Neutral beam integrated efficiency for steady-state operation of a fusion neutron source design based on a spherical tokamak

Abstract High fusion performance and steady-state plasma operation are the key challenges for designing a compact fusion neutron source based on a spherical tokamak. Steady-state operation will be supported by non-inductive current drive (NBCD) produced by high-energy particles injected by neutral beam injection (NBI). The possibility of NBCD current enhancement and profile control is discussed. The majority of thermonuclear neutrons in the plasma are expected to come from beam–thermal fusion reactions. Overall NBI performance is reduced by the power losses in the injector beamline and by the fast particle losses in the plasma. Optimum fast ion spatial and energy profiles are evaluated for a wide range of NBI-driven plasma scenarios with low-aspect magnetic geometry features accounted for. Combined NBI optimization methodology allows tracking and reducing the fast particle losses throughout the entire beam lifetime—from the accelerated ion extraction at the injector ion source until the final thermalization of particles in plasma. This integrated approach has proved to be highly efficient in time and computing resources. Possible applications of the method are not limited to purposes specific to fusion neutron source devices. It can be applied to enhance NBI gain in magnetically confined plasmas designed for steady-state or long-pulse operation in high-, moderate- or low-aspect-ratio tokamaks.