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

16. Electrostatic fluctuations : associated transportII
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 fluxII
// 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 asymmetryII
// 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 asymmetryII
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 asymmetryII
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 consistencyII
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 asymmetryII
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 investigationII
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