan Collaboration: (2018 ~ present)◆JSPS grants-in-aid: (2016~2019), (2019~2024 J-PARC, UEC, KEK)
◆ JAEA director’s Houga fund: 2018-2020, 2020-2022, 2023-2025 6

Setup for POP demonstration at 400 MeV
Place: J-PARC L-3BT (Linac to the 3-GeV
Beam Transport)

H−H−

Laser transport system from
L-3BT PS room to ACC tunnel.
Under construction in 2023pCompleted: 2022

Vacuumchamber

In 2023
H− 400 MeV

IP90-deg BD (H0)

◆ Each step can be separately studied and optimized.

BendingmagnetP.K. Saha

◆ All charge fractions can be separately measured◆ BPM electrode signals will be used (already tested)
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YAG Laser system developed at the UEC
20mW CW, 10 GHz BW
1064nmSeedlaser

EO
modulator
RFgenerator

FI＆BPPD

Yb-doped
Single mode
fiber

Yb-doped
Single mode
fiber

Yb-doped
Single mode
fiber

FI＆BP
700mW
CW pump

FI＆BP
900mW, 1.5ms
pulse operation

3mmΦrod
FR

P.K. Saha

Beamcombiner

FI＆BP

900mW, 1.5ms
pulse operation

AWG clock frequency
AWG 24.948087198GHz(±0.1kHz)③LD-Excited
Nd:YAG Rod Amplification
System2mmΦ
clock

Multi reflection
cavityYb-doped
Double clad fiber①Fiberamplification

Pulsed10W pump
FI＆BP
Pulsed10W pump
Photonic
Crystal Fiber
(PCF)

120W,
Pulse pump
PCF

120W,
Pulse pump
②PCF
amplification

FRPASJ 2023

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Multi-reflection cavity system
to reduce the seeder energy
3.1 ns..

32
2 1
Laser output
pulsesImage relay optics
f=100Virtual imaging
relay point◆ The R&D of the laser and the cavity systems have been done

through 3 MeV H- neutralization studies at J-PARC RFQ-TF.
◆ Also demonstrated non-destructive H- beam diagnostic systems.

P.K. Saha

Key features:I) Image transfer optics,II) Focusing at the IP
 Maximize photon flux & Minimize mirror damage!

PASJ 2023

1+2+…+32
Reduction of the seeder energy: ~1/32
Further reflections possible.
Goal for seeder pulse: mJ  ~µJ9

Study of 3 MeV H- neutralization
Bendingmagnet

FCT2

FCT1

H0H-FCT3J-PARC RFQ-TF
40µs

・Pulse energy: 150 mJ @ 40 µs.
・Energy/micro pulse: 24µJ
 380µJ @ 32 passes (50%)
・Micro pulse width : 100ps
・Spot size : 0.1mm

Typical laser pulse
P.K. Saha

PASJ 2023

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Results of 3 MeV H- neutralization
3.1 ns・H- beam interval: 3.1 ns
・Laser pulse interval: 6.2 ns
 Interaction @ alternate H- pulses

Neutralization every alternate pulse Neut. freq. : 162 MHz
 Unique!

BeamLaser pulse

32 passes

・Obtained 18% neutralization.
・Flat over 40µs.
P.K. Saha

FCT time domain data

1 pass324 MHzFFT spectrum
162 MHzOffline analysis

AWG tunning
precision: ～10-4 GHz
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Cavity pass dependence & longer pulse neutralization
Longer pulse neutralization

Pass dependence

300 µsNeutralization

Achieved cavity gain: ×16

・Photon losses, especially at the vacuum windows
are relatively high.
・R&D of low loss windows are manufactured.

P.K. Saha

PASJ 2023

・Lower pulse energy, but approaching toJ-PARC full injection pulse length of 500µs.
・Next step: Higher pulse energy

12Low loss vacuum window & seeder power reduction scheme◆ Low loss windows (loss rate 1%  <0.1%) are manufactured.

Duty: 1/80

x 614
Duty: 1/1.79
x 148● ~1/32 reduction of the seeder pulse.

J-PARC beam structure.
 Laser pulse should be at 324 MHz 0.15 mJ/pulse  4 mJ @ 32 reflections 90% neut.
● Reflections 32  64 and more are also in consideration.

 Seeder: 0.1 mJ/pulse  Easily achievable!
P.K. Saha

PASJ 2023

For > 90% efficiency
ProcessLaser

Pulse energy (mJ)H-  H0YAGH0  H0*UVH0*  H+YAG

 0.0910  0.16
11  0.17
66 mJ ×324 MHz = 2 MWWith duty ratio at 25 Hz:
2 MW×1/80 = 25 kW
With pulse recycling: 32
25 kW ×1/32 = 0.8 kW
With pulse recycling: 64
25 kW ×1/64 = 0.4 kW
More essential for the UV laser!

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Demonstration of non-destructive H- beam diagnostics
Longitudinal:

Laser time scan through the H- micro pulse.
 Laser time vs. neutralization yield Longitudinal H- micro bunch profile

Transverse:

Laser position scan through the H- micro pulse.
 Laser position vs. neut. Yield
 Transverse H- beam profile
Vertical(Preliminary)

σ1 = 277 ps
σ2= 263 ps

P.K. Saha

PASJ 2023

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Summary and outlook◆ To overcome the issues and limitations associated with a stripper foil, a laser stripping

H- CEI is under studied at J-PARC RCS.

◆ A POP demonstration of 400 MeV H- stripping by using only lasers is under preparation.
◆ A prototype YAG laser system and also a multi-reflection cavity system to significantly

reduce the seed laser power has been developed.

◆ The laser and the cavity systems have been successfully tested for 3 MeV H- beam

neutralization and further upgrades are in progress.

◆ High-reflective vacuum windows have been developed to achieve a negligible light losses.

 We expect ~3 orders of magnitude reduction of the seeder power.

◆ We have also demonstrated non-destructive H- beam diagnostics at 3 MeV.
◆ The laser system setup for 400 MeV test are in progress.

◆ The POP test will be started in 2024.

P.K. Saha

PASJ 2023

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