ELM filament structure in the National Spherical Torus Experiment R. J. Maqueda Nova Photonics Inc., New Jersey R. Maingi Oak Ridge National Laboratory, Tennessee R. E. Bell, B. P. LeBlanc, J. E. Menard, D. P. Stotler, S. J. Zweben Princeton Plasma Physics Laboratory, New Jersey K. Tritz Johns Hopkins University, Maryland S. A. Sabbagh Columbia University, New York and the NSTX Research Team 50 th APS-DPP Meeting Nov , 2008 Dallas – Texas TI2.03 NSTX

2 Outline Short review of ELM filaments: experiment and modelling The NSTX experiment and ELM characteristics ELM filaments in NSTX: primary and secondary filaments* - high speed movies - gas puff imaging diagnostic Characteristics of secondary filaments Summary and discussion * J. L. Terry et al., J. Nucl. Mater (2007) 994.

3 ELM structure and dynamics key for ITER D. N. Hill, JNM (1997) 182. JET Short-timescale loss of energy and particles from edge plasma (H-mode transport barrier). Power density to plasma facing components limits lifetime of material walls. ITER has adopted 0.5 MJ/m 2 for the maximum allowed energy load in 250  s. Yet, structure and dynamics of energy and particle losses during ELMs in current experiments not completelly understood. ELM filament structure and dynamics (this talk) contributes to this knowledge base. ELM: Edge Localized Mode

4 ELM evolves into filamentary structure in scrape-off layer A. Kirk et al., PPCF 49 (2007) MAST Filaments aligned with local magnetic field. Filaments propagate radially and toroidally. Visible imaging DIII-D J. Boedo et al., JNM (2005) 771. Reciprocating probe ELM

5 Models show formation of filaments Peeling-ballooning invoked as driving mode for ELM. DIII-D P. B. Snyder et al., PoP 12 (2005) ELITE (linear) BOUT (nonlinear) pure n=20 seed“broadband n” seed -> nonlinear mode coupling 20.1  s 36.4  s 36.8  s 17.6  s 34.2  s 37.4  s Toroidal angle (1/5 torus) Radius Intermediate mode number

6 Primary vs. secondary filaments Basic difference: origin. Primary filaments: high density (temperature) originating from main instability driving the ELM. Secondary filaments: lower density (temperature) due to confinement changes, secondary consequence of ELM event.

7 Some open questions regarding ELMs Experimental support for peeling-balloning mode comes from: - (Some) Experimental data consistent with stability diagram. - “Observation” of filaments during ELMs. …are all the filaments observed during the ELM due to the driving mode? Energy content in observed primary filaments only <25% of losses. No shortage of possible mechanisms. …what other mechanism for the energy losses can be of importance?

8 National Spherical Torus Experiment Center stack Carbon tiles Typical NSTX parameters R ~ 0.85 m a ~ 0.67 m R/a ~ 1.3  ~ B axis = 4.5 kG I p = MA P NBI < 7 MW T e (0) ~ 1 keV n e (0) ~ 6 x m -3

9 Type I and Type III ELMs in NSTX R (m) Z (m) time (s) I p (MA) W MHD (kJ) D  lower div. (a.u.) D  midplane (a.u.) “Bolometer” SOL (a.u.) “Bolometer” Ped (a.u.) USXR hdown #13 USXR hdown #11 USXR hdown #13 USXR hdown #14 Type I (4.8 MW NBI)Type III (2.0 MW NBI) (0.30 s) (0.24 s) ELM “types” assigned based on scaling with P NBI and

10 Type I and Type III ELMS in NSTX (cont.) freq. (Hz)  W W tot  W W ped e * Type IType III ~120~460 ~6%~2% ~18%~5% ~1.3 ~0.8 nengneng ~0.5 n e (10 19 m -3 ) T e (keV) R - R sep (m) MPTS Region of interest extends from ~3 cm inside separatrix to far SOL Notes:1) Parameter space broader than indicated for both ELM types. 2) Collisionality generally higher for Type III ELMs. ELMs in NSTX: R. Maingi et al., Nucl. Fusion 45 (2005) Inter-ELM profiles

11 Type I ELMType III ELM t ELM = mst ELM = ms No interference filter ~ frames/s ELM filamentation Primary filaments Type III ELMs usually have rotating structure (low n) before onset of primary filaments. Wide angle view Primary filaments are “bright” filaments present during early onset of ELM event. t - t ELM (ms) Edge D  (a.u.)

12 Primary filaments show no periodic structure t - t ELM (  s) Number of filaments Type I ELMs Filaments aligned with magnetic field. Filaments evolve at different times with no periodic structure. Increasing number of filaments with time. Non-linear coupling of unstable modes appears to be of relevance for primary filament development. -16  s -8  s 0  s +9  s +17  s+25  s t ELM = ms Field lines at  R sep = 9 cm

13 ELM filamentation Secondary filaments t ELM = ms Secondary filaments seen over an extended period of time after the main ELM event. Are the secondary filaments just “more visible” due to an increased neutral density due to the ELM? Wide angle view No interference filter Edge view – low field side, ~ midplane D  filter 24 cm Type I ELM t - t ELM (ms) Edge D  (a.u.) separatrix limiter shadow

14 ELM filamentation Gas Puff Imaging Wide angle view No interference filter Edge view – low field side, ~ midplane D  filter 24 cm Type I ELM t ELM = ms D2puffD2puff Locally increase neutral density above ELM level. Diagnostic puff injected in poloidal plane within field of view. Gas puff does not perturb plasma nor filaments (blobs). Degas 2 simulation of D  emission (D. Stotler) indicates overall structure is preserved, despite non-linear dependence on n e and T e. separatrix limiter shadow t - t ELM (ms) Edge D  (a.u.)

15 ELM filamentation Gas Puff Imaging (cont.) t ELM = ms Secondary ELM filaments observed independent of increased neutral density in scrape-off layer due to ELM. Wide angle view No interference filter Edge view – low field side, ~ midplane D  filter 24 cm t - t ELM (ms) Edge D  (a.u.) Type III ELM D2puffD2puff separatrix limiter shadow

t - t ELM (  s)  I/I Rel. fluctuation level (RMS) t - t ELM (  s) k pol (cm -1 ) Poloidal spectrum  s+8  s -17  s-8  s -33  s-25  s -50  s -41  s Type III ELMs D 2 puff, D  filter t ELM = ms 24 cm Primary filament formation similar to that seen in simulations Perturbation growth observed in RMS fluctuation level and poloidal FFT spectrum. FFT spectrum shows increase in the 2 cm -1 range, broadband. Non-linear coupling of many modes may be important.

17 t - t ELM (ms) Type III ELMs Type I ELMs Primary filaments Secondary filaments Ion diamagnetic drift (plasma rotation) v r (km/s) v pol (km/s) Primary filaments move faster (radially) than secondary filaments Primary ELM filaments are distinguished by their higher radial velocity, reaching ~8 km/s. Within edge field-of-view, primary filaments seen only during early ELM stages ( t - t ELM < 50  s ). Secondary ELM filaments have similar characteristics to turbulent blobs (v r ~ 1 km/s and v pol predominantely in ion diamagnetic drift direction)*. * S. J. Zweben et al., Nucl. Fus. 44 (2004) 134. Velocities at R – R sep = 2.5 cm

18 I  / inter-ELM Type III Type I SOL (R – R sep = 5.5 cm) Timescales for scrape-off layer activity different between ELM types Early scrape-off layer activity (D  emission) higher for Type III ELMs. Activity maintained for ~300  s for Type I ELMs. Characteristic decay times longer for Type I ELMs compared to Type III (300  s vs. 100  s). Emission composed of fine structured filaments with poloidal auto-correlation lengths of ~4 cm …similar to that seen in turbulent blobs. FWHM L pol (cm) t - t ELM (ms)

19 Secondary ELM filamentation similar to L- mode turbulence and blobs FFT amplitudes increase without the appearance of modes, neither in time nor in poloidal spatial domains. Broad spectra similar to edge turbulence and “blobs”. [S. J. Zweben et al., Nucl. Fus. 44 (2004) 134]. 505 FFT ampl. (rel.) ELM Inter-ELM 50.5 FFT ampl. (rel.) SOL (R – R sep = 5.5 cm) Type I ELM 10 1 Frequency (kHz) k pol (cm -1 )

20 Evaluation of density (and temperature) within filament Following work in J. R. Myra et al., Phys. Plasmas 13 (2006) # The D  image intensity (I  ) is given by: I  = n o A L* F(n e,T e ) wheren o is the neutral deuterium density Ais the radiative decay rate L*is the line of sight integration length (and calibration factor) F(n e,T e )is the population ratio for the emitting energy levels, obtained from Degas2 modeling (D. Stotler) The product (n o A L*) is assumed constant in time, even as filaments move by. Assume filament convects plasma from its birth place, within the filament: T e = T e (n e ) (pedestal profile) Unique pair of n e and T e values obtained for each emitting filament.

21 n e (10 19 m -3 ) R - R sep (m) MPTS n e (10 19 m -3 ) v r (km/s) Type I ELMs Radial velocities present continuum between primary and secondary (blob) filaments secondary primary The radial velocity of the filaments, both primary and secondary (blobs), have a positive scaling with their density (and brightness). The radial velocities present a continuum, with no break separating primary from secondary filaments. Primary originate from top part of pedestal, while secondary originate from foot of pedestal. Inter-ELM

blob size â v r exp (km/s) secondary primary v min th (km/s) collisionality  resistive balloning resistive X-point sheath- connected Secondary filament velocity consistent with blob model J. R. Myra et al., Phys. Plasmas 13 (2006) # Filaments marginally in sheath-connected regime. Radial velocity of secondary filaments consistent with predictions from blob model v r exp ~. Primary filaments (high T e ) have lower velocities than predicted by model. Sheath-connected regime

23 Summary Primary and secondary filaments during ELM event are distinguished by their timing, velocity, T e and n e. ↔ Origin Secondary filaments have all the characteristics of turbulence born blobs: velocities, time and poloidal spectra, birth location, etc. Formation of primary filaments qualitatively consistent with non- linear evolution predicted by models. Coupling of unstable modes an important aspect of evolution.

24 Discussion Study of modes driving the ELM made difficult by non-linear evolution. “Filament counting” may be complicated by the interaction between modes and presence of secondary filaments. Other experimental data (DIII-D, C-Mod, JET, ASDEX, TCV, etc) also point to the presence of secondary filaments during ELM event. These secondary filaments (L-mode edge phase) represent an energy and particle loss mechanism that needs to be considered as possible loss channel.