Vacuum field optimization and the effects of field errors on the flux surfaces of the H-1 Heliac

The H-1 heliac configuration has been optimized for low aspect ratio and minimum B ripple by modulation of the toroidal spacing of the toroidal field coils, and by optimization of the vertical field magnitude and index. Two numerical studies of symmetry breaking error configurations are presented, a displacement of the central conductor and a stray field modelled by a dipole. The rate of growth of island size with the error magnitude Δx is found to increase from the expected (Δx)1/2 scaling to Δx scaling within the range of errors to be expected for H-1. It is expected that the island size in the central 75% of the plasma volume can be limited to 10mm. Presented is a preliminary investigation of compensation of the toroidal coil position errors by adjustment of the position of the ring conductor. 6 figs