Speaker
Description
Future tokamak power-plant prototypes will rely on alpha heating to sustain burning-plasma conditions. However, the transport and loss of a minority of energetic alpha particles can lead to significant heat loads on plasma-facing components (PFCs), which must withstand unprecedented steady-state particle and power fluxes during operation.
The development and validation of reliable workflows to predict alpha-particle heat loads and assess the viability of associated diagnostics are therefore essential. To quantify these effects, the LOCUST[1] (Lorentz Orbit Code for Use in Stellarators and Tokamaks) full-orbit code is employed to model steady-state heat loads on PFCs. LOCUST tracks fast-ion trajectories in the presence of three-dimensional (3D) magnetic perturbations and includes Monte Carlo collisions with the background plasma.
The workflow is compared against fast-ion loss detector (FILD) measurements of alpha particles from deuterium–tritium (DT) discharges in JET [2]. The contributions from both beam–target and thermonuclear reactions are modelled during the discharge. While the steady-state heat flux on the poloidal limiters and other structures is found to be low and difficult to distinguish from other sources in infra-red measurements. Synthetic diagnostic signals generated for the FILD show strong agreement with experimental observations.
The workflow is subsequently applied to STEP[3] (Spherical Tokamak for Energy Production), where the impact of error fields arising from toroidal-field (TF) coil misalignments is analysed alongside magnetic perturbations generated by edge-localised-mode control coils (ELMcc). The results show that, for compact outboard configurations with an external coil radius of 9 m, relatively small TF-coil misalignments of only a few centimetres can produce excessive alpha-particle power fluxes exceeding 5 MWm⁻². In contrast, larger misalignments can be tolerated when the coil radius is increased to at least 10.5 m, for which peak power fluxes remain below 0.2 MWm⁻². While different ELMcc current configurations result in varying levels of alpha-particle transport, the total loss of alpha-heating power remains low, with peak heat fluxes reaching up to 0.9 MWm⁻² on the lower divertor and 0.32 MWm⁻² on the first wall. These results also demonstrate the strong influence of the plasma response to externally applied magnetic perturbations. Furthermore, the presence of high-density, low-temperature plasma in the scrape-off layer is found to provide a significant protective effect to the first wall. Finally, preliminary simulations exploring the implementation of a FILD diagnostic for monitoring alpha-particle losses in STEP are presented.