Role of Edge Stochastic Layer in Density Pump-out by Resonant Magnetic Perturbations

Formation of an edge stochastic layer is shown to cause the pump-out of density as observed in DIII-D plasmas with applied Resonant Magnetic Perturbations (RMPs) but without exhibiting the suppression of Edge Localized Modes (ELM). In these plasmas RMP penetration near the zero-crossing of the radial electric field E_r at the plasma foot leads to density pump-out. Using an analytical model that adds the stochastic parallel transport of electrons to the fluid equations, the ambipolar E_r and particle flux are calculated simultaneously. In this model the nonambipolar electron flux, driven by the stochastic magnetic field, is predominantly balanced by the nonambipolar perpendicular ion flux, driven by anomalous viscosity, across a narrow stochastic layer in the last ≈ 2% of the plasma minor radius (0.98<ѱ_n<1). Stochasticity driven enhanced transport causes the flattening of the density profile in the pedestal foot, which leads to the pump-out at the pedestal top. Using the viscosity coefficient calculated by TRANSP, and RMP amplitudes calculated by the TM1 code, the model reproduces the level of RMP driven density pump-out for three plasma discharges for which the pedestal foot is at medium to high collisionality (2≤ν^∗_sep≤ 30). The experimentally observed inverse density dependence of density pump-out is accurately captured by the model: an increase in collisionality with density in the pedestal foot results in a decrease in stochastic diffusivity, and hence a decrease in the level of pump-out. This improved understanding of pump-out can enhance the capability for predicting the effect of RMP on particle confinement in ITER.