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

Centre de Recherches en Physique des PlasmasEPFL, Association Euratom-Fédération Suisse, Lausanne,

Switzerland

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

Geodesic acoustic modes:simultaneous observation of density,

magnetic-field, and flow components

in the TCV tokamakS. 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

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

1LPP, CNRS-Ecole Polytechnique, Palaiseau, France 2Forschungszentrum Jülich, Germany

S. Coda, U.S.-E.U. Joint Transport Task Force Workshop, Santa Rosa, CA, 9-12 April 2013GAMs in TCV2

• Geodesic acoustic modes• Multi-diagnostic measurements of GAMs in

TCV• Modeling of GAMs in TCV• Summary and outlook

Outline


• Electric potential perturbations: symmetric over flux surface (m=n=0), low-frequency (w0)

• Nonlinearly generated by broadband drift-wave turbulence

• Associated poloidal, sheared (kr≠0) EB flows break apart turbulent eddies and effectively regulate turbulence self-organization

Zonal flows


• Finite-frequency (wcs/R) zonal-flow component

• n=0, m=1 standing-wave density fluctuation• n=0, m=2 standing-wave magnetic

component (recent prediction, Wahlberg 2009)

Geodesic acoustic modes

recent proposal to excite GAM with external magnetic perturbation (Hallatschek 2012)


• Flow and density components observed on several devices(Doppler backscattering, reflectometry, beam emission spectroscopy, heavy ion beam probe)

Geodesic acoustic modes




Outline


• Density: tangential phase contrast imaging• Magnetic field: Mirnov coils• Flow: Doppler backscattering• Radiative temperature: correlation ECE

GAMs in TCV• Unique, correlated multi-diagnostic

observation• First sighting of magnetic-field component for

turbulence-driven GAM• Axisymmetry unambiguously determined


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)


TCVR = 0.88 m, a =

0.25 mIp < 1 MA, BT < 1.54

Tk < 2.8, -0.6 < d <

0.9

×4

×2

4.5 MW ECRH power, 7 steerable launchers


• Established technique for measuring line-integrated density fluctuations

• Tangential geometry + spatial filtering adds spatial resolution

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



Current specs:

1 cm-1 < k < 9 cm-1

line-integratedmeasurement only

1.5 MHz bandwidth



• k is radial TPCI signal comes from tangency point

• scan r by moving plasma vertically

TPCI provides GAM’sspatial distribution and radial

wavelength


TPCI provides GAM’sspatial distribution and radial

wavelength


22-40 kHz

peaks near edge

kr 1.7-2.1 cm-1 (mainly

outward)krs 0.4-0.5


Magnetic component of the GAM


Bq (r,q,t) q2b sin(2q) sin(krr-wt)

Theory: magnetic component of the GAM

• 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 n=0


At GAM node location,residual signal dominated by n=1

toroidal mode number n


Magnetic component of GAM has m=2



antinodes and LFS phasing consistent with sin(2q)



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


GAM scales with sound speed


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)


Doppler backscattering on TCV


Oscillating EB 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 ECESix-channel tunable X2 system, LFS detection


Strong correlation between TPCI & ECE


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

• 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




Outline


• 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)

Gyrokinetic modeling


Good, semi-quantitative agreement between experiment and modeling



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



kr 2.3 cm-

1

coherent over several wavelengths

f 33 kHz

peak amplitude 3 km/s rms

peaks at outermost properly simulated radius (r=0.85)




Outline


• 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

Summary


• 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)

Outlook

Documents

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