ELM propagation in Low- and High-field-side SOLs on JT-60U

Nobuyuki Asakura1)

N.Ohno2), H.Kawashima1), H.Miyoshi3), G.Matsunaga1), N.Oyama1), S.Takamura3), Y.Uesugi4), M.Takechi1), T.Nakano1), H.Kubo1)

1)Japan Atomic Energy Agency, Naka 2)EcoTopia Science Institute, Nagoya Univ., Nagoya 3)Graduate School of Engineering, Nagoya Univ., Nagoya 4)Faculty of Engineering, Kanazawa Univ., Kanazawa

8th ITPA SOL and Divertor Physics TG meeting, Toronto, Canada, 6-9 Nov. 2006

CONTENTS

1. ELM (and fluctuation) study in SOL

and parallel transport at LFS

2. Radial propagation in Low-Field-Side SOL

3. ELM propagation in High-Field-Side SOL

4. SummaryRef. Thermal conductivity of deposition layers (at HFS target)

Ref. SOL fluctuation characteristics between ELMs, and in L-mode

1. ELM study in SOL and divertor• Understanding of ELM dynamics is important to evaluate transient heat and

particle loadings to the first wall as well as the divertor: ELM plasma propagation along and perpendicular to the field lines was inve

stigated at High- and Low-field-side SOLs.• Fluctuation characteristics of SOL plasma was studied, using statistic analysis (p.d.f.).

ELMy H-mode plasma:Ip=1MA, Bt=1.87T, PNB=5.5MWne=1.8-2.1x1019m-3(ne/nGW=0.5-0.54), fELM~20-40HzTe

ped~700 eV, Tiped~900 eV, WELM/Wped =10-12%

Main SOL/divertor diagnostics: (1) Probe measurement (500kHz sample): Ion flux (js) and floating potential (Vf) at 3 poloidal locations and divertor target(2) Fast TV camera (6-8kHz) Visible light image in divertor (similar to D) All sampling clocks are synchronized.

R (m) 2 3 4

-1

0

Z (m)

SOL

SOL

Midplane Mach probe

X-point Mach probeHigh-field-side Mach probe

Target probe array (18 probes)

23cm

Probes and fast TV in JT-60U

Fast TV camera

・ Plasma is exhausted at large Bp turbulence start of first large Bp peak: t0

MHD is defined.

・ Plasma flux at midplane Mach probe: jsmid

　 Large peaks appear during Bp turbulence ELM plasma reaches Both sides of Mach probe:

mid (~20s)

Parallel propagation of ELM at LFS (similar result)

⊥ ~ 20μs for Δrmid= 1cm

mid

Divertor probe

LFSSOL

τ// ~160μsdiv

for Lc = 30mmid-div

midplaneMach probe

・ Plasma flux at LFS divertor: jsdiv

　 starts increasing after large Bp turbulence ELM flux reaches divertor: //

div (90-160s) which is comparable to parallel convection time: //

conv = Lc

mid-div/Csped(2.7x105 m/s) ~110s.

jsdiv base-level increases during ~500s.

Parallel convection of ELM at LFS (similar result)

Power fraction of convective heat flux to LFS divertor is 50-100% of heat flux density measured by IRTV.

Example:qdiv

convection heat flux to the divertor = q// x pitch (=0.04)convconv

Here, q// =[ kTi+ kTe+ mi(M//Cs2) ](js/e)25

21

25conv

when pedestal plasma (Ti=900eV, Te=700eV) is exhausted to SOL (Cs =2.7x105m/s, M//~1), js =0.4 MA/m2 (at LFS divertor) corresponds to qdiv = 85 MW/m2 (61% of qdiv ~140 MW/m2) IRTV

conv

2. Radial propagation at LFS SOL

・ Delay of jsmid peak: mid(peak) increases with rmid in near-SOL. -- Delay of large Vf is also observed. jsmid peak propagates towards first wall, faster than parallel convection:

Magnetic turbulence and D increase start almost simultaneously jsmid : large peak and/or “multi-peaks” with large Vf (~800V):Te, Ti ~ a few 100eV (peak duration: tpeak =10-25s) “base-level” of jsmid increases:

mid(peak) < // conv

~ //div ≤ mid(base)

base-level enhancement time, mid(base), is longer than parallel convection time, //

conv

(~110s).

V⊥ i

LFSSOL

iplneMch probe

rpek~V ⊥

i xpek

Large peak flux, js(peak), appears at LFS midplane

Peak particle flux, jsmid(peak): 20-50 times larger than jsmid btw. ELMs jsmid(peak) propagates up to the first wall shadow (rmid >13cm) with large decay length: lpeak ~7.5cm (~2.5 x lSS ~3 cm)

Max. base-level, jsmid(base): 10-20 time larger than jsmid btw. ELMs Decay length of jsmid(base) is comparable to lSS.

Peak particle flux near X-point, jsXp(peak), is decreased. X-point (LFS )

jsXp (peak) Note: jsmid(peak) profile is “an envelope of peaks”

X-pointMach probe

Propagation velocity of ELM particle flux • Delay of peak particle flux, jsmid(peak): mid(peak) 　 increa

ses with rmid at near-SOL (< 5cm) Average radial velocity: V

mid(peak) = rmid/mid(peak) = 0.4-1.5km/s Radial scale of peak is estimated: rpeak

=Vmid(peak)xpeak (10-25s) ~0.5-4cm

Characteristic length of radial propagation (during parallel convection time): rpeak= V

mid(peak)x // conv = 4-15cm

・ Delay of base-level flux, jsmid(base): mid(base)

is ranged in 100-300s with low Vf (<150V). heat load is small due to low Te &Ti.

rpeak

Peak particle flux (temperature of a few 100eV) reaches LFS Baffle or First wall.

At far-SOL(rmid > 6 cm), mid(peak) = 40-90s:

Vmid(peak) = 1.5-3km/s becomes faster.

2.6 2.8 3.0 3.2 3.4 3.6

-1.2

HFS strike-point

High-field-side Mach probe

LFS D chnnelsHFS

3. ELM propagation in HFS SOLD increase start almost simultaneously both at HFS and LFS divertors

Enhancement of jsHFS base-level andSOL flow towards HFS divertor are observed after parallel convection time from LFS to HFS: //

conv = LcLFS-HFS(50m)/Cs

ped ~185 s Parallel convection towards HFS divertor Only near separatrix (rmid < 0.4cm), fast jsHFS and/or heat load to Mach probe is measured: heat flux may be carried by fast el./ conduction neutrals are released due to large Ttarget rise. "flow reversal " (SOL flow away from divertor).

rmid=0.3cm

Radial distribution of ELM plasma in HFS SOL

・ Large peaks are observed occasionally: jsHFS(peak) and Vf (~100V) are smaller than those in LFS SOL.

Fast SOL flow (M// up to 0.4) is produced towards HFS divertor. Parallel convection from LFS to HFS.・ jsHFS(base) enhancement near separatrix is comparable to that in LFS SOL, while HFS lbase (~2cm) is smaller than LFS lbase (~3.5cm). "SOL flow reversal" is generated over wide area in HFS SOL (rmid<3.5cm).

・ Conductive heat flux/ fast electrons may be transported near separatrix.

Flow reversal will play an important role in particle and impurity transport/ re-deposition (potentially, Tritium retention) at HFS divertor.

Fast TV (up to 8kHz) views divertor from tangential port:HFS divertor:D emission is enhanced immediately Flow reversal is generated.LFS divertor: 3-4 filament-like structures are observed above divertor and baffle for ~1ms.Radial scale of the filament: r ~3-5 cm

Viewing Divertor region tangentially (512x1025 pixels, 3kHz)

Particle flux is deposited locally, but extended over wide area: LFS baffle as well as divertor plate

t = -0.33ms

HFS baffle

Viewing port edge

LFS baffle

LFS divertor

HFS separatrix

HFS divertor

Dome top

t = 0.0ms t = 0.33ms

Filament-like image is observed in LFS divertor

LFS baffle

LFS divertor

HFS divertor

t = 0.33ms(512x512 pixels, 6kHz)

Fast TV (up to 8kHz) views divertor from tangential port:LFS divertor: 3-4 filament-like structures are observed above divertor and baffle plates during ~1ms. Radial scale of the filament: r ~3-5 cm

(512x1025 pixels, 3kHz)

Particle flux is deposited locally, but extended over wide area: LFS baffle as well as divertor plate

ELMs

(512x512 pixels, 6kHz)

Filament-like image is observed in LFS divertor

t = 0.33ms

HFS baffle

Viewing port edge

LFS baffle

LFS divertor

HFS separatrix

HFS divertor Dome top

LFS baffle

LFS divertorHFS divertor

t = 0.33ms

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4. SummaryTime scale and radial distribution of Type-1 ELM (fELM = 20-40 Hz) were inves

tigated at HFS and LFS SOLs with synchronizing sampling-clocks.

(1) ELM peak heat/particle flux appeared dominantly at LFS midplane: Large jsmid peaks (high Vf ) propagated with V

mid = 1.5-3 km/s:

mid (= 40-90s) was faster than parallel convection to divertor (~110s). fast peak flux (a few 100eV) will cause local heat and particle loadings. "Filament-like structures" were observed in LFS div. during ELM events. Local deposition of particle flux on LFS baffle were sometimes observed.

(2) ELM heat and particle flux in HFS SOL and divertor: Fast heat/particle transport was seen near separatrix (rmid < 0.4cm) mayb

e by conduction/ fast electrons producing large neutral desorption and flow reversal.

Convective plasma flux was transported towards HFS divertor, but (maybe) small heat deposition.

　　 Laser flash device(LFA427/G, NETZSCH )Nd GGG 1.064m Pulse width 0.3 〜 1.2msLaser power 〜 10mJIR detector InSb

Sam

ple

Pulsed laser IR-detectorFurnace

Ref.1 Thermal diffusivity measurement was performed (2004) Ishimoto et al. PSS (2005)

Laser Flash method

0

1

2

3

-5 0 5 10 15 20 25

IR-DetectorLaser pulse shape

Detector signal ( V )

Time ( ms )

Sample temperature 22.1oC

5DV3bq-t06_RTTest_detector.csv

Scattered laser

Half rise time t1/2

€

α= 0.1388l2t1/ 2

Comparison with previous measurementsHeat conductance "heat transmission coefficient" was used

heat conductance:dkh= k : thermal conductivity

d: thickness

Device h ( kW/m2 K ) Reference

ASDEX Upgrade 100 A. Herrmann, EPS2001

JET 3~300*) P. Andrew et al., PSI15

JET 15~50 E. Gauthier et al., PSI16

JT-60U 10 This study

In the case of JT-60U,( )( ) KmkWmmKWh 2/10

200/2 ==μ

*) lower value of h is needed on the inner target.

Estimation of ELMs heat loads (WELMIR vs WELM

dia)

W/O considering thermal property: WELM

IR was 6.8xWELMdia

Difference was dominant at HFS

Assuming thermal conductivity at HFS target (using lowest value): WELM

IR was 1.7xWELMdia

where thermal properties of LFS divertor (erosion dominant) are equal to those of CFC.

- poloidal/ toroidal distribution of deposition layer should be considered.

0

150

300

450

600

750

900

0 50 100 150

y = 6.8y = 1.7

The loss of the plasma stored energy (kJ)

Not considering redeposit

Considering redeposit

Net divertor heat loads estimated from the IR-camera as a function of the loss of the stored energy by ELMs.

Net

div

erto

r hea

t loa

d (k

J)W

ELM

IR

WELMdia

Probability Distribution Function (p.d.f.) is applied to js fluctuationsBetween ELMs in H-mode and L-mode plasmas (Nagoya Univ.)

p.d.f. moment represents fluctuation property away from random:

Ref.2 Fluctuation characteristics by statistic analysis (2006)Ref.2 Fluctuation characteristics by statistic analysis (2006)

normalized 3rd moment: Skewness = <x3p>/<x2p>3/2

2ms (sampling rate: 500kHz)

Large positive bursts

Gaussian distribution

Asymmetry in p.d.f.

Positive bursts S > 0

Gaussian distribution S=0

Negative bursts S < 0

asymmetry in p.d.f.

L-mode at LFS midplane

Fluctuation property is different in H- and L-modesL-mode: Large asymmetry in js/<js> : 30~40% at LFS midplane, and bursty events extend to far-SOL(<10cm).

ELMy H-mode (between ELMs): js/<js> near separatrix (20-30%) is similar. bursty events are localized near-SOL(<3cm).

Summary of SOL study in 21st IAEA Time scale and radial distribution of ELM propagation for Type-1 ELM (fELM = 20-40 H

z) were investigated at HFS and LFS SOLs with synchronizing sampling-clocks.

(1) ELM peak heat/particle flux appeared dominantly at LFS midplane: Large jsmid peaks (high Vf ) propagated towards first wall with V

mid = 1.5-3 km/s:

mid (= 40-90s) was faster than parallel convection to divertor (~110s). fast peak flux (with a few 100eV) will cause local heat and particle loadings.

(2) ELM heat and particle flux in HFS SOL and divertor: Fast heat/particle transport was seen near separatrix (rmid < 0.4cm) maybe by co

nduction/ fast electrons producing large neutral desorption and flow reversal. Convective flux was transported towards HFS divertor, but small heat deposition.

(3) Fluctuations Between ELMs: statistical analysis (P.D.F.) determined js/<js> (20-30%) was comparable at three poloidal positions bursty events are localized in near-SOL (rmid < 3 cm). On the other hand, in L-mode, bursty events extend to far-SOL (rmid < 10cm) only at LFS Midplane.

Measurements for fast deposition of ELM heat flux and wide 2D view on the first wall and divertor will improve evaluation of power load deposition on PFC.