Interplay of turbulent density and momentum transport

• Applicant: Mirko Ramisch, Ph.D.
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 471314753

Turbulent dynamics, e.g. in fluid-like systems, are a multi-scale problem, in which the understanding of structure formation through self-organization processes is still one of the great challenges of today’s sciences. Turbulent systems characterized by efficient mixing properties and, thus, material transport, can undergo spontaneous transitions, where energy is transferred to macroscopic structures, away from scales dominating turbulent transport. The geometry of the system in particular determines the effect of non-linear drive as opposed to damping, and thus the dynamics’ characteristics. Understanding in detail, how geometry affects the capability for this self-organization processes, provides a route to active control of turbulent transport.Besides the band-like structures formed on Jupiter as a natural example, another concrete consequence of such processes is the formation of zonal flows in magnetically confined plasmas. Since micro-instabilities degrade the confinement through turbulent transport, the transfer of energy to zonal shear flows is associated with the formation of a transport barrier, leading to a transition into a regime of improved confinement. Inhomogeneities in the magnetic field can favor the formation of zonal flows. Understanding the underlying physics is important for shaping the magnetic cage as subject of future numerical optimization for the confinement of hot fusion plasmas.In this project, the interplay between turbulent density and momentum transport is investigated, with regard to their spatial dependence on the magnetic-field structure. To this end, measurements are envisaged in the magnetic plasma-confinement experiment TJ-K, which features unique access options. The magnetized low-temperature plasmas are diagnosed with arrays of Langmuir probes, providing high spatial and temporal resolution to address simultaneously density and plasma-potential fluctuations, both governing the plasma ExB-drift dynamics, on multiple scales. The momentum transport, as a measure of vortex tilt, determines the Reynolds-stress drive of zonal flows. In previous studies, both transports have been found to peak in coinciding regions. While density transport relies on polarization of density perturbations, i.e., decoupling of density and plasma-potential perturbations, momentum transport is based on a strong coupling between the perturbations. This project will resolve this contradictory behavior by correlating the dynamical interrelation of transports with the temporal density-potential cross-coupling in conjunction with the occurrence of zonal structures. The spatial structure of the turbulent quantities will be correlated with numerically calculated characteristics of the magnetic field.

• DFG Programme: Research Grants