Masami Okamura

Specific Heat of ErNi in Magnetic Fields and Its Performance as Regenerator Material in Gifford-McMahon Refrigerator at Cryogenic Temperatures

Specific heat of the rare earth compound ErNi is measured in magnetic fields up to 5 T in the temperature range 4.2 to 35 K. The specific heat under applied magnetic field of 1 T is larger than that of Er3Ni below 16 K. The numerical simulation of a Gifford-McMahon cycle shows that the refrigeration capacity using an ErNi regenerator is larger than that using one of Er3Ni or lead. It is shown for the first time that the refrigeration capacity can be increased by application of a weak magnetic field to the ErNi regenerator.

Specific heat behavior for pseudobinary compounds ErxDy1−xSb

To search the new magnetic regenerator materials in the temperature range near 4 K, specific heat behavior of pseudobinary compounds ErxDy1−xSb(x=0, 0.2, 0.6, 0.8, and 1) was studied at temperatures from 2 to 40 K. The ordering temperature shifts from 3.5 to 9.5 K with increasing the concentration of the Dy+3 ion. The compounds of 1≳x≳0 show the gradual magnetic phase transition. The change of entropy associated with the magnetic phase transition is close to R ln 2 for all compounds. The change of magnetic entropy up to 40 K increases rapidly with decreasing the concentration of the Dy+3 ion, which implies that more excited states exist. The compounds of lower concentration of the Dy+3 ion have properties well suited for magnetic regenerative material around the temperature range of 4 K.

Evaluation of mechanical properties for spherical magnetic regenerator materials fabricated by rapid solidification process

Various magnetic regenerator materials, such as Er3Ni, Er3Co and ErNi, are fabricated in the form of a spherical particle by a rapid solidification process. 4 K level refrigeration has been obtained by a GM refrigerator using these materials. However, the magnetic regenerator materials are considered brittle, as they are intermetallic compounds.

Multilayer Magnetic Regenerators with an Optimum Structure around 4.2K

In order to obtain high regenerative effectiveness, the heat capacity of the regenerator materials must be larger than that of helium as the working gas. For a magnetic regenerator material to be effective in the low temperature range, its transition temperature must be within the range where the helium regenerative operation is performed.

A novel thermal insulation method for a next-generation conduction : cooled superconducting magnet

We have developed a new thermal insulation method for use with superconducting magnets or in applications where temperature needs to be maintained within a limit. Termed as Multi-shell-insulation (MSI) method, this method uses several thermal-energy storage shields to intercept heat flowing into the superconducting magnet. The shields are arranged in a vacuum vessel so that each shield is inside another slightly bigger shield and the superconducting magnet is inside the innermost shield. To use MSI, the shields are first cooled to about the same temperature as the superconducting magnet and then they are thermally separated from the cooling means. The temperature of the outermost shield would rapidly rise, but those of inner shields would rise only slowly; the closer the shield to the center, the slower the temperature rise. To demonstrate MSI effect, we used 10 copper shields, five of which were cooled to 30K and the other five to 4K by a two-stage Gifford-McMahon refrigerator. The 3kg-innermost shield could be maintained below 30K for a one-month period: a promising result for the realization of a cryocooler-detached, high-Tc superconducting magnet. It was also possible to maintain the inner shield below 10K for 8 days by coating the three inner copper shields with HoCu2 and Er3Ni.

Magnetic Regenerator Materials for Sub-2 K Refrigeration

The heat capacities of rare-earth intermetallic compounds RAlxGa2-x (R=Nd,Ho,Er,Dy; 0≤x≤2) and rare-earth garnets Gd3Ga5012, Dy3Al5012 were investigated for regenerator use below 4 K region. The ErAl0.5Ga1.5 sample had a comparable heat capacity to that of HoCu2 below 2 K. Gd3Ga5O12 and Dy3Al5012 materials have a larger heat capacity than HoCu2 below 2 K, but the narrow temperature width of the half peak value in the heat capacity requires a mixture of several rare-earth oxides which have different magnetic transition temperatures.

A new Amorphous saturable core for a magnetic amplifier at high frequency

In view of the increased use of higher frequencies for switching power supply, a new amorphous saturable core for a high frequency magnetic amplifier has been developed. Its core loss is about one half of that for a conventional amorphous saturable core, and one third of that for an 80 Ni permalloy core with a high rectangular ratio. The newly developed amorphous saturable core has been actually operated as a magnetic amplifier in a 220 kHz dc-to-dc converter. It is found that the converter efficiency increases by about 1%, core temperature rise decreases by 12°C, and the reset current reduces to about one half, in comparison with a conventional amorphous saturable core. These improvements are more conspicuous, when compared with an 80 Ni permalloy core.