In this Letter, we report analyses of spatiotemporal dynamics of turbulence and structure in the limit-cycle oscillation (LCO) that precedes an L-to-H transition. Zonal flows are not observed during LCO, and the oscillation is the periodic generations or decays of barrier with edge-localized mean flow. Oscillatory Reynolds stress is found to be too small to accelerate the LCO flow, by considering the dielectric constant in magnetized toroidal plasmas. Propagation of changes of the density gradient and turbulence amplitude into the core is also observed.
Heavy ion beam probe measurements of the interior of the TEXT-U tokamak plasma have been used to discover a density fluctuation spectrum that exhibits poloidal asymmetry with ballooning characteristics. The asymmetry was eliminated at the smallest radii during the electron cyclotron heating phase, which has a flatter density profile and a higher electron temperature
Electron temperature fluctuations with long-distance correlation have been discovered in LHD. This paper reports the extended observations recently made on the spatiotemporal structure of the long-range fluctuations both in quasi-stationary and transient plasmas. The detailed characteristics or spatiotemporal characteristics of long-range temperature fluctuations are revealed successfully using correlation analysis. Particularly, the dynamics of the long-range fluctuations is investigated to find that the amplitude of the fluctuations decreased and their radial correlation lengths shortened during the transient phase induced by pellet injection. Temporal changes of radial correlation structure and amplitude of fluctuations at the onset of change in the plasma state are discussed.
The role of zonal flows in the formation of transport barrier in helical plasmas is analysed using the transport code. A set of one-dimensional transport equations is analysed, including the effect of zonal flows. The turbulent transport coefficient is shown to be suppressed when the plasma state takes the strong positive radial electric field. This bifurcation of the turbulent transport is newly caused by the change of the damping rate of zonal flows. It is theoretically demonstrated that the damping rate of zonal flows plays an important role in global confinement in toroidal plasmas.
In this paper we present space potential profiles directly observed in a toroidal helical plasma of the Compact Helical System ͑CHS͒ ͓K. Matsuoka et al., Proceedings, 12th International Conference on Plasma Physics and Controlled Nuclear Fusion, Nice, 1988 ͑International Atomic Energy Agency, Vienna, 1989͒, Vol. 2, p. 411͔, using a 200 keV heavy ion beam probe. The potential profiles exhibit widely varied characteristics, including positive and negative polarities for electron cyclotron and neutral beam-heated plasmas, respectively. The behavior of high-energy particles in the CHS plasmas are deduced from loss cone diagrams evaluated from the observed potential profiles.
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