Speaker
Florin Spineanu
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1104.pdf
The helicity and the generation of large scale flows in confined plasma
F. Spineanu, M. Vlad
National Institute of Laser, Plasma and Radiation Physics, Bucharest, Romania
The formation of quasi-coherent, large scale structures in turbulent plasma is known from
numerical simulations and from experimental observations. In particular, formation of a layer of
sheared quasi-laminar poloidal rotation which acts as a transport barrier (ITB, Internal Transport
Barrier) has been observed. It is not always clear what are the conditions which trigger ITB
formation and in gyrokinetic simulations some additional factor, beyond the Reynolds stress,
appears to be necessary.
We prove that an essential element leading to the formation of coherent, sheared poloidal
flows, is the turbulent helicity. Currently, this term is absent from the statistical analysis of the
turbulence, with the consequence that the Reynolds stress seems ineffective to convert the tur-
bulence into Internal Transport Barrier. In a basic statistical analysis of turbulence the two-point
correlations must include a term which breaks the parity invariance and in spectral representa-
tion is purely imaginary. It is not possible to include this contribution in a renormalization of
the linear propagator.
We propose a fundamental justification and a detailed description of the helical content of the
fluctuating field for current turbulent models, like ITG.
The connection between laminar sheared flow (like in the H-mode rotation layer) and the
filamentary structures (like in Edge Localized Modes or, more generally, in Kelvin-Helmholtz
fluid instability) is examined as a conversion of the uniform vorticity into topological linking of
helical streamlines. We will provide a quantitative description of this process. Conversely, the
effect of turbulent distribution of Gaussian linking is shown to contribute to an inverse cascade.
The approach developed in this work is inspired by neutral turbulent fluids. However, the
plasma instabilities have a natural helical content at the scale of the vorticity advection (of the
order of sonic Larmor radius).
