Stabilization of the resistive wall mode using a fake rotating shell

The goal of an advanced tokamak design is to simultaneously maximize the fusion reactivity (i.e., the plasma β), the plasma energy confinement, and the non-inductive bootstrap current. The eventual aim is, of course, to achieve a steady-state fusion reactor in which all of the toroidal current is maintained in the plasma by non-inductive means. The β limits in advanced tokamak designs are invariably set by low mode-number external-kink modes. The full potential of such designs is only realized when the stabilizing effect of a close-fitting conducting shell is incorporated into the MHD stability calculations. This is only reasonable provided that the so-called open-quotes resistive wall modeclose quotes is stabilized. Stabilization of this mode has been achieved in present-day tokamaks by spinning the plasma rapidly. Unfortunately, such a scheme is not reactor relevant. A more promising approach is to apply external feedback in order to make a resistive shell placed around the plasma act like a perfect conductor. A scheme is outlined by which a network of feedback controlled conductors surrounding the plasma can be made to act like a rotating shell, as far as its interaction with the plasma is concerned. This open-quotes fakeclose quotes rotating shell, when combined with a stationary conventional shell (e.g., the vacuum vessel), can completely stabilize the resistive wall mode. The gain, bandwidth, current, and power requirements of the feedback amplifiers axe extremely modest. A previously proposed stabilization scheme (the intelligent shell) is also investigated, and is compared with the fake rotating shell concept. The main disadvantage of the former scheme is that it requires extremely high gain amplification