Events in Physics
Wednesday, June 19, 2013
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Jay Holberg (University of Arizona)PS0.17aLarge Samples and Ground Truth: The 25 Parsec Local White Dwarf Population J. B. Holberg Lunar and Planetary Laboratory, University of Arizona, Tucson AZ White dwarf stars are studied both as individual objects and as collective members of large surveys. The former emphasizes the unique aspects of the stars while the latter emphasizes population characteristics such as high proper motions or uv-excesses. Although large surveys enhance our statistical understanding of degenerate star populations (and turn up intriguing new object types) they are often too narrowly focused to simultaneously include all white dwarf classes. The enumeration of all white dwarfs within a limited volume of space offers the best opportunity for both the systematic and detailed study of individual stars, and at the same time realizing a significantly large sample size. Such a volume-limited study achieves very high levels of completeness and permits drawing inferences that can be related directly to larger surveys. We have extended the detailed survey of the local white dwarf population from 20 pc to 25 pc, effectively doubling the sample volume. The present 25 pc sample contains 226 white dwarf stars having an estimated completeness of 70% (the corresponding 20 pc sub-sample is now 85% complete). The space density of white dwarfs remains at 4.8 ± 0.5 x 10-3 pc-3. We have also produced a white dwarf mass distribution and luminosity function based on the 25 pc sample. A discussion of likely sources for likely new members of the local sample is provided. |
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On the physics of the first wall in fusion devicesPLTTitle: On the physics of the first wall in fusion devices By: Sergei Krasheninnikov (University California San Diego) Date: Wednesday 19th June 2013 Where: PLT Time: 16.30pm Abstract: Over the years, and in particular with the beginning of the ITER era, it became clear that the fusion has, at least, two major issues: core plasma confinement and the performance of the first wall. Different aspects of the plasma confinement were the subjects of intense theoretical and experimental studies for a long time and the main plasma physics issues are rather familiar to the theoretical part of fusion community. On the contrary, the studies of the physics of the first wall in fusion devices, in particular some puzzling experimental observations and theoretical issues, approaches, models, etc., are not very familiar to fusion plasma theorists. Meanwhile, already existing experimental observations and theoretical models show that the irradiation of the first wall by fusion relevant plasmas results in interesting and rich physics of i) first wall material evolution, and ii) behavior of hydrogen/helium gas absorbed by the wall. Both these issues often exhibit strong synergistic effects and have direct and important implications for all future fusion reactors. In this talk we start with reviewing main issues of the first wall in fusion devices, experimental technique, and the most interesting and, in many cases, puzzling experimental observations, including the formation of “swamp” of helium nano-bubbles, “fuzz”, hydrogen outgassing, etc. Then, we outline the main theoretical approaches used to study the physics of the first wall, including Molecular Dynamic (MD) simulation, continuum Reaction-Diffusion (RD) models, etc. We illustrate the outcomes of these approaches with some particular examples, trying to link them to the experimental data. We will also emphasize some common features of the models that describe transport processes in the wall, magnetized plasmas and beyond. In the conclusions, we will discuss the existing gaps of our understanding of the physics of the first wall.
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