In an ultimate torsatron, magnetic surfaces are formed entirely from a single periodically modulated helical coil. Separatrix size and shape are determined by the winding poloidal multiplicity ℓ, the number of field periods and the coil aspect ratio. The coil modulation principally determines the magnetic axis positioning in the radial direction, but does not directly affect the surface shape. An optimized ℓ = 2 design demonstrates the desired magnetic properties of stable surfaces which are insensitive to winding perturbations, have high shear transform profiles and non-zero transform at the magnetic axis in addition to ample access to the plasma.
Flexibility of LHD configuration with multilayer helical coils