Turbulence characterization in the outer region of fusion plasmas: Flow measurements in the edge region of the RFX-mod experiment 11/01/20151 Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Gianluca De Masi

Experimental EquipmentInterpretation ModelsFlow measurement resultsDynamic Variations Outline: 11/01/20152 Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Experimental setup 11/01/20153 Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Gundestrup (  =217° 30’) U-probe (  =247° 30’) r (mm) 0a = 459  z Plasma radius= m Major radius = 2 m Plasma current= 2 MA

B≈B θ r φ δ Gundestrup probe Electrode n° δ angle w.r.t. equatorial plane δ angle w.r.t. equatorial plane 1 22,5° 2 67,5° 3 112,5° 4 157,5° 5 202,5° 6 247,5° 7 292,5° 8337,5° The 8 external electrodes radius is about 2 mm (comparable with ion Larmor radius) with 4.8 mm 2 area Gundestrup probe can collect two types of data: Ion Saturation Current The probe has to be connected to a resistor Floating Potential The probe has to be connected to a voltage divider 11/01/20154

U-probe 5 Measurements of n, Te, Vp at 6 radial positions (Δr=6mm) 17 measurements of floating potential Vf at several radial positions (Δr=6 mm) e toroidal positions (Δφ=24 mm and 72 mm) 10 triaxial magnetic probes (Δr min =6 mm, Δφ=95 mm )

Interpretation Models (1) Gundestrup probe can collect two types of data: Ion Saturation Current Magnetized models Unmagnetized models Floating Potential Model proposed by Jachmich 11/01/20156 Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Interpretation Models (2) Magnetized Model: Hutchinson (1987) presheath two fluids equations diffusivity and viscosity The presheath is described with two fluids equations taking into account particle diffusivity and viscosity. upstream and downstream directions αncS e βncS The ion saturation current densities collected in the upstream and downstream directions are given by the expressions αncS e βncS The coefficients α e β are functions of the Mach number: M=vpar/cs which in turn can be inferred from the ratio of the two currents upstream and downstream MacLatchy et al (1992) perpendicular drift In presence of a perpendicular drift, the ion saturation current collected by a probe oriented at an angle δ w.r.t. the magnetic field is: 11/01/20157 Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Interpretation Models (3) Unmagnetized Model: Hudis and Lidsky (1970) Collinsionless Collinsionless plasma; the Mach number can be written in the form: M=K ln(Isu/Isd) where K is only function of Te and Ti streaming velocity The ion saturation current collected by probe (accounting for the arbitrary angles between streaming velocity and the probe surface) is: 11/01/20158 Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Interpretation Models (4) Jachmich Model (2000) floating potential Vfl The voltage difference arising between the probe tip and a reference probe is defined as the floating potential Vfl. This potential is related to the plasma potential Vp and the electron and ion saturation current:, the difference ΔVfl of floating potential between 2 opposite electrodes parallel flow Mach number By means up/downstream saturation current, the difference ΔVfl of floating potential between 2 opposite electrodes, can be related to the parallel flow Mach number: 11/01/20159 Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Main plasma parameters in several experimental campaigns considered (insertion probe range [409,479] mm) Plasma current< 400 kA Electron density( ) x m -3 Electron temperature (that we assume equal to Ti) 30 eV (at the deepest radial insertion, r = 409 mm) Ion Larmor radius4 mm (at r = 409 mm) Ion thermal velocity5.4 x 10 4 m/s (at r = 409 mm) Ion sound velocity7.6 x 10 4 m/s (at r = 409 mm) 11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Flow measurements results (1) Ion saturation current configuration Time behaviour of a typical ion saturation current signal, collected by a probe electrode during one of considered discharges [Shot = 22920; Probe= 5 (δ = 202,5°); r = 449 mm] Angular behaviour of i(δ) in 3 different shots corresponding to different radial positions Shots=[22368, 22369, 22373];F= /01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Flow measurements results (2) Radial Profiles In both cases the profiles are comparable The parallel drift velocity inside the plasma reaches values of 0.5 c S (corresponding to (3-4) x 10 4 m/s) The perpendicular drift velocity is of the order of 0.1 c S (corresponding to 3 x 10 3 m/s) in the tile shadow, and it reaches values of –0.2 c S (corrisponding to 1.5 x 10 4 m/s) inside the plasma 11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Ion saturation current configuration

Flow measurements results (3) Floating Potential configuration Time behaviour of a floating potential signal, collected by a probe electrode during one of considered discharges [Shot = 22326; Probe= 3 (δ =112,5°); r = 439 mm] Angular behaviour of V(δ) in 3 different shots corresponding to different radial positions Shots=[22413, 22345, 22411]; F= /01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Electron flux effect B

Flow measurements results (4) Radial profiles Velocity profiles based on floating potential measurements are comparable with the ones based on ion saturation current measurements Parallel drift velocity reaches values of 0.4 c S (corresponding to 3 x 10 4 m/s) inside the plasma Perpendicular drift velocity is of the order of 0.3 c S (corresponding to ( ) x 10 4 m/s) in tile shadow, up to -0.2 c S (corresponding to (1-2) x 10 4 m/s) inside the plasma 11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Floating Potential configuration Ion saturation current configuration

11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Flow measurements results (5) Gundestrup and U-probe

11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Several plasma parameters have been investigated during different experimental campaigns at different radial positions, in order to create a database and study the edge plasma flow variations w.r.t. global discharges parameters Flow measurements results (6) Reversal parameter F = B tor (a)/ Greenwald density n G = I P / πa 2

11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Flow measurements results (7) Flow profiles comparison w.r.t. several plasma parameters

11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Flow measurements results (8) Velocity radial profiles In the region of strong gradient, fluctuations coupling between flow contributions and could be transfer momentum to the perpendicular drift

Work in progress Plasma dynamics fluctuations F-Crashes: Correlation between fast variations of reversal parameter F and drift velocity fluctuations? 11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

Work in progress Plasma dynamics fluctuations (2) Coherence and phase of parallel and perpendicular drift velocity fluctuations 11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering Work in progress Parallel drift velocity transient due to pressure gradient? Pellet injection Some tokamak results link the L-H mode transition to an edge flow gradient. O.D.Gurcan et al.,’Intrinsic rotation and electric field shear’,Physics of Plasmas,14 (2007)

Summary -For the ion saturation current measurements we found that the two applied interpretation models give comparable estimates of the parallel and perpendicular drifts -We have verified the reliability of floating potential measurements in order to evaluate the Mach number. A good agreement exists between the plasma flow and the ExB drift flow -The perpendicular velocity comparison between Gundestrup and U-probe reveals a double shear (across the r=a surface and deeper into the plasma) -Very important informations about mechanisms that regulate the transport in the RFP edge could be obtained by studying plasma dynamic fluctuations and further research is now in progress 11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and Engineering

11/01/ Centro Ricerche Fusione Consorzio RFX Università degli studi di Padova International Doctorate in Fusion Science and EngineeringReferences -G. Serianni, ‘Struttura della regione esterna di un plasma confinato in una configurazione Reversed Field Pinch’, Tesi di dottorato in Fisica, Università degli studi di Padova, Anno Accademico 1994/1995; -V. Antoni, et al., Nuclear Fusion, 36 (1996) 435; -P.C. Stangeby, G.M. McCracken, Nucl. Fusion, 30 (1990) S. Jachmich, et al.,’Influence of plasma flow on the floating potential and an ensuing novel technique for measuring parallel flows’, (27th EPS Conference on Contr. Fusion and Plasma Phys. Budapest), ECA vol. 24B (2000) 832 -I.H. Hutchinson, ‘A fluid theory of ion collection by probes in strong magnetic fields with plasma flow’, Phys. Fluids 30 (1987) C.S. MacLatchy, et al., ‘Gundestrup: a Langmuir/Mach probe for measuring flows in the scrape off layer of TdeV’, Rev. Sci. Instrum. 63 (1992), M. Hudis, L.M. Lidsky, ‘Directional Langmuir probe’, J. Appl. Phys. 41 (1970) M. Zuin, et al., ‘Fast dynamics of relaxation events in RFX-mod’, (2006) -O.D.Gurcan et al.,’Intrinsic rotation and electric field shear’,Physics of Plasmas,14 (2007)