Fast particle effects and microturbulence: Stability, transport and size scaling

In the process of producing fusion grade plasmas, Tokamaks are expected to become highly populated with energetic or fast particles produced by auxiliary heating schemes and/or fusion born alpha particles. For efficient heating of a fusion grade machine, it is important that these energetic particles stay in the system for long enough time to be able to heat the background thermal plasma. In the past, while the interaction of the energetic particles with Magnetohydrodynamics (MHD) modes has been understood to some extent, there has been little rigorous numerical study on the interaction of the fast particles with microturbulence, especially, the one driven by the temperature gradient of the thermal ions and trapped electrons. In the following, we describe our recent studies on the interaction of fast particles on microturbulence in particular ion temperature gradient-driven modes (ITGs). Using a nonperturbative global code EM-GLOGYSTO we address the linear stability of global ITGs in the presence of fast particles. We further demonstrate, using a fully nonlinear, global gyrokinetic code GTS, that the transport of hot ions exhibits subdiffusive nature at smaller system size which eventually becomes diffusive as the system size increases, with its magnitude tending to saturate at larger system size. We also show that the transport of hot ions with lower energy remains virtually independent of temperature gradient of thermal ions, while that of the hot ions with higher energy increases with the temperature gradient of thermal ions. Some of the open areas where more work needs to be done is also addressed.