Space telescope phase B definition study. Volume 2A: Science instruments, astrometer

The analysis and design of an astrometer instrument for the space telescope are discussed. The design concepts utilize the astrometric multiplexing area scanner and the OTA fine guidance sensor

Computational analysis of space transfer vehicle engine turbopump diffuser design for deep-throttling

Space Transfer Vehicles (STV) are expected to perform missions (orbital transfer, Lunar/Mars transfer and descents) which will require deep-engine throttling thrust capability. To accomplish this, turbopumps which can efficiently provide a wide range of flow outputs are needed. The current state-of-the-art cryogenic fuel and oxidizer turbopump designs do not perform efficiently at off-design flow rates mainly due to stall and flow separation in the diffuser. This thesis examines how boundary layer control techniques can be used to control, reduce or eliminate diffuser flow separation. A finite element based code, FIDAP, was used to study flow in a diffuser by developing a 2-D diffuser model and establishing conditions for flow separation. Various rates of suction (removing decelerating fluid particles) and blowing (reenergizing decelerating fluid particles) were tested for their effectiveness in suppressing or eliminating the flow separation at an off-design flow rate. The results showed that FIDAP can be used effectively to model boundary layer control by suction and blowing in a diffuser operating at 60% of design flow. Suction was implemented at various flow rates through the top of the diffuser and shown to be effective at a rate of 15% in counteracting the incidence effects at the inlet of the diffuser and reduce the region of flow separation. Blowing was also shown to be effective in reenergizing the boundary layer to reduce the region of flow separation

Impact of shorter wavelengths on optical quality for laws

This study explores parametrically as a function of wavelength the degrading effects of several common optical aberrations (defocus, astigmatism, wavefronttilts, etc.), using the heterodyne mixing efficiency factor as the merit function. A 60 cm diameter aperture beam expander with an expansion ratio of 15:1 and a primary mirror focal ratio of f/2 was designed for the study. An HDOS copyrighted analysis program determined the value of merit function for various optical misalignments. With sensitivities provided by the analysis, preliminary error budget and tolerance allocations were made for potential optical wavefront errors and boresight errors during laser shot transit time. These were compared with the baseline 1.5 m CO2 laws and the optical fabrication state of the art (SOA) as characterized by the Hubble Space Telescope. Reducing wavelength and changing optical design resulted in optical quality tolerances within the SOA both at 2 and 1 micrometer. However, advanced sensing and control devices would be necessary to be tightened by a factory of 1.8 for a 2 micrometer system and by 3.6 for a 1 micrometer system relative to the baseline CO2 LAWS. Available SOA components could be used for operation at 2 micrometers but operation at 1 micrometer does not appear feasible

Impact of shorter wavelengths on optical quality for laws

This study explores parametrically as a function of wavelength the degrading effects of several common optical aberrations (defocus, astigmatism, wavefront tilts, etc.), using the heterodyne mixing efficiency factor as the merit function. A 60 cm diameter aperture beam expander with an expansion ratio of 15:1 and a primary mirror focal ratio of f/2 was designed for the study. An HDOS copyrighted analysis program determined the value of merit function for various optical misalignments. With sensitivities provided by the analysis, preliminary error budget and tolerance allocations were made for potential optical wavefront errors and boresight errors during laser shot transit time. These were compared with the baseline l.5 m CO2 LAWS and the optical fabrication state of the art (SOA) as characterized by the Hubble Space Telescope. Reducing wavelength and changing optical design resulted in optical quality tolerances within the SOA both at 2 and 1 micrometers. However, advanced sensing and control devices would be necessary to maintain on-orbit alignment. Optical tolerance for maintaining boresight stability would have to be tightened by a factor of 1.8 for a 2 micrometers system and by 3.6 for a 1 micrometers system relative to the baseline CO2 LAWS. Available SOA components could be used for operation at 2 micrometers but operation at 1 micrometers does not appear feasible

Larval cannibalism, time constraints, and adult fitness in caddisflies that inhabit temporary wetlands

The fitness of non-feeding adult insects depends on energy accumulated during the larval stage. Larvae of the caddisfly Asynarchus nigriculus primarily feed on plant detritus, but supplement their diet with animal material obtained through cannibalism. Habitat drying constrains development in many populations of this species, and we hypothesized that cannibalism should accelerate development to facilitate timely metamorphosis. We manipulated larval diets in a field experiment by supplementing detritus with animal material, and in a laboratory experiment by varying animal material and detritus quality (conditioned vs unconditioned). We measured the effects of dietary manipulation on larval and pupal growth and development, the timing of metamorphosis, and adult fitness correlates. The results of the laboratory experiment suggest that this species can metamorphose with a detritus-only diet, but development is extremely protracted. In the field experiment, individuals with animal material in their diet had higher larval survival, shorter larval and pupal development times, and earlier emergence dates (7–10 days), than those without a supplement. This delay in emergence should have important effects on survival in natural populations where the difference between desiccation and successful emergence can be only a few days. Dietary supplementation also affected adult body mass (30–40% increase), female fecundity (30% more eggs), and proportional allocation to different adult body parts. Our results are consistent with recent growth-development models that predict coupled (earlier emergence and larger adults) rather than tradeoff responses (earlier emergence and smaller adults) to pre-threshold manipulation of larval diets. Many detritivorous aquatic insects supplement their diets with animal material, and our data provide evidence that this supplementation can have strong effects on fitness. This type of dietary supplementation should be especially important for taxa that do not feed as adults, and in temporary habitats that impose time constraints on larval development.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47698/1/442_2003_Article_1397.pd

Effect of bulk inversion asymmetry on the Datta-Das transistor

A model of the Datta-Das spin field-effect transistor is presented which, in addition to the Rashba interaction, takes into account the influence of bulk inversion asymmetry of zinc-blende semiconductors. In the presence of bulk inversion asymmetry, the conductance is found to depend significantly on the crystallographic orientation of the channel. We determine the channel direction optimal for the observation of the Datta-Das effect in GaAs and InAs-based devices.Comment: 4 pages, Revtex4, 4 EPS figure

Spin Orientation and Spin Precession in Inversion-Asymmetric Quasi Two-Dimensional Electron Systems

Inversion asymmetry induced spin splitting of the electron states in quasi two-dimensional (2D) systems can be attributed to an effective magnetic field B which varies in magnitude and orientation as a function of the in-plane wave vector k||. Using a realistic 8x8 Kane model that fully takes into account spin splitting because of both bulk inversion asymmetry and structure inversion asymmetry we investigate the spin orientation and the effective field B for different configurations of a quasi 2D electron system. It is shown that these quantities depend sensitively on the crystallographic direction in which the quasi 2D system was grown as well as on the magnitude and orientation of the in-plane wave vector k||. These results are used to discuss how spin-polarized electrons can precess in the field B(k||). As a specific example we consider GaInAs-InP quantum wells.Comment: 10 pages, 6 figure

Electron Spin Decoherence in Bulk and Quantum Well Zincblende Semiconductors

A theory for longitudinal (T1) and transverse (T2) electron spin coherence times in zincblende semiconductor quantum wells is developed based on a non-perturbative nanostructure model solved in a fourteen-band restricted basis set. Distinctly different dependences of coherence times on mobility, quantization energy, and temperature are found from previous calculations. Quantitative agreement between our calculations and measurements is found for GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb quantum wells.Comment: 11 pages, 3 figure

Magnetotransport in Two-Dimensional Electron Systems with Spin-Orbit Interaction

We present magnetotransport calculations for homogeneous two-dimensional electron systems including the Rashba spin-orbit interaction, which mixes the spin-eigenstates and leads to a modified fan-chart with crossing Landau levels. The quantum mechanical Kubo formula is evaluated by taking into account spin-conserving scatterers in an extension of the self-consistent Born approximation that considers the spin degree of freedom. The calculated conductivity exhibits besides the well-known beating in the Shubnikov-de Haas (SdH) oscillations a modulation which is due to a suppression of scattering away from the crossing points of Landau levels and does not show up in the density of states. This modulation, surviving even at elevated temperatures when the SdH oscillations are damped out, could serve to identify spin-orbit coupling in magnetotransport experiments. Our magnetotransport calculations are extended also to lateral superlattices and predictions are made with respect to 1/B periodic oscillations in dependence on carrier density and strength of the spin-orbit coupling.Comment: 8 pages including 8 figures; submitted to PR