B. Biswas (York Plasma Institute, University of York. York, UK)

"Predictions of efficient electron Bernstein wave current drive in STEP"

Abstract: The UK’s Spherical Tokamak for Energy Production (STEP) will rely on electron cyclotron (EC) and electron Bernstein waves (EBW) for fully non-inductive steady-state operation. While EC is lower-risk, EBW enables a higher efficiency, and therefore higher Q_eng, device1. To assess EBW current drive (CD) performance, an extensive modelling program is underway to predict wave coupling, propagation, damping, and the electron response. Analytic and numeric full-wave modelling is conducted to optimise O-X-B mode-conversion at the plasma edge. Reduced models of parasitic losses, including collisions and parametric decay, are also being investigated. This talk discusses these efforts, with a particular emphasis on the core microwave physics where ray-tracing and Fokker-Planck models are being used to optimise wave propagation and current-drive performance.

At reactor-relevant temperatures (T_e>5keV), relativistic effects can significantly modify EBW propagation and polarisation. In particular, rays that deeply penetrate the hot plasma cannot be simulated in the non-relativistic limit (due to breakdown of the weak-damping approximation). These rays could be of interest for near-axis current drive. Kramers-Kronig relations are exploited to efficiently evaluate the fully-relativistic dispersion relation for arbitrary wave-vectors2, leading to a >50x speed-up compared to previous efforts3 at relativistic ray-tracing. A recently verified linear adjoint model4 is used to estimate CD efficiency. Thus, for the first time, large parametric scans of fully-relativistic EBW CD simulations are performed. In STEP, relativistic physics are found to severely alter CD performance if rays are able to propagate sufficiently far into the core (ρ<0.7). In contrast, rays that damp strongly far off-axis are sufficiently short and “cold” such that relativistic effects are unimportant. These discoveries are factored into the design of STEP’s microwave launchers.

STEP will utilise multiple launchers - totalling ~150MW of microwave power - for steady-state operation. In this regime, strong quasilinear effects are expected to impact wave absorption and CD efficiency. A quasilinear Fokker-Planck solver is coupled to the fully-relativistic ray-tracer, enabling high-fidelity predictions in reactor-relevant conditions. Modelling shows that quasi-linear effects can be expected on MAST Upgrade, and plans for experimental validation of this physics will be discussed.
References

[1] S. FREETHY, et al., EPJ Web of Conferences. 277, 04001 (2023)
[2] S. PAVLOV and F. CASTEJON, Nuclear Fusion. 58, 126030 (2018)
[2] E. NELSON-MELBY, et al., Plasma Physics and Controlled Fusion. 49, 1913–1929 (2007)
[3] B. BISWAS, et al., Nuclear Fusion. 63, 126011 (2023)