Top Launch for Higher Off-axis Electron Cyclotron Current Drive*

Efficient off-axis current drive is crucial for economic, steady-state tokamak fusion power plants. ’Top Launch’ ECCD is a promising off-axis CD method to achieve the desired broad current profile. By launching the waves downwards (or upwards) parallel to the resonance plane with a large toroidal steering, high CD efficiency can be obtained at large radii owing to the large Doppler shift, wave-particle interactions on HFS of the plasma, and the long absorption path.

Previous modeling for FNSF predicts Top Launch can provide significantly higher (~50%) off-axis ECCD efficiency with broader profile peaked off-axis than standard LFS launch. In this work, a systematic study of Top Launch for CFETR, using the applied EC frequency and launch location as free parameters, finds a >35% improvement in off-axis ECCD over LFS launch. In addition, its physics potential in various DIII-D scenarios is investigated along with the sensitivity to launch location, magnetic field, density, beam aiming and divergence, etc. Enhancement in off-axis ECCD efficiency of up to a factor-of-2 seems possible. A prototype fix-injection Top launch system is being designed and planned to be installed on D3D to characterize and evaluate effects of this injection scheme.