Bradley C. Edwards

DESIGN AND DEPLOYMENT OF A SPACE ELEVATOR

Abstract The space elevator was first proposed in the 1960s as a method of getting into space. The initial studies of a space elevator outlined the basic concept of a cable strung between Earth and space but concluded that no material available at the time had the required properties to feasibly construct such a cable. With the discovery of carbon nanotubes in 1991 it is now possible to realistically discuss the construction of a space elevator. Although currently produced only in small quantities, carbon nanotubes appear to have the strength-to-mass ratio required for this endeavor. However, fabrication of the cable required is only one of the challenges in construction of a space elevator. Powering the climbers, surviving micrometeor impacts, lightning strikes and low-Earth–orbit debris collisions are some of the problems that are now as important to consider as the production of the carbon nanotube cable. We consider various aspects of a space elevator and find each of the problems that this endeavor will encounter can be solved with current or near-future technology.

Measurements of optical refrigeration in ytterbium-doped crystals

We measured anti-Stokes fluorescence cooling (optical refrigeration) in ytterbium-doped yttrium aluminum garnet (YAG). Pumping the 2.3% Yb3+:YAG crystal with 1.8 W of 1030 nm laser light produced a temperature drop of 8.9 °C from room temperature. The high thermal conductivity and ruggedness of this crystal make it an attractive material for use in optical refrigerators. Our spectral studies show that pure crystals of this material could be efficient for optical refrigeration at temperatures above ∼100 K. Photothermal deflection measurements show that our current crystals can cool at ∼250 K. Additionally, we measured optical refrigeration in a 5% Yb3+:Y2SiO5 crystal when pumped at 1050 nm.

Demonstration of a solid-state optical cooler: An approach to cryogenic refrigeration

We report the successful operation of an optical cooler system. This device achieved 48 °C of cooling from room temperature and a heat lift of 25 mW when it was pumped with 1.6 W of laser light. Its performance as a function of pump laser wavelength and chamber temperature agrees well with theoretical models. This device validates the physics needed for exploiting the laser cooling of solids to develop practical optical refrigerators.

Development of the Los Alamos solid-state optical refrigerator

Laser-induced cooling of a solid by net anti-Stokes fluorescence, first experimentally demonstrated in 1995, can be the basis of a new type of cryocooler, an optical refrigerator. This article describes the physics and design issues of a practical optical refrigerator for operation at 77 K. In particular, the Los Alamos Solid-State Optical Refrigerator (LASSOR) which we are developing would have an operating efficiency comparable to commercial small cryocoolers, be completely vibration-free and operate for years without maintenance.

Spectroscopic evaluation of Yb/sup 3+/-doped glasses for optical refrigeration

The absorption and emission properties of Yb/sup 3+/-doped ZBLANP, BIGaZYT and QX/Yb phosphate glasses are studied to evaluate their potential for laser-induced fluorescent cooling or optical refrigeration. The efficiency of optical refrigeration increases with pump wavelength in the anti-Stokes region. The cooling efficiencies of the three glasses as a function of temperature are evaluated at the wavelength /spl lambda/p corresponding to the absorption coefficient of 10/sup -3/ cm/sup -1/. For temperatures <110 K, the cooling efficiency of the BIGaZYT glass may be more than twice that of the ZBLANP.

INTERNAL LASER COOLING OF YB3+-DOPED GLASS MEASURED BETWEEN 100 AND 300 K

Laser cooling of a solid can occur when it emits photons of higher mean energy than those it absorbs. Photothermal deflection spectra of a fluorozirconate glass (ZBLANP) doped with 1 wt% Yb3+ show cooling in an internal volume of the sample at temperatures between 100 and 300 K. A cooling efficiency of ∼1% relative to the absorbed laser power at 1015 nm is maintained at all temperatures. The results show promise for solid-state cooling of bulk materials at temperatures below 150 K.

Laser-induced fluorescent cooling of rare-earth-doped fluoride glasses

Continual cooling of a material by anti-Stokes luminescence has been experimentally observed in a solid material. Net cooling was achieved by pumping to the lower levels of the 2F5/2 manifold of Yb3+, followed by population redistribution across this manifold via thermalization and fluorescence to the ground state. Fluoride glass hosts in the ZBLAN and BaF2–InF3–GaF3 (BIG) families have been used in these experiments. Temperature decreases of 0.33 K in a bulk glass sample and 16 K in a fiber sample were obtained in ZBLANPb:Yb3+. Net cooling was achieved when an electrochemical purification stage was incorporated into the glass fabrication process to decrease the extrinsic absorptive component of the host glass in the 1.0 μm region. Resonant laser ablation experiments showed the positive and negative effects of electrochemical processing on fluoride glasses in reducing transition metal content which is detrimental to successful fluorescent cooling. Reduction in copper, iron, and chromium content of 18, 4.2, and 5.5 were determined for the ZBLANPb sample via RLA. However, reductions in transition metal content were only 1.9 for copper and a negligible amount for iron and chromium in the BIG glasses.

Electrochemical purification of heavy metal fluoride glasses for laser-induced fluorescent cooling applications

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. An electrochemical purification stage has been incorporated into the conventional fabrication process of heavy metal fluoride glasses. This change was undertaken to reduce the absorption losses associated with residual transition metal impurities, particularly the Fe 2+ band at 1.0 μm. Purified samples doped with ytterbium exhibited net cooling due to anti-stokes fluorescence and a relative cooling efficiency of about 2% was observed via photothermal deflection spectroscopy. Pumping to the lower Stark levels of the 2 F 5/2 manifold, followed by population redistribution across this manifold and fluorescence to the ground state, results in net cooling. This cooling can be achieved only if energy transfer and the extrinsic absorptive component of the host glass are suppressed.

Compositional investigation of Yb3+-doped heavy metal fluoride glasses for laser-induced fluorescent cooling applications

Abstract A full-scale compositional analysis of Yb3+-doped heavy metal fluoride glasses (HMFG) has been undertaken to determine potential hosts for use in the development of a first-generation optical cryocooler for space-borne remote sensing applications. By pumping to the lower levels of the Yb3+ 2F5/2 manifold, followed by a thermally-driven population redistribution to higher levels within the manifold, net cooling can be achieved by anti-Stokes fluorescence back to the ground state. This paper reports on fluoride glasses in the ZBLAN and BaF2–InF3–GaF3 (BIG) families which have shown promise for fluorescent cooling. ZBLANPb:Yb3+ is the first solid to actually exhibit net cooling due to anti-Stokes fluorescence. The BIG-derived hosts have been determined to contain similar mean emission photon energies and larger long-wavelength absorption tails than those of ZBLANPb:Yb3+. Low-temperature absorption and fluorescence spectra have indicated that the Yb3+-containing BIG glasses should have a cooling efficiency more than twice that of ZBLANPb at temperatures below 80 K and may attain a minimum temperature of 45 K compared to 55 K expected for the ZBLANPb.

Expected extreme ultraviolet spectrum of the lunar surface

The moon was recently observed to be a source of very soft x-ray emission. The emission was most intense at wavelengths longer than 62 {angstrom} and was attributed to Thomson scattering of solar x-rays. This observation prompted the authors to study the emissions expected from the lunar surface in the wavelength range between 90 and 500 {angstrom}. Photons in this wavelength range scatter inefficiently. Instead, the solar x-rays are absorbed in the first several microns of lunar regolith. The absorbed x-rays can excite the surface elements and result in fluorescent emission. The authors find that much of the L- and M-shell extreme ultraviolet fluorescence, in the wavelength range between 90 and 500 {angstrom}, have higher peak intensities than the scattered solar spectrum. The total integrated fluorescent emission is also higher than the total scattered solar radiation. The L-shell fluorescent radiation can be an indicator of the surface abundances of Si, Al, Mg and other major lunar elements.