Tritium burn efficiency in deuterium-tritium magnetic fusion

The controlling parameters regarding tritium burn efficiency (TBE) are derived from first principles and shown to depend fundamentally on the permitted He gas fraction in the divertor and effective pumping speeds of He ash and unburned hydrogenic fuel. The analysis is generic to any equilibrated magnet fusion plasma using a divertor for particle exhaust. The He gas fraction in the plasma limits the maximum TBE due to the link between ash dilution effects in the core plasma and fusion performance. High TBE in magnetic fusion devices is counter-correlated to achieving high gain and power density for commercial fusion. The impact of TBE on fusion performance for several figures of merit are derived, including power density, required n − τ e product, and plasma energy gain Q p . The TBE formulation presented here is applied to existing devices, based on published data of enrichment and τ He ∗ from research tokamaks. This assessment strongly motivates exploration of technologies that would enhance the effective pumping speed of He to fuel out of the plasma.