高出力レーザー (+α) を用いた宇宙物理実験：磁気リコネ
クションの電子ダイナミクス
境 健太郎
プラズマ量子プロセスユニット
核融合科学研究所
sakai.kentaro at nifs.ac.jp
研究部セミナー2023/10/13

Kentaro Sakai (境 健太郎)
• National Institute for Fusion
Science• PhD in Engineering in July2023
• Advisor: Yasuhiro Kuramitsu• Research interest: laboratory
astrophysics with high-power
and intense lasers

Summary• Experimental investigations on space andastrophysical plasmas with high-power/intenselasers• Magnetic reconnection driven by electron
dynamics• Laser +α: multiscale observations of magnetic
reconnection with laser + magnetic device• Others (if I have time)• Collective Thomson scattering to measure waves
and instabilities• Scattering in intense laser beam toward relativisticreconnection experimentMagnetic reconnection• Releases magnetic
energy into plasma• Changes macroscopic
magnetic field topology• Role of microscopic
electron dynamics• Multiscale nature
B

Solar flare

A.O. Benz, Living Rev. Sol. Phys. (2017)

Earth’s magnetosphereJδ𝛿 > 𝑟!"→ ion&electron current
𝑟!" > 𝛿 > 𝑟!# → electron current𝛿 < 𝑟!#→ no current→reconnection

S. A. Fuselier and W. S. Lewis, Space Sci. Rev. (2011)

Magnetic reconnection• Releases magnetic
energy into plasma• Changes macroscopic
magnetic field topology• Role of microscopic
electron dynamics• Multiscale nature

Solar flare

A.O. Benz, Living Rev. Sol. Phys. (2017)

Earth’s magnetosphere(Electron) outflow
at (electron) Alfvén speed
S. A. Fuselier and W. S. Lewis, Space Sci. Rev. (2011)• No resolution for electrons in astrophysicalplasmas• Local observations with MMS revealed electron
dynamics in magnetic reconnection• No global observations• Experiment to obtain global/local information

J.L. Burch et al., Science (2016)

Laboratory experiments• Reproduce space and astrophysicalphenomena in laboratory
Space (magnetosphere)

Astro (SNR)

Laboratory (laser)

D. Burgess+ SSR (2012)

NASA/NRAO/NOAO

Y. Kuramitsu+ PRL (2011)

Laboratory experiments• Active/passive diagnostics• Global structures• Local quantities• Controllability• Why laser? – easy to accessto electron-scale
Controlled B field
R. Yamazaki et al., Phys. Rev. E (2022)

Global structure of collisionless shock
Y. Kuramitsu et al., Phys. Rev. Lett. (2011)

Local velocity field in shock transition
T. Morita et al., Phys. Plasmas (2013)

Experimental setup• GEKKO XII HIPER laser• Wavelength : 351 nm• Energy : 110 J/beam• Pulse duration : 500 ps• Spot size : 300 μm
CTS ed
tter
sca

Ma
in l
110
ase
J×
r
3
500
ps

• CH foil (thickness 10 μm)• Ambient gas• 5 Torr or 10-4 Torr nitrogen• Magnetic field

Local plasma
parameter

CTS probe
Targety• Target• 3 kG or null B field~ 5 cmB1

B3

r
B-dot pB2

obe

Local B fieldmeasurement

X point

Global opticalimaging
x

z

Setup

Plasmoid
MagnetSchematic image of reconnection

Global observations
Self-emission image

Interferometry• Plasmoid & cusp

• rge ~ 9.5 μm, rgi ~ 17 mm→ electron-magnetized plasma

35 ns

15 ns

𝑛! ∼ 2×10"# cm$%𝑣 ∼ 500 km/s

Streaked self-emission
16 km/s• Electron Alfvénic outflow
115 km/s

Δv ~ vAe = B (4πneme)-1/2 ~ 40ｰ63 km/s

First global observations ofreconnection at electron scale
No local observations
Y. Kuramitsu et al., Nat. Commun. (2018)Collective Thomson scattering
Y. Kuramitsu et al., Nat. Commun. (2018)• Local observation• Ion acoustic wave
• Electron & ion
velocities• Different spectra indifferent positions

Wavelength

Spectrumvi

Spectrumi-vve

Spectrum

Probe laser
wavelength

If it is electron outflow

Wavelength

Wavelengthve < vive > vi

Observed spectrum
With B

Profiles

50 ns

No B
50 ns

Asymmetry(large ve-vi)

Symmetry(small ve-vi)

K. Sakai et al., Sci. Rep. (2022)

Velocity difference
K. Sakai et al., Sci. Rep. (2022)

electron

Δv = vwithB - vnoBelectron• Drastic change of electron velocity whereas
unchanged ion motion• Electron exhaust at d~0-0.5 mm with the external B• Electron outflow without ion outflow

Gyrofrequency: Ω = |q|B / (mc)Gyroradius:
rg = mvc / (qB)
Magnetization: σ = B2 / (4πnmv2)

Magnetization• vflow ~ 100 km/s, ne ~ 1017 cm-3 from CTS.
• Scale length of reconnection: L ~ 2 mm
• Electrons are magnetized but ions are not
ElectronProtonCarbonTemperature (T)

10 eV

50 eV

50 eV

Mean velocity[vflow2 + 2T/m]0.5

2000 km/s

140 km/s

100 km/s+1+3

Ionization stateElectrons are magnetizedGyroradius (rg)

36 µm ≪ L

4.9 mm > L

14 mm > L

Gyrofrequency (Ω)

8 GHz

5 MHz

1 MHz

Magnetization (σ)

0.22

8.7×10!" ≪ 1

1.3×10!" ≪ 1

Ions are rarely influenced by B fieldElectron Alfvénic outflow
Y. Kuramitsu et al., Nat. Commun. (2018)• Plasma separates at thetwice of outflow velocity• Electron outflow velocity
vout ~1250 km/s
• Electron Alfvén speed:vAe = B (4πneme)-0.5~900 km/s• Electron-only Alfvénic
outflow• Magnetic energy isreleased only to kinetic
energy of electron flow

K. Sakai et al., Sci. Rep. (2022)

2vout ~ 2500 km/s

B field measurements• 3-axis B-dot probe• Time evolution of local B• ~ 5 cm away from thereconnection regionm~5c• Magnetic oscillation at~ 400 ns with B
B1
With B
B2
B3
Plasmoid – evidenceof B reconstruction

No B

B field measurements• 3-axis B-dot probe• Time evolution of local B• ~ 5 cm away from thereconnection regionm~5c• Magnetic oscillation at~ 400 ns with B
B1
With B
B2
B3
EM waves generated
by reconnection

No B

Spectrogram• B field in the timefrequency domain• Wavelet analysis• ~10 MHz at ~400 ns in
B2 & B3 with B• Perpendicular to
background B• Little signal in B1 with B• Parallel to background B• No clear signal in no B
• Ωi < ω < Ωe→ whistler
mode frequency

Phase difference of B2&B3• ~ 90 deg. phase difference• Right-hand polarization• Falling tone – “whistler”• Model calculation explains
the frequency chirp• Whistler waves comingfrom the reconnection
region• Evidence of electron-scale
dynamics

Whistler propagationt = 50 ns

Plasma expansion u = x/t

x=0x = 5 cmx

TargetReconnection
occurs near here

Probe
Whistler propagation vg• Modeling the arrival timing of whistler waves• Group velocity of whistler wave (when Ω% ≪ 𝜔 ≪ Ω&);
𝑐 𝜔Ω(
𝜔𝐵
𝑣' ∼
∝
2𝜔)(
𝑛(
• Wavefront propagation velocity in the laboratory frame;𝑑𝑥 𝑥= + 𝑣'
𝑑𝑡 𝑡• Initial conditions from observation; ne ~ 1017 cm-3,
B ~ 3 kG, t0 = 50 ns, x0 = 0 cm

Phase difference of B2&B3• ~ 90 deg. phase difference• Right-hand polarization• Falling tone – “whistler”• Model calculation explains
the frequency chirp• Whistler waves comingfrom the reconnection
region• Evidence of electronscale dynamics

Summary–1• Magnetic reconnection driven by electron dynamics• Electron Alfvénic outflow without ion outflow• Magnetic field inversion relevant to the plasmoid• Whistler waves associated with electron dynamics
Magnetic energy is released into only electron
kinetic energy at the scale less than ion gyroradius
Y. Kuramitsu et al., Nat. Commun. (2018)

K. Sakai et al., Sci. Rep. (2022)

Acknowledgements• We appreciate GXII laser facility at ILE, Osaka
19K21865, JSBP120203206, 20KK0064,
21J20499, 22H01195, 18H01232, 22H01251,
JSCCA2019002, JSCCB20190003
Y. Kuramitsu, T. Minami, S. Egashira, Y. Sakawa, N. Ozaki,
R. Kodama, H. Takabe
K. Sakai, T. Moritaka, M. Ota
T. Morita, S. Matsukiyo

K. Tomita

R. Yamazaki, S. J. Tanaka

T. Takezaki

M. Koenig, B. Albertazzi, P. Mabey

N. Woolsey

M. Hoshino

Perspectives• Multiscale structures in reconnection• How microscopic (electron-scale) structuresconnect to macroscopic (MHD) ones?
• Laser + magnetic device• Wave excitations in reconnection• Where and what mechanism is the origin of waves
such as whistlers?
• Energy partition• Collective Thomson scattering• Toward relativistic reconnection• Intense laser experiment• Scattered intense laser beam for diagnostics

What we have investigated• Very small spatial and temporal scales less than ion
gyromotion – electron-scale• Both global/local observations but no multiscale ones
Y. Kuramitsu et al., Nat. Commun. (2018)

Less than ion gyroradius

Solar flareLess than 1 pixel(not scaled)

Laboratory experimentControlled manner
Global structure

A. O. Benz, Living Rev. Sol. Phys. (2017)

What we have investigated• Very small spatial and temporal scales less than ion
gyromotion – electron-scale• Both global/local observations but no multiscale ones
Y. Kuramitsu et al., Nat. Commun. (2018)

Less than ion gyroradius

Solar flareLess than 1 pixel(not scaled)

Local quantities
A. O. Benz, Living Rev. Sol. Phys. (2017)
K. Sakai et al., Sci. Rep. (2022)

Energy partition in multiscale
Electron scale

EM
field

Magnetic
field

Ion-kinetic scale

MHD scale

Wave-wave interactions?

Current Electron-scale waves Ion-scale waves
research Instabilities?

Bulk
Electron outflow
kinetic
Shocks?
ThermalMHD waves

Ion outflow

Electron thermal

Ion thermal

Wave-particle interactions?
Non-thermal

Electron non-thermal

Scale

Ion non-thermal

Energypartitioninmultiscale
Current experimentElectron scale

EM
field

Magnetic
field

Ion-kinetic scale

MHD scale

Wave-wave interactions?

Current Electron-scale waves Ion-scale waves
research Instabilities?

Bulk
Electron outflow
kinetic
Shocks?
ThermalMHD waves

Ion outflow

Electron thermal

Ion thermal

Wave-particle interactions?
Non-thermal

Electron non-thermal

Scale

Ion non-thermal

Limitations of laser exp.
K. Sakai et al., Sci. Rep. (2022)• Laser-produced fast plasma
flow enlarges gyroradii• Limit of B field strength(technical issue)• Small system size (< 1 cm)compared to ion gyroradiiB ~ 3 kG, rgi ~ 2 cm, L ~ 1 cm
R. Yamazaki et al., Phys. Rev. E (2022)• Short lifetime (<100 ns)
compared to ion gyroperiod• Difficult to magnetize ions
• No MHD scales
B ~ 40 kG, rgi ~ 3 mm, L ~ 1 cm

Electron to MHD scales• Larger spatial
and temporalscales toreach MHD
• Magneticallyconfined
plasmas• Electron-scalemeasurements• Lasers• “Fusion” ofmagneticallyconfined
plasmas withlasersPlanning experiment• The same setup as the previous laser-plasma
experiment but the ambient plasma is amagnetically-confined one• We are starting with a relatively
small laser• YAG laser, ~ 1 J, 10 ns, ~1012 W/cm2/se.g., torus plasma

~

40
0

km

1. Target injection

2. Laserirradiation

3. Diagnostics
Radiation hydrodynamic simulation

Perspectives• Multiscale structures in reconnection• How microscopic (electron-scale) structuresconnect to macroscopic (MHD) ones?
• Laser + magnetic device• Wave excitations in reconnection• Where and what mechanism is the origin of waves
such as whistlers?
• Energy partition• Collective Thomson scattering• Toward relativistic reconnection• Intense laser experiment• Scattered intense laser beam for diagnostics

Waves in reconnections• Free energy of electron outflow
can excite waves by instabilities• Such as whistler waves• Waves can heat plasmas andaccelerate particles• Important to understand energy
partition during reconnection• How can we observe waves andinstabilities?
• Collective Thomson scattering

S. Wang et al., Geophys. Res. Lett. (2023)

Thomson scattering• Light scattering by charged particles (mainly electrons)• Spectral shape corresponds to fe(v)• λI > λD → collective Thomson scattering (CTS)• CTS analysis in non-equilibrium plasmas accompaniedwith wave excitations is not establishedWaves/instabilities in CTS𝑣# − 𝑣" = 0(Maxwellian)𝑣,(

kSL－ kI

0
w

≈

𝑓( 𝑣

I

≈

• Electron two-stream
instability as an example• Landau damping• Peak associated with theexcited wave

kSR－ kIΔk

Plasma 1
Plasma 2𝑣*, 𝑣+

𝑣

S. Matsukiyo et al., J. Phys. Conf. Ser. (2016)
K. Sakai et al., Phys. Plasmas (2020)kWaves/instabilities in CTS𝑣# − 𝑣" > 𝑣$%

𝑣,(

𝑣*

kSL－ kI

Enhance

0
w

≈

𝑓( 𝑣

I

≈

• Electron two-stream
instability as an example• Landau damping• Peak associated with theexcited wave

kSR－ kIΔkPositive slope
Two-stream instability𝑣+

𝑣

S. Matsukiyo et al., J. Phys. Conf. Ser. (2016)
K. Sakai et al., Phys. Plasmas (2020)

Two-stream
instability
ω / k ~ vbk

Numerical simulation• Spectrum from arbitral distribution function• Scattered wave solving wave equation

+
+
𝜔
1
𝜕
4𝜋𝑒 𝜕
)
+
−∇ + + + + + 𝑬𝑺 = +𝒗𝑰𝒆 𝛿𝑛( − 𝒗𝑰𝒊 𝛿𝑛%
𝑐 𝜕𝑡
𝑐
𝑐 𝜕𝑡

S. Matsukiyo et al., J. Phys. Conf. Ser. (2016)• ne from particle-in-cell simulation
Electron distribution function

Density fluctuation

Numerical simulation• Spectrum from arbitral distribution function• Scattered wave solving wave equation

+
+
𝜔
1
𝜕
4𝜋𝑒 𝜕
)
+
−∇ + + + + + 𝑬𝑺 = +𝒗𝑰𝒆 𝛿𝑛( − 𝒗𝑰𝒊 𝛿𝑛%
𝑐 𝜕𝑡
𝑐
𝑐 𝜕𝑡

S. Matsukiyo et al., J. Phys. Conf. Ser. (2016)• E field of scattered wave in k-ω space

Numerical simulation• Spectrum from arbitral distribution function• Scattered wave solving wave equation

+
+
𝜔
1
𝜕
4𝜋𝑒 𝜕
)
+
−∇ + + + + + 𝑬𝑺 = +𝒗𝑰𝒆 𝛿𝑛( − 𝒗𝑰𝒊 𝛿𝑛%
𝑐 𝜕𝑡
𝑐
𝑐 𝜕𝑡

S. Matsukiyo et al., J. Phys. Conf. Ser. (2016)

Electron feature

Ion feature

Electron feature• Pick up E field on dispersion relation of lightSimulated CTS spectrum• Electron two-stream instability• Enhanced spectrum
associated with excited waves• Temporal evolution of electron
distribution function explains
the spectrum

Right peak

Left peakElectron distribution function
v2 - v1 > vte

Density fluctuation

CTS spectrum
Excitedwave

Positive
slope

K. Sakai et al., Phys. Plasmas (2020, 2023)

×~10Simulated CTS spectrum• Electron two-stream instability• Enhanced spectrum
associated with excited waves• Temporal evolution of electron
distribution function explains
the spectrum

Right peak

Left peakElectron distribution function
v2 - v1 > vte

Density fluctuation

CTS spectrum
Excitedwave

PlateauK. Sakai et al., Phys. Plasmas (2020, 2023)

×~10

Right peak

Left peak

Simulated CTS spectrum• Asymmetries in ion acoustic feature• Landau damping rate of ion acoustic
wave is different for two peaks
One can identify two-stream instability
by observing excited waves andasymmetries in electron and ion features

K. Sakai et al., Phys. Plasmas (2020, 2023)

• Ongoing project at NCU 100-TW
facility• High repetition rate (10 Hz)• 3.3 J, 150 ps, ~1×1015 W/cm2• Ablation plasma and ambient gas• Preliminary results

Wavelength filter

Proof-of-principle experiment

CTS
spectrum• Low flow speed• No ion feature spectrometer

K. Sakai et al., Phys. Plasmas (2020)

Perspectives• Multiscale structures in reconnection• How microscopic (electron-scale) structuresconnect to macroscopic (MHD) ones?
• Laser + magnetic device• Wave excitations in reconnection• Where and what mechanism is the origin of waves
such as whistlers?
• Energy partition• Collective Thomson scattering• Toward relativistic reconnection• Intense laser experiment• Scattered intense laser beam for diagnostics

Role of magnetic reconnection
as a particle accelerator• Multiple reconnection outflows
kicking particles many times→ stochastic acceleration
M. Hoshino, Phys. Rev. Lett. (2012)• Stochastic electron
acceleration by multiple
reconnections in a shock
Y. Matsumoto et al., Science (2015)• We are planning a modelexperiment with intense lasers
Y. Kuramitsu et al., Phys. Plasmas (2023)• “relativistic” laboratory
astrophysics

Toward “relativistic”
laboratory astrophysics• High power lasers• Generate a large-scale plasma with high flow speed• e.g., Gekko XII (ILE, Osaka)• Intense lasers• Generate a plasma flow close to the speed of light• e.g., LFEX (ILE, Osaka), J-KAREN (KPSI, QST),
NCU 100-TW (National Central Uinv.)• Limited diagnostics• Due to short pulse
duration• Scattered light from
intense laser itself

Intensity

Intense lasers• High peak intensity• Duration of fs - ps
High power lasers• High total energy• Duration of ~nsTimeExperimental setup• 以下未発表データにつき省略

Summary• Experimental investigations on space andastrophysical plasmas with high-power/intenselasers• Magnetic reconnection driven by electron
dynamics• Laser +α: multiscale observations of magnetic
reconnection with laser + magnetic device• Collective Thomson scattering to measurewaves and instabilities• Scattering in intense laser beam toward
relativistic reconnection experiment

Acknowledgements
Y. Kuramitsu, T. Nishimoto, K. Himeno, T. Minami, Y. Abe,K. Oda, S. Suzuki, F. Nikaido, K. Kuramoto, T. Yasui, T. Pikuz,
H. Kohri, S. Egashira, Y. Sakawa, N. Ozaki, R. Kodama, H. Takabe
K. Sakai, T. Moritaka, M. Ota, R. Yasuhara
T. Morita, S. Matsukiyo, S. Isayama
Y. Fukuda, H. Kiriyama, K. Kondo, N. Nakanii, A. Kon
R. Yamazaki, S. J. Tanaka

K. Tomita

M. Kanasaki, T. Asai

T. Takezaki

M. Koenig, B. Albertazzi, P. Mabey

N. Woolsey

A. O. Tokiyasu, H. S. Kumar

M. Hoshino

Y. L. Liu, C. S. Jao

N. Bolouki

W. Y. Woon, C. M. Chu, K. T. Wu