Investigation of electron Bernstein wave (EBW) coupling and its critical dependence on EBW collisional loss in high-β, H-mode ST plasmas

摘要

High-β spherical tokamak (ST) plasma conditions cut off propagation of electron cyclotron (EC) waves used for heating and current drive in conventional aspect ratio tokamaks. The electron Bernstein wave (EBW) has no density cutoff and is strongly absorbed and emitted at the EC harmonics, allowing EBWs to be used for heating and current drive in STs. However, this application requires efficient EBW coupling in the high-β, H-mode ST plasma regime. EBW emission (EBE) diagnostics and modelling have been employed on the National Spherical Torus Experiment (NSTX) to study oblique EBW to 0-mode (B-X-O) coupling and propagation in H-mode plasmas. Efficient EBW coupling was measured before the L-H transition, but rapidly decayed thereafter. EBE simulations show that EBW collisional damping prior to mode conversion (MC) in the plasma scrape off reduces the coupling efficiency during the H-mode phase when the electron temperature is less than 30 eV inside the MC layer. Lithium evaporation during H-mode plasmas was successfully used to reduce this EBW collisional damping by reducing the electron density and increase the electron temperature in the plasma scrape off. Lithium conditioning increased the measured B-X-0 coupling efficiency from less than 10% to 60%, consistent with EBE simulations.