Edge Zonal Flows and Blob Propagation in Alcator C-Mod P5.073 EPS 2011

Transcription

1 Edge Zonal Flows and Blob Propagation in Alcator C-Mod S.J. Zweben 1, J.L. Terry 2, M. Agostini 3, B. Davis 1, O. Grulke 4,J. Hughes 2, B. LaBombard 2 D.A. D'Ippolito 6, R. Hager 5, J.R. Myra 6, D.A. Russell 6 1 Princeton Plasma Physics Laboratory, Princeton NJ Massachusetts Institute of Technology, Cambridge MA Consorzio RFX, Associazione EURATOM, I-35127, Padova Italy 4 Max Planck Institute for Plasma Physics, D-17489, Greifswald, Germany 5 Max Planck Institute for Plasma Physics, D Garching, Germany 6 Lodestar Research Corporation, 2400 Central Ave., Boulder CO P5.073 EPS

2 Outline of this Poster Motivations GPI diagnostic Edge zonal flows Blob propagation Further directions 2

3 Motivations for Studying Zonal Flows Radial turbulence velocity is probably the main cause of radial transport in the edge and SOL Poloidal turbulence velocity can reduce radial transport by suppressing or shearing turbulent structures Big issue at present is whether steady mean flow or fluctuating zonal flow is more important Prior mean turbulence velocity measurements in C-Mod: - Velocity fields from PSI-5 GPI camera (Terry, J. Nucl. Mat 2005) - Radial velocity from probe ExB (Smick & LaBombard, RSI 2009) - Blob radial and poloidal velocity (Grulke, PoP 2006) - Poloidal velocity from APD array (Cziegler, POP 2010) - Radial velocity from Phantom 7.3 GPI camera (Zweben, JNM 2010) - Velocity fields vs. density (Agostini, Nucl. Fus. 2011) 3

4 Motivations for Studying Blob Motion Try to clarify mechanism of blob formation, i.e. where and why are blobs formed Compare radial motion of blobs to theory and simulation to better understand their effect on the SOL width Compare blob velocities to velocities derived from crosscorrelation technique Prior blob motion measurements on C-Mod: - Blob radial velocity from GPI-probe correlations (Grulke, PoP 2006) - Blob radial velocity vs. analytic blob models (Zweben, JNM 2010) - blob birth and radial speed vs. density (Agostini, Nucl. Fus. 2011) 4

5 Gas Puff Imaging Diagnostic GPI view using Phantom 710 camera data at 400,000 frames/sec select shots with good B-field alignment q 95 ~ 3.4 ± 0.4 select shots with good gas puff contrast (both D and He) => 30 shots in the database for this poster 5

6 Interpreta(on of GPI- Inferred Velocity GPI views local emission of D α ~ n o f(n e,t e ) within window where D α is emitted in plasma edge, where T e ~ ev Can measure 2-D turbulence structure and motion even if response of D α is nonlinear (like contrast knob on a TV) Can not directly measure fluid (ion) flow or ExB flow, but measures turbulence or blob velocity, as done previously in studies of edge zonal flow * * McKee et al, PoP 03 using BES on DIII-D Conway et al, PPCF 05 using Doppler reflectometry on AUG 6 6

7 Examples of GPI Camera Images raw data first normalized to a time-averaged frame movies can be seen at CMod2010/CMod2010.html MA, 3.6 T no RF MA, 5.3 T 1.6 MW RF 7 7

8 Zonal Flow Algorithm one frame 6 cm poloidal 6 cm radial (outward) - for each pixel in each frame, make a short time series of the GPI signal at that pixel over ±5 frames or ~25 µs total - find best match to this time series in pixels of the next frame and get V rad and V pol from displacement to best match - average 4 cm poloidally to get zonal flow velocity 8 8

9 Cross-Correlation Analysis Outputs Finds time-resolved, poloidally-averaged velocities vs. radius V pol frequency response ~30 khz (1/e of amplitude at f~0) V pol and V rad vs. time at ρ=0.3 cm frequency response 9

10 Spatial Variation of Poloidal Velocity Significant poloidal variation of V pol in all cases, thus the poloidal average of V pol is not just an m=0 zonal flow Radial profile of (poloidally averaged) V pol shows timeaveraged structure which varies slowly with radius 10

11 Mean Velocities vs. Radius Poloidal velocities vary considerably, but generally in IDD Radial velocities outward in SOL but slightly negative inside separatrix (similar to Terry JNM 05 and Agostini NF 11) V pol vs. radius V rad vs. radius 11

12 Spectra of Velocity Fluctuations Radial velocity Poloidal velocity MA, 3.6 T ρ= cm P rf = MA, 5.3 T ρ= cm P rf = 1.6 MW 12

13 Velocity Spectra vs. Radius vs. Time Radial velocity Poloidal velocity MA, 3.6 T ρ= cm P rf = MA, 5.3 T ρ= cm P rf = 1.6 MW 13

14 Correlation of Velocity and MHD Shots with incoherent (broadband) spectra of V pol do not have a clear correlation of V pol & MHD Shots with a clear coherent spectra of V pol have a partial correlation of V pol & MHD (e.g. at 6.5 khz in ) 14 (thanks to David Pace for help with InSpect)

15 Theoretical GAM Frequency for C-Mod GAM frequency f = G c s /(πr) with G=geometric factor, R = R o +r, and c s = [γ(t i +T e )/m i ] 1/2 where G~ (2-1/2 ) (2/(1+κ) (1+1/(2A 2/3 ) (1+1/(4q 2 )) For C-Mod with A=3, κ=1.6, q=3, T e =T i =50 ev, γ=4/3 and m i =2 f GAM ~ 20 khz These analytic values (from R. Hager) can still deviate a factor-of-two from experiment (Hallatschek PPCF 2007, Conway PPCF 2008) 15

16 L-H Transition and ELMs Coherent V pol at ~ 6.5 khz disappears at L-H transition Both V pol and V rad increase transiently at ELMs V sec sec Time (sec) 16

17 Radial Correlation of V pol Fluctuations Find maximum of <V pol (r,t) V pol (r,t) > at (r -r) = 0.35 cm Increased poloidal velocity correlation at lower <n e > 17

18 Are These Zonal Flows? Poloidal correlation of V pol over GPI view in poloidal angle (~ 5 L pol ) and radius (~ 1 L rad ) is a fairly good indicator of zonal flow (similar to BES and single-ch. reflectometry) Frequency range of V pol fluctuations similar to other zonal flow fluctuations (e.g khz in DIII-D, 3 khz in NSTX) Amplitude of V pol fluctuations is comparable to mean flows, similar zonal flows to other devices (e.g. AUG) Magnetic component may be from EGAM or zonal field => Tentative answer is yes 18

19 Blob Motion Algorithm In normalized images, find regions with closed contours Define blob as any region above 1.2 normalized value Track blob centers over time to determine blob velocity Blob motion over µs/frame (# ) 19

20 Examples of Blob Tracking Blob center trajectories calculated over time in all images Proximity criteria set to insure continuity of blob motion blob tracks starting within radial box over 2 msec period vertical pixel vertical pixel horizontal pixel horizontal pixel 20

21 Distribution of Radial Blob Speeds Radial speed distribution of blobs is broad at all radii in SOL Not much variation in speeds over ρ = cm in this case Blob speed similar to Grulke POP (2006), Agostini (NF 2011) 21

22 Cross-Correlation vs. Blob Velocity averaged over 20 msec (error bars are velocity fluctuations) comparison with velocity codes of M. Agostini in progresss 22

23 Some Open Issues (Experimental) Look for global correlation of δv pol with other diagnostics Look at correlations <δv pol δv rad > for Reynolds stress Look at correlations <δv pol δi GPI > for transport effects Evaluate poloidal m-spectrum of δv pol within GPI box Reconcile poloidal flows with poloidal phase speeds Look at shots in H-mode and with various types of MHD Cross-correlate δv pol and MHD vs. poloidal angle Look for scaling of blob speed with SOL parameters 23

24 Some Open Issues (Theoretical) Compare with expected frequency of GAMs and EGAMs Compare with zonal flows in SOLT calculation (Lodestar) Evaluate effect of collisional damping on edge zonal flow Understand physics of zonal flows on open field lines Evaluate edge poloidal flows expected for MHD modes Evaluate whether magnetic zonal fields are expected Compare blob speed distributions with blob theory Compare blob formation process with blob theory 24