Description
The Spherical Tokamak for Energy Production (STEP) is a proposed prototype fusion power plant designed to demonstrate net electrical power generation [1]. Among the plasma operating scenarios under consideration for STEP, a high radiation fraction ($f_{rad}=70\ \%$) electron cyclotron heated scenario has been selected as the focus of this modelling work. The flat-top phase of this scenario has already been modelled for both the initial STEP design point (SPP-001, $R_{geo}=3.6\ m$) and the latest design point (SPP-002, $R_{geo}=4.3\ m$). Simulations were performed using the 1.5D integrated core plasma model JETTO in “assumption integration” mode, in which energy confinement is prescribed rather than calculated from first-principles transport models, as described in detail in [2]. Electromagnetic micro-instabilities are expected to dominate anomalous transport in STEP, in particular micro-tearing modes (MTMs) and hybrid kinetic ballooning mode (KBM)-like instabilities, as predicted by gyrokinetic simulations [3]. However, such simulations are computationally too demanding for application within integrated modelling frameworks. For this reason, efforts have focused on coupling JETTO with the reduced quasilinear model for turbulent transport TGLF, which uses saturation rules based on nonlinear gyrokinetic calculations [4]. In this work, preliminary results of the JETTO-TGLF(Sat1) integrated modelling are presented for the flat-top phase of a STEP plasma scenario. The transient ramp-up and ramp-down phases have also been simulated, although with different levels of modelling fidelity, and require careful optimisation of plasma parameters to remain within operational constraints. The ramp-up phase has been modelled using a TGLF neural network surrogate to improve on the “assumption integration” mode while reducing computational cost with respect to TGLF. In addition, at five selected time points, the pellet injection, ablation, and deposition code HPI2 has been employed to improve the fidelity of pellet deposition modelling. The use of HPI2 enables the evaluation of changes in the density profile before and after pellet injection and allows the characterisation of pellet injector frequency requirements. The ramp-down phase has also been optimised, with particular emphasis on maintaining a low internal inductance to mitigate the risk of vertical instabilities. For this phase, turbulent transport has been modelled using the semi-empirical Bohm/gyro-Bohm transport model.
References
[1] Meyer H. 2024 Plasma burn—mind the gap. Phil. Trans. R. Soc. A 382 20230406, [2] E. Tholerus et al 2024 Nucl. Fusion 64 106030, [3] M. Giacomin et al 2024 Plasma Phys. Control. Fusion 66 055010, [4] G. Staebler et al 2024 Nucl. Fusion 64 103001
Acknowledgements: 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.