Dependence of Perpendicular Viscosity on Magnetic Fluctuations in a Stochastic Magnetic Field

In a magnetically confined plasma with a stochastic magnetic field, the dependence of the perpendicular viscosity on magnetic fluctuation amplitude has been measured for the first time [PRL 120, 225002 (2018)]. With a controlled, ∼ 10-fold variation in the fluctuation amplitude, the viscosity increases ∼100-fold, exhibiting the same (b/B)^2 dependence as the predicted rate of stochastic field line diffusion. The absolute value of the viscosity is well predicted by a model [Finn et al., 1992] based on momentum transport in a stochastic field, the first in-depth test of this model. Derived for the tokamak, we tested this model in MST RFP plasmas. The viscosity in MST plasmas reaches about 55 m^2/s and can exceed the Braginskii prediction by several-hundred-fold. Measurements of the viscosity in a stochastic topology are rare but can be of critical importance. Viscosity is a key parameter in visco-resistive nonlinear MHD modeling, which is being applied to stochastic scenarios like tokamak disruptions. Viscosity, like the resistivity, is likely to increase substantially during a disruption. And while the modeled resistivity, or at least the electron temperature, is sometimes based on experimental data, the viscosity is often assumed.