Dependence of Spatial Scale of Quasi-Coherent Mode on Collisionality in Electron Cyclotron Resonant Heating Assisted Low-Confinement Plasmas

Abstract

Quasi-coherent mode (QCM) is a widely studied ion-gyroscale micro-instability with characteristics similar to those of trapped electron mode in tokamak plasmas. The effect of the normalized collisionality on the QCM amplitude is quantitatively studied in low-confinement plasmas assisted by electron cyclotron resonant heating in the KSTAR machine using the microwave imaging reflectometer (MIR). To study the evolution of the QCM amplitude by the increased collisionality, coherence lengths are obtained from coherence spectra of signals from multiple poloidal or radial channels of the MIR system. Compared to the other definition of the QCM amplitude 'the energy of QCM' extracted from power spectrum of MIR signals, the coherence length is more sensitive to small-amplitude fluctuations, providing more detailed dependence of the mode amplitude on the collisionality especially near the critical level for full suppression.