Features of spherical torus plasmas

Abstract

The spherical torus is a very small aspect ratio (A < 2) confinement concept obtained by retaining only the indispensable components, such as the toroidal field coils, inboard to the plasma torus. MHD equilibrium calculations show that spherical torus plasmas with an edge safety factor qa > 2 are characterized by high toroidal beta (βt > 0.2), low poloidal beta (βp < 0.3), naturally large elongation (κ ≅ 2), large plasma current with Ip/(aBt0) up to about 7 MA·mT−1, strong paramagnetism (Bt/Bt0 > 1.5), and strong magnetic helical pitch (Θ comparable to F). A large near-omnigeneous region is seen in the large major radius, bad curvature region of the plasma in comparison with the conventional tokamaks. These features combine to engender the spherical torus plasma in a unique physics regime which permits compact fusion at low field and modest cost. Because of its strong paramagnetism and helical pitch, the spherical torus plasma shares some of the desirable features of spheromak and reversed-field pinch (RFP) plasmas, but with tokamak-like confinement and safety factor q. The general class of spherical tori, which includes the spherical tokamak (q>1), the spherical pinch (1>q>0), and the spherical RFP (q<0), have magnetic field configurations unique in comparison with conventional tokamaks and RFPs.