Turbulence and Transport in Simple Magnetized Toroidal Plasmas

Full text: Progress in understanding turbulence and related cross-field transport of fusion relevance is achieved in the simple magnetized plasmas of the TORPEX toroidal device, which provides a test bed for code benchmarking and theory validation. In TORPEX (R = 1 m, a = 0.2 m), a small vertical magnetic field (Bz < 4 mT) is superposed on a toroidal magnetic field Bt < 100 mT to form open helicoidal magnetic field lines, which, similarly to Scrape-Off Layers of fusion devices, feature grad-B and magnetic field curvature. Plasmas of different gases are produced by microwaves (2.45 GHz, P < 10 kW) with typical electron temperatures ∼ 5 - 15 eV and densities ranging from ∼ 3 x 1016 m-3 for hydrogen to ∼ 3 x 1017 m-3 for neon. For large Bz amplitudes, the ideal interchange instability is observed and characterized. The existence of a critical pressure gradient needed to drive the ideal interchange instability is experimentally demonstrated using a scan in the neutral gas pressure. The dynamics of density blobs born from ideal interchange waves is studied using a novel spatiotemporal pattern recognition method, allowing the determination of statistical observables. We investigate the scaling of the cross-field blob velocity with blob size, connection length and neutral gas pressure for different gases. An analytical expression for the blob velocity including cross-field ion polarization currents, parallel currents to the sheath, and ion-neutral collisions is derived and shows good quantitative agreement with the experimental data. We report on the link between toroidal flows and blobs: toroidal momentum is transferred from an ideal-interchange mode to density blobs. Depending on the phase between the toroidal flow and the density in the mode region, blob-induced flows are either dipolar structures or monopolar perturbations, so large that the toroidal flow gets transiently reversed. The turbulent toroidal momentum flux is dominated either by the nonlinear flux or by the convective part but not by the Reynolds stress component. We present also 3D global fluid nonlinear simulations, which reveal the existence of a resistive interchange instability at low Bz while the drift wave regime is almost inaccessible at realistic collisionality. The experimental validation of these predictions is under way. (author)