Role of 3D neoclassical particle flux in density pump-out during ELM control by RMP in DIII-D

The application of three-dimensional (3D) resonant magnetic perturbations, for the purpose of controlling edge localized modes (ELMs) in high confinement (H-mode) tokamak fusion operations, routinely leads to a significant reduction of the plasma density during the discharge, a phenomenon termed 'density pump-out'. Understanding the density pump-out phenomenon due to 3D fields is a longstanding challenging issue. This work reports a self-consistent toroidal model that allows initial value simulation of the density pump-out effect. The model successfully reproduces pump-out phenomenology in DIII-D ELM suppression experiments. The radial particle flux, associated with the 3D neoclassical effects (the neoclassical toroidal viscosity, NTV), is found to play an important role in producing the observed large density pump-out. The key physics is associated with the drift kinetic Landau resonances between the toroidal precession of trapped thermal ions and electrons from one side, and the magnetic field perturbation from the other side. Such resonances, occurring near the top of the pedestal region, substantially enhances the radial particle flux associated with NTV. As a general consequence of the resonant NTV flux, larger density pump-out occurs in plasmas with lower collisionality and slower toroidal rotation.