TJ-II:Investigation of turbulence spreading and information transfer in the TJ-II stellarator: Difference between revisions

Motivation. Turbulence spreading or radial transport of turbulence energy occurs because inhomogeneous turbulence generically tends to redistribute its energy from regions where dominant free energy sources are located to regions which do not access external free energy sources directly. This causes the so call spreading of turbulence from regions that are strongly driven (i.e., unstable regions) into regions which are only weakly driven [1 and references therein].

Plasma edges in stellarators can be quite different than edges in tokamaks. In particular, configurations with the long connection lengths in stellarators means that cross-field transport can compete with parallel transport along open field lines. Clarifying whether the SOL width is dominated by local effects at the SOL region or/and by transport driven in the plasma edge is a relevant question. Recent experiments in the TJ-II stellarator have shown that SOL profiles are coupled with edge plasma parameters and that the strength of this coupling depends on the magnitude of the edge shearing rate and level of edge fluctuations.  Consequently optimizing SOL power exhaust would require considering transport in the edge region [Wu Ting et al., Master Thesis 2016].  

Experimental plan and goals. The goal of this research work is to investigate the influence of isotope mass and sheared flows on the coupling between core / edge and SOL regions including the following actions:

• Characterization of the structure of edge ExB flows and edge turbulence during the electron-ion root transition.

• Analysis of these edge/core perturbations using the Transfer Entropy [2], to quantify the flow of information from the edge to the SOL

• Characterization of core-edge-SOL turbulence spreading.

Key diagnostics: Edge probes to characterize edge-SOL coupling and Dual HIBP to characterize core – edge coupling