Spatial characterization of edge zonal flows in the TJ-II stellarator: the roles of plasma heating and isotope mass

Abstract

In this article, we report the latest results for the impacts of the isotope mass and the plasma heating scenario on the spatiotemporal characteristics of the low-frequency zonal flow (ZF)-like electrostatic oscillation found in the plasma edge of the TJ-II stellarator. The radial profile of the toroidal cross-coherence in low-frequency floating potential fluctuations (1 < f < 25 kHz), known as long-range correlations (LRCs), was explored along the plasma boundary region by a dual Langmuir probe system. The study was performed using electron cyclotron resonance heating (ECRH)- and neutral beam injection (NBI)-heated plasmas with two different isotope concentrations: pure hydrogen plasmas and plasmas dominated by deuterium ( ≈ 70 % D ). The highest toroidal coherence levels are detected for floating potential oscillations at less than 10 kHz in hydrogen and deuterium plasmas, in both the ECRH and NBI heating schemes. The radial extent of the LRC profile is clearly larger in the ECRH than in the NBI-heated plasma scenarios. While the maximum amplitude of the LRC does not show a significant dependence on isotope mass, the relation between the LRCs’ radial sizes in plasmas and different isotope masses increases with the isotope mass in NBI-heated scenarios. These results provide the first experimental observation of ZFs with a larger radial size in deuterium than in hydrogen plasmas.

Characterization of isotope effect on ion internal transport barrier and its parameter dependence in the Large Helical Device

“…The effect of fast ions on ion mass dependent core thermal transport has been investigated [11]. Turbulence driven symmetric plasma flow, so-called zonal flow, is suggested to be a candidate for an essential player in the isotope effect, and experimental [12][13][14] and theoretical [15] examinations have been intensively performed. The isotope effect is also categorized in two main classes: the direct one and the indirect one.…”

Observation of low frequency electrostatic coherent oscillation in the proximity of the density limit in the TJ-II stellarator

“…As shown in figure 4, the LRC value is strongly intermittent and varies for different probes, being maximum when LOP05 is correlated with LOP106 or LOP107, probes that are located at a similar radial position. This shows that the global structures are radially localized, radial size of these structures depends on plasma heating scenario and ion mass [31]. The high level of the cross-coherence with near zero phase and the radial localization points towards the existence of low frequency structures with ZF like properties.…”

H-mode transition in the TJ-II stellarator plasmas

“…Pure hydrogen versus deuterium dominated (up to 70%) plasmas did not show any noteworthy difference when comparing Er profiles, amplitude of ZFs or plasma turbulence reduction during the L-H transition [47]. Interestingly, the radial width of the ZF structure is affected by the ion mass, being about 1.5 times larger in D than in H plasmas [48].…”