Spectral treatment of gyrokinetic shear flow

Sheared E×B flow in a tokamak, driven by external torque from neutral beam injection, is known to have an important stabilizing effect on drift-wave turbulence. In gyrokinetic codes, flow shear can be implemented directly on a radial mesh with nonperiodic boundary conditions. The mesh-based implementation is straightforward, but carries the possibility of spurious effects related to simulation boundaries. Alternatively, flow shear has been implemented in spectral solvers using a wavenumber shift method. Although the spectral representation has numerous computational benefits, the wavenumber shift method for treating flow shear is of questionable accuracy. Efforts to compare mesh-based solutions with spectral ones have met with limited success. In particular, significant differences in the critical shear required to stabilize turbulence are sometimes observed. We outline a new approach to treat flow shear spectrally. The method is simple to implement, matches the nonperiodic results more closely, and predicts a critical shear that is less sensitive to radial wavenumber resolution.