Development of 2D implicit particle simulation code for ohmic breakdown physics in a tokamak

A physical mechanism of an ohmic breakdown in a tokamak has not been clearly understood due to its complexity in physics and geometry especially for a role of space charge in the plasma. We have developed a 2D implicit particle simulation code BREAK, to study the ohmic breakdown physics under a realistic complicated situation considering the space charge and kinetic effects consistently. The ohmic breakdown phenomena span a broad range of spatio-temporal scales, from picoseconds order of the electron gyromotion to milliseconds order of the plasma transport. It is impossible to employ a typical explicit particle simulation method to see the slow plasma transport phenomena of our interest, because a time step size is restricted to be smaller than a period of the electron gyromotion in the explicit scheme. Hence, we adopt several physical and numerical models, such as a toroidally symmetric model and a direct-implicit method, 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. The performance of BREAK is verified with several test cases so that BREAK is expected to be applicable to investigate the ohmic breakdown physics in the tokamak by considering 2-dimensional plasma physics in the RZ plane, self-consistently.