A simple construction of initial data for multiple black holes

We consider the initial data problem for several black holes in vacuum with arbitrary momenta and spins on a three space with punctures. We compactify the internal asymptotically flat regions to obtain a computational domain without inner boundaries. When treated numerically, this leads to a significant simplification over the conventional approach which is based on throats and isometry conditions. In this new setting it is possible to obtain existence and uniqueness of solutions to the Hamiltonian constraint.Comment: 4 pages, LaTeX (RevTeX), minor changes, improved presentation, to appear in PR

Cactus: Issues for Sustainable Simulation Software

The Cactus Framework is an open-source, modular, portable programming environment for the collaborative development and deployment of scientific applications using high-performance computing. Its roots reach back to 1996 at the National Center for Supercomputer Applications and the Albert Einstein Institute in Germany, where its development jumpstarted. Since then, the Cactus framework has witnessed major changes in hardware infrastructure as well as its own community. This paper describes its endurance through these past changes and, drawing upon lessons from its past, also discusses futureComment: submitted to the Workshop on Sustainable Software for Science: Practice and Experiences 201

Control of charged particle dynamics and electron power absorption dynamics utilizing voltage waveform tailoring in capacitively driven radio-frequency plasmas

In this work, experimental measurements and analysis of numerical simulations are performed for capacitively coupled plasmas driven by tailored voltage waveforms under conditions which examine complicating factors present in industrial processes, including the influence of resonance effects, electronegative gases or gas mixtures, and plasma-surface interactions at a changing plasma-surface interface. Furthermore, the influence of different tailored voltage waveforms on the spatio-temporal electron power absorption, the generation of a DC self-bias, and on process relevant plasma parameters like ion energy distribution functions is investigated to provide a more complete understanding of the underlying fundamental plasma physics responsible for sustaining the discharge. It is found that these complicating factors can dramatically alter the operation of discharges under conditions that are highly relevant to many industrial processes. First, it is demonstrated that tailored voltage waveforms provide improved control over the charged particle dynamics and process-relevant plasma parameters of electropositive argon discharges. The self-excitation of the plasma series resonance and its subsequent influence on the charged particle dynamics is then analyzed using numerical simulations of geometrically symmetric but electrically asymmetric argon discharges. The influence of negative ions and electronegativity on the charged particle dynamics produced by various tailored voltage waveforms is investigated for tetrafluoromethane discharges and argon-tetrafluoromethane gas mixtures. It is found that the discharge electronegativity and the presence of the drift-ambipolar heating mode dramatically alter the operation of the discharge. Lastly, the dependence of secondary electron emission on the surface characteristics (surface roughness, film thickness) of aluminum and aluminum oxide surfaces is demonstrated to be non-negligible and hypotheses for the underlying physical mechanisms behind these dependencies are presented. Thus, several important factors frequently used in industrial processing which are usually omitted from fundamental studies of capacitively coupled plasmas are shown to significantly modify the associated spatio-temporal charged particle dynamics and should not be neglected in future research

On the self-excitation mechanisms of Plasma Series Resonance oscillations in single- and multi-frequency capacitive discharges

The self-excitation of plasma series resonance (PSR) oscillations is a prominent feature in the current of low pressure capacitive radio frequency (RF) discharges. This resonance leads to high frequency oscillations of the charge in the sheaths and enhances electron heating. Up to now, the phenomenon has only been observed in asymmetric discharges. There, the nonlinearity in the voltage balance, which is necessary for the self-excitation of resonance oscillations with frequencies above the applied frequencies, is caused predominantly by the quadratic contribution to the charge-voltage relation of the plasma sheaths. Using PIC/MCC simulations of single- and multi- frequency capacitive discharges and an equivalent circuit model, we demonstrate that other mechanisms such as a cubic contribution to the charge-voltage relation of the plasma sheaths and the time dependent bulk electron plasma frequency can cause the self-excitation of PSR oscillations, as well. These mechanisms have been neglected in previous models, but are important for the theoretical description of the current in symmetric or weakly asymmetric discharges

Attribute Elicitation: Implications in the Research Context1

Three different methods of attribute elicitation for two different paper-based products were compared in this study. The three methods used were free elicitation (FE), hierarchical dichotomization (HD), and Kelly's repertory grid (RG). The two paper-based products used in this study were bathroom tissue and paper towels. The methods were compared by abstraction, efficiency in data collection, convergent validity, and respondents' reaction to the task. The results from this comparison indicated that the level of abstraction did not significantly differ between methods or products. However, a rank order analysis revealed that a substantial difference existed with 18 to 20% of the attributes being rated significantly different between the elicitation methods for paper towels and bathroom tissue, respectively. Convergent validity was exhibited between all the methods, although was found to be highest between HD and RG. These findings suggest that all three elicitation methods elicit very similar information from the consumers' knowledge base. The efficiency in data collection revealed that for both products FE took significantly less time to complete the task, as well as to elicit the individual attributes. Furthermore, HD was identified as being the least efficient of the methods for either product. For the comparison of the reaction to task, FE was found to be the least difficult of the three methods and also allowed the respondents to more freely express their opinion

Shared memory parallelism in Modern C++ and HPX

Parallel programming remains a daunting challenge, from the struggle to express a parallel algorithm without cluttering the underlying synchronous logic, to describing which devices to employ in a calculation, to correctness. Over the years, numerous solutions have arisen, many of them requiring new programming languages, extensions to programming languages, or the addition of pragmas. Support for these various tools and extensions is available to a varying degree. In recent years, the C++ standards committee has worked to refine the language features and libraries needed to support parallel programming on a single computational node. Eventually, all major vendors and compilers will provide robust and performant implementations of these standards. Until then, the HPX library and runtime provides cutting edge implementations of the standards, as well as proposed standards and extensions. Because of these advances, it is now possible to write high performance parallel code without custom extensions to C++. We provide an overview of modern parallel programming in C++, describing the language and library features, and providing brief examples of how to use them