Speaker
Description
The success of next-generation tokamaks such as ITER relies on minimizing the turbulent heat transport that limits confinement and on avoiding macroscopic instabilities that can lead to disruptions. Toroidal plasma rotation is known to be beneficial in both regards: It stabilizes the resistive wall mode and suppresses turbulence. Many current experiments induce rotation by applying an external torque, but this will be less efficient in large, dense plasmas such as ITER. However, even in the absence of external torque, tokamak plasmas spontaneously rotate. This `intrinsic' rotation is a generic feature of tokamak plasmas, but it is not understood well enough to predict reliably the rotation in experiment. A major reason for this uncertainty is that the models used to simulate fusion plasmas are not of a sufficient physical fidelity in order to capture the effects---such as large-scale variations in the pressure and magnetic geometry profiles---needed to describe intrinsic rotation correctly.
To that end, we develop a novel approach to gyrokinetics where multiple flux-tube simulations are coupled together in a way that consistently incorporates global profile variation while retaining spectral accuracy. By doing so, the need for Dirichlet boundary conditions, where fluctuations are nullified at the simulation boundaries, is obviated. These conditions, which are typically employed in global gyrokinetic simulation, prevent convergence to the local periodic limit unless large simulation domains are utilized. Thus, our method of global-local gyrokinetics is appropriate for simulations of the pedestal region where the generation of intrinsic momentum is expected to commence and the details of boundary physics are important. Preliminary results from simulations with equilibrium flow shear using this approach are compared to simulations using conventional global methods and to local flux-tube simulations with wavenumber-remapped flow shear. Progress is also reported on implementing profile variation in both the plasma pressure and magnetic geometry.
