Flexible Booms, Momentum Wheels, and Subtle Gravity-Gradient Instabilities

A gravity-gradient boom and a momentum wheel provides a passive, three-axis attitude control system for a small satellite requiring 10° Earth-oriented pointing In a low Earth orbit. The Polar BEAR satellite Is a small satellite using just such a system that has experienced unexpected attitude instabilities during some of Its full-sun orbit periods. This paper examines the attitude dynamles and disturbances associated with gravlty-gradientlmomentum-wheel systems In an attempt to identify potential destabilizing mechanisms common to the configuration. Polar BEAR is not the only such configuration to experience problems In full sun. and several other examples are briefiy discussed. Although we place particular emphasis on trying to understand Polar BEAR\u27s anomaly, Its performance may be symptomatic of problems with the Dexible-boom/momentum-wheel configuration

Unique continuation for solutions to the induced Cauchy-Riemann equations

AbstractLet M be a real infinitely differentiable closed hypersurface in X, a complex manifold of dimension n ⩾ 2, and let \̄t6M denote the induced Cauchy-Riemann operator on M. The problem considered in this paper is unique continuation for distribution solutions to the equation \̄t6Mu = 0 (these solutions are called CR distributions). In a local version of the problem it is shown that a CR distribution u in an open set U ⊂M which vanishes on one side of a C1 hypersurface S ⊂U which is noncharacteristic at a point p ϵ S necessarily vanishes in a neighborhood of p. If the CR distribution u is a continuous function on U, then it is only necessary to assume that u vanishes on S in order to prove that u vanishes in a neighborhood of p in M. It is also proved that if u is a CR distribution on M, then the boundary of the support of u is foliated by complex hypersurfaces. Thus a global unique continuation theorem is obtained by assuming that such a set is not contained in M

Hopping Transport in the Presence of Site Energy Disorder: Temperature and Concentration Scaling of Conductivity Spectra

Recent measurements on ion conducting glasses have revealed that conductivity spectra for various temperatures and ionic concentrations can be superimposed onto a common master curve by an appropriate rescaling of the conductivity and frequency. In order to understand the origin of the observed scaling behavior, we investigate by Monte Carlo simulations the diffusion of particles in a lattice with site energy disorder for a wide range of both temperatures and concentrations. While the model can account for the changes in ionic activation energies upon changing the concentration, it in general yields conductivity spectra that exhibit no scaling behavior. However, for typical concentrations and sufficiently low temperatures, a fairly good data collapse is obtained analogous to that found in experiment.Comment: 6 pages, 4 figure

Relationship between dynamical heterogeneities and stretched exponential relaxation

We identify the dynamical heterogeneities as an essential prerequisite for stretched exponential relaxation in dynamically frustrated systems. This heterogeneity takes the form of ordered domains of finite but diverging lifetime for particles in atomic or molecular systems, or spin states in magnetic materials. At the onset of the dynamical heterogeneity, the distribution of time intervals spent in such domains or traps becomes stretched exponential at long time. We rigorously show that once this is the case, the autocorrelation function of the renewal process formed by these time intervals is also stretched exponential at long time.Comment: 8 pages, 4 figures, submitted to PR

Sodium atoms and clusters on graphite: a density functional study

Sodium atoms and clusters (N<5) on graphite (0001) are studied using density functional theory, pseudopotentials and periodic boundary conditions. A single Na atom is observed to bind at a hollow site 2.45 A above the surface with an adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates a flat potential energy surface. Increased Na coverage results in a weak adsorbate-substrate interaction, which is evident in the larger separation from the surface in the cases of Na_3, Na_4, Na_5, and the (2x2) Na overlayer. The binding is weak for Na_2, which has a full valence electron shell. The presence of substrate modifies the structures of Na_3, Na_4, and Na_5 significantly, and both Na_4 and Na_5 are distorted from planarity. The calculated formation energies suggest that clustering of atoms is energetically favorable, and that the open shell clusters (e.g. Na_3 and Na_5) can be more abundant on graphite than in the gas phase. Analysis of the lateral charge density distributions of Na and Na_3 shows a charge transfer of about 0.5 electrons in both cases.Comment: 20 pages, 6 figure

Macrosegregation During Dendritic Arrayed Growth of Hypoeutectic Pb-Sn Alloys: Influence of Primary Arm Spacing and Mushy Zone Length

Thermosolutal convection in the dendritic mushy zone occurs during directional solidification of hypoeutectic lead tin alloys in a positive thermal gradient, with the melt on the top and the solid below. This results in macrosegregation along the length of the solidified samples. The extent of macrosegregation increases with increasing primary dendrite spacings for constant mushy zone length. For constant primary spacings, the macrosegregation increases with decreasing mushy zone length. Presence of convection reduces the primary dendrite spacings. However, convection in the interdendritic melt has significantly more influence on the spacings as compared with that in the overlying melt, which is caused by the solutal buildup at the dendrite tips

Methods and approaches to facilitate inclusion of the views, perspectives and preferences of people with moderate-to-severe dementia in research: a mixed-methods systematic review

YesBackground: The perspectives of people with moderate-to-severe dementia are rarely directly elicited in research studies. Objectives: This systematic review will explore methods and approaches for including the perspectives and preferences of people with moderate-to-severe dementia in research. Methods: AgeLine, CINAHL, Embase, PsycINFO, PubMed, Social Policy and Practice and Web of Science were searched until June 16 2022. Study quality was assessed using the 16-item Quality Assessment Tool. We described specific communication tools, reviewed the evidence for their effectiveness and considered their strengths and limitations. We examined the more general communication skills and techniques applied to support the use of these tools using thematic synthesis. The review protocol was registered with PROSPERO CRD42019130386 and the review was conducted and reported according to PRISMA guidelines. Results: Seven studies reported in 11 publications were included. In these studies five specific communication tools were used: Talking Mats, Augmentative and Alternative Communication Flexiboard, generic photographs in combination with a preference placement board, consultation ballot and personalised communication prescriptions. Each tool identified had advantages and disadvantages depending on dementia severity, verbal or physical ability, expense, researcher training requirements and ease of use. Thematic synthesis identified five general approaches to optimising communication that were employed to support use of the tools: ensuring conversations are individual and person-centred, managing external influences, engaging others, creating structure and facilitation skills. Conclusion: All tools had some utility and there was no clear evidence to support the recommendation of any one specific tool; therefore, researchers are advised to select the tool most appropriate to their context. Implications for Practice: The findings offer general guidance for researchers and practitioners on how to facilitate communication with people with moderate-to-severe dementia.Alzheimer's Society. Grant Number: AS-PR2-16-001. National Institute for Health and Care Research. Grant Number: ES/L001853/2. UK Research and Innovation. Grant Number: ES/L001853/

A glassy contribution to the heat capacity of hcp 4^4He solids

We model the low-temperature specific heat of solid $^4$He in the hexagonal closed packed structure by invoking two-level tunneling states in addition to the usual phonon contribution of a Debye crystal for temperatures far below the Debye temperature, $T < \Theta_D/50$. By introducing a cutoff energy in the two-level tunneling density of states, we can describe the excess specific heat observed in solid hcp $^4$He, as well as the low-temperature linear term in the specific heat. Agreement is found with recent measurements of the temperature behavior of both specific heat and pressure. These results suggest the presence of a very small fraction, at the parts-per-million (ppm) level, of two-level tunneling systems in solid $^4$He, irrespective of the existence of supersolidity.Comment: 11 pages, 4 figure