Alpha confinement in the quasi-axisymmetric Helios fusion power plant

Achieving good confinement of alpha particles from deuterium-tritium (“D-T”) fusion is necessary for the power balance of magnetic confinement fusion and desired to avoid excessive heat loads on the first wall. This study presents the first detailed analysis of alpha particle confinement in the “Helios” fusion power plant physics design. Monte Carlo simulations of collisional alpha particle confinement using ASCOT5 have provided critical feedback during the Helios equilibrium design process, both during the choice of the coil set and in subsequent coil current reoptimization to improve alpha confinement and divertor properties. These simulations predict losses of < 6 % of fusion-born alpha power and an alpha heat load on the first wall < 4 MW/m 2 . The former satisfies power balance requirements while the latter presents an engineering challenge that motivates further optimization. A classification of collisionless alpha trajectories with FIRM3D identifies the dominant mechanism of alpha deconfinement. Greater than 60% of lost orbits are banana-trapped alphas undergoing diffusive drift motion due to resonances of the bounce and precession motion. Further efforts in improving the alpha particle confinement and reducing the heat load on the first wall will target the minimization of these resonances.