Observation of electron driven quasi-coherent modes and their connection with core intrinsic rotation in KSTAR ECH and ohmic L-mode plasmas

Abstract

Quasi-coherent (QC) modes, known as a type of the trapped electron mode (TEM) turbulence, have been measured in the outboard core region of low-density electron cyclotron resonant heating (ECH) injected and ohmically heated L-mode plasmas on the Korea Superconducting Tokamak Advanced Research (KSTAR) device. It appears that QC modes of 20-60 kHz occur or strengthen with an increase of the electron temperature to ion temperature ratio, but weaken or are fully suppressed by increased density/collisionality. Toroidal rotation shear, which is strongly related to the density/collisionality, also seems to stabilize the QC modes. Linear gyrokinetic simulations indicate that TEM is the most unstable mode at low densities where the QC modes are observed for both the ECH and ohmic plasmas. At high densities where the QC modes are suppressed, the most unstable mode is the ion temperature gradient (ITG) mode for the ECH plasmas but still TEM for the ohmic plasmas. In the ECH plasmas, it is found that the direction of the core toroidal intrinsic rotation is gradually reversed from the co-current to counter-current direction and the QC mode is suppressed as the line-averaged density increases, which can be explained by the transition of dominant micro-turbulence (TEM to ITG). However, in the ohmically heated plasmas, the acceleration of the core toroidal rotation is not fully explained by the TEM-ITG transition since the direction of the core toroidal rotation is counter-current direction when the QC mode is observed. Furthermore, the core toroidal rotation is accelerated to the counter-current direction even the line-averaged density decreases.