Design of heliac vacuum magnetic fields

The design of vacuum magnetic fields for helical axis stellarators (HELIACS) is investigated. The aim is to produce realistic configurations possessing magnetic wells (for stability), favourable transform with low shear (to avoid resonant rational surfaces), and small variation of Q= ∫ dℓ/B (to minimize parallel currents). An asymmetric toroidal domain whose boundary is assumed to be an outer magnetic surface is created by helically winding a poloidal cross-section about a circle. The magnetic scalar potential inside the domain is computed numerically and field-line integrations are performed in order to evaluate the field properties. The magnetic field is generated by skin currents flowing along the equipotential contours on the boundary. These currents are then discretized into a modular coil system, whose resulting magnetic field is compared with that of the corresponding continuum. Skin current configurations having a shallow well and favourable beta limitations and Q-variation are presented. Discretization of the skin current usually results in a magnetic field which preserves the transform and Q-variation, but with less favourable well properties.