CONTROL OF TOROIDAL-LIKE ASYMMETRIES IN A HELIAC

A simple modification to the heliac coil configuration [A. H. Boozer et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1966), Vol. 1, p. 103] is described that reduces the toroidal perturbation to the magnetic field structure and significantly restores the helical symmetry. This is accomplished by shifting the internal current-carrying conductor (hardcore) of the heliac radially inward from its normal position at the geometric center of the external l = 1 coils. It is shown by computations that a large-aspect-ratio toroidal heliac is similar to a helically symmetric linear heliac in which the hardcore has been shifted. This similarity is seen in the analytic form of the magnetic potential, the flux surface shape, and in the harmonic spectra of the field lines. It is then shown that the toroidicity perturbations can be reduced in a toroidal heliac by combining these effects-using a radially inward hardcore shift in the toroidal configuration. In the experiments reported here the toroidal-like effects are created in the linear High Beta Q Machine (HBQM) heliac [C. M. Greenfield et al., Phys. Fluids B 2, 133 (1990)] by shifting the hardcore off the geometric center of the l = 1 coils. The induced toroidal-like effects are seen from an asymmetry in the axial excluded flux; however, no decrease in global beta is seen as compared to the unshifted symmetric case (G. G. Spanjers, Ph.D. dissertation, University of Washington, 1992) indicating the absence of large-scale magnetohydrodynamic instabilities at high beta.