In a fusion device such as a tokamak, it is well known that an unstable external kink mode, driven either by the plasma current or pressure, produces an external magnetic field perturbation, which induces stabilizing image currents in the surrounding conducting structures, such as the vacuum vessels made of low resistivity materials. The image currents, flowing in the conducting walls, can effectively reduce the growth rate of the external kink mode, from the Alfvénic time scale microseconds to the characteristic current decay time of the wall, which is typically a few milliseconds in present tokamaks, and a fraction of a second in ITER.
