A new quasilinear saturation rule for tokamak turbulence

The confinement of tokamak plasmas is largely determined by the presence of turbulence, as
a result of its associated radial transport of energy and particles. This level of transport is observed
to vary with isotope mass, however with an experimental scaling relation in opposition
to that suggested by simple theory. A greater understanding of the isotope dependence of
confinement is therefore required for the development of operating scenarios in future devices.
Integrated plasma simulators can be used to make predictions of confinement, which calculate
transport via quasilinear turbulence models. The simplifications employed in these models
to enable their required computational efficiency can cause their transport predictions to
deviate from those of the more accurate but prohibitively expensive framework of nonlinear
gyrokinetics, motivating their continued verification and improvement.
Steep gradients in the plasma density characteristic of the tokamak edge coupled with the
non-adiabatic response of the electrons can drive turbulence dominated by the trapped electron
mode. Nonlinear gyrokinetic simulations demonstrate that the resulting local transport
exhibits isotope scaling reversal, which contemporary quasilinear models are unable to replicate
due to the absence of the relevant physics in their description of the turbulence. Through
analysis of gyrokinetic spectra this work attributes the physical origin of this discrepancy to
the saturation level of the fluctuating electrostatic potential. A new quasilinear rule SAT3 is
constructed to extend previous descriptions of turbulent saturation via the incorporation of
differing saturation levels depending on the turbulence regime. This enables the isotope scaling
reversal of the trapped electron mode to be described quasilinearly for the first time, whilst
retaining the more established description of ion temperature gradient driven turbulence in
the appropriate parameter space.
The new model is validated against data from the recent JET isotope experiments in H, D
and T. SAT3 is seen to perform well in integrated modelling simulations, however further
investigation into the relative contributions of the various physical effects present is required.