Cascade Joule-Thomson refrigerators

The design criteria for cascade Joule-Thomson refrigerators for cooling in the temperature range from 300 K to 4.2 K were studied. The systems considered use three or four refrigeration stages with various working gases to achieve the low temperatures. Each stage results in cooling to a progressively lower temperature and provides cooling at intermediate temperatures to remove the substantial amount of parasitic heat load encountered in a typical dewar. With careful dewar design considerable cooling can be achieved with moderate gas flows. For many applications, e.g., in the cooling of sensitive sensors, the fact that the refrigerator contains no moving parts and may be remotely located from the gas source is of considerable advantage. A small compressor suitable for providing the gas flows required was constructed

Four K refrigerators with a new compact heat exchanger

Two refrigerators have been developed which have nominal cpacities of 0.25M and 0.5W at 4.2K. These use standard two stage Displex sup R expanders and compressors combined with a new compact heat exchanger which is concentric with the expander cylinder. These refrigerators can be used to cool superconducting electronic devices by direct attachment to the 4K heat station, or they can be plugged into the neck of a liquid helium superconducting magnet cryostat where they can cool the radiation shields and reliquefy helium

The Berkeley tunable far infrared laser spectrometers

A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10–100 cm^–1 and exhibits a maximum absorption sensitivity near one part in 10^6. Each laser line can be tuned by ±110 GHz with first-order sidebands. Applications of this instrument are detailed in the preceding paper

Tunable far infrared laser spectrometers

The state of the art in far infrared (FIR) spectroscopy is reviewed. The development of tunable, coherent FIR radiation sources is discussed. Applications of tunable FIR laser spectrometers for measurement of rotational spectra and dipole moments of molecular ions and free radicals, vibration-rotation-tunneling (VRT) spectra of weakly bound complexes, and vibration-rotation spectra of linear carbon clusters are presented. A detailed description of the Berkeley tunable FIR laser spectrometers is presented in the following article

Design and Performance of a Magnetic Refrigerator and Heat Engine

The initial operation of the refrigerator/heat engine is very encouraging. The analysis indicates that the magnetic field profile and inefficient water pump are the main reasons for the limited ..delta..T. The next experiments will be performed with soft iron shielding to increase ..delta..H and also with a more efficient pump to reduce the heat load. The prognosis is very good. The basic advantages of magnetic refrigerators/heat engines are that the energy density in the working material is very high, 25 kJ/l of Gd at 300 K; the heat exchange is between a solid and a liquid which makes the regenerative stages of the cycle more efficient; and in some cases the expensive external heat exchangers could be eliminated. The disadvantage of magnetic refrigeration is that a superconducting magnet is required. An example of an application at room temperature is a magnetic heat pump for utilization of low-grade waste heat such as in nuclear power plants. For applications such as this, where external heat exchangers may be eliminated, the economics look attractive. The prospects for magnetic refrigeration at low temperatures (< 77K) look much better than at room temperature because the ratio of magnetic entropy to lattice entropy increases rapidly as the temperature decreases

Infrared Observations During the Secondary Eclipse of HD 209458b: I. 3.6-Micron Occultation Spectroscopy Using the VLT

We search for an infrared signature of the transiting extrasolar planet HD 209458b during secondary eclipse. Our method, which we call `occultation spectroscopy,' searches for the disappearance and reappearance of weak spectral features due to the exoplanet as it passes behind the star and later reappears. We argue that at the longest infrared wavelengths, this technique becomes preferable to conventional `transit spectroscopy'. We observed the system in the wing of the strong nu-3 band of methane near 3.6 microns during two secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution of 3300. Our analysis, which utilizes a model template spectrum, achieves sufficient precision to expect detection of the spectral structure predicted by an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium model for the exoplanet atmosphere. However, our observations show no evidence for the presence of this spectrum from the exoplanet, with the statistical significance of the non-detection depending on the timing of the secondary eclipse, which depends on the assumed value for the orbital eccentricity. Our results reject certain specific models of the atmosphere of HD 209458b as inconsistent with our observations at the 3-sigma level, given assumptions about the stellar and planetary parameters.Comment: 26 pages, 8 figures Accepted to Astrophysical Journa