Nonlinear excitation of particle fluxes by turbulence spreading in tokamak plasma

Sustainment of a high-density plasma is an essential issue in fusion reactors, and penetration of particles deep into the core is necessary after injecting solid fuel pellets. One of the promising methods is to utilize a radial inward particle flux induced by plasma turbulence. In this paper, a global model is used to simulate the plasmas after post-ablation by introducing a peaked density profile as an initial condition. Nonlinearly sustained inward fluxes was observed, so nonlinear analyses are carried out to evaluate the energy balance of the fluctuation modes, which identifies dominant nonlinear couplings that drive the inward particle flux. The sustainment includes two fundamental mechanisms: (i) nonlinear mode couplings associated with local linear unstable modes and nonlinearly excited modes, and (ii) nonlocal turbulence spreading from the strong gradient region. This study provides the basis to increase an inward turbulent flux in the inverted particle gradient region.