Simulations within density functional theory (DFT) are a common component of research into the physics of materials. With the broad success of DFT, it is easily forgotten that computational DFT methods invariably do not directly represent simulated properties, but require careful construction of models that are computable approximations to a physical property. Perhaps foremost among these computational considerations is the routine use of the supercell approximation to construct finite models to represent infinite systems. Pitfalls in using supercells (k-space sampling, boundary conditions, cell sizes) are often underappreciated. We present examples (e.g. vacancy defects) that exhibit a surprising or significant dependence on supercells, and describe workable solutions. We describe procedures needed to construct meaningful models for simulations of real material systems, focusing on k-space and cell size issues.
A new approach to optical time-domain reflectometry (OTDR) is proposed that will enable distributed fault monitoring in singlemode VCSEL-based networks. In situ OTDR uses the transmitter VCSEL already resident in data transceivers as both emitter and resonant-cavity photodiode for fault location measurements. Also valuable at longer wavelengths, the concept is demonstrated here using an 850 nm oxide-confined VCSEL and simple electronics. The dead times and sensitivity obtained are adequate to detect the majority of faults anticipated in local- and metropolitan-area networks.
This highly interactive workshop is designed to elicit from the participants a vision of an ideal future analytic environment for intelligence analysis, the components of such a system that are already in place or in development and the identification of needed future developments. It will cover processes and tools for enabling effective individual analysts, teams of analysts, computer mediated analysis teams and management of tasks and teams.
Leaders around the world and across the ideological spectrum agree that the global nonproliferation regime is facing a serious test. The emergence of sophisticated terrorist networks, black markets in nuclear technology, and technological leaps associated with globalization have conspired to threaten one of the most successful examples of international cooperation in history. The rampant proliferation of nuclear weapons that was predicted at the start of the nuclear age has been largely held in check and the use of those weapons avoided. Nonetheless, with the thirty-fifth anniversary of the Treaty on the Nonproliferation of Nuclear Weapons (NPT), the threat of nuclear proliferation seems more serious than ever. Although experts readily concede that there exist many pathways to proliferation, the threat posed by the misuse of the civilian nuclear fuel cycle has received considerable recent attention. While the connection between nuclear energy and nonproliferation has been a topic of discussion since the dawn of the nuclear age, world events have brought the issue to the forefront once again. United States President George W. Bush and International Atomic Energy Agency (IAEA) Director General Mohammad ElBaradei are among those who have highlighted proliferation risks associated with civilian nuclear power programs and called for revitalizing the nuclear nonproliferation regime to address new threats. From the possibility of diversion or theft of nuclear material or technology, to the use of national civilian programs as a cover for weapons programs - what some have called latent proliferation - the fuel cycle appears to many to represent a glaring proliferation vulnerability. Just as recognition of these risks is not new, neither is recognition of the many positive benefits of nuclear energy. In fact, a renewed interest in exploiting these benefits has increased the urgency of addressing the risks. Global energy demand is expected to at least double by the middle of the century and could increase even more quickly. Much of the new demand will come from the rapidly expanding economies in China and India, but much of the developing world stands poised to follow the same path. This growth in demand is paralleled by concerns about global warming and the long-term reliability of carbon-based fuel supplies, concerns which expanded use of nuclear power can help to address. For these reasons and others, many countries in Asia have already clearly signaled that nuclear energy will play a key role for years to come. Numerous proposals have been made in the last two years for reducing the proliferation risk of the civilian nuclear fuel cycle. These range from a ban on export of enrichment and reprocessing technology to countries not already possessing operational capabilities to multinational management of the nuclear fuel cycle and strengthening existing monitoring and security mechanisms. The need for international willingness to enforce nonproliferation commitments and norms has also been emphasized. Some of these proposals could significantly impact the production of nuclear energy. Because the successful strengthening of the nonproliferation regime and the expansion of nuclear energy are so closely related, any successful approach to resolving these issues will require the creative input of experts from both the nuclear energy and nonproliferation communities. Against this backdrop, Sandia National Laboratories organized its 14th International Security Conference (ISC) around the theme: Strengthening the Nuclear Nonproliferation Regime: Focus on the Civilian Nuclear Fuel Cycle. The goal of the conference was to begin a constructive dialogue between the nuclear energy and nuclear nonproliferation communities. The conference was held in Chantilly, Virginia, just outside Washington, D.C. on April 4-6, 2005, and was attended by approximately 125 participants from fifteen countries. The ISC agenda was structured to produce a systematic review of the connection between civilian nuclear energy programs and the proliferation of nuclear weapons and to identify constructive approaches to strengthen the nonproliferation regime. The conference began by reviewing the energy and security context that has, once again, raised the profile of this issue. A discussion of the risks associated with the civilian nuclear fuel cycle was then used to inform the analysis of several potential risk-management tools. The conference concluded by looking for lessons from the past as well as looking forward to future opportunities, with a particular focus on East Asia. In this paper we summarize the debates and ideas that emerged during the conference. Although we have drawn on material presented by speakers and comments made by participants, we do not quote or cite the specific contributions of individuals.
We propose a new class of Discontinuous Galerkin (DG) methods based on variational multiscale ideas. Our approach begins with an additive decomposition of the discontinuous finite element space into continuous (coarse) and discontinuous (fine) components. Variational multiscale analysis is used to define an interscale transfer operator that associates coarse and fine scale functions. Composition of this operator with a donor DG method yields a new formulation that combines the advantages of DG methods with the attractive and more efficient computational structure of a continuous Galerkin method. The new class of DG methods is illustrated for a scalar advection-diffusion problem.
This paper analyzes the accuracy of the shift-invert Lanczos iteration for computing eigenpairs of the symmetric definite generalized eigenvalue problem. We provide bounds for the accuracy of the eigenpairs produced by shift-invert Lanczos given a residual reduction. We discuss the implications of our analysis for practical shift-invert Lanczos iterations. When the generalized eigenvalue problem arises from a conforming finite element method, we also comment on the uniform accuracy of bounds (independent of the mesh size h).
A unique nanocrystalline, mesoporous PdO-SnO{sub 2} film exhibiting high sensitivity and selectivity to hydrogen gas at room temperature has been developed.