Amplitude Modulation And Nonlinear Self-Interactions of the Geodesic Acoustic Mode at the Edge of MAST

Bogdan Hnat,Nicholas Walkden

doi:10.3390/plasma2020013

Bogdan Hnat, Nicholas Walkden

Open Access

https://doi.org/10.3390/plasma2020013

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Abstract

We studied the amplitude modulation of the radial electric field constructed from the Langmuir probe plasma potential measurements at the edge of the mega-ampere spherical tokamak (MAST). The Empirical Mode Decomposition (EMD) technique was applied, which allowed us to extract fluctuations on temporal scales of plasma turbulence, the Geodesic Acoustic Mode (GAM), and those associated with the residual poloidal flows. This decomposition preserved the nonlinear character of the signal. Hilbert transform (HT) was then used to obtain the amplitude modulation envelope of fluctuations associated with turbulence and with the GAM. We found significant spectral coherence at frequencies between 1–5 kHz, in the turbulence and the GAM envelopes and for the signal representing the low frequency zonal flows (LFZFs). We present the evidence of local and nonlocal, in frequency space, three wave interactions leading to coupling between the GAM and the low frequency (LF) part of the spectrum.

Highlights

The edge region of tokamaks, defined by the steep pressure gradient, is dominated by turbulent structures of density, temperature, and the electrostatic potential arising from resistive and/or interchange plasma instabilities
We found that the amplitude modulation of the turbulence and the Geodesic Acoustic Mode (GAM) had a similar behavior at low frequency (LF), between 2–5 kHz
A strong oscillatory component has been previously identified in mega-ampere spherical tokamak (MAST) edge plasma density and electrostatic potential fluctuations measured by the reciprocating Langmuir probes [15,20]

Summary

Introduction

The edge region of tokamaks, defined by the steep pressure gradient, is dominated by turbulent structures of density, temperature, and the electrostatic potential (see, for example, Reference [1] and references therein) arising from resistive and/or interchange plasma instabilities. These fluctuations are responsible for the turbulent radial transport, which drives anomalous heat and particle losses and has a detrimental impact on the global energy confinement. The anisotropic shear amplification of micro-turbulence Reynolds stress produces radially localised, toroidally and poloidaly symmetric flows, called zonal flows (ZFs) These flows are distinct from the residual poloidal flows, often called zero frequency ZFs [2,3]. Shearing associated with both types of ZFs can nonlinearly modify stability threshold of unstable plasma modes [4,5,6] and reduces turbulence level by vortex stretching [7,8,9]

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