Department of Physics and Astronomy
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Item Open Access A free-standing space elevator structure: a practical alternative to the space tether(Elsevier, 2009-04-19) Quine, Brendan; Seth, R. K.; Zhu, Z. H.Space tethers have been investigated widely as a means to provide easy access to space. However, the design and construction of such a device presents significant unsolved technological challenges. We propose an alternative approach to the construction of a space elevator that utilizes a free-standing core structure to provide access to near space regions and to reduce the cost of space launch. The structure is comprised of pneumatically inflated sections that are actively controlled and stabilized to balance external disturbances and support the structure. Such an approach avoids problems associated with a space tether including material strength constraints, the need for in-space construction, the fabrication of a cable at least 50,000 km in length, and the ageing and meteorite-damage effects associated with a thin tether or cable in Low Earth Orbit. An example structure constructed at 5 km altitude and extending to 20 km above sea level is described. The stability and control of the structure, methods for construction and its utility for space launch and other applications are discussed.Item Open Access Global Evolution of Spiral Galaxies by Means of Cumulative Oxygen Abundances(2011-08) Dack, StuartStudying the global evolution of spiral galaxies requires determining their overall chemical compositions. However, since spirals tend to possess gradients in their chemical compositions, determining their overall chemical abundances poses a challenge. In this study, the framework for a newly proposed method for determining the overall oxygen abundance of a disk is established. By separately integrating the absolute amounts of hydrogen and oxygen out to large radii, the overall oxygen abundance is shown to reach an asymptotic value. In this manner, a reliable account of the overall chemical state is revealed. Knowing the chemical state of spirals then allows for their study alongside other galaxies, such as dwarf irregulars, whose chemical states are better understood. The relationship between the gas fraction and the overall oxygen abundance is established, and a comparison is made to the corresponding relationship for dwarf irregular galaxies. It is concluded that the effective oxygen yield for spirals is consistent with recent closed-box model estimates of yields for dwarf irregulars. Thus, it appears that the galaxy type does not have an effect on the formation of stars or the enrichment process associated with them, and no relation between yield and metallicity exists.Item Open Access Characterization and applications of auto-locked vacuum-sealed diode lasers for precision metrology(AIP Publishing, 2019-08-30) Kumarakrishnan, Anantharaman; Beica, Hermina; Pouliot, Alexander; Carew, A.; Vorozcovs, A.; Afkhami-Jeddi, N.; Vacheresse, T.; Carlse, G.; Dowling, P.; Barron, B.We demonstrate the performance characteristics of a new class of vacuum-sealed, autolocking diode laser systems and their applications to precision metrology. The laser is based on adaptations of a design that uses optical feedback from an interference filter and it includes a vacuum-sealed cavity, an interchangeable base-plate, and an autolocking digital controller. A change of the base-plate allows operation at desired wavelengths in the visible and near infrared spectral range, whereas the autolocking ability allows the laser to be tuned and frequency stabilized with respect to atomic, molecular, and solid-state resonances without human intervention using a variety of control algorithms programmed into the same controller. We characterize the frequency stability of this laser system based on the Allan deviation (ADEV) of the beat note and of the lock signal. We find that the ADEV floor of 2 × 10−12 and short-term linewidth of ∼200 kHz are strongly influenced by current noise and vacuum sealing. Reducing the current noise and cavity pressure decreases the ADEV floor and increases the averaging time at which the floor occurs, which is a signature of long-term stability. We also show that evacuating the cavity to ∼1 Torr reduces the range of the correction signal of the feedback loop by approximately one order of magnitude, thereby increasing the lock range of the controller. The long-term stability allows the laser to be incorporated into a commercial gravimeter for accurate measurements of gravitational acceleration at the level of a few parts-per-billion, which are comparable to values obtained with an iodine-stabilized He–Ne laser. The autolocking and pattern-matching features of the controller allow the laser to be tuned and stabilized with respect to a temperature tunable transmission spectrum of a fiber-Bragg grating. This capability may be suitable for the development of a differential absorption LIDAR transmitter that can generate data at both on-line and off-line lock points using a single laser.