1 Results and analysis of Gas Puff Imaging experiments in NSTX: turbulence, L-H transitions, ELMs and other phenomena R.J. Maqueda Nova Photonics S.J. Zweben, T. Munsat, T. Biewer Princeton Plasma Physics Laboratory C. Bush, R. Maingi Oak Ridge National Laboratory Laboratory J. Boedo UC-San Diego N.A. Crocker, S. Kubota, X.V. Nguyen, W.A. Peebles UC-Los Angeles and the NSTX Team

2 Abstract The 2-D structure of the plasma edge is measured in the National Spherical Torus Experiment (NSTX) by imaging its spectral line emission with 4-10  s time resolution. In the Gas Puff Imaging (GPI) diagnostic a poloidally elongated gas puff is used to localize the emission region in the poloidal-radial plane. The typical L-mode edge is turbulent with toroidally elongated filaments moving poloidally and radially. This is contrast with the H-mode edge where a more quiescent emission is observed. By using a PSI-5 ultra-fast framing camera developed by Princeton Scientific Instruments, capable of frame rates of up to frames/sec, the transitions between L- and H-modes are recorded. In addition, ELMs and MHD activity with high poloidal modes numbers are also observed with this diagnostic. Analysis of data obtained in recent GPI experiments will be presented, as well as comparison with other edge diagnostics. This includes spatial mode number, time series and velocity field analysis. Plans for future research/experiments will also be presented. Work supported by DoE grant DE-FG02-04ER54520.

3 Motivation There are very many reasons why the edge plasma is important... Core confinement depends on edge gradients and gradient driven instabilities. The transport at the edge and across the SOL depends on the level and characteristics of the local edge turbulence, either directly or through their effect on the edge profiles. The edge plasma interacts with the wall (divertor or limiters), liberating impurities. Impurity transport along SOL and across the edge determines impurity concentration in the core. Edge plasma interacts with injected auxiliary RF heating power. Gas Puff Imaging (GPI) provides access to many of the important phenomena at the edge: turbulence, L-H transitions, ELMs, RF coupling, etc.

4 GPI Diagnostic Imaging camera used to view visible emission from edge. Gas puff is injected to increase image contrast and brightness. Gas puff does not perturb local (nor global) plasma. Emission filtered for D  (or HeI) light from gas puff: I  n o n e f(n e,T e ) D a 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.

5 GPI Diagnostic (cont.) Viewing area located just above midplane on outer edge of NSTX. 25 cm x 25 cm area imaged with 1-2 cm resolution. PSI-5 camera used to record 300 frames, 64 x 64 pixels/frame, typically at 250,000 frames/s -> 1.2 ms of discharge coverage Measurement complemented with 13 discrete chords having 200 kHz resolution and 128 ms of discharge coverage, each viewing a 2 cm spot of the emission cloud.

6 Typical NSTX parameters Shot at 0.18s Core B(0) = 3.5 kG I = 900 kA T e (0) = 1 keV n e (0) = 2.5 x cm -3 = 2 x cm -3 Outer edge (R mid = 1.46 m) n e ~ 5 x cm -3 T e ~ 13 eV  ~ 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 “High-speed imaging of edge turbulence in NSTX”, S.J. Zweben et al., Nucl. Fusion 44, p. 134, 2004.

7 Movie clips available from: RF limiter shadow separatrix 24 cm GPI Movie Clips Gas injection

8 Movie clips Shot L-mode Shot H-mode Shot H-mode Shot L-H transition Shot L-H transition Shot L-mode, just before L-H transition Shot H-L transition – dither Shot H-L transition – dither Shot Type II/III ELM Shot Type V ELM Shot MHD – “Breathing” Shot MHD – “Bouncing” Shot MHD – “Blinking”

9 Image analysis L-mode vs. H-mode comparison R mid -R sep (cm) Emission profile (a.u.) Relative fluctuation level  I/I Poloidal correlation length (cm) L-mode H-mode Emission region is narrower in H-mode Fluctuation level is greatly reduced in H-mode Poloidal correlation length does not change over main emission region Turbulence is qualitatively similar in Ohmic and L-mode discharges. No differences have been (yet) observed in lower single null, upper single null, double null and limited discharges.

10 Pre-LH transition dithers t-t LH (ms) Filament frequency (filaments/ms) Transient H-mode like periods can occur up to 10 ms before the main L-H transition Although turbulence is much reduced in H-mode, with quiet periods lasting up to 100 ms, filaments (“blobs”) and waves can be occasionally seen. dithers Filaments on 2 cm diameter spot

11 L-H transition in other diagnostics Fluctuations in 30 GHz reflectometer decrease rapidly during L-H transition. For details see Crocker et al. JP

12 Transition observations L-H transition “dithers” can precede the main transition. The main L-H transition appears like a continuous evolution from turbulent filaments to the characteristic quiescent state in H-mode. This evolution lasts < 100  s without any apparent new spatial features or flows. Filaments just appear to “drain out”. Similar suppression of turbulence observed in other diagnostics during L-H transition. The H-L back transition generally appears as a high-n poloidal perturbation that evolves into radially moving filaments.

13 New analysis tool Velocity fields Tobin Munsat Do shear or zonal flows develop at the time of the L-H transition?...work in progress.

14 ELMs Type I Mid  W MHD with very fast or no pre-cursor, P heat >> P L-H GPI images best described as a momentary transition back to L-mode characteristics. Type II/III Small  W MHD with low frequency, long lived pre-cursor Increased filament activity respect to quiescent H-mode discharges. Type V Very small  W MHD with n=1 pre-cursor Modest difference (if any) with background H-mode turbulence (filament frequency). ELMs in NSTX: Maingi et al., CO3.005

15 Compound ELMs Divertor D  (a.u.) GPI fast chord (a.u.) Auto- correlation Power spectrum H-mode L-mode Compound ELM No substantial differences observed between L-mode turbulence and that during compound ELMs.

16 Summary and open questions Measurements Open Questions Turbulence is similar in Ohmic and L-mode discharges. (Differences are observed during H-modes.) H-modes might be preceded by short quiescent phases. H-modes have short periods of L- mode type activity (filaments). H-L transitions (generally) appear first as high-n poloidal modes. Are there differences in Ohmic, L- mode, upper single null, lower single null, double null, and inner wall limited? Is the main transition, and any pre- transition dithers, associated with poloidal flows (shear or zonal)?. What triggers filament formation during H-modes? Is there a consistent instability pattern leading to the H-L transition?

17 Measurements Open Questions Fast divertor D  light appears to start decreasing before the midplane GPI images show H-mode characteristics. ELMs are seen with similar characteristics as L-mode blobs. “Breathing”, “bouncing” and “blinking” are seen associated with MHD activity? Where are the first signs of the L-H transition observed? Candidate: X- point region. Are ELMs and L-mode filaments similar entities? Are the sporadic H- mode filaments minute ELMs? What causes breathing, bouncing and blinking? Summary and open questions

18 Next steps Use new fast framing camera, capable of recording > frames/s and whole discharge coverage. Observe emission from X-point region during L-H and H-L transition.. Use wide view of NSTX plasma to determine location of L-H transition and ELM birthplace. Study effects of low Z (lithium) and medium Z (neon), radiatively cooled edge, on the edge turbulence. Is there a relation with impurity enhanced modes? Search for poloidal flows (shear, zonal, etc.) during L-H transition, both at midplane and at the X-point region.