1. High speed images of edge plasmas in NSTX IEA Workshop Edge Transport in Fusion Plasmas September 11-13, 2006 Kraków, Poland GPI outer midplane – shot 118152 – 208.762 ms to 208.837 ms 2cm R. J. Maqueda Nova Photonics Inc., USA in collaboration with R. Maingi, T. Munsat, J. R. Myra, D. P. Stotler, A. E. White, S.J. Zweben and the NSTX Research Team

2. 2 Outline Introduction: National Spherical Torus Experiment (NSTX) and diagnostics Edge turbulence: Gas Puff Imaging (GPI) images Edge Localized Modes (ELMs) Summary

3. 3 Typical NSTX parameters General R ~ 0.85 m a ~ 0.7 m B axis = 4.5 kG I p = 0.7-1.2 MA P NBI < 7 MW T e (0) ~ 1 keV n e (0) ~ 2.5 x 10 13 cm -3 ~ 2 x 10 13 cm -3 Outer edge (R mid = 1.46 m) n e ~ 5 x 10 12 cm -3 T e ~ 13 eV  ~ 10 -3 L n ~ 2 cm  s ~ 0.2 cm ei ~ 6 x 10 6 s -1 L c ~ 5 m (connection length to divertor) ei /L c ~ 0.05 q = (B T /B pol )(R/A) ~ 2 L RBM ~ 1 cm (shot 108332 at 0.18s) Center stack (18.5 cm radius) Carbon tiles

4. 4 Time (ms) Contrast enhanced -50 to 200 scale Divertor D  (a.u.) Phantom 7 camera Frame rate: ≤68000 frames/s at 128 x 128 pixels ≤ 120000 frames/s at 64 x 64 pixels Minimum frame exposure: 2  s Digitization: 12-bit Full discharge coverage (2 GB of on-board memory) Complex filament structure and dynamics Good performance, long pulse at 1 MA Clip:no filter 2  s exposures 7 ms at 100000 frame/s playback at 150  s/s Raw images 0 to 400 scale 1 MA - 4.0 MW NBI - Double null Click on image above to play movie clip. (Caution: 29 MB file)

5. 5 GPI Diagnostic Camera used to view visible emission from edge just above midplane. Gas puff is injected to increase image contrast and brightness. Gas puff does not perturb local (nor global) plasma. Emission filtered for D  (He) light from gas puff: I  n o n e f(n e,T e ) (  n e  T e  ) with 0.5 < ,  < 2 D  (He) emission only seen in range ~ 5 eV < T e < 50 eV View aligned along B field line to see 2-D structure  B. Typical edge phenomena has a long parallel wavelength, filament structure. For more details: “Gas puff imaging of edge turbulence”, R.J. Maqueda et al., Rev. Sci. Instrum. 74(3), p. 2020, 2003.

6. 6 GPI: L-H Transition Transition takes place at ~192.1 ms L-mode Separatrix Antenna limiter shadow 23 cm radial 23 cm poloidal Spontaneous transition into quiescent H- mode “Blobs” Ohmic H-mode ~8  s between frames 0.65 ms mosaic D 2 puff D  filter

7. 7 GPI: “Quiescent” H-mode Separatrix Antenna limiter shadow 23 cm radial 23 cm poloidal Ohmic H-mode L-H transition takes place at ~192.1 ms ~8  s between frames 0.65 ms mosaic D 2 puff D  filter

8. 8 GPI: Active H-mode H-mode edge with “blobs”...micro-ELMs? Separatrix Antenna limiter shadow 23 cm radial 23 cm poloidal “Blobs” Active Quiet 4.5 MW NBI ~8  s between frames 0.65 ms mosaic D 2 puff D  filter

9. 9 Quiescent vs. active H-modes Gas puff imaging D 2 puffs FOV: 23cm x 23 cm Quiescent 900 kA Ohmic Lower single null Active 1 MA 4.7 MW NBI Lower single null poloidal R D 2 puff Clip:D  filter 3  s exposures 5 ms at 120000 frame/s playback at 125  s/s Click on image above to play movie clip. (Caution: 23 MB file)

10. 10 Turbulence/blob activity during H-mode The characteristics of the H-mode turbulence and blobs present a continuum from a turbulence level just above that measurable (a “quiescent” H-mode) to that approaching L-mode level (an “active” H- mode), at least for brief periods of time. The level of activity correlates well with the pedestal n e or P e. Blob activity (a.u.) Pedestal n e (10 13 cm -3 )Pedestal T e (eV)Pedestal P e (kPa)

11. 11 232.291 ms 232.366 ms Shot 118152 Radial Poloidal 75  s Blob history 1.Blob birth (frames 28156-28158) 2.Detached blob: polarization and ExB drift (frames 28159-28163) 3.Blob dissipation (frames 28164-28165)...at the same time the blob “trail” gives rise to a secondary blob that flows poloidally CAUTION: Inflection points in the blob trajectory can be seen at all radial positions Secondary blob 2cm Separatrix Antenna limiter shadow 8.3  s

12. 12 SOL flows 2cm 208.853 ms Separatrix Antenna limiter shadow SOL flows (“wind”) visible Shot 118152 208.738 ms Blob “shread” upward 2cm 309.898 ms 310.014 msShot 116107 ~8  s between frames D 2 puff/D  filter

13. 13 Raw images 0 to 300 scale Contrast enhanced -50 to 200 scale Divertor D  (a.u.) Time (ms) Small ELM (“Type V”) filament: propagating ionization front 800 kA 6.5 MW NBI Lower single null Type V ELMs 23 cm poloidal 23 cm radial Clip:D  filter 3  s exposures 5 ms at 120000 frame/s playback at 125  s/s 119318 @ 0.668417 s separatrixlimiter shadow Type V ELM filament ribbon Tangential edge imaging NO gas puff Crossfield (poloidal) width: ~12 cm Crossfield (radial) width: ~3- 4 cm Plasma within filament similar to that on pedestal. Type V ELM:  W/W < 1% Click on image to play movie clip. (Caution: 23 MB file)

14. 14 Filament coincident in time with divertor signature Time (ms) 312.0312.5313.0313.5 113665 Interferometer K.C. Lee (UC-Davis) Mirnov array J. Menard (PPPL) E. Fredrickson (PPPL) 1 2 3 7 USXR arrays Filament also carries current ~400 A IpIp Line average density (10 19 m -3 ) USXR Divertor light I USXR (a.u.) I vis (a.u.) Toroidal angle (deg.) 300 200 100 0 Chord #1 Chord #2 Chord #3 Chord #7 Midplane chord BII 0 100 200 300 NSTX plan view (midplane) K. Tritz (JHU) R. Maingi (ORNL)

15. 15 Small ELM (“Type V”) filament: no detachment Toroidal velocity: ~8 km/s (~0.9 kHz at R~1.45 m)...counter I P and plasma rotation Radial velocity: ≤0.2 km/s Current: ~400 A (~100 kA/m 2 )...co-I P Lifetime: 0.5 to 1 ms Filament coincident in time with divertor signature Plasma within filament similar to that on pedestal Filament detachment not observed “Soft” ELM crash due to enhanced transport on perturbed flux surfaces Blob characteristics during H-mode different from small Type V ELMs: magnetic signature, characteristic sizes, propagation, detachment

16. 16 After large ELMs edge similar to L-mode edge Gas puff imaging Field of view 23 cm x 23 cm L-mode 800 kA 2 MW NBI Lower single null H-mode 1 MA 4.7 MW NBI Lower single null Clip:D  filter 3  s exposures 5 ms at 120000 frame/s playback at 125  s/s poloidal R D 2 puff ELM at ~221.1 ms Click on image above to play movie clip. (Caution: 23 MB file)

17. 17 Summary Edge of toroidally confined plasma (like NSTX) show a complex filament structure and dynamics: blobs and ELMs. Fast-frame imaging is a very useful tool to study these phenomena. Gas Puff Imaging (GPI) enhances the usefulness of fast imaging for edge turbulence studies. While “blob” (and turbulent) activity is much reduced in H-mode compared to L-mode, H-modes present a continuum from “quiescent” to “active” edges. H-mode blob activity increases with edge pedestal density (and pressure). Long-lived “Type V” ELM filaments have very different characteristics and dynamics than blob filaments. Type V ELM crash associated with enhanced transport during filament lifetime. Large ELMs revert edge turbulence characteristics to L- mode like.

18. 18 Blob, GPI and ELM structure related NSTX papers...and references within “High-speed imaging of edge turbulence in NSTX”, S. J. Zweben et al., Nucl. Fusion 44 (2004) 134. “Three-dimensional neutral transport simulations of gas puff imaging experiments”, D. P. Stotler et al., Contrib. Plasma Phys. 44, 294 (2004). “Structure and motion of edge turbulence in the National Spherical Torus Experiment and Alcator C-Mod”, S. J. Zweben et al., Phys. Plasmas 13, 056114 (2006). “Bispectral analysis of low- to high-confinement mode transitions in the National Spherical Torus Experiment”, A. E. White et al., Phys. Plasmas 13, 072301 (2006). “Characterization of small, Type V ELMs in the National Spherical Torus Experiment”, R. Maingi et al., accepted Phys. Plasmas (2006). “Blob birth and transport in the tokamak edge plasma: analysis of imaging data”, J. R. Myra et al., accepted Phys. Plasmas (2006). “Structure of MARFEs and ELMs in NSTX”, R. J. Maqueda et al., submitted J. Nucl. Mater. (2006). “Derivation of time depedent 2-D velocity field maps for plasma turbulence studies”, T. Munsat and S. J. Zweben, submitted Rev. Sci. Instrum. (2006).