ELM propagation in Low- and High-field-side SOLs on JT-60U Nobuyuki Asakura 1) N.Ohno 2), H.Kawashima 1), H.Miyoshi 3), G.Matsunaga 1), N.Oyama 1), S.Takamura.

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ELM propagation in Low- and High-field-side SOLs on JT-60U Nobuyuki Asakura 1) N.Ohno 2), H.Kawashima 1), H.Miyoshi 3), G.Matsunaga 1), N.Oyama 1), S.Takamura 3), Y.Uesugi 4), M.Takechi 1), T.Nakano 1), H.Kubo 1) 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. Summary Ref. 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 investigated at High- and Low-field-side SOLs. Fluctuation characteristics of SOL plasma was studied, using statistic analysis (p.d.f.). ELMy H-mode plasma: I p =1MA, B t =1.87T, P NB =5.5MW n e = x10 19 m -3 (n e /n GW = ), f ELM ~20-40Hz T e ped ~700 eV, T i ped ~900 eV,  W ELM /W ped =10-12% Main SOL/divertor diagnostics: (1) Probe measurement (500kHz sample): Ion flux (j s ) and floating potential (V f ) 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. Fast TV camera

・ Plasma is exhausted at large B p turbulence  start of first large B p peak: t 0 MHD is defined. ・ Plasma flux at midplane Mach probe: j s mid Large peaks appear during B p turbulence  ELM plasma reaches Both sides of Mach probe:   mid (~20  s) Parallel propagation of ELM at LFS (similar result) ・ Plasma flux at LFS divertor: j s div starts increasing after large B p turbulence  ELM flux reaches divertor:  // div (  s) which is comparable to parallel convection time:  // conv = L c mid-div /C s ped (2.7x10 5 m/s) ~110  s.  j s div base-level increases during ~500  s.

Parallel convection of ELM at LFS (similar result) Power fraction of convective heat flux to LFS divertor is % of heat flux density measured by IRTV. Example:

2. Radial propagation at LFS SOL ・ Delay of j s mid peak:   mid ( peak ) increases with  r mid in near-SOL. -- Delay of large  V f is also observed.  j s mid peak propagates towards first wall, faster than parallel convection: Magnetic turbulence and D  increase start almost simultaneously  j s mid : large peak and/or “multi-peaks” with large  V f (~800V):T e, T i ~ a few 100eV (peak duration:  t peak =10-25  s)  “base-level” of j s mid increases:   mid ( peak ) <  // conv ~  // div ≤   mid (base) base-level enhancement time,   mid (base), is longer than parallel convection time,  // conv (~110  s).

Large peak flux, j s (peak), appears at LFS midplane Peak particle flux, j s mid (peak): times larger than j s mid btw. ELMs j s mid (peak) propagates up to the first wall shadow (  r mid >13cm) with large decay length: peak ~7.5cm (~2.5 x SS ~3 cm) Max. base-level, j s mid (base): time larger than j s mid btw. ELMs Decay length of j s mid (base) is comparable to SS. Peak particle flux near X-point, j s Xp (peak), is decreased. Note: j s mid (peak) profile is “an envelope of peaks”

Propagation velocity of ELM particle flux Delay of peak particle flux, j s mid (peak):   mid (peak) increases with  r mid at near-SOL (< 5cm)  Average radial velocity: V  mid (peak) =  r mid /   mid (peak) = km/s Radial scale of peak is estimated:  r peak =V  mid (peak) x   peak (10-25  s) ~0.5-4cm Characteristic length of radial propagation (during parallel convection time):  r peak = V  mid (peak) x  // conv = 4-15cm ・ Delay of base-level flux, j s mid (base) :   mid (base) is ranged in  s with low V f (<150V).  heat load is small due to low T e &T i.  r peak  Peak particle flux (temperature of a few 100eV) reaches LFS Baffle or First wall. At far-SOL(  r mid > 6 cm),   mid (peak) =  s: V  mid (peak) = 1.5-3km/s becomes faster.

3. ELM propagation in HFS SOL D  increase start almost simultaneously both at HFS and LFS divertors Enhancement of j s HFS base-level and SOL flow towards HFS divertor are observed after parallel convection time from LFS to HFS:  // conv = L c LFS-HFS (50m)/C s ped ~185  s  Parallel convection towards HFS divertor Only near separatrix (  r mid < 0.4cm), fast j s HFS and/or heat load to Mach probe is measured: heat flux may be carried by fast el./ conduction  neutrals are released due to large T target rise.  "flow reversal " (SOL flow away from divertor).

Radial distribution of ELM plasma in HFS SOL ・ Large peaks are observed occasionally: j s HFS (peak) and  V f (~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. ・ j s HFS (base) enhancement near separatrix is comparable to that in LFS SOL, while HFS base (~2cm) is smaller than LFS base (~3.5cm). "SOL flow reversal" is generated over wide area in HFS SOL (  r mid <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 Filament-like image is observed in LFS divertor (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

4. Summary Time scale and radial distribution of Type-1 ELM (f ELM = Hz) were investigated at HFS and LFS SOLs with synchronizing sampling-clocks. (1) ELM peak heat/particle flux appeared dominantly at LFS midplane: Large j s mid peaks (high V f ) propagated with V  mid  = km/s:   mid (=  s) was faster than parallel convection to divertor (~110  s).  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 (  r mid < 0.4cm) maybe 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 GGG1.064  m Pulse width 0.3 〜 1.2ms Laser power 〜 10mJ IR detectorInSb Sample Pulsed laser IR-detector Furnace Ref.1 Thermal diffusivity measurement was performed (2004) Ishimoto et al. PSS (2005) Laser Flash method

Comparison with previous measurements Heat conductance "heat transmission coefficient" was used heat conductance: k : thermal conductivity d: thickness Deviceh ( kW/m 2 K )Reference ASDEX Upgrade100A. Herrmann, EPS2001 JET3~300* ) P. Andrew et al., PSI15 JET15~50E. Gauthier et al., PSI16 JT-60U10This study In the case of JT-60U, *) lower value of h is needed on the inner target.

Estimation of ELMs heat loads (  W ELM IR vs  W ELM dia ) W/O considering thermal property:  W ELM IR was 6.8x  W ELM dia Difference was dominant at HFS Assuming thermal conductivity at HFS target (using lowest value):  W ELM IR was 1.7x  W ELM dia where thermal properties of LFS divertor (erosion dominant) are equal to those of CFC. - poloidal/ toroidal distribution of deposition layer should be considered. 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 divertor heat load (kJ)  W ELM IR  W ELM dia

Probability Distribution Function (p.d.f.) is applied to j s fluctuations Between 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) normalized 3rd moment: Skewness = / 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-modes L-mode: Large asymmetry in  j s / : 30~40% at LFS midplane, and bursty events extend to far-SOL (<10cm). ELMy H-mode (between ELMs):  j s / 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 (f ELM = Hz) were investigated at HFS and LFS SOLs with synchronizing sampling-clocks. (1) ELM peak heat/particle flux appeared dominantly at LFS midplane: Large j s mid peaks (high V f ) propagated towards first wall with V  mid  = km/s:   mid (=  s) was faster than parallel convection to divertor (~110  s).  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 (  r mid < 0.4cm) maybe by conduction/ 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  j s / (20-30%) was comparable at three poloidal positions  bursty events are localized in near-SOL (  r mid < 3 cm). On the other hand, in L-mode, bursty events extend to far-SOL (  r mid < 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.