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Arnas Volcokas (EPFL, Switzerland): Self-interaction impact on turbulent transport in tokamaks at very low magnetic shear

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Location: PS128

AbstractIn our work, we use local nonlinear gyrokinetic simulations of tokamaks to demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns when the magnetic shear is very weak or zero. Their length is limited only by critical balance — the distance that electrons can travel along the field line within the lifetime of a turbulent eddy. Such "ultra long" eddies can have significant consequences on turbulent transport due to parallel self-interaction. Moreover, it makes correctly treating the field line topology, in particular whether a flux surface has a safety factor that is integer, rational, near rational, or irrational, all the more important. To this end, we will show that field line topology can cause transitions between different turbulent modes and completely stabilize Ion Temperature Gradient (ITG) turbulence, both linearly and nonlinearly. Empirically, very weak or zero magnetic shear has been identified as being one of the key conditions for facilitating Internal Transport Barriers (ITBs). We present standard local gyrokinetic simulations that exhibit weak ITBs caused by the magnetic topology, which may inform a long-standing experimental observation that it is often easier to trigger ITBs where the safety factor has a low-order rational value. Lastly, we observe a novel physical effect termed ``poloidal eddy squeezing’’ — when eddies become ultra long they can cover the full flux surface and, for specific values of the safety factor, strongly interact with themselves in the perpendicular direction. This can squeeze them, reducing their perpendicular size and ability to transport energy, thereby embodying an intriguing new strategy to improve confinement in tokamaks.

Tags: CFSA Seminar

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