Equilibrium, Stability, and Deeply Trapped Energetic Particle Confinement Calculations for l = 2 Torsatron/Heliotron Configurations

Abstract

Confinement properties of l-2 torsatron/heliotron configurations with number of toroidal field periods, M, in the range of 10 to 14 are studied. This involves the calculation of zero-current and flux-conserving equilibria; stability against Mercier modes and low-n ideal modes, with n denoting the toroidal mode number; and orbit confinement of deeply trapped energetic particles. Optimization of both mag-netohydrodynamic (MHD) and transport properties is pursued under the condition of plasma aspect ratio A = R/a ≥ 7, with R denoting the major radius and a the average plasma radius. For configurations with M ≤ 12, an average MHD beta limit of 4 to 5% is possible. The addition of a quadrupole field improves the confinement of trapped particles at zero pressure, but particle losses increase with increasing beta. This loss is less severe if the vacuum magnetic axis is shifted slightly inward. A configuration with M = 10, a coil pitch parameter p_c in the range 1.25 to 1.30, and an added quadrupole field satisfies the beta and energetic particle confinement requirements for the next generation of large torsatron/heliotron devices.