Speaker
Mr
Gustavo Grenfell
(Consorzio RFX)
Description
Turbulence spreading is the transfer of free turbulent energy from strongly (i.e., unstable regions) to weakly driven locations \cite{garbet:1994}. The net effect of this phenomenon is the radial distribution of energy, which couples different turbulent regions and changes local plasma features. It has been pointed out that spreading may be important in setting the Scrape-Off Layer (SOL) width. Since the peak divertor heat load is intimately related to the SOL width, the comprehension of the mechanisms setting the SOL properties is essential in the framework of the prediction of the SOL width in ITER. In this work, we report measurements of the turbulence drive and turbulence spreading profile, as defined by Manz, P. et al \cite{Manz:2015}, from the near edge to the far SOL region of the TJ-II stellarator, for ECRH and NBI heated plasmas. A 2-D Langmuir probe array \cite{Alonso:2015} was used to measure both parameters as well as the profiles of floating potential, plasma density, radial turbulent particle flux, effective radial velocity, turbulence correlation time and phase velocity of the fluctuations. In addition, the plasma edge was modified by means of a biasing electrode, inserted into the edge of the plasma ($\rho \approx 0.85$) and delivering a voltage $\pm$ 350 V (with respect to the wall), in a square waveform, with a frequency of 40 Hz. The transition from ion to electron root (when the heating is changed from ECRH to NBI) deeply impacts the plasma parameters, due to the establishment of a shear layer close to the Last Close Flux Surface (LCFS). The local turbulence drive is mainly reduced in the shear layer, while the turbulence spreading decreases in the SOL, where its value is higher than in the edge for both regimes. On the other hand, in the NBI at -350 V, the velocity shear reached its maximum, resulting in a strong suppression of turbulent transport and the effective fluctuating radial velocity, not only in the shear layer, but also in the far SOL. Moreover, the ion saturation profile got steeper near the shear layer and more truncated in the SOL. Finally, both the local turbulence drive and turbulence spreading are impacted by the biasing. The driving term is strongly reduced in the shear layer, and only slightly reduced in the SOL. However, turbulence spreading is mainly modified in the SOL in the vicinity of the LCFS, when the $E_r\times B$ shear reaches values close to the inverse of the turbulence correlation time. In short, biasing was found to reduce edge-SOL coupling by decreasing turbulence spreading, thus affecting the ion saturation current profile, which may have an impact in the SOL width.
Primary author
Mr
Gustavo Grenfell
(Consorzio RFX)
Co-authors
Bing Liu
(Southwest Jiaotong University, Chengdu, China)
Boudewijn van Milligen
(CIEMAT)
Carlos Hidalgo
(CIEMAT)
Mr
Carlos Silva
(IST)
Monica Spolaore
(Consorzio RFX)
Ulises Losada
(CIEMAT)
Wu Ting
(Southwestern Institute of Physics, Chengdu, China)
