On turbulence driven stationary electric currents in a tokamak

Abstract

This paper investigates the influence of turbulent dynamics on the neo-classical equilibrium in a tokamak, with an emphasis on the turbulence driven stationary electric current. The neo-classical solution is evaluated using the Hirschmann-Sigmar formalism, in which the turbulent dynamics enter as a forcing term. The latter forcing terms are evaluated through time averages of gyro-kinetic turbulence simulations and are linked with the velocity non-linearity in the gyro-kinetic equation. The time averaged turbulent forcing terms connected with the velocity non-linearity provide a non-negligible current drive, despite being a correction of second order in the normalized Larmor radius. For ITG turbulence, the force exerted due to the heat flux balance is the dominant contribution to the current. The parallel fluctuations of electron density/temperature and the electrostatic potential drive the majority of the current, which is in magnitude comparable to the bootstrap current in the kinetic cyclone base case and increases the total current by a few percent in cases with an experimentally relevant heat flux. An up-down symmetry breaking mechanism is required for turbulent current drive, which is provided in this study by a background rotation or rotation gradient. Consequently, the current is nearly linear in the plasma rotation or its gradient.