1
Investigation of ELMs on Alcator C-Mod J.L Terry, A.E. Hubbard, I. Cziegler, J.A. Snipes, J.W. Hughes, M. Greenwald, B. LaBombard, Y.Lin, P. Phillips*,P.B. Snyder + , L. Sugiyama, S.M. Wolfe, S. Wukitch Plasma Science and Fusion Center, MIT, Cambridge, MA, *Univ. of Texas at Austin - Fusion Research Center, + General Atomics Motivation and Introduction l Understanding ELMs is of crucial importance both because of they limit the pedestal pressure gradient and because of danger they pose to the first wall and divertor l Large, discrete ELMs are typically NOT present in C-Mod discharges - EDA H-mode with the "Quasi-Coherent Mode" as the pedestal relaxation mechanism is typical l Recently, a new region of operational space has been accessed where discrete, relatively large ELMs are common l We present here results of an investigation of those ELMs: - "Global" characteristics: plasma parameters, ELM energetics, edge stability - ELM dynamics: precursor oscillation, pedestal/SOL perturbation, filament ejection, filament propagation, relative timing of ELM perturbations 1050628014 @0.9 s typical time (s) 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 H ITER98(y,2) factor T eo (keV) GPC (R=0.67) - 1060503022 MHD - 1060503022 2 1 0 n eo (10 20 /m 3 ) TS (R=0.68) - 1060503022 6 4 2 0 1.5 1.0 0.5 0 - these discharges exhibit other interesting transport properties, see M. Greenwald in C-Mod Oral Session Tues. afternoon - T eo > 4 keV n e typically low with n eo ~2x10 20 m -3 non-typical shape for C-Mod d lower > 0.75, d upper ~0.15 - Confinement can be good in these ELMing H-modes (H ITER98(y,2) > 1) ~ P rf (MW) 6 4 2 0 D a Core Plasma Characteristics Edge Operational Space for Pedestal Relaxation in "Non-typical" Shape - ELMs occur at relatively low n* (0.2<n*<1) - discrete ELMs, variable size, probablyType I - steady EDA, QC mode, no ELMs - small ELMs with EDA, at high power & pressure for q 95 ~3.5, d lower >0.7 Z eff taken to be 1 1000 800 600 400 200 0 T e ped (eV) n e ped (10 20 m -3 ) 1 2 0 3 ELM Pedestal Energetics l Energy loss per ELM typically 10-25% of W ped -100 0 100 200 t-t crash (ms) 0.42 0.46 0.50 T e (keV) 1060503022:ECE ch 6 t c =0.84349s 12 ELM avg ped l Pedestal is not destroyed by ELM DW ELM /W ped C-Mod (w strong shaping) l 10% loss in T pedestal l n e pedestal loss is less certain, but in range 0 < Dn e < 15% 0.01 0.1 1 10 0.00 0.05 0.10 0.15 0.20 0.25 n* ITER n* (neoclassical) } JET JT-60U DIII-D ASDEX-U } l Consistent with multi-machine observations for Type I ELMs from A. Loarte, et al., JNM 313-316 (2003) 1050628014 @0.9 s nozzle Edge TS B dot B dot ELM Dynamics Investigated using Magnetic-pickup-coils, Da Gas-Puff-Imaging, and ECE views for GPI Toroidal view of Outboard Edge limiter Top View gas puff B dot coils l GPI localizes emission toroidally - primarily sensitive to changes in n e , with some sensitivity to changes in T e B dot coils very close to plasma Fine Time-scale ELM Evolution end with ~10% of W ped lost H-mode pedestal close to stability boundary peeling/ballooning mode growth "filaments" ejected dB q /dt -100 0 100 88 89 90 91 92 Major Radius [cm] Shot 1050628018; to=916082 ms 0 -50 50 t-t filament start (ms) radial filament ejection t-t filament start (ms) steady precursor growth fil. ejection steady precursor growth fil. ejection ELM Precursor GPI D a Emission from ped. 0 -100 50 1060503022 t=0.934113 s;1TABK - frequencies: 200-400 kHz, slowing to 70-100 kHz - toroidal mode number is around 10, constant during growth phase - precursor oscillation begins growing - often with large growth rate - osc. seen on outboard GPI emission with radial location in and at top of pedestal, radial structure seen hi-pass-filtered dB q /dt 0.55 0.60 0.65 T e ped (keV) time at which filaments are first seen in outboard SOL - at filament ejection, a higher freq. perturbation (0.5-1 MHz) is seen, the "hi-freq mag. osc." - use as t=0 from now on dB q /dt 2 8 D a monitor 1050623033 t=0.784215 s;view 2 Dt from onset of "hi-freq. mag. osc." pert. (ms) ELM Dynamics in Radial Dimension - GPI Da emission - much more complex time history with multiple ejections apparent from the radial array of views - "primary" ejection occurs after ped. is perturbed time (ms) sepx limiter shadow r (cm) 1050628014: t=0 is t=0.918943 s -1 0 1 2 3 0 700 time (ms) 2 6 Da monitor 2 frames from (6ms/fr) movie -50 0 100 200 1 cm time ave. subtracted 1050628019 frame 229 Radial Vertical during filament ejection (12 ms after frame on left) before filament ejection primary - "primary" propagates rapidly outward with radial velocities ~0.5 to >6 km/s time (ms) sepx r (cm) -1 0 1 2 3 V R =3.2 km/s radial velocity (km/s) # of ELM primaries in velocity interval 0 1 2 3 4 5 6 7 0 10 20 30 40 time of "primary" ejection onset of outboard perturbation time (ms) 2 6 Da monitor 1050628014: t=0 is t=0.918943 s 0 700 -50 0 100 200 - multiple "secondaries" occur after "primary" - typically slower than "primary" time (ms) sepx limiter shadow r (cm) -1 0 1 2 3 # of ELMs # of secondaries 0 1 2 3 4 5 6 7 0 10 20 30 l likely that not all secondaries are seen in limited view time (ms) 2 6 Da monitor 1050628014: t=0 is t=0.918943 s 0 700 -50 0 100 200 large case - a MHD =8.8 - n >15 unstable dP dr note: no modes on inboard side ped dP dr ped medium case - a MHD =3.9 - all n stable ELITE: both cases unstable to n=6,12, 20 modes mode structure from ELITE a MHD =8.8 case ped M3D: prelim. conclusions: - ELMing pedestals are close to and sometimes exceed peeling/ballooning instability boundary, - for unstable case, modes are predominantly ballooning with weaker peeling component - including previous ELITE analyses of equilibria of typical C-Mod shape, these atypical shapes are less stable for similar pedestal profiles Pedestal Stabilty Analysis using MHD codes ELITE (ideal MHD) and M3D (extended MHD) press. pert. (n=12) *see L. Sugiyama (Thurs. morn.) 2 atypically-shaped EFIT equilibria analyzed (meas. ped. press. profiles used) 0 100 dB q /dt hi-pass-filtered dB q /dt f=0,Z=0.1m 1050628014;t0=0.997845 s rel. Fourier amp. 30 20 10 0 freq. (MHz) 0 0.2 0.4 0.6 0.8 1.0 1.2 1050628014; t=0.99785 s Freq. spectrum of hi-pass-filtered dB q /dt sig. The "Hi-Frequency Magnetic Oscillation" l Onset of "hi-freq. mag. osc." nearly coincident with onset of filament ejection l Onset of "hi-freq. mag. osc." is nearly simultaneous on coils separated toroidally and poloidally - with systematic differences of up to ~10 ms time difference relative to onset of "hi-freq. mag. osc." -60 -40 -20 0 Dt (ms) 0.0 0.2 0.4 0.6 0.8 1.0 normalized PDF (# of ELM primaries) onset of filaments 10 -10 -30 -50 Dt from onset of "hi-freq. mag. osc." pert. (ms) ECE Observations during ELM 0.85 0.90 0.95 1.00 1060503022: t o =962252 ms -100 0 100 200 Dt from onset of "hi-freq. mag. osc." (ms) norm. change in signal -100 -50 0 50 100 Dt from onset of "hi-freq. mag. osc." pert. (ms) steady precursor growth filament ejection 0 50 100 -50 -100 -2 0 2 4 R-R sep (cm) Edge ECE emission 1060503022: t o =962252 ms l Onsets of T e pert. and "hi-freq. mag. osc." are nearly coincident l Non-thermal emission present during filament ejection since effective T rad > T e fil ped ped dB q /dt ECE eff.T rad Effective T rad (keV) far SOL Dt from onset of "hi-freq. mag. osc." (ms) near SOL mid-ped. steady precursor growth fil. ejection T e (mid-ped.) 1060503022 to=0.976846 s -100 -50 0 50 100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Observations at Inboard Edge l No mode observed on inboard edge l No filaments observed in inboard SOL l However, inboard edge is perturbed l onset of pert. occurs at or during precursor growth phase, before outboard filament ejection l perturbation propagates into the inboard SOL and appears to be a relaxation 1060719016: t o =861856 ms -100 0 100 200 change in GPI D a emission due to ELM 8 0 4 0 3 0 1.0 0 1.0 0 1.5 0 r=-0.47cm r=-0.15cm r=0.15cm r=0.50cm r=0.85cm r=1.19cm Dt from onset of "hi-freq. mag. osc." pert. (ms) steady precursor growth filament ejection inner wall Time-Sequence of ELM Pertubations l Onset of (outboard) filament ejection: ~coincident Dt (ms) normalized PDF (# of ELM primaries) l t=0 taken to be onset of "hi-freq. magnetic oscillation" l Onset of rapid growth of magnetic precursor amplitude: ~40 ms before l Onset of density (GPI) perturbation on outboard edge: ~10-20 ms before l Onset of density (GPI) perturbation on inboard edge: ~10 ms before l Onset of (outboard) T e pertubation: ~coincident -60 -40 -20 0 20 0.0 0.2 0.4 0.6 0.8 1.0 filaments inboard pert. outboard pert. precursor growth Te pert. Speculations about ELM Filament Ejection * acknowledgement to A. Boozer - Columbia Univ. and V. Naulin - JET for helpful discussions; see also W. Fundamenski, et al. submitted to PPCF (2007) l ECE sometimes shows "filaments" of non-thermal emission with effective T rad > 1 keV, a possible result of MR causing parallel electron acceleration l non-thermal emission follows the onset of "hi-freq. magnetic oscillation", with the oscillation showing a broadly peaked 0.5-1 MHz spect. Evidence for ejection as electrostatic ExB convection Possible Evidence for Magnetic Reconnection (MR) l Similar to "blob" radial convection (likely elect. stat. ExB) l V rad /C s ~1% l Observe poloidal variation in outboard filament ejection l ELITE & M3D simulations show unstable modes with poloidal variation within flux surface - MR near X-point causes Alfven wavepacket to bounce back and forth between high-shear X-pt regions, i.e. along field line of length=2pRq 95 bounce freq=V Alfven /2pRq 95 =0.5 MHz for C-Mod ELM Evolution Paradigm - Summary end with ~10% of W ped lost H-mode pedestal close to stability boundary peeling/ballooning mode growth "filaments" ejected l Complex structure of filaments observed in single ELM event l "Primary" filaments propagate radially thru outboard SOL with V r of 0.5-7 km/s; radial size ~1 cm; poloidal size >4.5 cm l Multiple "secondary" filaments follow l "Primary" ejection coincident with 0.5-1MHz hi-freq magnetic oscillation & onset of T e ped decrease l Non-thermal ECE emission observed in outboard SOL coincident with "primary" l Precursor oscillation, n~10, cntr-I p rotation, localized in outboard ped., grows rapidly before filament ejection l Rapid mode growth precedes filament ejection by ~40 ms l No mode evident at inboard SOL - yet inboard SOL responds to perturbation before ejection l Modeling with ELITE and M3D yield unstable modes for n >15 (ELITE), for n=6,12, 20 (M3D) Implication: non-typical shape unfavorable for ped stability Summary and Conclusions l Discrete, relatively large ELMs occur reproducibly at low n* in an non-typical C-Mod shape (d lower >0.7,d upper <0.2) l DW ELM /W ped =10-25% - consistent with multi-machine scaling - but pedestal is not destroyed l Both inboard and outboard pedestals are perturbed before "primary" filament ejection l Timing of "primary" ejection and the 0.5-1 MHz "hi-freq. mag. oscillation", as well as the non-thermal T rad observations, suggest that magnetic reconnection may be occurring in ejection process l ELITE MHD stability code shows closeness of pedestal profiles to peeling/ballooning instability boundary - for higher gradient ped. profiles, modes with n>15 are unstable and predominantly ballooning with weaker peeling component
