Confinement of fusion alpha-particles and Alfven eigenmode stability in STEP

The Spherical Tokamak for Energy Production (STEP) programme is focused on designing and building a prototype fusion power plant that will generate approximately 1.5–1.8 GW of deuterium-tritium fusion power. To achieve this, the α -particles generated through fusion must be adequately confined to maintain the necessary high temperature in the core of the plasma and to protect the wall from excessive damage. Microwaves will be used for both external heating and current drive, making α -particles the only significant fast-ion species. The purpose of this work is to model the confinement of α -particles and the toroidal Alfvén eigenmodes (TAEs) driven by these particles in a variety of scenarios to help determine the best configuration. The scenarios examined here have been identified by the STEP team as potential flat-top operating configurations. We use LOCUST (Lorentz Orbit Code for Use in Stellerators and Tokamaks) to model the α -particle confinement and heat-load distribution on the wall, and HALO (HAgis LOcust) to model the TAEs. The results indicate that acceptable confinement in terms of power loading can be achieved in candidate flat-top operating points, but the results are sensitive to some of the system parameters. For example, a change in the phase difference between the upper and lower edge localised mode suppression coils can increase the maximum power load on the first wall due to α -particle losses by a factor of 10.