Images acquired by cameras installed in tokamaks are difficult to interpret because the three-dimensional structure of the plasma is flattened in a non-trivial way. Nevertheless, taking advantage of the slow variation of the fluctuations along magnetic field lines, the optical transformation may be approximated by a generalized Abel transform, for which we propose an inversion technique based on the wavelet-vaguelette decomposition. After validation of the new method using an academic test case and numerical data obtained with the Tokam 2D code, we present an application to an experimental movie obtained in the tokamak Tore Supra. A comparison with a classical regularization technique for ill-posed inverse problems, the singular value decomposition, allows us to assess the efficiency. The superiority of the wavelet-vaguelette technique is reflected in preserving local features, such as blobs and fronts, in the denoised emissivity map.
We describe experiments with a biased electrode inserted into the scrape-off layer (SOL) of the CASTOR tokamak. The resulting radial and poloidal electric field and plasma density modification are measured by means of Langmuir probe arrays with high temporal and spatial resolutions. Poloidally and radially localized stationary structures of the electric field (convective cells) are identified and a related significant modification of the particle transport in the SOL is observed.
Nonlinear drift-waves are experimentally studied in a cylindrical magnetized laboratory plasma. Low-dimensional chaotic regimes are controled using the time-delay autosynchronization method [K. Pyragas, Phys. Lett. A 170, 421 (1992)]. A significant reduction of spectral width in the power spectrum is obtained.
This paper describes recent experimental investigations of the nonlinear dynamics of collisional current-driven drift waves in a linear low-β discharge. It is shown that the bias of an injection grid leads to rigid-body rotation of the cylindrical plasma column that strongly destabilizes the drift waves, thus providing a control parameter for the drift-wave dynamics. In the nonlinear regime, when the control parameter is increased, the transition scenario from stability to weakly developed turbulence is studied. Two successive Hopf bifurcations, a modelocked state and its gradual destabilization to chaos and finally turbulence follow the classical Ruelle-Takens transition scenario known from neutral fluids. In addition to the temporal dynamics, the spatiotemporal evolution of drift waves is studied by means of circular Langmuir probe arrays with high spatial and temporal resolution. With each Hopf bifurcation, a drift-mode onset is associated and the bifurcation from quasi-periodicity to mode locking corresponds to the transition from non-resonant to resonant mode interaction. The mode-locked state forms a persistent spatiotemporal pattern that is destabilized by the occurrence of defects. In contrast, the turbulent state is a fully disordered, intermittent state.
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