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 Qeng 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 (Te > 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 approx-

imation). 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>50 times 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.

[1] S. FREETHY, et al., EPJ Web of Conferences. 277, 04001 (2023).

[2] S. PAVLOV and F. CASTEJON, Nuclear Fusion. 58, 126030 (2018)

[3] E. NELSON-MELBY, et al., Plasma Physics and Controlled Fusion.

49, 1913–1929 (2007)

[4] B. BISWAS, et al., Nuclear Fusion. 63, 126011 (2023)