Description
The UK’s Spherical Tokamak for Energy Production (STEP) programme, aiming to provide a prototype fusion power plant (SPP) based on the spherical tokamak concept targeting 2040 [1], has now moved from the conceptional to preliminary design phase. Key objectives of the SPP, which will drive the creation of a UK fusion economy, are to deliver net electric power $P_\mathrm{net}\gt 100 \mathrm{MWe}$ and demonstrate tritium self-sufficiency. Choosing a fully non-inductive flat top operating point without inboard breeding drives the design to low aspect ratio A~1.8, high elongation κ~3 and high normalised plasma pressure $β_N\sim 4$ [2] as fusion power scales like $P_\mathrm{fus} \propto (β_N\cdot B_t )^4 κ^5 (R_\mathrm{geo} ⁄A)^3$ at high bootstrap current fraction $f_\mathrm{BS} =I_\mathrm{BS} ⁄I_\mathrm{p}\sim 0.8-0.9$. The high $f_\mathrm{BS}$ reduces the demand on the auxiliary microwave based heating and current drive system on STEP. In addition to the usual electromagnetic electron cyclotron wave current drive (ECCD), the use of electrostatic electron Bernstein waves current drive (EBCD) in combination is considered[3]. EBCD provides a 3 times higher normalised current drive efficiency than ECCD and opens opportunities to potentially access scenarios at commercial power plant relevant fusion gain of $Q=P_\mathrm{fus} /P_\mathrm{aux}\sim 30$. The ECCD scenario operates at Q~11. Since publishing the original design base (SPP-1) ($R_\mathrm{geo,1} =3.6 \mathrm{m}$ , $B_{t,1}=3.2 \mathrm{T}$ , $I_{p,1}\sim 20 \mathrm{MA}$ , $1.5 \mathrm{GW}\leq P_\mathrm{fus}\leq 1.8 \mathrm{GW}$ in 2025) [1], to reduce the risk the centre column, the design has moved to a larger baseline (SPP-2) with $R_\mathrm{geo,2} =4.3 \mathrm{m}$ , $B_{t,2}=3 \mathrm{T}$ but with the same fusion power range and aspect ratio, leading to a similar $I_p$. Plasma solutions for the larger design have proven to be more challenging in some areas, while making it easier to find an exhaust solution. To handle the heat load in the divertor at this high fusion power a core radiation fraction $f_\mathrm{rad} =P_\mathrm{rad} ⁄P_\mathrm{heat}\sim 0.7$ is adopted. Recent efforts to explore solutions with lower recirculating power have led to plasma flat-top operating points at lower $f_\mathrm{rad} \geq 50\%$ and lower fusion power $P_\mathrm{fus} \geq 1\mathrm{GW}$ that open attractive opportunities for the plasma and the machine design, whilst still meeting the objectives. The physics basis for STEP has grown considerably, reducing uncertainties in plasma core performance, divertor performance, control and operations without type-I ELMs. The onset of high core transport fluxes in high β regimes with low torque input due to the dominant hybrid kinetic ballooning mode (hKBM) turbulence is now understood. The competition between the Reynolds and Maxwell stresses with respect to zonal flow generation leading to these high fluxes is governed by a a threshold $q^2 β_e=f(\hat{s},\gamma_E)$ ($\hat{s}$: magnetic shear, $\gamma_E$: E×B flow shear). This has been used to optimise the flat-top solution in SPP-2. Although, most of the scenario exploration is done with using JETTO or RAPTOR with $β_N$ feedback used to achieve the required $P_\mathrm{fus}$ , codes like an ST optimised version of TGLF or a newly developed quasilinear model for hKBM transport provide flux driven predictive capabilities. Machine learning has been used to provide surrogates for faster scenario evaluation. An optimised divertor design has been developed to provide good detachment access and efficient He exhaust at the same time. The first coupled core edge simulations are being used to understand the impact of He and Ar on the scenario performance. A novel scheme for vertical control in double null has been developed and tested on TCV. A runaway electron (RE) beam mitigation coil (REMC) has been incorporated into the design to provide a feasible solution to dissipate the RE current in combination with shattered pellet injection. This reduces a key risk to the STEP plasma scenario by deconfining >75% of the REs in less than 0.1 ms . A manageable R&D programme to develop key diagnostic systems for STEP is being established. This paper will give an overview of the most important aspects in which risk and uncertainty in of the STEP plasma scenario has been reduced. Considerable progress has been made in STEP scenario modelling capability but still many challenges remain. The burning plasma solution is driving physics development at the forefront of fusion research.
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. To obtain further information on the data and models underlying this paper please contact PublicationsManager@ukaea.uk.
[1] C. Waldon et al., Phil. Trans. R. Soc. A 382, 20230414 (2024).
[2] H. Meyer et al., Phil. Trans. R. Soc. A 382, 20230406 (2024).
[3] S. Freethy et al., Nuclear Fusion 64, 126035 (2024).