206 research outputs found
Prediction of temperature profiles in helical plasmas by integrated code coupled with gyrokinetic transport models
No full text0000-0002-0712-8811Transport simulation is performed by integrated code using reduced transport models (Toda S et al 2019 Phys. Plasmas 26 012510) in a kinetic electron condition for turbulent heat transport including the effect of zonal flows in helical plasmas. A reduced model can be formulated for the heat diffusivity using only the linear properties, or can be constructed by considering the expression of the quasilinear flux. These reduced models reproduce nonlinear gyrokinetic simulation results for ion temperature gradient mode turbulence by a linear growth rate and zonal flow decay time. Temperature profiles can be obtained when the turbulent heat transport is evaluated by reduced models at each time step in the evolution of integrated simulation. Computational cost using the reduced models where linear gyrokinetic simulation is performed at each time step in the integrated simulation is about two orders of magnitude lower than that using nonlinear gyrokinetic simulation. Stationary temperature profiles are predicted by simulation, in which, the linear simulation is performed at each time step in the integrated simulation for steady heating power. The density profile and the edge temperature are needed in this simulation.journal articl
Research of turbulent transport due to dissipative trapped electron mode in tokamak plasmas
No full textORCID 0000-0002-0712-8811The purpose of this article is to study turbulent transport for laboratory plasmas in toroidal devices by gyrokinetic analyses. Linear analysis is performed to clarify the dominant mode for tokamak plasmas. The dissipative trapped electron mode (d-TEM) and the ion temperature gradient (ITG) mode are predicted using the Sugama collision model operator [Sugama et al., Phys. Plasmas 16, 112503 (2009)]. Nonlinear gyrokinetic analysis is used to quantify turbulent transport. The nonlinear simulation results show the levels of particle and energy transport, where the d-TEM and ITG mode are unstable. The effect of zonal flows is studied by the linear and nonlinear simulation results. The results of the analysis are compared when two types of model collision operator, which are the Sugama and Lenard–Bernstein [Phys. Rev. 112, 1456 (1958)] collision model operators, are used. In this study, the simulation results using the Sugama collision operator show a stronger effect of the zonal flows on the turbulent transport than those using the Lenard–Bernstein collision operator, as predicted by the linear simulation result such as the zonal flow decay time.journal articl
Marginal sea overflows and the upper ocean interaction
No full textAuthor Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 39 (2009): 387-403, doi:10.1175/2008JPO3934.1.Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume.
The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more.
Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentum flux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone.
Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger.SK’s support during the time of his Ph.D.
research in the MIT/WHOI Joint Program was provided
by the National Science Foundation through
Grant OCE04-24741. JP and JY have also received
support from the Climate Process Team on Gravity
Current Entrainment, NSF Grant OCE-0611530. JY has
also been supported by NSF Grant OCE-0351055
Transport Analysis of Dynamics of Plasma Profiles in Helical Devices
No full textPlasma dynamics and structure are studied using a one-dimensional theoretical model for the anomalous transport diffusivities. In this analysis, the high collisional Pfirsch-Schl?ter regime is examined and the anomalous particle diffusivity is employed. The reduction of the anomalous particle diffusivity and a steep gradient in the density profile can be obtained. This prediction may be the theoretical explanation for the internal diffusion barrier observed in super dense core plasmas of large helical device.journal articl
Study of the Effect of the Helical Ripple Transport on the Confinement via Zonal Flows in Helical Plasmas
No full texthe role of the effective helical ripple in the helical plasma confinement is analyzed using the transport code via the effect of zonal flows. One-dimensional coupled transport equations are calculated in the different cases of the effective helical ripple. The reduction of the effective helical ripple ratio lowers the criterion for the excitation of zonal flows in helical plasmas. It is demonstrated that the reduction of the turbulent transport can be obtained in the case of the smaller neoclassical transport when zonal flows are excited.journal articl
Theoretical Analysis of Transport Barriers in Helical Systems
No full textlntroduction The Abstract A set of one-dimensional model equation in helical system is analyzed including the electric field bifurcation. The spatial and temporal evolutions of the temperature and the electric field are examined. A spatial structure which is related with the edge transport barrier is duscussed. A self-generated oscillation of the edge temperature and the heat flux loss takes place under the constant heat flux from the core. The oscillation occurs near the transition boundary. The transport barrier in the inner region is found in the high confinement state
Study of the Effect of the Helical Ripple Transport on the Confinement via Zonal Flows in Helical Plasmas
No full text
Stable Self-homotopy Groups of Complex Projective Plane
No full textWe compute the stable homotopy group Gk(CP2) = ΣkCP2,CP2} of the complex projective plane CP2 for k ≤ 20 by using the exact sequence associated with canonical cofiber sequence and Toda bracket.</p
Overflows and upper ocean interactions : a mechanism for the Azores current
No full textThesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 155-162).The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows are a major source of the dense water of the global deep ocean, originating from only a few marginal seas. They enter the open ocean as dense gravity currents down a continental slope and play a crucial role in the deep ocean circulation. To understand the dynamics of these overflows, previous studies simplified their dynamics by treating the overlying ocean as inactive. This simplification may be a first approximation for the overflow but not for the overlying ocean. The Mediterranean overflow, for example, entrains about 2 Sv of overlying Atlantic water when it enters the Atlantic through Gibraltar Strait. The upper ocean must balance the mass loss and vortex stretching associated with entrainment. Thus for the upper ocean, overflows represent a localized region of intense mass and PV forcing. The simulations in this study show that in the upper layer, entrainment forces a cyclonic circulation along bathymetric contours. This is a topographic [beta]-plume and its transport depends on the entrainment region size and the topographic slope.(cont.) Baroclinic instability also develops and creates eddy thickness flux to the in-shore direction, forcing a double gyre topographic [beta]-plume near the strait due to eddy PV flux convergence on the in-shore side of the continental slope and divergence on the offshore side. When the upper oceanic response to overflows is examined specifically for the Mediterranean overflow, the upper ocean is found to establish two trans-Atlantic zonal jets, analogous to the Azores current and the Azores Counter current. These two zonal jets are an extension of the topographic [beta]-plume driven by the overflow. Because the eddies in the steep slope region near Cape St. Vincent drive a mean flow across the slope, the topographic [beta]-plume connects to the Atlantic Ocean to become a basin scale flow. This thesis shows that overflows can induce a significant circulation in the upper ocean, and for the Mediterranean overflow, this circulation is a basin scale flow.by Shinichiro Kida.Ph.D
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