Exploring experimental isotope scaling and density limit in tokamak transport
J. Weiland, T. Rafiq, E. Schuster
Plasma 2024, 7, 180-792
Abstract
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As it turns out, both isotope scaling and density limits are phenomena closely linked to fluid
closure. The necessity to include ion viscosity arises for both phenomena. Thus, we have added ion
viscosity to our model. The experimental isotope scaling has been successfully recovered in our fluid
model through parameter scans. Although ion viscosity typically exerts a small effect, the density
limit is manifested by increasing the density by approximately tenfold from the typical experimental
density. In our case, this increase originates from the density in the Cyclone base case. Notably, these
phenomena would not manifest with a gyro-Landau fluid closure. The isotope scaling is nullified
by the addition of a gyro-Landau term, while the density limit results from permitting ion viscosity
to become comparable to the gyro-Landau term. The mechanism of zonal flows, demonstrated
analytically for the Dimits upshift, yields insights into the isotope scaling observed in experiments.
In our approach, ion viscosity is introduced in place of the Landau fluid resonances found in some
fluid models. This implies that the mechanism of isotope scaling operates at the level of fluid closure
in connection with the generation of zonal flows. The strength of zonal flows in our model has
been verified, particularly in connection with the successful simulation of the nonlinear Dimits
shift. Consequently, a role is played by our approach in the temperature perturbation part of the
Reynolds stress.