Connections between Particle Transport and Turbulence Structures in the Edge and SOL of Alcator C-Mod

Transcription

1 Connections between Particle Transport and Turbulence Structures in the Edge and SOL of Alcator C-Mod I. Cziegler J.L. Terry, B. LaBombard, J.W. Hughes MIT - Plasma Science and Fusion Center th 19 Plasma Surface Interactions San Diego, CA - May 2010

2 Background and motivations: Edge-SOL transport schematic Open field line region, Scrape-Off-Layer limiter Closed flux surfaces, plasma edge Material surfaces

3 Background and motivations: Edge-SOL transport schematic limiter In the SOL the turbulence is manifested as filaments (blobs) Blobs propagate rapidly radially, carrying plasma to the main walls Dominate transport 0 LCFS Z (cm) cm APD spot size s major radius (cm)

4 Background and motivations: Edge-SOL transport schematic limiter In the SOL the turbulence is manifested as filaments (blobs) Blobs propagate rapidly radially, carrying plasma to the main walls Dominate transport through SOL Source of SOL plasma Wavelike edge fluct. Why is SOL transport the way it is? 0 LCFS Z (cm) cm APD spot size s major radius (cm)

5 How does cross-field particle transport work? Observations in Ohmic L-modes 1) As central density is raised, density profiles flatten in the SOL 2) Fluxes across the LCFS increase exponentially with density Diagnostics Lá Array ionization profiles: ionization source Fluxes are then estimated from a simple 1D balance [1] What is increasing the flux so dramatically? [1] B. LaBombard et al., Nucl. Fusion 40 (2000) m m -2 s L-mode profiles Near SOL Far SOL Distance into SOL(mm) 10 Cross-Field Particle Flux 1mm outside LCFS Line-Averaged Density (10 20 m -3 ) factor of ~20 n e /n G

6 Observations: Enhanced D-Alpha H-mode QC mode related to cross-field transport (m 2 /sec) D eff EDA H-mode spectrogram QC mode 0 QC mode D = Ã/ n eff Ä e 16 /m 2 ) D not meant to imply diffusive transport: eff à is measured in the same way as before[2] [2] J. L. Terry et al., Nucl. Fusion 45 (2005) Time (s) The Quasi-Coherent Mode is an EM edge oscillation Characteristic to EDA H-modes Frequency range ~ khz Propagates in the EDD in lab frame Provides edge relaxation instead of large ELM s

7 Experimental setup Gas-Puff-Imaging diagnostics Side View Top View? gas puff injects? neutral D2, sensitive to ne, Te limiter Toroidal view of Outboard Edge? small toroidal extent (~5cm) allows localization? 90 channels cover ~ 5cm x 5cm nozzle? views coupled to APD arrays, 2MHz? limiter gas puff

8 Full radial profiles of turbulence spectra in wavenumber and frequency space - routinely recorded Ohmic L-modes poloidal arrays + time history: S( k í)=s( k, í)/s( í) dispersions are very nearly linear: k (í)> v = í/ k è è è ñ = -1.1cm ñ = 0.1cm ñ = 1.2cm EDGE ph IDD : EDD IDD : EDD IDD : EDD v ph è SEPARATRIX REGION S(k,í) è k pol (cm -1 ) SOL shot S(í)=S (í)+s (í) Relative fluctuation power k<0 k>0 EDD propagating part edge: dominated by electron diam. drift (EDD) prop. feature SOL: dominated by blobs in the ion diam. drift (IDD) direction shot t= s k pol (cm -1 ) k pol (cm -1 ) k pol (cm -1 )

9 Velocities in edge and far SOL are what we expect for drift wave turbulence and blobs Measured pol. velocity (km/s) I. Cziegler et al, Phys. Plasmas (2010) B =6.8T t I =1.0MA p Distance into SOL (cm) IDD : EDD Poloidal velocity in the far SOL where blobs are seen matches ExB velocity estimated from probe measurements EDD propagation in the edge is close to the electron diamagnetic flow velocity estimated from TS profile Normalized total power Normalized total power Distance into SOL (cm) e. prop. feature I. prop. feature n e /n G Distance into SOL (cm)

10 Wavenumber filtered spectra on underlying dynamics S k>0 (í) can yield information Relative fluctuation power EDD propagating part shot t= s

11 Wavenumber filtered spectra on underlying dynamics S k>0 (í) can yield information Relative fluctuation power EDD propagating part ã shot t= s Spectra are often clear power laws Spectral indices contain information about the spectral transfer dynamics similar to Kolmogorov cascades

12 Wavenumber filtered spectra on underlying dynamics S k>0 (í) can yield information Relative fluctuation power EDD propagating part ã = -1.3± ã = -4.6± ã 1 c í ã 2 crit through k (í) gives k è shot t= s Spectra are often clear power laws Spectral indices contain information about the spectral transfer dynamics similar to Kolmogorov cascades c Break-in-slope í may indicate: - dissipation scale - scale of energy input In the latter case: - ã 1 indicates inverse cascade - ã 2 indicates forward cascade

13 The total spectral power and the distribution in L-mode show a strong dependence on n e /n G n e/n G = 0.15 n e/n G = 0.16 n e /n G = n e /n G = Total relative fluctuation power clear peak at high freq part 1 10 low intensity broad plateau 1 10 med intensity pl. diminishing 1 10 ã = -4.6± forward transfer (features breaking up) is reproducible in the range of known theories - Itoh interchange: ã = -4 - Wakatani-Hasegawa (collisional drift): ã = -3 (Kolmogorov-Kraichnan 2D) - Hasegawa (1979): ã = -6 break-in-slope spectral position develops a peak at low densities, indicating hindered inverse dynamics k ñ s high intensity no plateau 1 10 c k ñ s~ n e /n G B t = 4.6T 5.4T 6.8T

14 Evidence that the EDD propagating edge turbulence is a major factor in edge particle transport and may be related to the Greenwald density limit EDGE REGION SOL densities Total relative fluctuation power (x10-5 ) EDD propagating part m L-mode profiles Near SOL Far SOL Distance into SOL(mm) m -2 s Cross-Field Particle Flux 1mm outside LCFS n e /n G factor of ~ n /n e G Line-Averaged Density (10 20 m -3 )

15 The physical variable â p seems to parametrize the turbulence better than the Greenwald fraction Total relative fluctuation power (x10-5 ) EDGE REGION EDD propagating part I. Cziegler et al, PoP (2010) n /n e G âp- normalized pressure better to the free energy source p and perhaps an interchangeballooning type drive? â p

16 Significant reduction in EDD turb. power at L-H transition and suggested connection to the QC mode n e (10 m ) L-mode ELMfree EDA shot n e n e n e n e IDD, SOL EDD, edge L-mode ELM free EDA H-mode Time (s) Relative fluctuation power k pol (cm -1 ) EDD propagating part k ñ =0.11 s k pol (cm -1 ) shot t= s k pol (cm -1 ) Edge turbulence drops at L-H transition No change in norm. SOL fluct. level Further evidence to transport-relevance of EDD edge turbulence critical wavenumber is at the same physical scale as QCM

17 Conclusions Found evidence that the edge turbulence is responsible for cross field particle transport, ie creating blobs, which hit the walls - located in the last 2 cm inside the separatrix, the same location the H-mode pedestal forms - turbulence power in goes up rapidly as n e/n G and the perpendicular particle flux increases - in ELMfree H-mode EDD edge, rather than IDD SOL turbulence normalized power is much reduced The possible energy input scale is k ñ s~0.1 showing likely connection to QCM (resistive ballooning drive) and possibly a drift-interchange mixed regime (Scott) Turbulence power may be better organized by grad-p source â indicating further connection to a p The spectral distribution responds sensitively to n e/n G, becoming more developed as n e/ng increases (more inverse cascade dynamics) - Why? What theory predicts the right spectra and what consequences does the right model have for transport?