Breakdown of Quasilinear Theory in the Tokamak Edge

A newly developed Lagrangian Gyrocenter Tracking code (LGT1) is used to analyze the turbulent transport by launching and tracking test-particles into the time-varying saturated turbulence, which is pre-calculated by the CGYRO Eulerian gyrokinetic code. In the edge of an L-mode tokamak plasma, particle transport and ion energy transport are shown to follow a strong microturbulence (SMT) scaling, which has a linear dependence of diffusivity on potential fluctuation amplitude [A. Ashourvan and J. Candy PRL 132, 205101 (2024)]. Conversely, in the plasma core, the transport is shown to follow quasilinear turbulence scaling, which has a quadratic dependence on potential amplitude. The transition to strong microturbulence results from larger E×B drift velocities in the edge compared to the plasma core. At these larger velocities, ions traverse the spatially correlated range faster than the stochastic evolution of the electric potential. Hence, these particles do not experience a time-stochastic field as required by the quasilinear approximation. Instead, scattering of particles in the SMT regime is caused by spatial stochasticity. In contrast, electron energy transport remains quasilinear due to decorrelations caused by collisions and fast parallel motion. Improved understanding of transport beyond quasilinear theory opens the path to more accurate modeling of transport in the tokamak plasma edge.