Analysis of lower hybrid current drive experiments on EAST in the presence of electron cyclotron current drive*

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 phase space. For Discharge #106904, a ray-tracing and Fokker-Planck code package, GENRAY/CQL3D, is utilized to evaluate a self-consistent electron distribution function. The model predicts a synergy current of ~15 kA, about one-third of the modeled EC current (45 kA). Experimentally, the application of 0.5 MW of EC power reduces the LH power requirement by 0.3 MW from 1.28 MW to 0.98 MW to sustain a non-inductive plasma. The electron temperature profile remains steady, ruling out heating effects that can increase the LH current drive (CD) efficiency. The model also shows an enhanced plateau in the distribution function and perpendicular broadening. A scan in the ECCD injection angles suggests that off-axis ECCD can provide maximum synergy currents by further aligning its power deposition to that of LHCD at ρ = 0.15 for the scenario considered. No radial transport loss is assumed. New experiments proposed with off-axis ECCD will be discussed with modeling predictions.