Reactors with Stellarator Stability and Tokamak Transport

The discovery of quasiaxially symmetric stellarators whose magnetic spectrum has approximate two-dimensional symmetry opens up the possibility of designing fusion reactors that have tokamak transport and stellarator stability. Prototypes with two or three field periods have asymmetries almost as small as the coefficients for a typical tokamak that are associated with ripple from the toroidal coils or helical excursion of the magnetic axis resulting from instability. We have found modular coils that are only moderately twisted and produce robust flux surfaces that do not deteriorate when changes are made in the magnetic field. This work is bolstered by recent stellarator experiments that have exceeded stability limits predicted by linear theory. The problem may be that force balance and stability are lost across islands if the equilibrium equations are not in conservation form.