turbulence characterization in the outer region of fusion plasmas: flow measurements in the edge...

Turbulence characterization in the outer region of fusion plasmas:

Flow measurements in the edge region of the RFX-mod experiment

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Centro Ricerche Fusione Consorzio RFXUniversità degli studi di Padova

International Doctorate in Fusion Science and Engineering

Gianluca De Masi

Outline:Outline:

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Experimental setupExperimental setup

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Gundestrup (=217° 30’)

U-probe (=247° 30’)

r (mm)

0 a = 459

z

Plasma radius= 0.459 m

Major radius = 2 m

Plasma current= 2 MA

1

8

2

76

5

43

B≈Bθ

r

φ

δ

Gundestrup probeGundestrup probe

protruding electrode flush electrode

23

mm

graphite electrodes

boron nitride case 4

0 m

m

Electrode n°Electrode n° δ δ angle w.r.t. equatorial plane 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°

8 337,5°

The 8 external electrodes radius is about 2 mm (comparable with ion Larmor radius) with 4.8 mm2 area

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U-probeU-probe

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•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 (Δrmin=6 mm, Δφ=95 mm )

Interpretation Models (1)Interpretation Models (1)

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MacLatchy et al (1992)

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Main plasma parameters in several experimental campaigns considered (insertion probe range [409,479] mm)

Plasma current < 400 kA

Electron density (1.5-2.8) x 1019 m-3

Electron temperature(that we assume equal to Ti)

30 eV (at the deepest radial insertion, r = 409 mm)

Ion Larmor radius 4 mm (at r = 409 mm)

Ion thermal velocity 5.4 x 104 m/s (at r = 409 mm)

Ion sound velocity 7.6 x 104 m/s (at r = 409 mm)

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Flow measurements results (1)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= -0.05

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Flow measurements results (2)Flow measurements results (2)Radial Profiles

In both cases the profiles are comparableThe parallel drift velocity inside the plasma reaches values of 0.5 cS (corresponding to (3-4) x 104 m/s)

The perpendicular drift velocity is of the order of 0.1 cS (corresponding to 3 x 103 m/s) in the tile shadow, and it reaches values of –0.2 cS (corrisponding to 1.5 x 104 m/s) inside

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Ion saturation current configuration

Flow measurements results (3)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= -0.2

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1

8

2

76

5

43

Electron flux effect

B


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 cS (corresponding to 3 x 104 m/s) inside the plasma

Perpendicular drift velocity is of the order of 0.3 cS (corresponding to (0.8-0.9) x 104 m/s) in tile shadow, up to -0.2 cS (corresponding to (1-2) x 104 m/s) inside the plasma

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Floating Potential configurationFloating Potential configuration Ion saturation current configuration

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Flow measurements results (5)Flow measurements results (5)Gundestrup and U-probe

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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


Reversal parameterReversal parameterF = Btor (a)/<Btor>

Greenwald densityGreenwald densitynG= IP / πa2

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Flow measurements results (7)Flow measurements results (7)Flow profiles comparison w.r.t. several plasma parameters Flow profiles comparison w.r.t. several plasma parameters

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Velocity radial profilesVelocity radial profiles

In the region of strong gradient, fluctuations coupling between flow contributions <vradM┴> and <vradM││> could be transfer momentum to the perpendicular drift

Work in progressWork in progress

Plasma dynamics fluctuations

F-Crashes:

Correlation between fast variations of reversal parameter F and drift velocity fluctuations?

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Plasma dynamics fluctuations (2)

Coherence and phase of parallel and perpendicular drift velocity fluctuations

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Parallel drift velocity transient due to pressure gradient?

Pellet injectionSome 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)

SummarySummary

-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

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ReferencesReferences-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) 1225

-S. Jachmich, et al.,’Influence of plasma flow on the floating potential and anensuing 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) 3777

-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), 3923

-M. Hudis, L.M. Lidsky, ‘Directional Langmuir probe’, J. Appl. Phys. 41 (1970) 5011

-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)

turbulence characterization in the outer region of fusion plasmas: flow measurements in the edge...

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