Overview of progress in establishing the plasma basis of STEP

The Spherical Tokamak for Energy Production (STEP) program has provided a conceptual design for a prototype steady-state fusion power plant targeting the year 2040 [1]. External heating and current drive (CD) is either pure electron cyclotron CD (ECCD) or a mix of ECCD and electron Bernstein wave CD (EBCD). EBCD is projected to have normalized efficiency three times higher than ECCD, providing possible access to Q_fus = P_fus/P_aux ~ 30 [1]. The published design point (SPP-001) [1], with geometric major radius R_geo = 3.6m, aspect ratio A = 1.8, toroidal field B_t(R_geo) = 3.2T and fusion power P_fus ~ 1.5-1.8GW, is technically challenging due to constraints on the inner build radius. This has led to a program pivot to a larger design (SPP-002) with R_geo = 4.3m but the same A and P_fus. I will report on recent work to extend the physics basis, explore control strategies [2], and develop whole-pulse operating scenarios. In integrated modeling Bohm/gyro-Bohm transport is scaled to achieve the desired P_fus. The scenario space is constrained by seven other conditions, e.g. a maximum divertor head load and Q_fus > 10. Gyrokinetic simulations show that the transport is dominated by hybrid kinetic ballooning modes [3]: the corresponding heat fluxes are sustainable in the presence of modest flow shear. α-particles do not drive toroidal Alfvén eigenmodes in these design points.

[1] H Meyer Phil. Trans. R. Soc. A 382, 20230406 (2024)

[2] M Lennholm Phil. Trans. R. Soc. A 382, 20230403 (2024)

[3] D Kennedy et al. Nucl. Fusion 63, 126061 (2023)