Stabilization of MHD-instabilities in a current-carrying-stellarator

Profiles that are stable against MHD instabilities are given in a cylindrical current-carrying stellarator. The comparison theorem, i.e. the guiding principle for stabilization, is obtained in the same way as in a tokamak. As the external rotational transform due to an l=2 helical field increases, the MHD properties become better than those of a tokamak, and the minimum value of q(a) providing simultaneous stabilization of MHD modes can be reduced to a value lower than two, even without a conducting shell. In an l=3 stellarator, however, as is shown from the Euler equation, the configuration becomes more unstable than in a tokamak, and strong current profile tailoring is necessary to stabilize the modes. (author)