Active feedback stabilization of toroidal external modes in tokamaks

Active feedback stabilization of pressure-driven modes in tokamaks is investigated by toroidal computations. Typically, the feedback does not strongly modify the plasma-generated magnetic field perturbation. Feedback with modest gain and a single coil array poloidally stabilizes substantially for a range of coil shapes. Optimum design uses narrow sensor coils not too far from the plasma and rather wide feedback coils, which may be outside the resistive wall. Complex gain, which makes the mode rotate, can decrease the gain required for stabilization, but real gain is more robust.