Dynamics of turbulence spreading in magnetically confined plasmas

A dynamical theory of turbulence spreading and nonlocal interaction phenomena is presented. The basic model is derived using Fokker-Planck theory, and supported by wave-kinetic and K-ε type closures. In the absence of local growth, the model predicts subdiffusive spreading of turbulence. With local growth and saturation via nonlinear damping, ballistic propagation of turbulence intensity fronts is possible. The time asymptotic front speed is set by the geometric mean of local growth and turbulent diffusion. The leading edge of the front progresses as the turbulence comes to local saturation. Studies indicate that turbulence can jump gaps in the local growth rate profile and can penetrate locally marginal or stable regions. In particular, significant fluctuation energy from a turbulent edge can easily spread into the marginally stable core, thus creating an intermediate zone of strong turbulence. This suggests that the traditional distinction between core and edge should be reconsidered.