Gifford-McMahon Cryocooler Research Articles

Liquid helium-free 15 T superconducting magnet at 4 K

We have successfully demonstrated a 15.1 T liquid helium-free superconducting magnet with a room-temperature bore of 52 mm using a Nb3Sn/NbTi hybrid coil, Bi2223 current leads and two Gifford-McMahon cryocoolers. The magnet has 830 mm outside diameter, 1221 mm height and 720 kg weight. The magnet was cooled to 3.6 K in 114 h. A central magnetic field of 15.1 T was achieved in 38 min. The temperature of the coil increased to 5.7 K due to ac losses during the excitation, but it decreased to 4.0 K before reaching 15.1 T. The temperature of the coil remained at a constant value of 3.8 K over the 24 h of operation. The demonstration indicated the usefulness of a liquid helium-free superconducting magnet to generate high magnetic fields up to 15 T.

Development of a 7 tesla superconducting wiggler

A 7 T three poles superconducting wiggler has been constructed to be used as a light source in AURORA-2D, a 700 MeV compact SR ring. The wiggler is cooled by compact Gifford-McMahon cryocoolers without liquid helium. The main components of the wiggler are three pairs of racetrack superconducting coils in an iron yoke, bismuth based oxide superconducting current lead unit and Gifford-McMahon cryocoolers. The coils generate a magnetic field of 7 T in a 40 mm coil gap within 360 seconds. The magnetic field period along the electron beam of the SR ring is 342 mm. The magnetic field integral is less than 4/spl times/10/sup -4/ Tm. The coils and the yoke are cooled by a Gifford-McMahon cryocooler with Joule-Thomson valve. The current lead unit and the thermal shield plate are cooled by separate Gifford-McMahon cryocoolers without Joule-Thomson valves. The current lead unit is designed to be easily removed for maintenance. The performance test in AURORA-2D was successfully completed. The maximum beam current was 19 mA at 700 MeV.

A cryocooler cooled 10 T split pair superconducting magnet for neutron scattering experiment

The authors have designed and constructed a cryocooler cooled 10 T split-pair superconducting magnet for a neutron scattering experiment with a vertical room temperature bore of 51 mm. The magnet, which is directly cooled by two 4 K Gifford-McMahon cryocoolers, consists of (Nb,Ti)/sub 3/Sn coils, NbTi coils, Bi(2223) superconducting bulk current leads and a cryostat. The inner coil with an inner diameter of 78 mm, an outer diameter of 191 mm, a height of 106 mm and a gap of 45 mm is made using (Nb,Ti)/sub 3/Sn conductor and a stainless steel bobbin. The outer coil with an inner diameter of 201 mm, an outer diameter of 300 mm, a height of 104 mm and a gap of 50 mm is made using NbTi conductor and a stainless steel bobbin. The Bi(2223) superconducting bulk current leads are thin walled sintered cylindrical tubes. The outer diameter, height and weight of the system are 600 mm, 760 mm and 280 kg, respectively. The magnet can be cooled down in approximately 52 hours. A continuous operation at 10 T, which is generated by an operating current of 151 A, has been performed. It takes about 15 minutes to excite the magnet up to 10 T.

１５Ｔ冷凍機冷却型超伝導マグネットの開発

We have designed and constructed a 15T cryocooled superconducting magnet with a vertical room temperature bore of 52mm. The main components of the magnet are four Nb3Sn superconducting coils, a NbTi superconducting coil, two pairs of bismuth-based oxide superconducting current leads (Bi2223 current lead) and two Gifford-McMahon (GM) cryocoolers. The coils were directly cooled down from room temperature to 3.6K in 114h by compact GM cryocoolers without liquid helium. The magnet generated a magnetic field of 15.1T at the center of the bore and was excited from 0 to 15.1T in 38min. The magnet succeeded in holding the field at 15.1T for 24h at the constant coil temperature of 3.8K.

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.

High gradient magnetic separation using a high temperature superconducting magnet

Abstract We report on the operation and testing of a high temperature superconducting (HTS) high gradient magnetic separator (HGMS). The separator magnet is made of 624 m of Silver/BSCCO HTS wire and has overall dimensions of 18 cm OD, 15.5 cm height and 5 cm ID. HTS current leads are used to reduce the heat leak to the magnet. The system operates in a vacuum and is cooled by a two stage Gifford–McMahon cryocooler. A series of HGMS experiments were performed using this system to demonstrate the performance of HTS magnetic separators.

A cryocooler cooled 5 T superconducting magnet with a horizontal and vertical room temperature bore

We designed and constructed a cryocooler cooled 5 T superconducting magnet with a horizontal room temperature bore of 50 mm and a vertical room temperature bore of 90 mm without liquid helium. The magnet, which is directly cooled by 4 K Gifford-McMahon cryocooler in vacuum, consists of NbTi coil, Bi(2223) bulk current leads and cryostat. The coil with an inner diameter of 130 mm, an outer diameter of 301 mm, a height of 66 mm and a gap of 80 mm is made using NbTi wires and Cu-plated SUS bobbin. Bi(2223) bulk current leads are thin-walled sintered cylindrical tubes. The outer diameter, height and weight of the magnet are 510 mm, 730 mm and 260 kg, respectively. The magnet is cooled down to 3.8 K in approximately 62 hours. A continuous operation at 5 T, which is generated by an operating current of 122 A, has been performed.

HTS high gradient magnetic separation system

We report on the assembly, characterization and operation of a high temperature superconducting (HTS) magnetic separator. The magnet is made of 624 m of Silver/BSCCO superconducting wire and has overall dimensions of 18 cm OD, 15.5 cm height and 5 cm ID. The HTS current leads are designed to operate with the warm end at 75 K and the cold end at 27 K. The system operates in a vacuum and is cooled by a two stage Gifford-McMahon cryocooler. The upper stage of the cryocooler cools the thermal shield and two heat pipe thermal intercepts. The lower stage of the cryocooler cools the HTS magnet and the bottom end of the HTS current leads. The HTS magnet was initially characterized in liquid cryogens. We report the current-voltage (I-V) on characteristics of the HTS magnet at temperatures ranging from 15 to 45 K. At 40 K the magnet can generate a central field of 2.0 T at a current of 120 A.

Advantages of a four-valve pulse tube cryocooler for high- T c sensor operation

In this contribution the results of our recent research on a special four-valve pulse tube cryocooler will be presented. We have performed control volume analysis in order to get information about the cooling power, the coefficient of performance and fundamental limitations of the ideal principle as a function of temperature, pressure ratio and the gas used for operation. High speed resistance thermometry at a prototype machine was used for measuring the temperature at the hot end and at the cold end of the pulse tube during operation for comparison with the predictions of theory. We will discuss a comparison about the possibility for continuous operation of a high- T c SQUID gradiometer for biomagnetic applications on our prototype cryocooler and on a Gifford-McMahon cryocooler using the results of noise measurements recorded during cryocooler operation.

Generation of 1 T, 0.5 Hz alternating magnetic field in room temperature bore of cryocooler-cooled Bi-2212 superconducting magnet

Alternating magnetic fields have been stably generated at frequencies up to 0.5 Hz in a room temperature (RT) bore of a Bi-2212 superconducting magnet cooled by a Gifford-McMahon (G-M) cryocooler. The magnet has generated alternating magnetic fields with peak magnetic fields up to 1.0 T. The present Bi-2212 superconducting coil heat-treated in an oxygen partial pressure ( pO 2) of 1 atm has shown an alternating current (a.c.) loss characteristic, reducing the intergranular hysteresis loss compared with the heat treatment in pO 2 of 0.2 atm.

A cryocooler cooled 6 T NbTi superconducting magnet with room temperature bore of 220 mm

We are developing a cryocooler cooled 6 T superconducting magnet without liquid helium and/or liquid nitrogen. This magnet, which is directly cooled by two 4 K Gifford-McMahon (GM) cryocoolers in vacuum, consists of an graded NbTi coil, Bi-based oxide superconducting current leads and a persistent current switch. The coil is made using two types of NbTi wires and a copper bobbin. The coil with an inner diameter of 260 mm, an outer diameter of 329 mm, and a height of 334 mm, is impregnated with grease to ensure a good heat conduction. The operating current for central field of 6 T in the 220 mm room temperature bore is 152 A. A mechanical persistent current switch is used to limit the load from the switch into the 4 K GM cryocooler. Designed temperature of the magnet is 4.9 K at the operating current of 152 A.

An open refrigerator cooled superconducting magnet for the special purpose of magnetic resonance imaging

A 10 K refrigerator cooled superconducting magnet with an open-ended, C-shaped iron core was developed and tested for the special purpose of magnetic resonance imaging. The magnet employs a single Nb3Sn tape superconducting coil and an open-ended iron loop. The superconducting coil Is cooled and maintained at the 10 K operational temperature by a Gifford-McMahon (G-M) cryocooler. The magnetic field in the iron open gap is used for magnetic resonance imaging. The field homogeneity is achieved by contoured iron pole faces and specially developed passive shimming procedures. We have achieved less than 1 ppm/hour field stability and a 29 ppm field homogeneity over a 10 cm diameter spherical volume. High quality images have been produced using this magnet.

A cryocooler-cooled 10 T superconducting magnet with 100 mm room temperature bore

A NbTi/Nb/sub 3/Sn superconducting magnet directly cooled by a 4 K cryocooler has been fabricated. It successfully attained a 10 T field at the center of a 100 mm room temperature bore. A 4 K Gifford-McMahon (GM) cryocooler, using Er/sub 3/Ni regenerator, cools the magnet without the use of liquid helium. Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ superconducting power leads were adopted to reduce heat input to the 4 K stage. The coil heat is removed through heat conducting copper cylinders. The coil winding is epoxy-impregnated without a bore tube to stabilize the magnet against quenching. Two diode shunt circuits are placed in the cryostat to protect the coils. A fast ramp of 10 T/20 min, was attained by reducing Nb/sub 3/Sn conductor hysteresis loss and by decreasing the contact resistance between the coil and the conduction cylinder.

磁性蓄冷材を使用した小型冷凍機 磁性蓄冷材を用いたＧＭ冷凍機における蓄冷器内の温度分布および流量分布

A two-stage GM cryocooler is commonly used in many cryogenic systems, such as a cryopump and an MRI. The lowest temperature for a conventional GM cryocooler, however, was limited to about 10K. Recently, magnetic regenerator materials, which have larger heat capacity below 10K than a conventional regenerator material of Pb, enable the GM refrigerator to achieve the liquid helium temperature level. In this temperature range, helium in the GM cryocooler cannot be regarded as an ideal gas. The purpose of this study is to investigate a regenerator, which is operated with non-ideal helium gas. The experimental results of the regenerator performance in a two-stage Gifford-McMahon (GM) cryocooler are described in this paper. Temperatures and pressure in the regenerator were measured and mass flow rate in the regenerator was calculated from these data. The temperature profile, temperature fluctuation and mass flow rate in the regenerator are compared between ideal gas operation and non-ideal gas operation. For ideal gas operation, the temperature profile was smooth and the temperature fluctuations at each positions are almost equal. For non-ideal gas operation, a steep temperature gradient and large temperature fluctuation, which almost covered the temperature difference of the regenerator were observed from the hot end to the middle of the regenerator. From the middle to the cold end, however, both the temperature gradient and the temperature fluctuation were very small. The mass flow rate in the regenerator for ideal gas operation decreased from the hot end to the cold end. For non-ideal operation, the mass flow rate increased. The difference between the regenerator for ideal gas operation and that for non-ideal gas operation was shown clearly.

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.

磁性蓄冷材を使用した小型冷凍機 １．５Ｗ級４Ｋ‐ＧＭ冷凍機の開発

A 1.5W-class 4K GM (Gifford-McMahon) cryocooler has been developed. The regenerator material ErNi0·9Co0·1, the cold end of the second cylinder made on copper and intake/exhaust valve having original timing and interval are used in the cryocooler. In this study, four sizes of cylinders and two types of compressor units were prepared and the cooling capacities were measured. As a result, cooling capacity of the first and the second stage depended on the ratio between the first and the second cylinder's volume. When the compressor was fixed and cylinders were changed, the bigger the second stage cooling capacity became and the smaller the first stage cooling capacity became. The maximum cooling capacity of the first stage at 40K was 53.7W, and that of the second stage at 4.2K was 1.74W.

Near-isothermal regenerator - A perturbation analysis

We develop a perturbation model for regenerators of the type used in Stirling and Gifford-McMahon cryocoolers. The model is valid in the near-isothermal limits, which is usually a quite good approximation for these devices, because typically, the flow passages are small compared to the conductive heat penetration depth in the gas, and because the thermal mass of the heat-storing matrix is large compared to the thermal mass of the gas. Our model is one dimensional and it uses a conventional, empirical model for heat transfer. Our approach can yield solutions for arbitrary heat transfer correlations, which makes it suitable for incorporation in computer models. We also present complete solutions for the important case corresponding to the laminar limit, when the Nusselt number is a constant. Results include the longitudinal temperature distribution and the enthalpy flux through the regenerator. In some conditions, which are more representative of pulse-tube refrigerators than of typical Stirling cryocooler operation, and for which the model remains valid, we obtain multiple solutions, some of which are associated with a sharp temperature peak, in a region in the regenerator where the amplitude of the mass flow rate oscillation exhibits a minimum

Signal-to-noise ratio improvement of HTSC bolometers for cryocooler operation using a thermal compensation principle

This paper reports on the reduction of thermal fluctuations in a high T c superconductor (HTSC) bolometer using a compensation technique. The HTSC bolometer consisted of two meander-like YBCO structures separated by a distance of 5 mm on a silicon substrate. A two-stage Gifford-McMahon cryocooler with an additional thermostat was used to cool the samples to a temperature of ≈88 K, with a 5 mK fluctuation peak to peak. A factor of 100 improvement in signal-to-noise ratio was achieved during the compensation experiments.

Critical current measurement unit utilizing Bi-based oxide superconducting current leads and cryocoolers

A measurement unit has been developed to investigate the dependence on temperature and magnetic field of superconducting cable critical current without the use liquid helium or liquid nitrogen. A test specimen, which is tested using the four probe method, is cooled by a Gifford-McMahon (GM) type cryocooler to a temperature of 20 K to 90 K in a vacuum vessel. Transporting direct current, up to 500 A, is supplied to the specimen through the Bi-based oxide superconducting current leads. The advantages of utilizing oxide superconducting current leads are that the leads have low thermal conductivity, and also that the leads create no Joule heating effect, so the heat input to the sample is minimized. The external magnetic field of up to 3 T, which is applied to the specimen, is generated by a superconducting magnet which also uses Bi-based oxide superconducting current leads and is also cooled by a GM cryocooler. Detail design of the unit, the results of operating test and an example of the measurement result on the Bi-based oxide bulk specimen are presented in the paper.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cryogen free Nb/sub 3/Sn magnet, operated at 9.5 K with High Tc BiSrCaCuO (2212) current leads

A superconducting Nb/sub 3/Sn magnet has been operated in the absence of liquid cryogen utilising the cooling power of a 2 stage Gifford-McMahon cryocooler. The magnet generated 2.0 T in a 17 cm bore at a current of 145 A and a temperature of 9.5 K. Current was supplied via high T/sub c/ BiSrCaCuO (2212) current leads. The magnet demonstrated long term stability and was successfully quenched without any damage to the system; recovery was complete within 40 minutes. >