ＬＨＤ周辺プラズマ揺動計測

N. Ohno, H. Miyoshi, S. Takamura (Nagoya Univ.)
S. Masuzaki, T. Morisaki, N. Ohyabu, A. Komori (NIFS)
V. Budaev (Kruchatov Institute)
N. Asakura (JAEA)

Introduction
Particle and heat transport in the SOL//B⊥B

Core
plasma

SOLD, χParticle and heat loads profile on the divertor plates

Fluctuation phenomena affect the transport.
Intermittent transport (Blobs) is found to affect thecross-field transport in tokamaks SOL.
In the LHD, magnetic structure in SOL is complicated in comparison withtokamak SOL.
--- How fluctuation phenomena affect the transport in the LHD SOL?
--- Is is different from that in tokamak SOL?

Comparison between the intermittent bursty fluctuations in theedge plasma of tokamaks and helical devices makes it possible
to understand the essential physics of the blob transport.

Typical Fluctuation Properties of Isat in Tokamak
L mode discharge in JT-60U
Shot Number : 44421 ,time = 5200-5800 ms

Slope of averaged Isat varies with radial position
The p.d.f of Isat is strongly skewed.
Skewness peaks at 60-70 mm

Fluctuation property is different in H- and L-modes
L-mode:Large asymmetry in δjs/<js> :30~40% at LFS midplane, andbursty events extend to far-SOL

1(Δrmid < 10 cm).

Ip=1MA, Bt=1.8T, ne=2x1019m-3 , PNB=5.5MW
LFS midplane
HFS SOL

0.1

0.01 X-point

noize level<js> (105Am-2 )<js> (105Am-2 )ELMy H-mode (between ELMs):δjs/<js> near separatrix (20-30%) is similar.
bursty events are localized in near-SOL(Δrmid < 3 cm).

X-point

0.01

noize level1

δjs/<js >δjs /<js>

0.10.1

Positive bursts

0.5
Gaussian-0.5 0Skewness

0.01
10.01
1Skewness

HFS SOL

0.1

10

Ip=1MA, Bt=3.2T, ne=1.7x1019m-3 , PNB=4MW
1LFS midplane

0.5
0-0.5

246810Distance from separatrix (cm)mapping to LFS midplane

Positive bursts

Gaussian
0246810
Distance from separatrix (cm)mapping to LFS midplane

Large Helical Device (LHD)
Cryostatvessel

Poloidal coils

Helical coils

Plasm
aSpecifications of LHD
Plasma Major radius
Plasma Minor radius
Plasma Volume
Coil minor radius
Magnetic field

3.5 – 4.0 m（mainly 3.6m）~ 0.6 m（average）~ 30 m30.975 m~ 2.9 T (at Rax=3.5m)

Heating power
ECH
2.1 MW
N-NBI
10.0 MW
ICRF
2.4 MW

Edge Magnetic Structure in the LHD
Edge magnetic structure can bedivided roughly three regions.
1. Island and stochastic layer region
2. Edge surface layers region

Profiles of connection length of fieldlines connected to a torus inboard side
divertor plate.

Connection Length (m)3. Divertor

Rax=3.9m

72

Inboard
LP#1

Outboard

LP#3
Thermocouples Botto
embedded m5LP#2red
LP#３blueLP#1LP#２0OutboardTop

Poloidal angle, θ (°)

36

φ=0

10 15 20 25

Langmuir Probe Array

Toroidal angle,φ (°)
Particle deposition profile (cal.)Divertor tile

R1mm6mm

Probe electrodes

Langmuir Probe Array（１６ｃｈ.）

Sampling frequency: 250kHz

Inside of the LHD vacuum vessel (view from outboard port)Calculated Magnetic Configuration
180(b)
Poloidal Angle[°]

(a)

900
#2-90
#3

#1-180036

Toroidal Angle[°]Schematics of calculated LHD
magnetic configuration in horizontallyelongated cross section for themagnetic axis of 3.53 m

Deposition profile of magnetic field lines
passing through the divertor plates72Edge Magnetic Structure near Probe Array(b)(a)
LFSHFS(c)HFSLFS

Fluctuation Properties at Rax=3.53 m

The connection length Lcof magnetic lines of force
varies from less than afew meters to over a few
kilometers.
Condition averaging of thepositive bursty events
indicates the intermittent
feature with a rapid increase
and a slow decay is similar tothat of plasma blobs
observed in tokamaks.

Large positive bursty
events were oftenobserved in the ion
saturation current
measured with adivertor probe near adivertor leg at which
the magnetic line offorce connected to thearea of a low-field sidewithashortconnection length.

positive spikes

negative spikes

PDF and Wavelet Analysis
#11

#9

#10

Low Field SideIntermittent positive spikes at probe tip 9
negative spikes at probe tip 10

blobHigh Field Side
Hole

Similar wavelet decomposition profile

In the LHD SOL

Profile of Isat and Fluctuation
Characteristics In the LHD SOL
A reciprocating probe
measurement also
shows positive bursty
events at a lower
magnetic field side of thedivertor leg.
These experimental
results agree with thetheoretical prediction ofplasma blob transport
although the second
SOL region with a flat
density profile was notclearly observed.

Density Dependence of Bursty Fluctuation
Bursty fluctuation
characteristics isstrongly influenced by
the magnetic fieldstructure around theprobe tips, but hasvery weak dependenceof plasma density andits gradient.

Multi-fractal Analysis
Not a Brownian-likeprocess:structure function ofincrements has nonlinear scalingPDF of increments
δlX=X(t+l)-X(t)PDF,l=300
PDF,l=1

M(q,l)= 〈δlX q 〉~lζ(q)ζ(q)=qH-λ2q2
T- λ2 diagramE(p-1)/p E 1/pE

Multifractal cascading
process – deviationfrom Kolmogorov’s
monofractal cascade

multifractality parameter λ2
Coarse time scale of ~50-100 µs in the process:coherent structures scaleEstimation of Correlation TimePDF forFFT of Isat in the LHD

No Apparent peaks
No SOC property

Waiting Time Statistics

self-organized criticality SOC model predicts

No SOC property

SummaryWe have investigated relation between edge magnetic
structure of LHD and fluctuation properties of ion saturation
current.
 The edge magnetic structure was calculated by KMAG andWatanabe codes. .
 Large positive skewness was obtained from measured ion
saturation currents at lower magnetic field side of diveror
leg and the negative skewness was observed at striking
point of the divertor leg.
 The fluctuation characteristics has very weak dependenceof plasma density and its gradient.
 A reciprocating probe measurement alsoshows positive burstyevents at a lower magnetic field side of the divertor leg.
 Waiting time statistics was analyzed to indicate that thewaiting time statistics is not Poisson process and there isno clear signature of SOC paradigm.

Statistical Analysis Based on P.D. F.

 Reconstruction of p.d.f. from Isat signal

 Calculation of moments from the p.d.f.
 1st moment → Averaged value
 2nd moment → Fluctuation level: Asymmetry of p.d.f.
 3rd moment → Skewness(S)Gaussian→S=０x˜ 3x˜2 3/2

Positive spikes → S>0
Negative spikes→ S<0In the LHD SOL