Theory and MHD simulation of fuelling by compact toroid injection

The process of fuelling by injection of a spheromak-like compact toroid (SCT) is investigated by using MHD numerical simulations, where the SCT is injected into a magnetized target plasma region corresponding to a fusion device. In a previous study, the authors proposed a theoretical model to determine the penetration depth of the SCT into the target region on the basis of simulation results; in that study the SCT is decelerated not only by the magnetic pressure force but also by the magnetic tension force. However, since both ends of the target magnetic field are fixed on the boundary wall in the simulation, the deceleration caused by the magnetic tension force would be overestimated. In the present study, the dependence of the boundary condition of the target magnetic field on the SCT penetration process is examined. On the basis of these results, the theoretical model previously proposed is improved to include the effect that the fluctuations of the target magnetic field propagate with the Alfven velocity. In addition, by carrying out the simulation with the torus domain, it is confirmed that the theoretical model can be applied to estimate the penetration depth of the SCT under such conditions. The dependence of the injection position (side injection and top/bottom injection) on the penetration process is also examined. (author)