Experimental study of selfreformation of collisionless shockusing high power laser
S. Matsukiyo (Kyushu Univ.)
Collaborators:
Higuchi, Murakami, Morita, Nishioka, Takagi, Kojima, Tomita (Kyushu Univ.)
Sakawa, Sano, Ohta, Egashira, Izumi (ILE at Osaka Univ.)
Yamazaki, Tanaka, Kakuchi, Sei, Ishizaka, Sugiyama, Aihara,Kuramitsu, Sakai, Minami (Osaka Univ.)Background

Burgess et al. [2012]

Observational dilemma in space
Extrasolar bodies--- remote sensing
 global/macroscale structures
Solar-Terrestrial system--- in-situ observation
 local/microscale structures
Multi-scale coupling plays essential roles in many astrophysical plasma phenomena
Laboratory astrophysics (potential advantages)
 High controllability and reproducibility
 Simultaneous accessibility to both global/macroscale and local/microscale
phenomena

Background & Purpose
Self-reformation of a collisionless shock A unique dissipation mechanism of acollisionless shock, contributing to
production of nonthermal particles, wave
emission, …
 Shock front is cyclically destroyed andreformed when Mach number is sufficiently
high, even if upstream plasma is completely
steady and homogeneous Predicted by numerical simulation in 1980s(e.g., Lembege & Dawson [1987])
 Not yet evidenced by in-situ observations
Purpose:Experimentally demonstrate cyclic nature of the shock reformationby using high power laser experimentGekko XII experiment on collisionless shock
Gekko XII@ILE~ 2m

Exp. setup
実験概要

yx

TS電子項

TSイオン項

磁場コイル
衝撃波

3mm
target
foil

雰囲気ガス
プラズマ
3cm

HIPER laser
磁場コイル

ターゲット
プラズマ

内径6 cm

TS計測
プローブ光 4 beams × 700 at ω (1053)
Gaussian shape with 1.3 duration
2.8spot, /15 for each beam
 target: Al foil (3×3×2)
 gas : N2 (5) magnetic field: capacitor bank (2 sets of 3× 4)→ > 3.5

Typical parameters of gas plasma & shock+ ⁄ " ~0.5
~6 × 10,- ~35 . ⁄Ω ~6 × 10,/0 ~200[. ⁄ ](~ " )~10[%&]12 3_" ~5[]
= 25704"⁄
563 _"" ~2[+ ]*~3Results: self-emission streakB = 0[T]noisyupstream(precursor)

Self-emission (snap shot)

B = 3.8[T]cleanupstream

B=0 Noisy upstream (precursor)
B = 3.8 [T] Clean upstream Small precursor in early timeStack plot
9 = =[<][>?]

9 = :. ;[<]

Effect of finite B: Relatively smooth transition(7~ + ⁄ 8 ) when B = 0
 Substructure in front of themain peak when B = 3.8[T]PIC simulation
Initial setting of PIC simulation=, ≠ 0, & ≠ 0@&⁄,-,8= "*"=, ≠ 0, & = 0

9: 1
8.7 : 0F8 = 1
100 : 1
2: 3200 : 100

B is a parameter-E =8,8 ⁄Ω ,8 = 10

Electron discontinuity vs density jump
Ω8GH I = 1.1 (~14)-E = 08 =0-E ≠ 08 =0Finite -E (≠ 0) is necessary to form jumpfor 8 = 0
Target (Al) plasma is magnetized probably
due to Biermann battery effect-E = 08 ≠0-E ≠ 08 ≠0Larger precursor for

8 =0

Precursor due to high speed ions
(Al ions at least in early time)Magnetic overshoot ?
@ obtained from PIC sim.
overshootexperiment Internal structure ofmagnetized shock isdetectedfoot edge ?

Thomson scattering measurement
Incident probe laser light is scattered by local plasma electrons
Scattered wave spectrum reflects local plasma stateestimate local quantities such as , , " , *, ,J , …

実験概要

yx

TS電子項

TSイオン項

磁場コイル
衝撃波

3mm
target
foil

雰囲気ガス
プラズマ
3cm

HIPER laser
磁場コイル

ターゲット
プラズマ

内径6 cm

TS計測
プローブ光downstream
detector
yksx

k

probe
path

upstream

3mmtargetfoil

shock

ks : scatteredk : plasma
densitykI : TS probekI
ks = kI + kωs = ωI + ω

Thomson scattering measurement
TS data at the time whenprecursor is visible

L~@ (.( ))35nsdownstream
transition
region?
upstream∆50

 Asymmetric spectrum in the precursor

Thomson scattering measurement
 Asymmetric distribution functionof electrons in precursor(PIC simulation)

L~@ (.( ))35nsasymmetryprecursor

0
In the foot similar electron∆5
distribution function is realized Self-reformation may lead to periodic change of the asymmetric TS spectrum

Summary Magnetized collisionless shock is reproduced by performing Gekko XII
experiment Internal structure of the magnetized shock is observed by streaked selfemission for the first time Precursor is detected by using TS measurement

Issues A number of snap shots of TS measurement at different timing to extract
cyclic feature of shock front (reformation)
 Improvement of accuracy of the PIC simulation for better comparison
with the experiment