Investigation of Electron Bernstein Wave ( EBW ) Coupling and its Critical Dependence on EBW Collisional Loss in High - β , H - Mode ST Plasmas

In conventional aspect-ratio tokamaks, electron cyclotron (EC) waves can provide local heating and current drive. However, the high-β spherical tokamak (ST) plasma conditions cut off propagation of EC waves. The electron Bernstein wave (EBW) does not experience a density cut off and is strongly absorbed and emitted at the EC harmonics, allowing EBWCD to be used in STs for current profile control to increase global stability, as well as possible NTM suppression. A critical challenge is to establish and maintain efficient EBW coupling in these high-β, H-mode ST plasmas. EBW emission (EBE) diagnostics and EBE modelling have been employed on NSTX to study oblique EBW to O-mode (B-X-O) coupling and propagation. Initial EBE measurements in H-mode plasmas exhibited strong emission before the L-H transition, but the emission rapidly decayed after the transition. EBE simulations show that collisional damping of the EBW prior to mode conversion (MC) can significantly reduce the measured EBE for Te < 30 eV, explaining the observations. Lithium evaporation was used to reduce EBW collisional damping near the MC layer. The evaporation rate was increased from 0 to 19 mg/min to reduce the electron density and increase the electron temperature outside the last closed flux surface (LCFS) in an H-mode plasma. With edge conditioning, an increase in Te near the fundamental MC layer from 10 eV (no Li) to 20 eV (with Li) was observed. As a result the measured B-X-O transmission efficiency increased from < 10% (no Li) to 60% (with Li), consistent with EBE simulations. This work has significant implications for future ST devices that use EBWCD, such as an ST-CTF, if the O-X-B MC layer is shifted outside the plasma to a region where Te < 30 eV. For these conditions, O-X-B heating and CD may become inefficient due to significant EBW collisional losses. This work has demonstrated that using edge conditioning to raise Te outside the LCFS can significantly reduce these losses.