Study of Core MHD Structures and Slow Crashes in Modified Sawtooth Patterns Driven by Localized Electron Cyclotron Heating and Current drive in KSTAR plasma

Abstract

In thermonuclear fusion tokamak plasma, fusion born alpha particles with the kinetic energy on the order of 1~10 MeV enhance not only fusion reaction via heating of the fuel ions, but can also drive a large amount of current and affect the plasma confinement. The core of tokamak plasma with large electric current become unstable to m = 1 internal kink mode, which results in sawtooth-like periodic collapses (relaxations) of the confinement of the plasma core region. The sawtooth oscillation has been considered as dangerous MHD instability because of fast collapse (10s of μs) driven by the m = 1 internal kink mode, despite its expected beneficial role in making steady-state burning plasma by preventing impurity accumulation in the core region. In KSTAR plasmas, we find that localized electron cyclotron heating and current (ECCD) drive close to the q = 1 surface can induce slow sawtooth relaxations with time scale comparable to the Sweet-Parker time scale (a few ms). In ECCD deposition scan experiments, two different types of slow relaxation are revealed: (1) post-damping of an m = 1 mode after partial crash of multiple m/n = 1/1 modes and (2) slow growth of a q = 1 island. For these two slow relaxations, the heating deposition is located just inside and just outside the q = 1 surface, respectively. In the heating power modulation experiment, we found that the slow relaxations of the second type initiated by a short ECCD beam blip continue for the duration of the redistribution time of the driven current, which exceeds the sawtooth period. In a set of experiments of ECCD deposition scan and power modulation, high-resolution quai-3D electron cyclotron emission (ECE) images showed detailed features of core MHD dynamics. We classified distinct features of spatial structure and temporal dynamics of the core MHD modes in four different sawtooth patterns, and explained the expected manifestation of such MHD dynamics in the details of waveform and oscillation pattern of 1-D ECE time traces. Since 1-D measurements of plasma parameters are essential to the real time diagnosis and control of the tokamak plasma, the present thesis work is expected to contribute to development of real-time sawtooth control in fusion plasma.