Turbulent energy transfer into zonal flows from the weak to the strong flow shear regime in the stellarator TJ-K

Abstract

The transition from low to high confinement in fusion experiments is accompanied by a reduction of turbulence in the strong shear regime. This work investigates the influence of the background shearing rate on the energy transfer between turbulence and zonal flows, which can serve as a loss channel of kinetic energy, in the different shear regimes using the k−ε model. To this end, plasma biasing is used to control the flow shear, which is categorized in terms of measured turbulent lifetime. The shearing rate scaling of Reynolds stress and zonal flow production is analyzed. A linear dependency of the Reynolds stress and a quadratic dependency of the energy transfer on the shearing rate are found. This is accompanied by a redistribution of the spectral power toward the zonal flow. The increase in relative zonal power is even higher beyond the transition to the strong shear regime.