Coil Configurations for Low Aspect Ratio Stellarators

Using the NESCOIL code, it is possible to find a surface current distribution which can create a given last closed flux surface. Thus, almost any stellarator configuration can be created with either helical or modular currents. In particular, we have succeeded in finding current distributions which generate the optimized ATF-II configuration described in Hirshman's paper. As the aspect ratio of the configuration is decreased, or the plasma-to-coil distance is increased, the harmonic content of the surface currents is increased. This makes it a challenge to cut the distribution into either discrete helical or modular coils which are attractive from an engineering point of view. Several approaches to this problem are discussed. There have been some hints of promising configurations, but to date, none of them are satisfactory. 2 refs., 7 figs.