Abstract
The mean tilt angle of turbulent structures is a key element for describing the turbulence and its interplay with plasma flows in magnetically confined plasmas. It is a quantity predicted by theories and gyrokinetic simulations, which can provide information on the type of the dominant micro-instability, and also on the turbulence anisotropy induced by sheared flows. A new method for measuring the tilt angle of turbulent structures using Doppler reflectometry has been recently introduced (Pinzón et al 2019 Nucl. Fusion 59 074002). It is based on the time delay of the cross-correlation between microwaves backscattered at radially displaced positions. In this paper, the method is presented in detail and is successfully applied on the ASDEX Upgrade tokamak and the TJ-II stellarator. Measurements of the tilt angle in the core of both machines are reported, in the TJ-II case, for the first time.
Highlights
Turbulence contributes significantly to the transport of energy and particles in magnetically confined plasmas, limiting the energy confinement time and the plasma performance in experiments
A new method for measuring the tilt angle of turbulent structures was introduced [28]. It is based on Doppler reflectometry and is non-invasive and applicable in the core region of fusion plasmas, where it was used to demonstrate the effect of sheared E × B flows in different turbulence regimes
Frozen turbulence has been assumed in the tilt angle measurement method. This is a valid assumption if the turbulent structures do not change appreciably while they propagate through the scattering region where the Doppler reflectometry measurements are performed
Summary
Turbulence contributes significantly to the transport of energy and particles in magnetically confined plasmas, limiting the energy confinement time and the plasma performance in experiments. A new method for measuring the tilt angle of turbulent structures was introduced [28] It is based on Doppler reflectometry and is non-invasive and applicable in the core region of fusion plasmas, where it was used to demonstrate the effect of sheared E × B flows in different turbulence regimes. It uses an obliquely injected microwave beam which propagates in the plasma and is reflected at the so-called cutoff layer, where the electric field of the microwave is maximum and backscattering at density fluctuations is strong This provides the diagnostic with a good spatial localization of measurements at perpendicular wavenumber k⊥ fulfilling the Bragg condition, k^ = -2ki where ki is the wavenumber of the probing wave at the cutoff.
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