1. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Current filaments and electrostatic structures measured in the edge region of the RFX-mod experiment N. Vianello, M. Spolaore, M. Agostini, R. Cavazzana, E. Martines, P. Scarin, G. Serianni, E. Spada, M. Zuin, V. Antoni Consorzio RFX, Padova, Italy

2. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Introduction and motivation The edge region of RFPs shares various characteristics with tokamaks and stellarators: the presence of coherent structures with eddy features or density blobs emerging from electrostatic turbulence background It is believed that these structures play a major role in driving the transport in the edge region. They contribute both through an enhancing of particle diffusivity or a direct convection of particle and energy towards the wall A characterization of structures observed in the boundary region of RFX-mod experiment will be presented based on detailed local measurements of both electrostatic and magnetic fluctuations in the cross field plane

3. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Experimental equipment: U-probe Two 2D arrays of electrostatic pins : 5 (toroidal) x 8 (radial) Δ r=6mm e Δφ =6mm; pin  =3 mm Two arrays of 7 (B r,B ,B  ) magnetic probes: size: 7mm X 6mm X 8mm Δ r=6mm 5 MHz sampling rate. High frequency bandwidth 50 mm probe Discharge parameters: Ip~350kA; n e ~2 10 19 m -3 r  BB An insertable edge probe for local measurements of both ELECTROSTATIC and MAGNETIC quantities has been used: high spatial and temporal resolution

4. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Experimental setup: GPI R.Cavazzana et al. Rev.Sci.Instrum 75 4152 (2004) M.Agostini et al., Rev.Sci.Instrum 77 10E513 (2006) G as P uff I maging diagnostics: non intrusive diagnostic Measures the visible light emission (neutral He) Three triaxial magnetic coils installed 32 Lines of sight looking at HeI emission line from a locally puffed He cloud Spatial resolution : 5 mm Viewed area : 70 mm x 50 mm Bandwidth: 2 MHz Sampling rate : 10 MSamples/s See P.Scarin talks this morning

5. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Some remarkfs on edge features: Flow and Reynolds stress In analogy to tokamak it has been established that perpendicular flow at the edge is sustained by electrostatic Reynolds stress Strong correlation between ExB flow time evolution and temporal evolution of Reynolds stress Gradient Strong gradient of Reynolds stress observed in the region of innermost ExB Shear N.Vianello et al, PRL 94 (2005) N.Vianello et al., PPCF 48 (2006)

6. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Density profile exhibits strong gradients located at the edge. (Lorenzini NF 47 2007) Also temperature gradient is located at the edge. With the improvement of magnetic control high temperature gradients obtained Particle flux mainly driven by electrostatic fluctuations at the edge ( Martines NF 50 1999) Edge Profiles

7. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Electrostatic features scaling Decreasing of m=0 fluctuations  increase the temperature gradient at the edge. Simultaneous increase of the flatness i.e. signal intermittency Edge Temperature profiles increases with the increasing of plasma current. Moving towards small values of R/L T =5-10

8. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Intermittent structures: Toroidal propagation Coherent structures associated to bursts in electrostatic fluctuations have been identified by statistical techniques [M. Farge, Annu. Rev. Fluid Mech., 24 (1992) 395]. A conditional average on these events is then applied Estimate of velocity propagation of structures v  ~ 22km/s Structures propagates toroidally with a velocity consistent with the ExB flow V ExB Conditional average on the toroidal array of 16 chords of GPI

9. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Electrostatic features of structures Structures of potential are associated to density and temperature structures structures on pressure are also evident Density and floating potential exhibits a phase shift of the order of  /2 Density and plasma potential exhibits a smaller phase shift

10. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Vorticity measurements Conditional averaging of bursts on a V f (r) array revealed of spatial structures on  plane Given the 2D array of Vf measurements an experimental estimate of vorticity can be provided [W. Horton et al. PoP 12, 022303 (2005)] [U. Frish, Turbulence: The Legacy of A. N. Kolmogorov (Cambridge University Press, Cambridge, England1995), p. 58] Potential structures are vortices and vorticity peaks agree with the average time behavior of V f Structures can be considered “frozen” while passing the measurement point and the Taylor’s hypothesis applies

11. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Pressure perturbation and vorticity These structures correspond to pressure perturbation A direct measurement of vorticity Shows the vortex like nature of these structures

12. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Radial Structure Dimension Radial dimension of the structures may be inferred from radially distributed measurements Conditional average with pressure perturbation at r=422.5,τ=4μs as trigger events

13. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Toroidal Dimension of the structures from GPI With GPI toroidal extent on plasma emissivity estimated without frozen-turbulence hypothesis (Serianni PPCF 49 2075 (2007)) At low plasma current (see P.Scarin talk)

14. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Electrostatic features scaling The toroidal extent is found to slightly decrease with increasing density P.Scarin et al, JNM 363-365 (2007) The perpendicular width increases non-linearly with  at a fixed n/n G The behavior of versus n/n G shows a slight decrease at a fixed time scale 

15. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Electromagnetic nature of the structures Electrostatic and magnetic fluctuations coherent within the range 50-300 kHz Cross correlation between GPI and local Br fluctuations Strong correlation indicates a clear link between density and magnetic fluctuations It is well know the existence of a relation between magnetic and electrostatic fluctuations (see for example works of A.M ö ller on T2)

16. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Filamentary structures observed at the edge The “U-probe” arrangement allows to measure the density current, if any, associated to electrostatic structures by using the Ampère’s law J  peaks result associated to electrostatic structures pressure peaks. Transverse components definitively smaller --> parallel current filament as postulated by J. Bergmans, T.J. Schep PRL 87 (2001) 195002 J. R. Myra PPCF 14 (2007) 102314

17. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Magnetic field hodogram Structures are elongated filaments with an associated parallel current (resembling ELM’s and blobs in tokamak) (Spolaore PSI2008) Filamentary structures observed at the edge Also emissivity structure associated to magnetic perturbation at all the range of current explored I p = 0.3 MA I p = 1.5 MA

18. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Currrent intensity scaling Current intensity associated to pressure perturbation exhibit a power law decay between 6 and 2  s with a maximum around 100 kHz. May be related to energy injection scale

19. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Conclusions Edge magnetic and electrostatic fluctuations investigated with high spatial and time resolution Coherent structures are detected within the turbulence background and exhibit both electrostatic, vortices, and magnetic, current density, features These structures are characterized by pressure peaks associated to current density filaments essentially aligned with the edge magnetic field (B  ) and travelling according to the ExB flow Structure dimension have been found to depend on plasma parameters and indications of possible source of free energy with characteristic time have been provided

20. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Comparison with diamagnetic current A comparison with the estimate of the diamagnetic current density can be performed The diamagnetic current density estimate results of the order of magnitude of the measured J  and larger than the measured J 

21. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Drift-Nature

22. N.Vianello 09 October 2008 13t h IEA/RFP Workshop Electrostatic feature scaling

23. N.Vianello 09 October 2008 13t h IEA/RFP Workshop  t=0  t=0.4  s  t=0.8  s  t=1.2  s Results of Tomographic Inversion  Motion and evolution of 2D coherent structures can be studied at high frequency (namely 0.1µs)  The mode number analysis gives also the toroidal velocity of emission structures with high frequency (until 1µs)  Linear time dependence of the phase of the dominant toroidal mode number is emphasized

24. N.Vianello 09 October 2008 13t h IEA/RFP Workshop M.Spolaore et al., 18 th Conference on Plasma Surface Interactions, 26-30 May 2008, Toledo, Spain Cross correlation between GPI signal and time derivative of the local B r fluctuations Strong correlation indicates a clear link between density and magnetic fluctuations Relation between GPI structures and time derivative of the magnetic field components Dashed lines: current filament moving both toroidally and radially Edge structures are linked with poloidal current filaments