VACUUM FLUX SURFACES PRODUCED BY INCLINED COILS

The detailed analysis of the vacuum magnetic field structure produced by the inclined toroidal field (TF) coils is presented. This configura tion has a potential for adding stellarator properties to the tokamak configuration while maintaining the simplicity of planar coils. Parame ters of the system are identified that result in significant stellarat or-like effects: large vacuum flux surfaces and appreciable rotational transform. Two sets of closed flux surfaces with opposite helicity ar e studied: the internal one and the external one. It is found that the external set of flux surfaces possesses a magnetic well and, hence, i s favorable for the magnetohydrodynamic (MHD) stability. Also, it has larger enclosed volume and rotational transform. It is, hence, prefere ntial in our studies, in comparison with the internal set that usually features a magnetic hill. Analysis of the flux surface structure and the helical harmonic spectrum yields optimization rules required for t he configuration to be of practical interest for possible fusion appli cations. In a few examples it is demonstrated what occurs if the param eters are set differently than optimal. It is found that toroidal inho mogeneity is a key factor and vacuum flux surfaces disappear in the li mit of a very high number of TF coils. The important role of the poloi dal field (PF) coil system is stressed, and the possibility of the com pensated PF system (with zero total current) is found. (C) 1995 Americ an Institute of Physics.