Electron Bernstein Wave heating and current drive for MAST Upgrade

Microwave-based current drive brings significant technological advantages in a reactor environment. However, the cyclotron harmonics in many spherical tokamak designs are covered by the plasma cut-off, prohibiting the use of ECCD. Electron Bernstein Waves (EBWs) offer a promising alternative. EBW-CD is predicted to be highly efficient, however EBW-CD has never been demonstrated on the spherical tokamak.

To investigate its use, MAST Upgrade will install a 2^nd harmonic EBW system as part of its enhancements program. The frequency is chosen to maximise the coupling efficiency for low-field-side launch, minimise the effect of density perturbations while maintaining efficient current drive. A low-field side launcher design allows coupling to EBW via O-X-B mode conversion, for near-axis co-current, counter-current and heating, as well as off-axis co-current drive. Predicted efficiencies of up to 0.144 A/W have been found (ξ_CD = 0.63). Full wave simulations have demonstrated coupling efficiency of ~96% in quiescent plasma conditions. The effect of turbulent perturbations on the coupling are also investigated, with scans in turbulent amplitude, position and scale informing the impact on coupling.