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S. Coda, U.S.-E.U. Joint Transport Task Force Workshop, Santa Rosa, CA, 9-12 April 2013 PowerPoint Presentation
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Geodesic acoustic modes: simultaneous observation of density, magnetic-field, and flow components in the TCV tokamak. S. Coda, C.A. de Meijere , Z. Huang, L. Vermare 1 , T. Vernay , V. Vuille , S. Brunner, J. Dominski , P. Hennequin 1 , A. Kr ä mer-Flecken 2 , G. Merlo, L. Porte.

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slide1

Geodesic acoustic modes:simultaneous observation of density,magnetic-field, and flow componentsin the TCV tokamak

S. Coda, C.A. de Meijere, Z. Huang, L. Vermare1, T. Vernay, V. Vuille, S. Brunner, J. Dominski,P. Hennequin 1, A. Krämer-Flecken2, G. Merlo, L. Porte

1LPP, CNRS-EcolePolytechnique, Palaiseau, France 2ForschungszentrumJülich, Germany

S. Coda, U.S.-E.U. Joint Transport Task Force Workshop, Santa Rosa, CA, 9-12 April 2013

outline
Outline
  • Geodesic acoustic modes
  • Multi-diagnostic measurements of GAMs in TCV
  • Modeling of GAMs in TCV
  • Summary and outlook
zonal flows
Zonal flows
  • Electric potential perturbations: symmetric over flux surface (m=n=0), low-frequency (w0)
  • Nonlinearly generated by broadband drift-wave turbulence
  • Associated poloidal, sheared (kr≠0) EB flows break apart turbulent eddies and effectively regulate turbulence  self-organization
geodesic acoustic modes
Geodesic acoustic modes
  • Finite-frequency (wcs/R) zonal-flow component
  • n=0, m=1 standing-wave density fluctuation
  • n=0, m=2 standing-wave magnetic component (recent prediction, Wahlberg 2009)
  •  recent proposal to excite GAM with external magnetic perturbation (Hallatschek 2012)
geodesic acoustic modes1
Geodesic acoustic modes
  • Flow and density components observed on several devices(Doppler backscattering, reflectometry, beam emission spectroscopy, heavy ion beam probe)
outline1
Outline
  • Geodesic acoustic modes
  • Multi-diagnostic measurements of GAMs in TCV
  • Modeling of GAMs in TCV
  • Summary and outlook
gams in tcv
GAMs in TCV
  • Unique, correlated multi-diagnostic observation
  • First sighting of magnetic-field component for turbulence-driven GAM
  • Axisymmetry unambiguously determined
  • Density: tangential phase contrast imaging
  • Magnetic field: Mirnov coils
  • Flow: Doppler backscattering
  • Radiative temperature: correlation ECE
gams in tcv1
GAMs in TCV
  • Initial study: L-mode, limited plasma with 1 MW central ECRH  magnetic analysis then extended to broad range of past shots (including Ohmic)
slide9

TCV

R = 0.88 m, a = 0.25 m

Ip < 1 MA, BT < 1.54 T

k < 2.8, -0.6 < d < 0.9

×4

×2

4.5 MW ECRH power, 7 steerable launchers

tangential phase contrast imaging tpci
Tangential phase contrast imaging (TPCI)
  • Established technique for measuring line-integrated density fluctuations
  • Tangential geometry + spatial filtering adds spatial resolution
tangential phase contrast imaging tpci1
Tangential phase contrast imaging (TPCI)

Ultimate specs:0.9 cm-1 < k < 60 cm-1 (0.2 < krs < 90)

spatial resolution down to 1% of minor radius

multi-MHz bandwidth

tangential phase contrast imaging tpci2
Tangential phase contrast imaging (TPCI)

Current specs:1 cm-1 < k < 9 cm-1

line-integratedmeasurement only

1.5 MHz bandwidth

tpci provides gam s spatial distribution and radial wavelength
TPCI provides GAM’sspatial distribution and radial wavelength
  • k is radial  TPCI signal comes from tangency point
  • scan r by moving plasma vertically
22 40 khz
22-40 kHz

peaks near edge

kr 1.7-2.1 cm-1 (mainly outward)krs 0.4-0.5

theory magnetic component of the gam
Theory: magnetic component of the GAM

Bq (r,q,t)  q2b sin(2q) sin(krr-wt)

  • short radial wavelength: faint signal outside plasma
  • nodes on LFS and HFS, so toroidal mode number should be measured away from equatorial plane
magnetic component of gam has m 21
Magnetic component of GAM has m=2

antinodes and LFS phasing consistent with sin(2q)

magnetic component of gam has m 22
Magnetic component of GAM has m=2

HFS phasing indicates presence of m>2 components (effect of shape?)

doppler backscattering on tcv
Doppler backscattering on TCV
  • Flow measurements performed with a 50-75 GHz tunable, heterodyne system on loan from LPP and Tore Supra
    • Collaboration with LPP (L. Vermare and P. Hennequin)1
  • Monostatic antenna = replica of ECRH launcher, can be oriented in real time

1L. Vermare et al, Nucl. Fusion 52, 063008 (2012)

oscillating e b gam poloidal flow is clearly seen in the edge region
Oscillating EB GAM poloidal flowis clearly seen in the edge region

GAM flow  0.7 km/s rms (background flow  2 km/s)

gam seen also by correlation ece
GAM seen also by correlation ECE

Six-channel tunable X2 system, LFS detection

gam on c ece vs tpci a few puzzles
GAM on C-ECE vs TPCI: a few puzzles
  • plasma is invariably optically thin (t<0.5): ECE measurement is unknown mix of ne and Te fluctuations
  • kr (TPCI)  1.7-2.1 cm-1, kr (C-ECE)  0.9 cm-1
  • predominantly outward-propagating on TPCI, propagation direction depends on location on C-ECE
global vs local gam
Global vs local GAM
  • All diagnostics on TCV see a single-frequency mode irrespective of location
  • Other devices have reported a single-frequency mode, several discrete modes, or a continuum over r
  • This variation in behavior is not well understood
outline2
Outline
  • Geodesic acoustic modes
  • Multi-diagnostic measurements of GAMs in TCV
  • Modeling of GAMs in TCV
  • Summary and outlook
gyrokinetic modeling
Gyrokinetic modeling
  • ORB5: global particle-in-cell ∂f code
  • Collisionless, electrostatic simulation using TCV experimental equilibrium and kinetic profiles: turbulence is dominated by trapped electron modes
  • Model breaks down for r > 0.85, so simulation restricted to inner region (fluctuation level artificially scaled down in edge)
good semi quantitative agreement between experiment and modeling1
Good, semi-quantitative agreement between experiment and modeling

Multiple discrete modes below a critical density gradient, single mode above (as in experiment)

good semi quantitative agreement between experiment and modeling2
Good, semi-quantitative agreement between experiment and modeling

kr 2.3 cm-1

f  33 kHz

coherent over several wavelengths

peak amplitude 3 km/s rms

peaks at outermost properly simulated radius (r=0.85)

outline3
Outline
  • Geodesic acoustic modes
  • Multi-diagnostic measurements of GAMs in TCV
  • Modeling of GAMs in TCV
  • Summary and outlook
summary
Summary
  • Initial study on TCV has revealed GAM in density, magnetic-field, and flow fields (plus ECE radiative temperature)
  • First multi-probe analysis of magnetic component has clearly confirmed axisymmetry
  • Frequency, radial wave number, poloidal and toroidal mode numbers, radial profile, direction of propagation have all been measured
  • Good agreement with gyrokinetic modeling
outlook
Outlook
  • Much more to come from the experiment: parametric studies (dependence on q profile, shape, collisionality, etc.), exploration of damping mechanism, etc.
  • Better diagnostics will be used: fully commissioned TPCI, C-ECE using movable antenna
  • Much more to come from modeling: synthetic diagnostics for TPCI and C-ECE, parametric studies, etc.
  • Further challenges to theory: e.g. m>2 magnetic GAM components (finite-b, toroidicity effects)