Conceptual design of spherical torus with TF-CS hybrid coils based on virial theorem

A conceptual design of a spherical torus (ST) device with a new type of toroidal field (TF) coils and a central solenoid (CS) whose stress is reduced to a theoretical limit determined by the virial theorem is proposed. A virial-limit coil (VLC), which is a helical-type hybrid coil of TF coils and CS, and its winding is modulated so that its winding direction is nearly vertical and more horizontal in the outer and inner sides of torus, respectively. Thus, the configuration of VLC is similar to that of CS and TF coil systems in conventional tokamaks. Recently, we have developed a VLC concept with a non-circular cross-section, and reduce the maximum stress to 30% of that of TF coils in a two-dimensional analysis. Moreover, the VLC configuration is remarkably similar to that of a ST with a low aspect ratio and a strongly elongated cross-section with a triangularity. In this work, we analyze three dimensional stress distributions, and propose a design of VLC ST.