Description
STEP (Spherical Tokamak for Energy Production) is a UK-led fusion programme that aims to demonstrate the commercial viability of fusion energy through the development of a prototype tokamak power plant. The plasma is expected to produce around 1.5 GW of fusion power, of which approximately 330 MW will be carried by fast alpha particles.
Fast particles are highly sensitive to three-dimensional (3D) magnetic fields, and alpha-heating power is vital for maintaining the STEP burning plasma condition. The characteristic time for a single alpha particle to transfer its energy to the background plasma is of the order of one second. In the presence of non-axisymmetric magnetic perturbations, a higher fraction of alpha particles follow unconfined trajectories and deposit their energy onto plasma-facing components. This reduces alpha-heating efficiency and can significantly shorten plasma-facing components lifetimes.
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 3D magnetic perturbations and includes Monte Carlo collisions with the background plasma. The impact of error fields arising from misaligned toroidal field (TF) coils is analysed, along with magnetic perturbations generated by edge-localised-mode control coils (ELMcc). Plasma response effects are calculated using both the MARS-F[2] and JOREK[3] codes, and differences between the vacuum approximation and plasma response models are explored.
The results show that relatively small TF-coil misalignments, of the order of a few centimetres, can lead to excessive alpha-particle power fluxes on STEP PFCs when the external coil radius is 9 m, with peak values exceeding 5 MWm⁻². In contrast, larger misalignments can be tolerated if the coil radius is increased to at least 10 m, for which the maximum power flux on the first wall remains below 1 MWm⁻². While different ELMcc current configurations result in varying levels of alpha-particle transport, the total loss of alpha-heating power remains negligible. Heat fluxes on all structures remain within acceptable limits, with peak values of up to 0.9 MWm⁻² on the lower divertor and 0.32 MWm⁻² on the first wall.
This work has been funded by STEP, a major technology and infrastructure programme led by UK Industrial Fusion Solutions Ltd (UKIFS), which aims to deliver the UK’s prototype fusion powerplant and a path to the commercial viability of fusion.
[1] S. H. Ward, R. Akers, et. al. Nuclear Fusion, 61(086029), 2021.
[2] Y. Q. Liu, et. al. Physics of Plasmas, 7(3681), 2000.
[3] Hoelzl M., et al, Journal of Physics: Conference Series, 401, (012010) 2012.