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P: Fachverband Plasmaphysik

P 7: Magnetic Confinement III

P 7.6: Talk

Tuesday, March 17, 2026, 18:30–18:55, KH 02.016

Geometry-dependent energetic bounds on gyrokinetic instabilities — •Paul Costello and Gabriel Plunk — Max-Planck-Institut für Plasmaphysik, Wendelsteinstraße 1, 17491 Greifswald, Germany

Outward turbulent transport of heat and particles largely dictates the necessary size of a viable tokamak or stellarator power plant. This turbulence is initiated by micro-scale plasma instabilities that are driven by the radial gradients of temperature and density in the plasma. Gyrokinetic theory forms the basis of our understanding of these instabilities, and gyrokinetic simulations are a relied-upon, albeit expensive, tool for computing their growth rates. Here, we present a complementary approach to understanding linear instabilities: constructing energetic upper bounds on instability growth. These upper bounds are computed via a variational principle by seeking perturbations that maximize the extraction of free energy from the plasma gradients. In many cases, these upper bounds are governed by a low-dimensional system of equations, while still capturing the dependence of the linear growth rate on key parameters. We demonstrate methods to include magnetic geometry into the upper bound analysis through two main pieces of work. Firstly, we consider the effect of including trapped electrons in the analysis, finding a new stabilising mechanism possessed by quasi-isodynamic stellarators. Secondly, we present a method by which the upper bound can systematically be made tighter to the linear growth rate. We highlight the potential applicability of this approach to the optimisation of stellarators to reduce turbulent transport.

Keywords: Turbulence; Gyrokinetics; Instabilities