Modeling study of synergistic effects between lower hybrid and electron cyclotron current drive on EAST

Recent radio frequency (RF) current drive experiments in EAST provide an opportunity to investigate RF synergy effects between electron cyclotron (EC) and lower hybrid (LH) current drive. Synergy current is the excess current that can arise from overlapping two wave-particle resonance bands in the velocity phase space. In this work, a ray-tracing and Fokker–Planck code package, GENRAY/CQL3D, is used to model and investigate the phase space interaction of two waves for EAST. The target plasma features a high central electron temperature of 6.5 keV, promoting Landau damping of the LH power on-axis that overlaps with the EC power resonance region. The GENRAY/CQL3D code evaluates a self-consistent electron distribution function in the presence of two RF quasilinear diffusion coefficients. A systematic scan in collisional dissipative LH power loss and fast-electron radial transport is performed. Including the divertor SOL in GENRAY and fast-electron radial diffusion in CQL3D can quantitatively provide model total RF currents in line with the experimental range. Radial transport operator broadens and smooths the radial region of synergistic current generation. The impact of LH wave scattering on the synergistic interaction is examined by introducing the LH wave scattering angle in a heuristic manner. The sensitivity analysis is presented. Furthermore, a numerical scan of the ECCD injection angles is conducted to evaluate the synergistic interactions. The roles of LH current drive and ECCD in synergistic current generation are further examined through a power scan of both power sources.