4K-GM Cryocooler Research Articles

Correlation between cooling power and heat quantity of Er-Ho binary nitride as regenerator of 4K-GM cryocooler

Advanced regenerator materials of erbium and holmium binary nitrides, Er x Ho1-x N (x = 0 - 1) were studied. Its specific heat vs. temperature curve, possesses a peak due to the magnetic phase transition at their Curie temperature. The cooling power at 4.2 K of a commercial 4K-GM cryocooler filled with the binary nitride of x = 0.625, 0.75, 0.875 or 1 was evaluated. It was found that the cooling power at 4.2 K has a good linear correlation with the heat quantity, which is calculated by integrating C with respect to T in the region of 4.2 - 7.0 K. Three binary nitrides (x = 0.625, 0.875 and 1) filled in series along the regenerator column gave 26% higher cooling power than that filled with HoCu2.

Improvement of cooling capacity of 4K GM cryocooler in magnetic fields

4K GM cryocoolers have been widely used for cooling superconducting magnets, such as, magnets in MRI, NMR, Magnetic field-applied Czochralski (MCZ) and electron accelerator systems. In general, high capacity cryocoolers are required to maintain a stable operation for these systems. Cryocoolers are inevitably exposed to the magnetic fields in these superconducting magnet systems. Therefore, it is important that the cooling capacity can be maintained in magnetic fields. Recently, we measured the cooling capacity of a Sumitomo Heavy Industries, Ltd., (SHI) new 4K GM cryocooler, RDE-412, in magnetic fields up to 3.0 T. It is found that the second stage temperature only increases about 0.3 K and the cooling capacity keeps in the specification of 1.25 W at 4.0 K up to 3.0 T.

Optimization of loading ratio of ErN as regenerator of 4K-GM cryocooler

High purity erbium nitride (ErN) spheres with the size range of 150-180 µm and 180-212 µm were prepared by nitriding Er metal spheres with low oxygen content. The initial regenerator material of HoCu2 on the cold end of the second regenerator column in 4K-GM cryocooler with nominal cooling power of 0.1 W at 4.2 K was replaced by ErN with different sizes. Higher cooling power was obtained when ErN of smaller size with lower oxygen content was used. We investigated the effect of partial replacement of HoCu2 by ErN in the cold end side of second stage regenerator column on cooling power of 4K-GM cryocoolers. When ErN were substituted for 20 % of HoCu2, the cooling power at 4.2 K reached 0.318 W. This value was 1.36 times as high as that of the cooling power of the GM cryocooler with commercially available regenerator arrangement. Therefore, use of ErN regenerator materials leads to the energy-saving and downsizing of 4K-GM cryocoolers.

Improvement of a Large Bore Cryogen-Free Superconducting Magnet for a Hybrid Magnet

A 360-mm room-temperature bore cryogen-free superconducting magnet (CSM), consisting of Nb 3Sn coils and NbTi coils, for a hybrid magnet (HM) has generated the maximum magnetic field of 9.5 T. However, the magnetic field of the CSM has been limited to 8.5 T in the hybrid magnet mode because of a cooling problem. As a result, the hybrid magnet composed of a 19-T water-cooled resistive magnet (WM) had the utmost field generation of 27.5 T. Therefore, we improved the CSM to generate higher magnetic fields. For the improvement of the cooling problem, Nb 3Sn coils were replaced, and thermal conduction was improved between coils and a 4K-GM cryocooler. Furthermore, support structures with a tensile strength over 80 kN and a spring support were adopted against the magnetic force to support the self-weight of coils and to absorb stress caused by thermal contraction difference between each coil. After the improvement, the CSM generated 9.5 T within 1 h and the maximum magnetic field of 9.7 T in a 360-mm room-temperature bore. The HM succeeded in generating 28 T in a 32-mm room-temperature bore with the CSM operated at 9.0 T.

Experimental Investigation of Cooling Capacity of 4K GM Cryocoolers in Magnetic Fields

4K GM cryocoolers are inevitably exposed to the magnetic field inMRI systems. The cooling capacity of a 4K GM cryocooler is strongly dependent on the heat capacity of the magnetic regenerator materials, such as HoCu2, Er3Ni and Gd2O2S(GOS). In order to clarify the effect of the magnetic field on acryocooler's performance, we measured the cooling capacity of Sumitomo Heavy Industries, Ltd. (SHI) 1W 4K GM cryocoolers in magnetic fields up to 2.0 T. It is found that the impact of a magnetic field on the cooling capacity with a HoCu2/GOS hybrid regenerator is much smaller than that with a HoCu2 regenerator.

Design and Demonstration of a 5-Bit Flash-Type SFQ A/D Converter Integrated With Error Correction and Interleaving Circuits

We have designed a fully integrated 5-bit flash-type single-flux quantum (SFQ) analog-to-digital converter (ADC), in which an error correction and a bit-interleaving circuit are integrated with complementary quasi-one-junction SQUID (CQOS) comparators, and we verified its operation in experiments. Two types of look-back error correction circuits with different clock schemes were designed for the integrated ADCs to avoid the gray zone in comparators and to convert gray code to binary code. The interleaving circuit was also used in the ADC to add one extra bit. A binary 5-bit A/D converter was integrated into an IC chip. Then, input waveforms at low frequencies were successfully retrieved from the binary data of the A/D converter without any errors. We also confirmed that the A/D converter operated properly by conducting tests in our cryocooling system using a 4K-GM cryocooler after the chip was flip-chip bonded onto a multi-chip-module (MCM) carrier. We also used the beat frequency method to test and confirm that the CQOS comparator operated effectively at high frequency, i.e., 3 bits at 15 GHz in the binary code operation and 4 bits at 15 GHz in the gray code operation. Furthermore, operations at sampling frequencies of up to 32 and 50 GHz with a low-frequency analog input signal were experimentally confirmed for a 4-bit comparator circuit with a critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 2.5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 10 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively.

Electrical and thermal characteristics of Bi2212/Ag HTS coils for conduction-cooled SMES

In this paper, we investigated the electrical and thermal performance of conduction-cooled Bi2212/Ag HTS coils with 4K-GM cryocooler system. First, we measured the critical current Ic for different ambient temperatures T0 at 4.2 K - 40 K. Experimental results revealed that Ic increased with the decrease in T0 and was saturated at T0 < 10 K. We carried out thermal analysis considering heat generation, conduction and transfer under conduction-cooling condition, and reproduced the electrical and thermal characteristics of the conduction-cooled HTS coil, taking account of temperature dependence of specific heat and thermal conductivity of the materials. We also measured the temperature rise of Bi2212/Ag HTS coil for different continuous current levels at T0 = 4.8 K. Experimental results revealed the criterion of thermal runaway, which was discussed in terms of heat generation and propagation in the test coil.

２１世紀へ伝えたい超伝導応用技術 ５ 冷凍機伝導冷却式超電導磁石の開発 スイッチポンで超電導，より使い易い超電導磁石を目指して

This paper describes the development of a conductive-cooled superconducting magnet that uses no cryogen, such as liquid helium or liquid nitrogen. First, the history of the development is introduced. A conductive-cooled superconducting magnet was realized in the late 1980s for the first time and was commercialized about 10 years ago. The technical points and construction for the magnet are explained. The important points were a 4K-GM cryocooler and an HTS superconducting current lead. A conventional GM cryocooler had not achieved 4K level, but by using magnetic regenerator material, we realized refrigeration at liquid helium temperature. An HTS current lead was used as a current lead between a thermal shield and a superconducting coil. Heat leakage by a superconducting current lead is less than 1/10th by a conventional copper current lead. Heat leakage to the 4K level was then dramatically reduced, and the total thermal load at 4K level becomes low enough for a 4K-GM cryocooler. In a conductive-cooled superconducting magnet, a superconducting coil is directly cooled by the second stage of a 4K-GM cryocooler at the 4K level via a good thermal conductive pass. The coil is surrounded by a thermal shield, which is cooled by the first cooling stage at around 50K. A conductive-cooled superconducting magnet has such features as simple operation, small size, and easy access to a magnetic field. Several companies have commercialized a conductive-cooled magnet, and the magnets have been applied not only to research, but also to industrial use, such as MRI, silicon crystal growth, and magnetic separation. The latest R & D and future aspects for a conductive-cooled superconducting magnet are also described.

４Ｋ−ＧＭ冷凍機の磁場中挙動

The behavior of a 4K-GM (Gifford-McMahon) cryocooler in a magnetic field has been investigated. Characteristics of the magnetic regenerator materials were researched. The specific heat of ErNi0.9Co0.1, ferromagnetic regenerator material decreased under a magnetic field of 1T, while that of HoCu2, an antiferromagnetic regenerator material, had little effect of the field. The SRDK-408BW 4K-GM cryocooler, made by Sumitomo Heavy Industries, Ltd., was installed in a cryocooler-cooled superconducting magnet. The cryocooler was operated in a magnetic field under 1T. The cryocooler with ErNi0.9Co0.1 rose to its reachable temperature after increasing the magnetic field. In contrast to ErNi0.9Co0.1, the cryocooler with HoCu2 has a constant cooling capacity under 1T.

磁性蓄冷材を使用した小型冷凍機 ４Ｋ‐ＧＭ冷凍機における吸排気バルブの最適化

The influence of intake/exhaust valve timing on performance of a 4K Gifford-McMahon (GM) cryocooler has been investigated for the optimization of intake and exhaust valves. The 4K-GM cryocooler employed for the present study was a standard two-stage type with a rotary valve for intake/exhaust. The second stage regenerator in the cryocooler had both lead spheres and spherical magnetic regenerator material, ErNi0·9Co0·1, with the latter arranged at the colder end. The experimental results described in this paper show that a reasonably early valve open timing not only produced much greater cooling capacities on both stages, but also was advantageous in terms of cycle speed, no load temperature and input power to the compressor. Under the optimum conditions, the cooling capacities, above 1.10W at 4.2K as well as close to 40W at 40K, were obtained with an input electric power of about 7kW. This paper discusses the results of cooling capacity with P-V diagrams, and indicates that the reduction in pressure drop at the intake and exhaust valves is important for a 4K-GM cryocooler.