Greifswald 2024 – scientific programme

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P: Fachverband Plasmaphysik

P 25: Poster III

P 25.14: Poster

Thursday, February 29, 2024, 16:30–18:30, ELP 6: Foyer

Reduction of maximum gyrokinetic instability growth in maximum-J stellarators — •Paul Costello, Gabriel G. Plunk, and Per Helander — Max-Planck-Institut für Plasmaphysik, Wendelsteinstraße 1, 17491 Greifswald, Germany

For the last decade, it has been known that the growth of many local gyrokinetic instabilities, which cause turbulence, is greatly reduced in stellarators which possess the maximum-J property [1]. This a result from linear, normal mode theory.

Here, we show that this benefit of the maximum-J property is also reproduced by gyrokinetic optimal mode theory [2,3]. In this analysis, upper bounds on the maximum allowable growth of instabilities are derived by finding the perturbations which can instantaneously maximise the growth of an energy measure of the gyrokinetic system. These optimal modes may not maintain their growth indefinitely, but their growth rate is greater than any normal mode in the system. Moreover, the optimal mode growth also bounds the total growth of instabilities in the nonlinear system. We apply this optimal mode analysis to a gyrokinetic system with gyrokinetic ions and bounce-averaged electrons. We construct the optimal modes of the generalised free energy [3] of this system in toy-model magnetic fields and find that those which possess the maximum-J property show a reduced optimal growth rate.

[1] J. H. E. Proll, P. Helander, J. W. Connor, and G. G. Plunk. PRL 2012.

[2] G. G. Plunk and P. Helander. JPP 2022.

[3] G. G. Plunk and P. Helander. JPP 2023.

Keywords: Gyrokinetics; Turbulence; Stellarators