Scoping study of lower hybrid current drive for CFETR

The China Fusion Engineering Test Reactor (CFETR) will require significant lower hybrid current drive (LHCD) power to supplement the bootstrap current in steady-state. A scoping study was performed using GENRAY/CQL3D ray-tracing/Fokker-Planck models to determine the appropriate antenna parameters for optimum current drive performance in the CFETR hybrid scenario. CFETR is a large (R₀ = 7.2 m, a = 2.2 m) steady-state tokamak with a toroidal magnetic field of 6.5 T, plasma current of 13 MA, core density of 1.2 × 10²⁰ m^-3, and core temperature of 30 keV. The key parameters scanned were the launched parallel refractive index of the primary lobe of the LH launcher (n_||) and poloidal angle (θ) of the antenna, with a LH source frequency of 4.6 GHz. A comparison of high field side (HFS) versus low field side (LFS) launch points shows similar current drive efficiency (1300 kA per 20 MW net power) from both the HFS and LFS. HFS launch yields a broad current profile in the outer part of the plasma (r/a = 0.6 - 0.9) as desired for supplementing the bootstrap current, while LFS launch yields a very narrow current profile near the top of the pedestal (r/a ∼ 0.9). A 5-D parameter scan (n_||1, n_||2, θ₁, θ₂, P₁/P₂) of synergy between HFS and LFS antennas at fixed total power of 20 MW shows a modest (∼5%) increase in efficiency when only cases with single-pass absorption are considered. Here the subscripts 1 and 2 refer to HFS and LFS respectively. Synergy simulations with very low n_|| (|n_||| ∼ 1.375) show higher current drive efficiency, but these scenarios suffer from poor wave accessibility, the multi-pass nature of the rays, and extremely far off-axis absorption (r/a ≥ 0.95). Parametric scans in these studies were enabled by the πScope workflow.