13 th RFP Workshop October Stockholm 1 P.Scarin, M.Agostini, N.Vianello, R.Cavazzana, F.Sattin, G.Serianni, M.Spolaore Consorzio RFX, Associazione Euratom-ENEA, Padova, Italy S.J.Zweben Princeton Plasma Physics Laboratory, Princeton, New Jersey, USA R.J.Maqueda Nova Photonics, Princeton, New Jersey, USA J.L.Terry Plasma Science and Fusion Center, MIT, Massachusetts, USA Y.Yagi, H.Sakakita, H.Koguchi, S.Kiyama, Y.Hirano National Institute of Advance Industrial Science and Technology, Japan Analysis of edge turbulence in RFX-mod from GPI data and comparison with other toroidal devices

13 th RFP Workshop October Stockholm 2 Introduction  Turbulent structures are detected in the edge region of several plasma devices: Tokamak, Stellerator, RFP, non fusionist machines  Structures are considered the main cause of transport and confinement degradation  Statistical analysis of edge turbulence reveals the presence of common properties that do not depend on the particular plasma device: indication of a similar physical driving mechanism  Statistical properties of edge signals of four different fusion devices are analysed and the common properties are pointed out  The different turbulent time and space scales between the four experiments seems to be linked with the edge pressure gradient scale  Indication of pressure-driven instability Introduction

13 th RFP Workshop October Stockholm 3 Edge Structures  t=0  t=12  s  t=24  s  t=36  s NSTX RFX-mod  t=0  t=0.4  s  t=0.8  s  t=1.2  s r [mm]  [mm] G.Serianni et al., PPCF (2007) In the edge region, potential, density and temperature signals are characterised by strong fluctuations Strong bursts in the signals are related with the presence of coherent structures (blobs) propagating in the plane perpendicular to the main magnetic field Introduction

13 th RFP Workshop October Stockholm 4 Main Diagnostic: GPI The edge regions of RFX-mod TPE-RX NSTX Alcator C-mod are studied with the Gas Puff Imaging diagnostic GPI is a non intrusive spectroscopic diagnostic which measures the local emission fluctuations of H  D  HeI lines in the plane perpendicular to the main magnetic field R.Cavazzana et al. RSI (2004) M.Agostini et al., RSI 77 10E513 (2006) P.Scarin et al., JNM (2007) Zweben S. et al., NF (2004) Terry J. et al., JNM (2001) Introduction

13 th RFP Workshop October Stockholm 5 PDF of the Signals NSTX RFX-mod TPE-RX Alcator C-mod PDF of raw signals are clearly non Gaussian in all the devices Deviation from Gaussianity increases outward Raw Signals

13 th RFP Workshop October Stockholm 6 Statistics Vs Radius NSTX Alcator C-mod  Flatness and skewness increase outside the separatrix  Edge fluctuations are more and more dominated by positive (S>0) bursts with high amplitude (high F), relative to the mean value of the signals, as one moves outward Raw Signals

13 th RFP Workshop October Stockholm 7 Universal Statistics F.Sattin et al. PPCF (2006) F.Sattin et al. submitted to PPCF Parabolic relation between F & S for the edge fluctuations in all the devices analysed: few conditions expected to hold for different environments B.Labit at al. PRL (2007) Schopflocher T.P. et al. Boundary-Layer Meteorology (2005) Sura P. and Sardeshmukh, J.Phys. Oceanogr (2008) Experimental PDF can be fitted with 2 Gamma functions: coefficient of the curve provide information about the physics of the system Raw Signals

13 th RFP Workshop October Stockholm 8 Power Spectra Power spectra show (typically) different regions with power law-decay f * f* should be linked to the turbulence injection scale Time Scales

13 th RFP Workshop October Stockholm 9 NSTX RFX-mod TPE-RX Alcator C-mod Intermittency The PDFs of the edge fluctuations depend on the scale of the fluctuations ** Time Scales  * assumes values similar to f* -1 Different time scales involved

13 th RFP Workshop October Stockholm 10 Flatness Comparison For a set of different discharges F(  ) has been evaluated for the different devices Time scale  multiplied for f*  For all the machines flatness reaches the Gaussian values of 3 for  f*=1  Coherent structures develop for  f *<1 τ f *=1 is linked with the largest spatial scale present in the system No structures Structures Time Scales Injection

13 th RFP Workshop October Stockholm 11 Edge Gradients RFX-mod The edge region of fusion devices are characterised by the presence of strong gradients that are possible sources of free energy for the development of turbulence [eV] NSTX  In tokamaks the edge region is fundamental in the rising and sustaining of the H-mode regimes Gradients

13 th RFP Workshop October Stockholm 12 Turbulence Vs Edge Gradients NSTX TPE-RX Link between electron pressure gradient and edge structures Area occupied by edge structures (f p ) increases from inward to outward Alcator C-mod Different experiments characterised by different pressure gradient scales L p Gradients Langmuir probes RFX-mod Langmuir probes

13 th RFP Workshop October Stockholm 13 Turbulence & Injection Scale Largest spatial scale present in the system: Autocorrelation time RFX-mod NSTX Alcator Clear relation between the largest structures and the characteristic pressure gradient scale L p Gradients v⊥τav⊥τa

13 th RFP Workshop October Stockholm 14 Conclusions Universal statistical properties have been detected in the edge region of four plasma fusion devices A common largest spatial scale present in the systems can be identified This spatial scale linked with the energy injection scale of the turbulence is clearly linked with the pressure gradient scale Pressure-gradient instabilities should play a role in the generation and evolution of the edge turbulence

13 th RFP Workshop October Stockholm 15  Flatness (F) as a function of Skewness (S) for the GPI fluctuations (about points)  The data can be fitted with a quadratic polynomial, and the result are very similar to the one reported for TORPEX machine in [B. Labit et al., PRL 98 (2007) ]  In the graphs different plasma conditions are considered (about 2000 different plasma discharges)  The numerical values of the coefficients of the curve provide information about the specific physics of the system  The analytical curve is a general consequence of a few conditions expected to hold for different environments [Schopflocher T.P. et al. Boundary-Layer Meteorology (2005), Sura P. and Sardeshmukh, J.Phys. Oceanogr (2008),……………] Statistical Properties of GPI signals in RFX-mod

13 th RFP Workshop October Stockholm 16 Statistical properties of GPI signals in RFP&Tokamak F = a S 2 +b for a large range of F and S values An estimate of coefficient a would bring information of underlying physical mechanism

13 th RFP Workshop October Stockholm 17 Pressure Driven Instabilities in TPE-RX and RFX-mod TPE-RX RFX-mod  In RFX-mod phase relation between V p and n e (measured at I  < 350kA) varies from 90° to 0°  Different pressure driven instabilities  In TPE-RX device phase relation between V f and GPI structures (link to n e ) is 90°  Indication of curvature driven instabilities [ M. Agostini et al., Plasma Phys. Control Fusion 50 (2008) to be published]

13 th RFP Workshop October Stockholm 18 GPI-Langmuir comparison TPE-RX GPI and Langmuir diagnostics are active simultaneously in RFX-mod at I  <350kA although at different locations The (S, F) dataset of n e from Langmuir overlaps with the GPI database The good overlap supports the assumption that GPI provides a measure with an equivalent statistic to plasma density fluctuations Time correlation between GPI and ion saturation current signals in TPE-RX taken simultaneously at almost the same location The high value of the correlation suggests that the 2 signals unveil the same dynamic of the turbulence

13 th RFP Workshop October Stockholm 19 …..about the shape of Power Spectrum S(ƒ) (1) S(ƒ) tails can switch from exp to power-law on the same plasma pulse Power-lawexp …it depends from the distribution of burst times ?

13 th RFP Workshop October Stockholm 20 …..about the shape of Power Spectrum S(ƒ) (2) … with the time scale analysis ones obtain different distributions, with different width: S(ƒ) power-law wide distribution S(ƒ) exp narrow distribution The S(ƒ) of edge turbulence may be interpreted as arising from superposition of signals with different coherent structures and the time width distribution could depend from the displacement of magnetic surface (radial component) τ [μs]