Investigating shear flow through continuum gyrokinetic simulations of limiter biasing in the Texas Helimak

Abstract

Previous limiter-biasing experiments on the Texas Helimak, a simple magnetized torus, have been inconclusive on the effect of flow shear on turbulence levels. To investigate this, the first gyrokinetic simulations of limiter biasing in the Helimak using the plasma physics code Gkeyll have been carried out, and the results are presented here. For the scenarios considered, turbulence is mostly driven by the interchange instability, which depends on gradients of steady-state density profiles. An analysis of both experimental and simulation data demonstrates that shear rates are mostly less than local linear growth rates, and not all requirements for shear stabilization are met. Rather, the mostly vertical shear flow has an important effect on bulk transport and experimental steady-state density profiles, and changes in the gradients correspond to changes in turbulence levels.