Novel Mechanisms of Ohmic Breakdown in a Tokamak by considering Plasma Responses

Ohmic breakdown is generally used to produce initial plasmas in tokamaks. However, the complex electromagnetic structure of tokamaks has obscured the physical mechanism of ohmic breakdown for several decades. Previous studies ignored plasma responses to external electromagnetic fields and adopted only the simplest Townsend avalanche theory. However, we found that the Townsend theory cannot explain mysterious experimental results. A theoretical model and a 3D particle simulation code, BREAK, are developed to study the previously unelucidated physical mechanism of the ohmic breakdown in a tokamak by considering plasma response self-consistently. The toroidally symmetric plasma model can facilitate a simple, clear understanding of breakdown physics in the RZ plane without considering unimportant fluctuations in the toroidal direction. The BREAK code is developed to verify the analytic modeling and to study detail mechanism of the ohmic breakdown under a realistic complicated situation considering the space charge and kinetic effects consistently. Since the ohmic breakdown physics is involved in various spatio-temporal scales, the conventional PIC-MCC method which requires many restrictions, such as Δt<ω^^(-1) and Δx<λ_De, cannot be applied due to incredibly large computation time. Various numerical schemes, such as a direct-implicit method, are adopted to relax or remove the spatio-temporal restrictions. In addition, coalescence strategies are introduced to control the number of numerical super particles within acceptable ranges to handle the exponentially growing plasma density during the ohmic breakdown. As a result, BREAK can allow reasonable computation time with much larger time step and cell size. The performance of BREAK is verified with several test cases and BREAK is applied to the artificial simplified scenarios and the reference breakdown scenario of KSTAR to investigate the ohmic breakdown physics in the tokamak by considering the 2-dimensional plasma physics in the RZ plane, self-consistently. In this dissertation, we propose a new theory of ohmic breakdown that consistently considers the plasma response and successfully explains experimental results. As the plasma response, self-electric fields produced by space-charges were found to be crucial for significantly reducing plasma density growth rate and enhancing anomalous perpendicular transport via ExB drifts. A three-dimensional particle simulation clearly captured these effects and provided a good reproduction of the mysterious experimental results in KSTAR. These new physical insights into complex electromagnetic topology provide general design guideline for a robust breakdown scenario in a fusion reactor