An approach to describe the plasma parameter variation with the effective minor radius across the edge region in the heliotron Large Helical Device (LHD) in configurations without and with a resonant magnetic perturbation (RMP) is elaborated, by averaging fluid equations for transport of particles, momentum and heat over the flux surfaces. Numerical solutions of one-dimensional, time-dependent equations derived and analytical estimates performed allow us to interpret the LHD experiments on the density limit. Calculations reproduce qualitatively the principal difference between situations without and in the presence of RMP: in the former case a thermal collapse of the discharge occurs immediately after the plasma detachment from the divertor target plates at a radiation level of 0.4–0.5; in the latter the radiating layer is localized at the plasma edge even if the power radiated exceeds 90% of the input power. A prominent role for such a behavior of plasma particle flows along magnetic field lines perturbed by RMP, leading to a positive radial gradient of the plasma density inside the magnetic island, is demonstrated.
Impact of plasma response on plasma displacements in DIII-D during application of external 3D perturbations