Laure Vermare, IRW9, Lisbon 3-6th May 1 Turbulence measurements during dimensionless parameters...

Laure Vermare, IRW9, Lisbon 3-6th May 1

Turbulence measurements during dimensionless parameters scans

on Tore Supra using Doppler reflectometry

L. Vermare, P. Hennequin, O. Gurcan, C. HonoréLaboratoire de Physique des Plasmas

Ecole Polytechnique, 91 128 Palaiseau, France

C. Bourdelle, T. Aniel, F. Clairet, R. Dumont, C. Fenzi, R.Sabot

and the Tore Supra teamAssociation Euratom-CEA,DSM/DRFC, Centre de Cadarache, France

TORE SUPRA


Outline

1. Doppler System on Tore Supra & fluctuations measurements

2. Dimensionless scaling experiments

k-spectrum shape in the medium k range : effect of * and * / comparison with theoretical model

Perpendicular velocity profiles during such scans

3. High k spectrum in presence of electron heating

Ultra-brief presentation of a new reflectometry project on Tore Supra


Doppler system on Tore Supra

Tiltable motorized antenna- Gaussian optics(beam waist 40mm)

- Low diverging beam (2.2° HPBW)

- Poloidal angle 0 to 20° (typically 3° to 9°)

=>k : 3 to 25 cm-1 =>ki ~0.5 to 5

O mode / V-bandr/a = 0.3 to 0.9

New channel :X mode / W-band r/a = 0.5 to 1.2

€

E0ei(ki .r−ωt )

Cut-off layer : N=0incident wave

kd = ki + kbackscattered kd=-ki => k=-2ki

ki

kd k

Selection in wavenumber :Bragg rule

),(~),(~)( 3. tknrdetrntE rki

V

d ≡∝ −∫Measurement of the backscattered electric field Ed (t) :


Density fluctuations measurements

Tore Supra => long pulse discharges : stationary phases long enough to scan the poloïdal angle =>k-spectrum

Steps of the probing frequency (∼200ms)


Scattering position and wave-number from ray tracing

Beam Tracing using density profiles from fast sweep reflectometry

(F, ) (r,k) from ray tracing

S(k)

±0.05


Frequency spectra

€

T(τ )∝ −k 2u2τ L2 τ

τ L−1+

−τ

τ Le ⎡

⎣ ⎢

⎤

⎦ ⎥

e

Best fitting function : Taylor function

2),(~),( ωω knkS ∝from the fit =>

Doppler shift => ∆ω ~ kv


Tore Supra experiments :* and * scans in L mode plasmas


Dimensionless scaling experiments in Tore Supra

Similarity principle =>

Scan en *

€

* ∝ B−2 / 3

€

T ∝ B2 / 3

€

n ∝ B4 / 3

Difficult to vary * in a wide range in the same machine

The density changes during the scan -> reflectometry accessibility also changes

Scan en *

€

T ∝ B2

€

* ∝ B−4

n constant

Motivations of this new scan -> reach lowest collisionnality regime

Significative variation in * achievable (limitation from the heating power)

The density remains constant during the scan -> reflectometry accessibility also

€

β ∝ nT /B2

€

*∝ n /T 2

€

*∝ T /B

Dimensionless profiles well matched

x 1.2

x 4


k-spectrum


Transition in the k-spectrum at high k : an universal feature?

Usual power law : S(k) ~ k-3 0.5 for ks < 1

Faster decrease at higher k

=>Observed with various diagnostics & conditions on Tore Supra + others machines[Gusakov EPS06, Basse PPCF05]

Laser scattering [Hennequin et al. PPCF 2004]Doppler [Hennequin et al, NF 2006]

=> Compared against gyrokinetic simulations :good agreement[A. Casati PRL2009]

[A. Casati et al. PRL2009] Various theories / models-2D inverse cascade (Kolmogorov Kraichnan)

-Finite Larmor effects(Hasegawa-Mima, Horton)

More recently :

Interaction with disparate scales: drift waves – ZF [O. Gürcan, s. PRL]


Shape of k-spectrum during * scan

Low *, high * High *, high *

-Similar shape between low and high *-Fair agremment with the shell model except at low and high k


Shape of k-spectrum during * scan

High *, high * high *, low *

Fair agreement for all cases with model taking into accountinteractions between disparate scales (drift-waves-zonal flows)


Perpendicular velocityprofiles


Perpendicular velocity profile

In Tore Supra, V profiles measured using Doppler are always counter-current direction (V <0), inside the separatrix

Hollow profiles => V decreases from the edge to the core

Perpendicular velocity in the laboratory frame:

with usually

=> direct access to the radial electric field

(see work by E. Trier NF08 )

€

V⊥ =VE×B ,⊥+ ω /k

fluc

€

ω /kfluc

<<VE×B ,⊥

O-mode

X-mode


Perpendicular velocity profile during * scan

High *, high *Low *, high *

V <0 ! Counter-current !


Comparison @ k=8cm-1

The velocity does not change significantlyduring the * scan

low *

high *


Perpendicular velocity profile during * scan

High *, high * High *, low *


Comparision @ k=10cm-1

The velocity increases with deacreasing *

low *

high *


High k-spectrum during electron heating


High k-spectrum with electron heating 2nd harmonic ICRH, B=2T electron heating

Observation 2nd Doppler componant at high k, v<0(electronic diamagnetic) => only for r/a > 0.7

Similar to the observation in FT-2 [Gusakov et al, IAEA 2008]

main component

2nd component


Which modes are instable ?

still to be confirmed withspecific experiments andtransport analysis …

2nd harmonic ICRH, B=2T, electron heating changesparameters governing TEM/ETG instabilities

R/LT compared to ETG theshold (Jenko PRL01) using

kinezero : unstable in the outer region=> observed by Doppler reflectometry for r/a > 0.7

fETG ~ 500 kHz (fmes 800kHz)

max at ke~0.7 => k s~50


Summary

Robust features observed in k-spectrum: departure from a classical k-spectrum power law with fast decrease for ki>1

Fair agreement with model taking into account interactions between disparate scales (drift-waves-zonal flows)

Perpendicular velocity increases with decreasing *

Additional component at higher frequency observed in some cases: related to ETG ?


New reflectometry project on Tore Supra


Vertical Doppler reflectometer

Main motivations

Role of turbulence driven flows and long-range correlations

Characterisation of long-range correlations of density fluctuation and of

their velocity in L mode plasmas using correlation between equatorialand vertical reflectometers (toroidal + poloidal)

Access to GAMs=> Accessibility in vertical line of sight (poloidal structure m=1)Has been observed on density fluctuations and velocity at the plasma

edge(L-mode) on AUG [Doppler measurements. conway]

Supported by EFDA

One channel identical to the equatorial Doppler system (0-mode, V-band)

The project has started and is planned to be ready for the end of this year …


The projet has started …

One channel identical to the equatorial Doppler system

O-mode polarisation, V-band Tiltable antenna with Gaussian optics

Same cut-off (O-mode) but poloidal angle will depend on the Shafranov shift :

r/a = 0.872k = 8.78 cm-1

z(cm) vertical

X(cm) radial

O-modeF=60GHz=-88°

DREVE

X(cm) radial

r/a = 0.872k = 8.55 cm-1

z(cm) vertical

O-modeF=60GHz=6.5°

DIFDOP


* with Te


ETG thershold

( )⎭⎬⎫

⎩⎨⎧

−⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

ni

eeffCriticTe L

R

q

s

T

TZLR 8.0,5.1191.133.11max/ ε


k-spectrum characterizes non-linear interaction between modes

• 3D Fluid Turbulence: – Kolmogorov direct energy cascade

Kinetic Energy E(k)=<vk2>

~k-5/3

• Magnetized plasma turbulence:– strong anisotropy induced by large B

2D. – 1 field isotropic 2D simplified

picture – inertial range

Log( k )

Log[ E (k) ]

Energy

Enstrophy

k

k

-5/3

-3

• 2D Fluid turbulence: – Inverse energy cascade leads to

the formation of large scale structures.

Laure Vermare, IRW9, Lisbon 3-6th May 1 Turbulence measurements during dimensionless parameters...

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