Electron Bernstein Wave (EBW) current drive profiles and efficiency for STEP

GENRAY and CQL3D were used to estimate the Electron Bernstein Wave (EBW) current drive profiles and normalised current drive efficiency ζCD (6) for several STEP reactor concepts with varying temperature, density, geometry and magnetic field. ζCD > 1.0 was readily found for ρ = 0.65 − 0.9 while ζCD > 0.5 was found for ρ ≥ 0.5. Okhawa is found to be the most efficient current drive mechanism due to the high trapped fraction in STEP. Optimal current drive was found for 2nd harmonic absorption for ρ ≤ 0.8 due to the higher ν⊥ at the wave-particle resonance. 1st harmonic absorption is required for ρ > 0.8 as there is no access to the 2nd harmonic at the launch frequencies examined. Rays with negative starting N∥ were found to penetrate furthest. Due to the high electron temperature in STEP the relativistic downshift of the harmonic becomes comparable to the Doppler shift, increasing access to the 2nd harmonic.