Roles of Particle Dynamics in Merging Processes of Two Spherical-Tokamak-Type Plasmoids

The merging processes of two spherical-tokamak-type plasmoids (STs), which are confined in a rectangular conducting vessel, is investigated by means of two-dimensional PIC simulation [1,2]. The reconnection current, which has an average electron velocity with an opposite sign to that of the equilibrium current inside two STs, grows in the interface region as two STs approach. When two STs merge, two-component electron distribution function with different velocity shifts along a toroidal magnetic field appears around a reconnection point, leading to the formation of a sharp peak of the electron parallel temperature profile. The same phenomenon was observed in the MAST merging experiments [3]. On the other hand, the convergent motions of two STs towards the merging region create a large ion average velocity perpendicular to the toroidal magnetic field. A phase-space mixing of the distribution functions takes place through stochastic particle motions in the merging region. In this way, the merging process of two STs leads to the increases in a parallel component of the electron temperature and a perpendicular component of the ion temperature while keeping the other components almost constant. A part of poloidal magnetic energy dissipates and is transferred to the ions and the electrons at an approximate rate of 2:1 through the merging process. It is also found that the energy dissipation process is mainly controlled by the electron dynamics, while the merging time scale is controlled by the ion dynamics.

[1] R. Horiuchi, et al., Plasma Fusion Res., 13, 3403035 (2018). [2] R. Horiuchi, et al., submitted to Physics of Plasmas. [3] H. Tanabe, et al., Nucl. Fusion 57, 056037 (2017)