Nonlinear Alfvén instability simulation of the ITER reversed shear regime*

The nonlinear stability characteristics and Alfvén instability driven transport are analyzed for an ITER steady-state case using the FAR3d gyro-Landau closure model. Significant levels of alpha and neutral beam ion transport are observed, at least for the duration of the simulations. This leads to localized flattening in the fast ion profiles. The Alfvén instabilities also drive inverse cascades of energy to low toroidal modes and to zonal flow shearing and zonal currents, which could play a role in setting transport barriers for the thermal plasma. The global multi-mode feature of these simulations provides detailed 2D images of the unstable modes and zonal flow structures. The energetic particle transport driven by the Alfvén instabilities is a collective effect related to the phase relations between the fluctuating fast ion density and the fluctuating E x B and magnetic perturbations. This transport is also nonlocal, with finite particle fluxes persisting in regions where the density gradients are zero. Understanding and minimizing alpha particle transport in the DT operation of ITER is an important goal of the ITER project; the maintenance of alpha confinement in reversed shear regimes is critical to demonstrate the success of steady-state tokamak operation.