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

1. 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

2. 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 CONTENTS

3. Fast TV camera 1. ELM study in SOL and divertor 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. • 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 lineswas 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: Ip=1MA, Bt=1.87T, PNB=5.5MW ne=1.8-2.1x1019m-3(ne/nGW=0.5-0.54),fELM~20-40Hz Teped~700 eV, Tiped~900 eV,WELM/Wped =10-12%

4. Parallel propagation of ELM at LFS (similar result) ・Plasma flux at LFS divertor: jsdiv starts increasing after large Bp turbulence  ELM flux reaches divertor://div (90-160s) which is comparable to parallel convection time: //conv= Lcmid-div/Csped(2.7x105 m/s) ~110s.  jsdiv base-levelincreases during ~500s. ・Plasma is exhausted at large Bpturbulence  start of first large Bppeak:t0MHD isdefined. ・Plasma flux at midplane Mach probe:jsmid Large peaks appear duringBp turbulence  ELM plasma reaches Both sides of Mach probe:mid (~20s)

5. 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:

6. mid(peak) <// conv~//div≤mid(base) 2. Radial propagation at LFS SOL 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 =10-25s) “base-level” of jsmid increases: ・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 thanparallel convection: base-level enhancement time,mid(base), is longer than parallel convection time, //conv (~110s).

7. Large peak flux, js(peak), appears at LFS midplane Peak particle flux near X-point, jsXp(peak), is decreased. Note: jsmid(peak) profile is “an envelope of peaks” Peak particle flux, jsmid(peak): 20-50 times larger thanjsmid btw. ELMs jsmid(peak) propagates up to the first wall shadow (rmid >13cm) with large decay length: peak ~7.5cm (~2.5 xSS ~3 cm) Max. base-level,jsmid(base):10-20 time larger than jsmid btw. ELMs Decay length of jsmid(base) is comparable toSS.

8. rpeak Propagation velocity of ELM particle flux  Peak particle flux (temperature of a few 100eV) reaches LFS Baffle or First wall. • Delay of peak particle flux,jsmid(peak): mid(peak)increases 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=Vmid(peak)xpeak (10-25s)~0.5-4cm Characteristic length of radial propagation (during parallel convection time): rpeak= Vmid(peak)x // conv = 4-15cm At far-SOL(rmid > 6 cm), mid(peak) = 40-90s: Vmid(peak) = 1.5-3km/s becomes faster. ・Delay of base-level flux, jsmid(base): mid(base) is ranged in 100-300s with low Vf (<150V).  heat load is small due to low Te &Ti.

9. 3. ELM propagation in HFS SOL D increase start almost simultaneously both at HFS and LFS divertors Enhancement of jsHFS base-leveland SOL flow towards HFS divertorare observed after parallel convection time from LFS to HFS: //conv= LcLFS-HFS(50m)/Csped~185 s  Parallel convection towards HFS divertor Only near separatrix (rmid < 0.4cm), fast jsHFS and/or heat loadto Mach probe is measured: heat flux may be carried by fast el./ conduction neutrals are releaseddue to large Ttarget rise. "flow reversal "(SOL flow away from divertor).

10. Radial distribution of ELM plasma in HFS SOL ・Conductive heat flux/ fast electrons may be transported near separatrix. ・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 base (~2cm) is smaller than LFS base (~3.5cm). "SOL flow reversal" is generated over wide area in HFS SOL (rmid<3.5cm). • Flow reversal will play an important role in particle and impurity transport/ re-deposition (potentially, Tritium retention) at HFS divertor.

11. (512x512 pixels, 6kHz) 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 Filament-like image is observed in LFS divertor Particle flux is deposited locally, but extended over wide area: LFS baffle as well as divertor plate Viewing Divertor region tangentially (512x1025 pixels, 3kHz)

12. 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 Filament-like image is observed in LFS divertor Particle flux is deposited locally, but extended over wide area: LFS baffle as well as divertor plate (512x512 pixels, 6kHz) (512x1025 pixels, 3kHz) ELMs

13. 4. Summary Time scale and radial distribution of Type-1 ELM (fELM = 20-40 Hz) were investigated at HFS and LFS SOLswith 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 (= 40-90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 (rmid < 0.4cm) maybe by conduction/ fast electrons  producing large neutral desorption and flow reversal. Convective plasma fluxwas transported towards HFS divertor, but (maybe) small heat deposition.

14. Ref.1 Thermal diffusivity measurement was performed (2004) Ishimoto et al. PSS (2005) Laser Flash method Sample IR-detector Pulsed laser Furnace Laser flash device (LFA427/G, NETZSCH ) Nd GGG 1.064m Pulse width 0.3〜1.2ms Laser power 〜10mJ IR detector InSb

15. Comparison with previous measurements Heat conductance "heat transmission coefficient" was used k : thermal conductivity d: thickness heat conductance: In the case of JT-60U, *) lower value of h is needed on the inner target.

16. Estimation of ELMs heat loads (WELMIR vs WELMdia) 9 0 0 y = 6 . 8 y = 1 . 7 7 5 0 6 0 0 4 5 0 3 0 0 1 5 0 0 0 5 0 1 0 0 1 5 0 Net divertor heat loads estimated from the IR-camera as a function of the loss of the stored energy by ELMs. W/O considering thermal property: WELMIR was 6.8xWELMdia Difference was dominant at HFS Assuming thermal conductivity at HFS target (using lowest value): WELMIR was 1.7xWELMdia where thermal properties of LFS divertor (erosion dominant) are equal to those of CFC. - poloidal/ toroidal distribution of deposition layer should be considered. Not considering redeposit Net divertor heat load (kJ) WELMIR Considering redeposit T h e l o s s o f t h e p l a s m a s t o r e d e n e r g y ( k J ) WELMdia

17. Large positive bursts Gaussian distribution Gaussian distribution S=0 Negative bursts S < 0 Positive bursts S > 0 Ref.2 Fluctuation characteristics by statistic analysis (2006) Probability Distribution Function (p.d.f.) is applied to jsfluctuations Between ELMs in H-mode and L-mode plasmas (Nagoya Univ.) Asymmetry in p.d.f. L-mode at LFS midplane 2ms (sampling rate: 500kHz) p.d.f. moment represents fluctuation property away from random: asymmetry in p.d.f. normalized 3rd moment: Skewness = <x3p>/<x2p>3/2

18. Fluctuation property is different in H- and L-modes ELMy H-mode (between ELMs): js/<js> near separatrix (20-30%) is similar. bursty events are localized near-SOL(<3cm). L-mode: Large asymmetry in js/<js> : 30~40% at LFS midplane, and bursty events extend to far-SOL(<10cm).

19. Summary of SOL study in 21st IAEA Time scale and radial distribution of ELM propagation for Type-1 ELM (fELM = 20-40 Hz) were investigated at HFS and LFS SOLswith 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 (= 40-90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 (rmid < 0.4cm) maybe by conduction/ fast electrons producing large neutral desorption and flow reversal. Convective fluxwas 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.