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Experimental investigation of turbulent transport

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1 Experimental investigation of turbulent transport
at the edge of tokamak plasmas Nicolas Fedorczak Thesis supervisor : A. Pocheau (IRPHE) CEA supervisor : P. Monier-Garbet J.P. Gunn Ph. Ghendrih

2 Magnetic confinement & transport
Magnetic confinement of ~10keV plasma Tokamak : !! gradients (no thermo-dynamical equilibrium) + curvature   transport - energy losses to the walls - pressure gradient in core plasma critical issue for reactor operation critical issue for reactor efficiency // direction : free motion   direction : constrained by B

3 Edge transport : turbulence & asymmetry
Current tokamaks : JET, DIII-D, ASDEX, Tore Supra, NSTX, TCV, Alcator C-mod … demonstrated the high level of turbulence ExB convection Experimental evidences : interchange P B free energy source drive  instability HFS : stable / LFS : unstable demonstrated  transport asymmetries Tore Supra, C-mod J.P. Gunn JNM2006 B. LaBombard NF2004 Garcia & Pitts NF2007 Picture from fast imaging on Tore Supra :

4 Edge plasma influenced by plasma/wall interaction
Boundary of confined plasma : open field lines region = Scrape-Off Layer delimited by the Last Closed Flux Surface Plasma facing components (PFC) : - perfect sink of particle - regulate charges & currents - spatial symmetry breaking  determine the SOL plasma equilibrium (balance between // and  fluxes) Heat load footprint, matter migration (flows) + boundary conditions for core rotation  flows

5 Open issues related to ITER project
ITER : next step toward fusion reactor - steady-state + fusion reactions - High performances discharges Critical issues related to edge plasma No robust model to predict: - Heat load on first wall - Pedestal gradient (shear flow & boundary conditions) shear-flow flow boundary Physics of blobs, ELM’s & transport barriers

6 Axis of my research on Tore Supra Tokamak
Build a consistent picture of particle transport at the edge Constrain models : Difficult to address from current experiments : C-mod, DIII-D, ASDEX, JET Multi-diagnostic investigation of edge turbulent transport Mach probe  // flows and density profiles Rake probes  electrostatic fluctuations Fast imaging & reflectometers fluctuation dynamics asymmetries

7 I. How do we diagnose the plasma phenomena
Experimental investigation of turbulent transport at the edge of tokamak plasmas I. How do we diagnose the plasma phenomena 1. Langmuir probes 2. Fast visible imaging II. The model drawn from experiments on Tore Supra 1. Consistency local / global measurements 2. 3D properties of edge particle transport : revisiting filaments III. Implications & conclusion

8 Illustration of data collection along the probe path
Reciprocating Langmuir probes 2 hydraulic systems installed at the plasma top  full radial profile collection Illustration of data collection along the probe path 200 ms !! experience strong heat flux ~MW.m-2

9 Improving Mach measurements
plasma collection by a biased electrode local plasma parameters : ne, Te, plasma Mach configuration : // flow velocity !! time averaged data !! calibration with effective collection area  sensitive to collector geometry Mach collectors of the rake probe Tunnel collector small cylindrical collector ?  large effect on flow velocity measurements Yet, tunnel probes used only on Tore Supra… Gunn, Dejarnac (poloidal rake probe)

10 Implementing rake probes for fluctuations
Requirements for SOL plasma fluctuations: ExB convection  potential & density 1 MHz acquisition rate ~mm spatial sampling  In charge of the final design, maintenance & use of a new diagnostic DTURB & the rake probes Improved control & electronics for a flexible & sensitive acquisition mode selection anti-aliasing dynamical sampling : SNR Collaboration with Gent University (G. Van Oost)

11 Issue on the interpretation of fluctuations
Turbulent flux : Discrete measurement : Effect of electric field under-sampling ? Tokam 2D Error calculated on turbulence simulation  non negligible error depends on potential eddy size (Never mentioned in the literature) Some experimental data are not exploitable for rturb !! experiment

12 local plasma electrons excite neutrals
Fast imaging of edge transport phenomena Use visible plasma emission to picture the density fluctuations local plasma electrons excite neutrals & & Rich qualitative understanding: geometrical + dynamical up to 50 kHz (effective  exp  turb ) wide opening angle Tore Supra top view Camera view Virtual view Collaboration with Nancy university + IRPHE

13 Qualitative information
LFS gas injection 50kHz Movies  resolve the main turbulence dynamics  extraction ? Collaboration LPMIA F. Brochard / G.Bonhomme LMD R. Nguyen / M. Farge Tangential projection of ~2D turbulence Tomographic reconstruction reconstruction? Velocity midplane (reduced projection artifact) Qualitative agreement with reflectometers Collaboration LPP L. Vermare camera

14 Transport model: diagnostics capabilities
II Transport model: diagnostics capabilities transport mechanisms + local amplitude Poloïdal rake probe  local ExB turbulence global particle balance + spatial asymmetries Tunnel Mach probe  // flows transport mechanisms + spatial asymmetries Fast visible imaging  spatial properties  3D transport description from experimental evidences

15 Electrostatic fluctuations : interchange-like
II Electrostatic fluctuations : interchange-like “bursty” transport Devynck, Boedo, Zweben & fluctuate in phase Radial convection of density bursts

16 Electrostatic fluctuations : associated transport
II Electrostatic fluctuations : associated transport Intermittent convection of density bursts : Vrblobs > 300 m.s-1 ( 1% cS ) qualitative agreement with fast imaging signature of filament convection? Averaged effect : Vrplasma ~ 30 m.s-1 ( 0.1% cS ) ordering consistent with density profiles but not quantitatively Consistency local vs. global measurements  asymmetries ?

17 // flow drivers in TS SOL : radial particle flux
II // flow drivers in TS SOL : radial particle flux What mechanisms for density profile // velocity ? Particle flux balance : // flow radial flux ? “ PS flows” limiter recycling  MC simulation probe data  ne EIRENE Y. Marandet  M// main driver ~10% ~15%

18 // flow velocity & radial flux asymmetry
II // flow velocity & radial flux asymmetry SOL plasma Particle flux balance Conservation law (pressure) Boundary limiters (Bohm) // flows  balance the particle source asymmetry quantify the global particle balance (r into the SOL) partial resolution of asymmetries

19 flows balance: quantify LFS / HFS asymmetry
II flows balance: quantify LFS / HFS asymmetry conservation laws applied to local // flow profiles  near top !! r() highly LFS ? consistency with local ExB flux ?  spatial mapping

20 Field line tailoring : resolve spatial asymmetry
II Field line tailoring : resolve spatial asymmetry Use movable limiters to tailor the flux distribution along field lines  quantify // flows response in term of radial flux distribution r outboard midplane in a narrow poloidal section (50°)

21 Local / global consistency
II Local / global consistency - Mach probe  spatial flux distribution (global) - Rake probe  ExB flux amplitude (local) Illustration for 3 different plasma scenarios : global global global local local local Fairly good agreement between both independent measurements ExB flux  highly asymmetric (?)

22 Fast imaging: evidence of asymmetry
II Fast imaging: evidence of asymmetry Gas injection performed on HFS and LFS  increase the visible emission pictured at 50kHz Plasma filaments are observed on LFS to propagate outward They are not observed on the HFS conciliate previous assumptions

23 Interpretation of multi-diagnostic investigation
II Interpretation of multi-diagnostic investigation Particle transport in SOL : - ExB convection of plasma “filaments” - highly asymmetric around the plasma : - outboard midplane - finite // extent  usual flute mode paradigm - consistent with interchange instability mechanisms (localization + extent) - drive near-sonic // flows around the confined plasma Necessity to consider a 3D model of filament dynamics

24 Picture of filaments in the core !
II Picture of filaments in the core ! Other experiment : stationary fully detached plasmas (3-4 sec.) --> emissive ring in the confined region (r/a ~0.5 ) + local conditions ( * , P ) similar to SOL Again, (largest) field aligned structures only on the Low Field Side filaments  k// > open / closed field lines

25 Implications of the results
3D description obtained on Tore Supra  applicable to divertor machines coherent with other evidences conciliate apparent incoherencies L-mode : LIMITER  DIVERTOR Revisiting the theoretical description & models of edge transport flute modes = 2D  full 3D (ESPOIR project) Database of strong evidences about edge plasma phenomena (flows, decay length, fluctuations) Case base for new code benchmarking (MISTRAL project) Coupling of turbulence & edge flows with core rotation  shear layers

26 Summary Experimental investigation of edge transport :
About diagnostics : - interpretation of experimental data improving probe geometry for // flow measurements critical issues on the spatial sampling of fluctuations projection artifacts with visible imaging - multi-diagnostic consistency local & global measurements About transport model : - mechanisms driving the // flows  radial flux - conservation laws  help from simulation (SOLEDGE2D) + a posteriori checking About experiments : - Experimental proposals dedicated to specific issues - Check the consistency of results with a variety of experiments

27 Ionization source in the SOL
Monte-Carlo simulation of recycling on main limiter : EIRENE simulation 3D domain ( toroidal symmetry) Initial input (experiment) : ion limiter plates (from Mach probe) ne + Te + Ti profiles (SOL + confined region) Complete database for Deuterium atomic reactions Self-consistent matter balance

28 Transversal drifts in SOL flux balance
Simplified flux balance : large aspect ratio Er independent of  |M//| ~ 1

29 Pressure conservation
radial transfer of // momentum Validation with simulation Spatial mapping of r SOLEDGE2D G. Ciraoloa & H. Bufferand spatial mapping of & (only viscosity) Computable Reynolds stress P/P < 10 % P/P < 15 % ! But only linear term !

30 Flow reversal experiment

31 3D filaments : revisiting momentum transfer
transfer of v// Issue in understanding SOL flow effect on core rotation computable from previous results average on flux surface : NO RESIDUAL TRANSFER // dynamic of a single “filament” // front expansion ? coupling with local ExB fluctuations along the field line ?

32 SOL flow and core rotation
flow reversal in SOL plasma for similar core plasma  change in core velocity fields co-IP V V ct-IP

33 More experimental implications for Tore Supra
Heat load asymmetry on the main limiter Y. Corre & al. Plasma environment of wave launchers (LH) M. Preynas, A. Ekedahl  coupling efficiency (gradients in front of antennae)  suprathermal electron generation V. Fuchs, J. Gunn, A. Ekedahl 3. More physics about fuelling by gas injection or pellets 4. Precise flow pattern in SOL plasma  Carbon migration & deposition

34 List of contributions First author publications :
N. Fedorczak, J.P. gunn, Ph. Ghendrih, P. Monier-Garbert, A. Pocheau Flow generation and intermittent transport in the scrape-off layer of the tore Supra tokamak Journal of Nuclear Materials 390–391 (2009) 368–371 N. Fedorczak, J.P. gunn, Ph. Ghendrih, G. Ciraoloa, H. Bufferand, L. Isoardi, P. Tamain, P. Monier-Garbet, Experimental investigation on the poloidal extent of the turbulent radial flux in tokamak scrape-off layer Journal of Nuclear Materials (2010) Oral contribution to international conferences : Ballooned like transport in the SOL of Tore Supra tokamak : evidences and properties Transport Task Force meeting (TTF2009) San Diego Poloidal mapping of turbulent transport in SOL plasmas Plasma Surface Interaction meeting (PSI2010) San Diego A first comparison between probes, fast imaging, and Doppler backscattering synchronous measurements of edge turbulence in Tore Supra European Plasma Society (EPS2009) Sofia (F. Brochard & N. Fedorczak)

35 Many thanks to Tore Supra pilots Technical support : Physicists
F. Saint-Laurent, P. Hertout, D. douai, Ph. Moreau Technical support : J.Y. Pascal, B. Vincent, F. Leroux, T. Alarcon, N. Seguin, V. Negrier Physicists Jamie Gunn, P. Hennequin, L. Vermare, P. Monier-Garbet, P. Devynck, F. Clairet C. Reux, D. Villegas, M. Kocan, X. Garbet, Ph. Ghendrih, Y. Sarazin, P. Tamain collaborators G. Ciraolo, L. Isoardi H. Buferand, E. Serre, G. Bonhomme, F. Brochard, M. Farge, R. Nguyen, A. Pocheau, G. Searby friends Matthieu, Sara, Sebastien, Vincent, Yannick, Matthieu, Clement, Sophie, Daniel, Gaëlle, Etienne, Cédric, Victor, Gwen, Ronan, Rémi, Matthieu, Mélanie, François, Joao, Tom, Magwa, Sparrow, Mélissa, Mai, Caro, Clemence, Dimitri, Vanessa, Julien, Lana, Alexis, Uron, Suk-ho, Timo and my family

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