Experimental investigation of geodesic acoustic mode spatial structure, intermittency, and interaction with turbulence in the DIII-D tokamak

Hillesheim, J. C.; Peebles, W. A.; Carter, T. A.; Schmitz, L.; Rhodes, T. L.

doi:doi:10.1063/1.3678210

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Nonlinear theory of mode conversion at plasma frequency

An electromagnetic (EM) wave at the mode conversion layer transfers its energy to the electrostatic (ES) wave whose amplitude can be greatly enhanced due to its slow group velocity near the cut off layer. The large amplitude ES wave produces the density compression, indispensable for the nonlinear coupling in the EM wave, leading to the localized vortex flow. A dc magnetic field, tantamount to the vorticity, is generated as a result of the conservation of canonical momentum.

Simulation studies of positron acceleration in a shock wave in a nonuniform external magnetic field

Positron acceleration in a shock wave in an electron-positron-ion plasma is studied with one-dimensional, fully kinetic, electromagnetic particle simulations, with particular attention paid to the effect of inhomogeneity of external magnetic field B0. First, acceleration to γ ∼ 104, where γ is the Lorentz factor, is demonstrated for a shock wave in a uniform B0 with the shock speed νsh close to c cos θ, where c is the speed of light and θ is the angle between B0 and the wave normal. The acceler...

Geodesic acoustic modes (GAMs) and zonal flows are nonlinearly driven, axisymmetric (m = 0andn = 0) E×B flows, which are thought to play an important role in establishing the saturated level of turbulence in tokamaks. Results are presented showing the GAM’s observed spatial scales, temporal scales, and nonlinear interaction characteristics, which may have implications for the assumptions underpinning turbulence models towards the tokamak edge (r/a>rsim0.75). Measurements in the DIII-D tokamak [Luxon, Nucl. Fusion 42, 614 (2002)] have been made with multichannel Doppler backscattering systems at toroidal locations separated by 180^∘; analysis reveals that the GAM is highly coherent between the toroidally separated systems (γ>0.8) and that measurements are consistent with the expected m = 0andn = 0 structure. Observations show that the GAM in L-mode plasmas with ~2.5-4.5 MW auxiliary heating occurs as a radially coherent eigenmode, rather than as a continuum of frequencies as occurs in lower temperature discharges; this is consistent with theoretical expectations when finite ion Larmor radius effects are included. The intermittency of the GAM has been quantified, revealing that its autocorrelation time is fairly short, ranging from about 4 to about 15 GAM periods in cases examined, a difference that is accompanied by a modification to the probability distribution function of the E×B velocity at the GAM frequency. Conditionally-averaged bispectral analysis shows the strength of the nonlinear interaction of the GAM with broadband turbulence can vary with the magnitude of the GAM. Data also indicate a wavenumber dependence to the GAM’s interaction with turbulence.

Article Outline

INTRODUCTION
GEODESIC ACOUSTIC MODE BACKGROUND
DIAGNOSTIC ARRANGEMENT
GAM IDENTIFICATION AND EXPERIMENTAL CONDITIONS
MEASUREMENTS OF GAM SPATIAL STRUCTURE AND RADIAL PROPAGATION
1. GAM damping
2. GAM frequency versus radius
3. GAM magnitude and E×B shear
QUANTIFICATION OF GAM INTERMITTENCY
BISPECTRAL ANALYSIS OF THE GAM
DISCUSSION AND SUMMARY

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KEYWORDS, PACS, and IPC

Keywords

differential geometry, Doppler broadening, plasma boundary layers, plasma diagnostics, plasma heating, plasma magnetohydrodynamics, plasma nonlinear processes, plasma toroidal confinement, plasma turbulence, Tokamak devices

PACS

52.55.Fa

Tokamaks, spherical tokamaks
52.35.Ra

Plasma turbulence
52.35.Mw

Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.30.Cv

Magnetohydrodynamics (including electron magnetohydrodynamics)
52.40.Hf

Plasma-material interactions; boundary layer effects
52.25.Os

Emission, absorption, and scattering of electromagnetic radiation

International Patent Classification (IPC)

H05H1/02
Arrangements for confining plasma by electric or magnetic fields; Arrangements for heating plasma

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3678210

PUBLICATION DATA

ISSN

1070-664X (print)

1089-7674 (online)

Publisher

$abstract.society.value is a Member of CrossRef

American Institute of Physics

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