Analysis of Magnetic Refrigerators With External Regeneration

The central idea of the magnetic refrigeration systems analyzed in this paper is that of a MR having a regenerator composed of an integral number of separate stages but having only a single magnetic stage. In principle, each stage is thermally isolated from the others, but the stages are accessed by a manifold and valve system which allows gas to flow between the MM and the individual stages. The concept therefore is distinctly unlike that of the unusual continuous regenerator of gas refrigerators. Although a small amount of gas in the manifold is responsible for the actual transfer of heat between elements of the REGMR, it is clear from the cycle description in the caption of Fig. 1 that the fixed mass of the MM is analogous to a fixed mass of a working fluid in a gas refrigerator or engine. Both, in effect, are carried thermally through all parts of their respective systems

Experiment to Determine Properties of Packed Particle Beds and Regenerators at Cryogenic Temperatures

The testing of the properties of packed-particle beds and regenerators at cryogenic temperatures as low as 4 K is an essential part of the magnetic refrigeration research and development program at the Los Alamos National Laboratory. We envision magnetic refrigeration and heat pump systems operating in various ranges from 4 K to ambient temperature and above. Only pressurized helium gas appears suitable as the heat exchange fluid for the low-temperature applications. Because published data on the properties of porous beds at low temperatures is sparse, we have found it necessary to develop an experimental test apparatus to study the properties of various configurations of beds and regenerators. Two of the well-known methods for such studies are the steady-enthalpy-flux method and the single-blow transient method. We have developed an experimental system in which gas flow can be suddenly switched to an alternate better (or colder) flow in step-function fashion at temperatures from 4 to 300 K. This apparatus will yield information on steady-state heat transfer and friction factors as well as on the transient behavior. Such information is very important to the design of high-efficiency magnetic refrigeration systems. This paper describes this experimental apparatus and presents the results and analysis of recent measurements on packed-particle beds in the liquid helium and liquid nitrogen temperature ranges

First results for a novel superconducting imaging-surface sensor array

A superconducting imaging-surface system was constructed using 12 coplanar thin-film SQUID magnetometers located parallel to and spaced 2 cm from a 25 cm diameter lead imaging-plane. Some measurements included two additional sensors on the back side of the superconducting imaging-plane to study the field symmetry for the system. Performance was measured in a shielded can and in the open laboratory environment. Data from this system has been used to: (1) understand the noise characteristics of the dewar-SQUID imaging plate arrangement, (2) to verify the imaging principle, (c) measure the background rejection factor of the imaging plane, and (4) compare superconducting materials for the imaging plane. A phantom source field was measured at the sensors as a function of phantom distance from the sensor array to verify the imaging theory. Both the shape and absolute values of the measured and predicted curves agree very well indicating the system is behaving as a gradiometer in accordance with theory. The output from SQUIDs located behind the imaging surface that sense background fields can be used for software or analog background cancellation. Fields arising from sources close to the imaging plane were shielded from the background sensors by more than a factor of 1000. Measurement of the symmetry of sensor sensitivity to uniform fields exactly followed theoretical predictions

A Self-Consistent Model for Positronium Formation from Helium Atoms

The differential and total cross sections for electron capture by positrons from helium atoms are calculated using a first-order distorted wave theory satisfying the Coulomb boundary conditions. In this formalism a parametric potential is used to describe the electron screening in a consistent and realistic manner. The present procedure is self consistent because (i) it satisfies the correct boundary conditions and post-prior symmetry, and (ii) the potential and the electron binding energies appearing in the transition amplitude are consistent with the wave functions describing the collision system. The results are compared with the other theories and with the available experimental measurements. At the considered range of collision energies, the results agree reasonably well with recent experiments and theories. [Note: This paper will be published on volume 42 of the Brazilian Journal of Physics

Moderation in management research: What, why, when and how.

Many theories in management, psychology, and other disciplines rely on moderating variables: those which affect the strength or nature of the relationship between two other variables. Despite the near-ubiquitous nature of such effects, the methods for testing and interpreting them are not always well understood. This article introduces the concept of moderation and describes how moderator effects are tested and interpreted for a series of model types, beginning with straightforward two-way interactions with Normal outcomes, moving to three-way and curvilinear interactions, and then to models with non-Normal outcomes including binary logistic regression and Poisson regression. In particular, methods of interpreting and probing these latter model types, such as simple slope analysis and slope difference tests, are described. It then gives answers to twelve frequently asked questions about testing and interpreting moderator effects

First results for a superconducting imaging-surface sensor array for magnetoencephalography

Magnetoencephalography (MEG) follows from the initial fundamental work of Cohen in 1968 and development by several groups, most notably at MIT and at NYU, based on the development of the Superconducting QUantum Interference Device (SQUID) using the Josephson effect. The SQUID`s incredible sensitivity to magnetic fields permits the measurement of the very weak magnetic fields emitted from the human brain due to intracellular neuronal currents. Current growth in MEG is dominated by multiple sensor arrays covering much of the head. These new large devices have primarily been developed and made commercially available by several companies including BTI in the US, CTF in Canada, and Neuromag in Finland. Large projects are also in place in Japan. These systems contain more than 100 sensors spaced at various intervals over the head using various configurations of magnetometers and gradiometers. The different designs available on the market are driven by factors such as detection efficiency, cost, and application. They now present a completely novel whole-head SQUID array system using a superconducting imaging-surface gradiometer concept derived at Los Alamos. Preliminary tests have demonstrated higher performance, lower noise, and additional shielding of background fields while using simpler fabrication techniques than existing whole-head MEG systems, which should reduce production costs