Study of the origin of quasi-coherent modes in low-density KSTAR ECH plasmas

Gradual or abrupt transitions from a coherent mode to quasi-coherent mode (QCM, a type of trapped electron mode turbulence) are observed in the core region of low-density plasmas in Korea Superconducting Tokamak Advanced Research. The coherent mode is initiated in the vicinity of the q = 1 surface by off-axis electron cyclotron resonant heating (ECH), and gradually or abruptly evolves into the QCM as the collisionality is slowly or rapidly increased through electron density control. When the collisionality further increases above a critical level, the QCM is changed into broadband turbulence, which has been preferentially observed in tokamak plasmas. The observed evolution of the QCMs suggests that (1) QCMs can originate from a coherent mode in ECH applied low-collisionality plasmas and (2) quasi-coherent spectra of QCMs are closely related with the collisionality in a finite range ('collisional broadening of the wavenumber and frequency'). The coherent mode appears to be a metastable electrostatic mode at the rational surface excited by off-axis ECH. (paper)