13,557 research outputs found
CubeSat Astronomy Mission Modeling Using the Horizon Simulation Framework
The CubeSat Astronomy Network is a proposed system of multiple CubeSat spacecraft capable of performing follow-up observations of astronomical targets of interest. The system is intended to serve as a space-borne platform that can complement existing systems utilized for astronomical research by undergraduate and high school students. Much research and development work has been performed to develop model-based system engineering methodologies and products for CubeSat missions, including the Horizon Simulation Framework. The Horizon Simulation Framework enables the development of system models using the Extended Markup Language (XML), and its simulation program can generate system simulations over model-specified timespans. System requirements and constraints, as well as subsystem dependencies and functions, can also be directly specified in these models. Previous work using the framework has been performed to characterize “day-in-the-life” operations for Earth-observing spacecraft. A similar goal is intended for modeling the CubeSat Astronomy Network: simulating mission operations during nominal conditions to validate system and subsystem requirements. By developing this model, system and subsystem requirements derived in the course of preliminary design for the Network can be analyzed, modelled, and evaluated for feasibility. These results can then be used to inform design decisions related to system architecture and concept of operations at the early stages of design, while the models themselves can grow and mature alongside project development and be re-used for future design work
Casimir forces in the time domain II: Applications
Our preceding paper introduced a method to compute Casimir forces in
arbitrary geometries and for arbitrary materials that was based on a
finite-difference time-domain (FDTD) scheme. In this manuscript, we focus on
the efficient implementation of our method for geometries of practical interest
and extend our previous proof-of-concept algorithm in one dimension to problems
in two and three dimensions, introducing a number of new optimizations. We
consider Casimir piston-like problems with nonmonotonic and monotonic force
dependence on sidewall separation, both for previously solved geometries to
validate our method and also for new geometries involving magnetic sidewalls
and/or cylindrical pistons. We include realistic dielectric materials to
calculate the force between suspended silicon waveguides or on a suspended
membrane with periodic grooves, also demonstrating the application of PML
absorbing boundaries and/or periodic boundaries. In addition we apply this
method to a realizable three-dimensional system in which a silica sphere is
stably suspended in a fluid above an indented metallic substrate. More
generally, the method allows off-the-shelf FDTD software, already supporting a
wide variety of materials (including dielectric, magnetic, and even anisotropic
materials) and boundary conditions, to be exploited for the Casimir problem.Comment: 11 pages, 12 figures. Includes additional examples (dispersive
materials and fully three-dimensional systems
Casimir repulsion between metallic objects in vacuum
We give an example of a geometry in which two metallic objects in vacuum
experience a repulsive Casimir force. The geometry consists of an elongated
metal particle centered above a metal plate with a hole. We prove that this
geometry has a repulsive regime using a symmetry argument and confirm it with
numerical calculations for both perfect and realistic metals. The system does
not support stable levitation, as the particle is unstable to displacements
away from the symmetry axis.Comment: 4 pages, 4 figures; added references, replaced Fig.
Achieving a Strongly Temperature-Dependent Casimir Effect
We propose a method of achieving large temperature sensitivity in the Casimir
force that involves measuring the stable separation between dielectric objects
immersed in fluid. We study the Casimir force between slabs and spheres using
realistic material models, and find large > 2nm/K variations in their stable
separations (hundreds of nanometers) near room temperature. In addition, we
analyze the effects of Brownian motion on suspended objects, and show that the
average separation is also sensitive to changes in temperature . Finally, this
approach also leads to rich qualitative phenomena, such as irreversible
transitions, from suspension to stiction, as the temperature is varied
A selective role for neuronal activity regulated pentraxin in the processing of sensory-specific incentive value
Neuronal activity regulated pentraxin (Narp) is a secreted neuronal product which clusters AMPA receptors and regulates excitatory synaptogenesis. Although Narp is selectively enriched in brain, its role in behavior is not known. As Narp is expressed prominently in limbic regions, we examined whether Narp deletion affects performance on tasks used to assess motivational consequences of food-rewarded learning. Narp knock-out (KO) mice were unimpaired in learning simple pavlovian discriminations, instrumental lever pressing, and in acquisition of at least two aspects of pavlovian incentive learning, conditioned reinforcement and pavlovian-instrumental transfer. In contrast, Narp deletion resulted in a substantial deficit in the ability to use specific outcome expectancies to modulate instrumental performance in a devaluation task. In this task, mice were trained to respond on two levers for two different rewards. After training, mice were prefed with one of the two rewards, devaluing it. Responding on both levers was then assessed in extinction. Whereas control mice showed a significant preference in responding on the lever associated with the nondevalued reward, Narp KO mice responded equally on both levers, failing to suppress responding on the lever associated with the devalued reward. Both groups consumed more of the nondevalued reward in a subsequent choice test, indicating Narp KO mice could distinguish between the rewards themselves. These data suggest Narp has a selective role in processing sensory-specific information necessary for appropriate devaluation performance, but not in general motivational effects of reward-predictive cues on performance
Unusual DNA Structure and DNA Damage Recognition: Structure and Dynamic Markers
Nucleic acids play a central role in many biological processes, including information storage, gene expression, serving as messengers or structural components and even catalysis. Their diverse roles have made them targets of interest to diagnose and treat an array of human disorders
such as infections, degenerative diseases and cancer. Nature has evolved proteins and ligands that recognize specific nucleic acid sequences or structures and control their function, demonstrating that this can be efficiently accomplished. This has led to the development of wide variety of
synthetic molecules that selectively bind to nucleic acids. In turn, this has precipitated numerous studies which showed that nucleic acid structures and their dynamic properties must be understood in order to efficiently target specific sequences or structures
Structural anisotropy and orientation-induced Casimir repulsion in fluids
In this work we theoretically consider the Casimir force between two periodic
arrays of nanowires (both in vacuum, and on a substrate separated by a fluid)
at separations comparable to the period. Specifically, we compute the
dependence of the exact Casimir force between the arrays under both lateral
translations and rotations. Although typically the force between such
structures is well-characterized by the Proximity Force Approximation (PFA), we
find that in the present case the microstructure modulates the force in a way
qualitatively inconsistent with PFA. We find instead that effective-medium
theory, in which the slabs are treated as homogeneous, anisotropic dielectrics,
gives a surprisingly accurate picture of the force, down to separations of half
the period. This includes a situation for identical, fluid-separated slabs in
which the exact force changes sign with the orientation of the wire arrays,
whereas PFA predicts attraction. We discuss the possibility of detecting these
effects in experiments, concluding that this effect is strong enough to make
detection possible in the near future.Comment: 12 pages, 9, figure. Published version with expanded discussio
Designing evanescent optical interactions to control the expression of Casimir forces in optomechanical structures
We propose an optomechanical structure consisting of a photonic-crystal
(holey) membrane suspended above a layered silicon-on-insulator substrate in
which resonant bonding/antibonding optical forces created by externally
incident light from above enable all-optical control and actuation of stiction
effects induced by the Casimir force. In this way, one can control how the
Casimir force is expressed in the mechanical dynamics of the membrane, not by
changing the Casimir force directly but by optically modifying the geometry and
counteracting the mechanical spring constant to bring the system in or out of
regimes where Casimir physics dominate. The same optical response (reflection
spectrum) of the membrane to the incident light can be exploited to accurately
measure the effects of the Casimir force on the equilibrium separation of the
membrane
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