9,412 research outputs found
A Search for Stars of Very Low Metal Abundance. V. Photoelectric UBV Photometry of Metal-Weak Candidates from the Northern HK Survey
We report photoelectric UBV data for 268 metal-poor candidates chosen from
the northern HK objective-prism/interference-filter survey of Beers and
colleagues. Over 40 % of the stars have been observed on more than one night,
and most have at least several sets of photometric measurements. Reddening
estimates, preliminary spectroscopic measurements of abundance, and type
classifications are reported.Comment: To Appear in the Astronomical Journal, October 200
Robustness of predator-prey models for confinement regime transitions in fusion plasmas
Energy transport and confinement in tokamak fusion plasmas is usually determined by the coupled nonlinear interactions of small-scale drift turbulence and larger scale coherent nonlinear structures, such as zonal flows, together with free energy sources such as temperature gradients. Zero-dimensional models, designed to embody plausible physical narratives for these interactions, can help to identify the origin of enhanced energy confinement and of transitions between confinement regimes. A prime zero-dimensional paradigm is predator-prey or Lotka-Volterra. Here, we extend a successful three-variable (temperature gradient; microturbulence level; one class of coherent structure) model in this genre [M. A. Malkov and P. H. Diamond, Phys. Plasmas 16, 012504 (2009)], by adding a fourth variable representing a second class of coherent structure. This requires a fourth coupled nonlinear ordinary differential equation. We investigate the degree of invariance of the phenomenology generated by the model of Malkov and Diamond, given this additional physics. We study and compare the long-time behaviour of the three-equation and four-equation systems, their evolution towards the final state, and their attractive fixed points and limit cycles. We explore the sensitivity of paths to attractors. It is found that, for example, an attractive fixed point of the three-equation system can become a limit cycle of the four-equation system. Addressing these questions which we together refer to as “robustness” for convenience is particularly important for models which, as here, generate sharp transitions in the values of system variables which may replicate some key features of confinement transitions. Our results help to establish the robustness of the zero-dimensional model approach to capturing observed confinement phenomenology in tokamak fusion plasmas
Avalanches in a Bose-Einstein condensate
Collisional avalanches are identified to be responsible for an 8-fold
increase of the initial loss rate of a large 87-Rb condensate. We show that the
collisional opacity of an ultra-cold gas exhibits a critical value. When
exceeded, losses due to inelastic collisions are substantially enhanced. Under
these circumstances, reaching the hydrodynamic regime in conventional BEC
experiments is highly questionable.Comment: 4 pages, 2 figures, 1 tabl
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