Role of Electron Temperature Control in Improved Safety Factor Profile Control in a Tokamak

Abstract

Advanced operation modes, promising tokamak plasma operation modes towards DEMO, can be characterized by a high plasma kinetic pressure, a large fraction of self-driven current, and a sufficiently good particle and energy confinement in a stationary condition. The advanced operation modes require control of plasma profiles to establish and to sustain the enhanced energy confinement and the non-inductive current fraction. Especially, the safety factor (q) profile control is essential. The advanced inductive operation mode requires a flat safety factor profile and the advanced tokamak operation mode requires a reversed q profile in the core region. For these reasons, most of the research have focused on the development and the improvement of q profile control for the advanced operation modes. The dissertation proposes a new way to improve the present q profile control methodologies by integrating the q profile control with the electron temperature (Te) profile control. The role of Te profile control is discovered for the first time which leads to the improvement of control performance and control stability in the q profile control. To develop this integrated control algorithm, firstly the property of the q and the Te control system is investigated which comes up with the finding that the former to be a variable delayed dynamics system and the latter to be a variable exhausted dynamics system. Secondly, experimentally relevant plasma response models are developed for each profile control. The physics based non-adaptive plasma profile control algorithms for q and Te are derived individually with the dual assumptions of linearity and time-invariance to develop the experimentally relevant plasma profile controller. Thirdly, the two profile controllers are integrated. Each profile control algorithm is adjusted by optimizing the proportional gain to improve the control. Then, these two control algorithms are integrated by introducing weights to consider the actuator sharing. The optimized gains and weights are obtained by a control optimization function. The control optimized function is defined with two terms

one is to integrate the amount of the control error during a control period and the other is to integrate the amount of the actuator action during a control period. In order to optimize the gains and the weights to design and evaluate the control algorithm proposed here, lastly, Plasma Profile Control Simulator (PPCS) has been newly developed based on the state-space equation. Background plasmas, control targets, and actuators are prepared based on the condition of KSTAR advanced operation in order to simulate and to evaluate the integrated control algorithm. The control gains and weights are optimized for this KSTAR condition and the consequent control simulation exhibits that the integrated plasma profile controller is experimentally relevant and robust in terms of the control performance. Based on the simulation result the Te profile control is found to be essential to improve the q profile control, the most significant conclusion in this dissertation work. Two roles of Te profile control are identified in the improvement of the q profile control. One is increase of the control performance and the other is intensification of the control stabilization in the q profile control. The mechanism of the improvement can be understood as follows. Firstly, the contributions from un-controlled inputs such as the ohmic current and the bootstrap current to control the q profile are minimized by considering the Te profile control. Secondly, the characteristic of the q profile control system is changed from a variable dynamic system to an invariable dynamic system by including the Te profile control. Therefore, the integrated q profile control with the Te control is clearly more promising in increasing the control performance and intensifying the control stabilization for the q profile control compared with the individual q profile control. This work is envisaged to contribute to realization of a stable and effective q profile control system in a Tokamak.