Radial cooling of a Spherical Torus (ST) toroidal field (TF) centerpost

It is best for fusion to operate STs at the highest feasible toroidal field. This may not be obvious since dissipated electrical power increases as the square of magnetic field strength. However, fusion power density increases even faster. For example, increasing TF by 10% increases electrical losses by 21% but it also may increase fusion power by 46.4%. The main impediment blocking increased toroidal field is centerpost heat removal. Heat deposited by resistive heating and by radiation from the fusioning plasma is removed by coolant flowing through holes cut in the centerpost. Conventional designs have used axial flow within spaced, vertically oriented holes, and they have optimized flow speed, temperature rise, and cooling hole size. However, with all flow paths having the same length, the conductor volume removed is just the product of centerpost height by total flow area. Radial flow cooling is a radically different scheme promising a factor of almost two improvement over axial flow designs in the volume of conductor removed for cooling. Its performance advantage stems from its shorter average flow path length while retaining the same total flow cross sectional areas for inflow, outflow, and internal flows. This is accomplished by cooling the upper centerpost from the top and cooling the lower centerpost from the bottom, with no coolant crossing the horizontal midplane. For a single-turn TF, high pressure coolant is supplied both from the top and from the bottom to a central manifold located radially in the middle of the centerpost conductor. Coolant flows outward through many small diameter radially oriented cooling holes in the centerpost conductor into a low pressure annular manifold surrounding the centerpost. The external membrane surrounding the low pressure manifold includes sealed penetrations for the centerpost electrical connections and mechanical supports. Radial cooling optimization includes tapering of the manifold cross sections over their axial length in conjunction with varying the density and size of the radial cooling holes so that coolant flow speed is spatially constant and local cooling matches local heating. Radial cooling may simplify single-turn TF centerpost fabrication since it eliminates the need for long, narrow cooling holes as required for the axial schemes.