n.vianello 09 october 2008 13t h iea/rfp workshop current filaments and electrostatic structures...

N.Vianello 09 October 2008 13th 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


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


Experimental equipment: U-probe

Two 2D arrays of electrostatic pins: 5 (toroidal) x 8 (radial)Δr=6mm e Δφ=6mm; pin =3 mm

Two arrays of 7 (Br,B,B) magnetic probes:size: 7mm X 6mm X 8mmΔr=6mm

5 MHz sampling rate. High frequency bandwidth

50 mm

rBθ

probeDischarge parameters:Ip~350kA; ne~2 1019m-3

r

B

An insertable edge probe for local measurements of both ELECTROSTATIC and MAGNETIC quantities has been used: high spatial and temporal resolution


Experimental setup: GPI

R.Cavazzana et al. Rev.Sci.Instrum 75 4152 (2004)

M.Agostini et al., Rev.Sci.Instrum 77 10E513 (2006)

Gas Puff Imaging diagnostics: non intrusive diagnostic

Measures the visible light emission (neutral He)

βαee TnI ∝ αβ ≤

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


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)


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


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/LT=5-10


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

VExB

Conditional average on the toroidal array of 16 chords of GPI


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


Vorticity measurements

Conditional averaging of bursts on a Vf(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 Vf

Structures can be considered “frozen” while passing the measurement point and the Taylor’s hypothesis applies


Pressure perturbation and vorticity

• These structures correspond to pressure perturbation

• A direct measurement of vorticity

Shows the vortex like nature of these structures


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


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)


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/nG

• The behavior of versus n/nG shows a slight decrease at a fixed time scale


Electromagnetic nature of the structures

Electrostatic and magnetic fluctuations coherent within the range 50-300 kHz

Cross correlation between GPI and local Br fluctuationsStrong 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.Möller on T2)


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) 195002J. R. Myra PPCF 14 (2007) 102314


Magnetic field hodogramStructures 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

Ip = 0.3 MA

Ip = 1.5 MA


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


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


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


Drift-Nature


Electrostatic feature scaling


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


M.Spolaore et al., 18th Conference on Plasma Surface Interactions, 26-30 May 2008, Toledo, Spain

Cross correlation between GPI signal and time derivative of the local Br 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

n.vianello 09 october 2008 13t h iea/rfp workshop current filaments and electrostatic structures...

Documents

edge features

edge profiles

electrostatic structures

edge region of rfps

electrostatic fluctuations

vianello et

insertable edge probe

edge martines nf