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Control of MHD instabilities in plasmas and liquid metals
Francesco Volpe
Columbia University, New York, USA
Fri, Mar. 24th 2017, 14:00-15:30
Salle René GRAVIER 506 rdc, CEA Cadarache

            Neoclassical Tearing Modes large enough to cause disruptions are also large enough to decelerate under the effect of the wall torque, and eventually lock to a residual error field. These locked modes with rotating precursors are, in fact, one of the most prominent causes of disruptions in present tokamaks, and a concern for ITER. With this motivation in mind, results obtained at the DIII-D tokamak will be presented in the areas of locked mode prediction (based on the analysis of a database of 22,500 discharges), avoidance (by pre-emptively applying rotating magnetic fields) and control (by a combination of static or rotating fields and continuous or modulated Electron Cyclotron Current Drive). In addition, possible mechanisms will be discussed, by which locked modes could initiate a thermal quench and, eventually, a disruption. The mode dynamics is simulated by simply solving the equation of motion for the island, subject to various electromagnetic torques. Good agreement with DIII-D and other experiments gives confidence in the model and in using it for predictions of mode locking and mode entrainment in ITER, with emphasis on the control-coil currents and frequencies needed for phase control. The aforementioned magnetic perturbations are exerted by arrays of control-coils, either in feedforward or in feedback with magnetic measurements.

            Related to that, it will be shown that Rayleigh-Taylor and Kelvin-Helmholtz instabilities occurring in liquid metals, as well as other non-uniformities (due for example to error fields) can be controlled by jxB forces exerted by an array of electrodes. Experimental results obtained at Columbia University in the absence of plasma will be presented, in which the thickness of free-surface liquid metal flows is resistively measured, and jxB forces are applied in feedforward. Preparatory work will also be presented about “closing the loop” between these sensors and actuators in the presence of plasma. This feedback stabilization technique could prevent that, in a fusion reactor, liquid metal walls “bulge” and enter in contact with the plasma, or deplete and expose the underlying solid walls.

Contact : SG165549

 

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