Effects of Three-Dimensional Magnetic Field Structure on MHD Equilibrium and Stability

The stellarator and heliotron are alternate candidates for magnetically confined fusion devices. A major difference is the source of the rotational transform [iota] = 1/q. In tokamaks, the rotational transform [iota] is produced by coupling the symmetric toroidal field and the poloidal field produced by the plasma current along the toroidal direction. Strictly speaking, the tokamak configuration can be assumed to be a two-dimensional (2-D) system. Note that the rotational transform does not exist for the vacuum. For stellarator and heliotron configurations, the rotational transform is produced by the shaping of flux surfaces. To shape flux surfaces, the vacuum magnetic field is produced by external coils with helical-winding laws. This means the vacuum magnetic field produced for the vacuum is intrinsically three dimensional (3-D). Thus, the plasma current is not required to make flux surfaces. This characteristic is an advantage. Since the plasma current is not necessary, disruptions do not appear and steady-state operation is possible. However, because of the 3-D plasma responses, experimental and theoretical studies become more complex.