Accounting for Saturation Efficiency in Quasilinear Transport Models

Ion Temperature Gradient instability (ITG) is a major contributor to turbulent transport in tokamaks. Plasma turbulence has numerous linear eigenmodes per wavenumber, most of which are stable. ITG saturates through nonlinear energy transfer involving zonal flows, scattering energy to higher radial wavenumber stable and ustable modes. Finite normalized plasma pressure beta significantly reduces ITG transport. We investigate a modified Cyclone Base Case where quasilinear estimates underpedict the stabilization due to the impact of nonlinear physics. We probe the cause of enhanced stabilization and measure of the importance of stable modes in energy dissipation and transport. At low toroidal wavenumber, stable modes increase energy production relative to the unstable mode by changing phase relations, while they cause energy dissipation at higher wavenumbers. Transport is also enhanced by stable modes at low wavenumber. This does not affect nonlinearly enhanced stabilization however. The transport reduction occurs because nonlinear energy transfer becomes more efficient with beta as the interaction time between modes becomes more resonant. This is measured by the triplet correlation time, given by the difference in frequencies between the unstable mode, stable mode and zonal flow. A quasilinear model scaled with triplet correlation time correctly matches nonlinear transport. We repeat this analysis with parameters from several experimental discharges, including a JET and an ASDEX Upgrade case. Stable mode effects were significantly different in each of them, but only varied significantly with beta for the JET case. In cases where beta did not change stable mode physics, including the triplet correlation lifetime improved transport estimates.