Analysis of observed tearing modes with 2D/3D ECE imaging diagnostics in KSTAR plasma

Abstract

Thermonuclear fusion performance of tokamak plasmas increases with the level of the confined plasma pressure, and many efforts will have been made to enhance the plasma confinement efficiently. Construction of international thermonuclear experimental reactor (ITER) is one of technical efforts, which intends to increase the plasma size for the slower diffusion and better confinement. However, for a successful high confinement operation of ITER, there are remaining physical issues/constraints to be addressed. It has been shown that the explosive growth of tearing mode instability can limit the achievable plasma beta by causing the disruption. In Korea superconducting tokamak advanced research (KSTAR), high resolution 2D/3D measurements of electron temperature fluctuations using the electron cyclotron emission imaging (ECEI) diagnostic have revealed the detail process of the fast (~100 µs) disruptive events with the tearing mode instability. The entire plasma can be disrupted when the m/n=2/1 magnetic island grows explosively toward the core region. In order to prevent the unwanted disruption, the tearing mode instability should be controlled thoroughly, and for the efficient control the evolution mechanism of the tearing mode should be understood clearly. Previous studies have found that the tearing mode stability is determined by the contributions from various physical mechanisms, and the nonlinear magnetic island evolution model based on the reduced MHD theory (so called the modified Rutherford equation or the MRE) has been developed for identification of the main driving/decaying mechanism of the tearing mode. However, those researches had used the linear theoretical/numerical approximations for some important stability parameters to describe the tearing mode evolution due to the complexity of the MRE (i.e. too many unknown stability parameters). For an accurate analysis of the tearing mode evolution, the experimental method to estimate the stability parameters has been developed based on the KSTAR ECEI diagnostics. The measured high resolution 2D ECE images have been compared directly with the synthetic ECE images from the transport relevant electron temperature model of the magnetic island to estimate two stability parameters, Δ' and w_d in the MRE. The accuracy of estimation has been improved greatly with the 2D data compared to the 1D data result, and the validity of the estimation method has been checked. As a result, the stability characteristic of tearing mode in the KSTAR plasma can be decided based on the ECEI measurement. One additional important fruit of the accurate stability parameter estimation is that a more reasonable form of the MRE can be obtained for the KSTAR plasma magnetic geometry, which will be useful for the efficient control of tearing mode in the high performance operation in future.