Determination of current densities for tokamak superconducting toroidal field coils

Abstract

A major goal of designing a tokamak is to minimize the size of the device and achieve lowest cost. Two key factors influencing the size of the device employing superconducting magnets are toroidal field (TF) winding current density and its nuclear heat load withstand capability. Lower winding current density requires larger radial build of the winding pack. Likewise, lower allowable nuclear heating in the winding requires larger shield thickness between the plasma and TF coils. In order to achieve a low-cost device, it is essential to maximize the winding's current density and nuclear heating withstand capability. A methodology for determining optimum current density is developed by using the Tokamak Fusion Core Experiment (TFCX) as an example. A winding current density of 3500 A/cm² is determined to be optimal at a peak field of 10 T and peak nuclear heat load limit of 1 mW/cm³. This study is based on employment of Nb₃Sn cable-in-conduit conductors cooled with forced-flow helium.