Investigation of MHD events and associated toroidal rotation variation in an EAST long-pulse discharge

Abstract The self-organizing regime has been studied in an Experimental Advanced Superconducting Tokamak (EAST) long-pulse discharge. Multiple magnetohydrodynamic (MHD) instabilities and associated change in the core toroidal rotation were observed. A m / n = 3/2 tearing mode (TM) persisted for ∼42 s, until being damped as a result of current profile evolution. The deposition of a lower-hybrid wave (LHW) was coupled with the current profile via the magnetic field, and gradually changed the current profile with a stable heating power in the progress. A m / n = 1/1 long-lived mode (LLM) co-existed with the TM, but appeared alternatively with a m / n = 1/1 high frequency mode (HFM). The HFM appeared when the normalized electron temperature gradient (ETG) R/L Te near the q = 1 surface exceeded ∼8.5, and its frequency chirped as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">∇</mml:mi> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>e</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> varied. Disappearance of the TM was followed by an increase in the core electron temperature and an acceleration of toroidal rotation in the counter-current direction. The neoclassical toroidal viscosity (NTV) torque driven by these modes was modeled by NTVTOK, and the results indicated that the TM drove a toroidal torque in the co-current direction which damped the intrinsic rotation. These results enhance the understanding of the self-organizing regime in long-pulse discharges and provide a prospective study on the physics under long-pulse conditions for future fusion reactors.