Electron Bernstein Wave (EBW) current drive profiles and efficiency for STEP
T. Wilson, S. J. Freethy, M. Henderson, Alf Köhn-Seeman, Ivan Konoplev, S. Saarelma, D. C. Speirs, R. G. L. Vann, the STEP team
Abstract
GENRAY and CQL3D were used to estimate the Electron Bernstein Wave (EBW) current drive profiles and normalised current drive efficiency ζ CD (6) for several STEP reactor concepts with varying temperature, density, geometry and magnetic field. ζ CD > 1.0 was readily found for ρ = 0.65 − 0.9 while ζ CD > 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 2 nd harmonic absorption for ρ ≤ 0.8 due to the higher ν ⊥ at the wave-particle resonance. 1 st harmonic absorption is required for ρ > 0.8 as there is no access to the 2 nd 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 2 nd harmonic.