Nondiffusive particle transport in the stellarator experiment TJ-K

Abstract

Current transport modeling follows a local, diffusive paradigm to describe the dynamics in the edge of magnetically confined plasmas. In the presence of steep gradients, the applicability of local transport theory can break down. In the low-temperature plasmas of the TJ-K stellarator, the extent and characteristics of nonlocal transport contributions are investigated experimentally. In this context, the convection–diffusion equation, which obeys a conventional diffusive formalism, is chosen. However, the predicted linear flux-gradient relationship differs significantly from the behavior observed in most discharges. A second method, which requires no previous assumptions on the diffusivity of particle transport, is applied. This model confirms the presence of significant nondiffusive contributions in the turbulent particle transport of TJ-K. Furthermore, three different regimes were identified at large, intermediate, and small scales. These could be governed by conventional, superdiffusive, and subdiffusive processes, respectively. All three regions persist across discharges at different gases and pressures.