Experimental investigations of LHW–plasma coupling and current drive related to achieving H-mode plasmas in EAST

TLDR

The study aims to achieve high‑confinement H‑mode plasmas in EAST by investigating how local gas puffing from the electron and ion sides of a lower hybrid wave antenna influences LHW–plasma coupling and current drive. The authors examine the effects of gas puffing on LHW–plasma coupling and conduct high‑density experiments with lower hybrid current drive to assess coupling and efficiency. Experiments show that electron‑side gas puffing enhances coupling, that coupling without puffing is sensitive to grill density causing L–H–L transitions, that strong lithiation improves LHCD efficiency above 2 × 10¹⁹ m⁻³, and that efficiency drops sharply due to parametric decay instability.

Abstract

Aimed at high-confinement (H-mode) plasmas in the Experimental Advanced Superconducting Tokamak (EAST), the effect of local gas puffing from electron and ion sides of a lower hybrid wave (LHW) antenna on LHW–plasma coupling and high-density experiments with lower hybrid current drive (LHCD) are investigated in EAST. Experimental results show that gas puffing from the electron side is more favourable to improve coupling compared with gas puffing from the ion side. Investigations indicate that LHW–plasma coupling without gas puffing is affected by the density near the LHW grill (grill density), hence leading to multi-transition of low–high–low (L–H–L) confinement, with a correspondingly periodic characteristic behaviour in the plasma radiation. High-density experiments with LHCD suggest that strong lithiation gives a significant improvement on current drive efficiency in the higher density region than 2 × 1019 m−3. Studies indicate that the sharp decrease in current drive efficiency is mainly correlated with parametric decay instability.