Closed cycle refrigerator for superconducting mm wave-mixers

We describe the construction and operation of a closed cycle refrigerator with an operating temperature of 2.8 K. Helium gas at a pressure of 1.2 MPa is precooled to 18 K by a Stirling refrigerator (CTI 350) and expanded in a Joule-Thomson stage to ≈10 kPa. A vacuum pump circuit and compressor bring the gas back to its original pressure. The refrigerator is designed to cool the first stages of a radio astronomical receiver with a superconducting (SIS) mixer. It fits into a small radio telescope and works widely independent of orientation. After a cooldown time of 12–14 h a cooling capacity of 200–250 mW is available.     

Dynamic simulation of the helium refrigerator/liquefier for LHD

A real-time dynamic simulation has been carried out for the 10 kW class helium refrigerator/liquefier of Large Helical Device (LHD) at National Institute for Fusion Science (NIFS). The refrigerator consists of eight screw compressors, seven expansion turbines, fourteen heat exchangers and a 20 m³ liquid helium reservoir. A simulation model was implemented to Cryogenic Process REal-time SimulaTor (C-PREST), developed as a platform for the plant process study and optimization. Validity of the simulation model has been confirmed based on the design values as well as the results of commissioning tests. This paper describes the cooldown process and expansion turbine trips during the operation. Difficulties of dynamic simulation for the large cryoplant are also discussed.     

Long flexible transfer lines for gaseous and liquid helium

Screened flexible four-fold coaxial transfer lines for gaseous and liquid helium with lengths of 5 to 50 m have been successfully built and tested. The lines for gaseous helium have to supply and return a mass flow of 5 to 10 g s⁻¹ at temperatures ranging between 350 and 20 K for cooldown or warmup of superconducting magnets. The lines for liquid helium have to supply up to 100 ℓ h⁻¹ for final cooldown or up to 25 ℓ h⁻¹ for normal operation of superconducting magnets. The hydrodynamic and thermal performance characteristics of the lines have been measured. The results are encouraging and it can be envisaged to use this kind of lines for superconducting magnets in high energy accelerators.     

Development and parametric study of the convection-type stationary adiabatic demagnetization refrigerator (ADR) for hydrogen re-condensation

The adiabatic demagnetization refrigerator (ADR) system in this paper is composed of a conduction-cooled current cycling high-temperature superconducting (HTS) magnet system, a magnetic bed assembly, its heat exchange parts and an auxiliary precooling stage (a commercial GM cryocooler and a liquid nitrogen vessel). The whole magnetic refrigeration system including the conduction-cooled HTS magnet is cooled by the precooling stage to absorb the rejection heat of the ADR cycle. The packed bed type magnetic bed consists of tiny irregular powders of Dy_0.9Gd_0.1Ni₂ enclosed in a thin walled stainless steel container (22.2 mm in O.D., 0.3 mm in thickness and 40.0 mm in height). The precooled heat transfer fluid (helium) travels through the magnetic material when heat rejection is required; otherwise the helium stagnates within its pores (pseudo-adiabatic process). Flow of the heat transfer fluid substitutes for the function of a traditional heat switch, creating, essentially, a forced-convection type heat switch. The magnetic bed assembly is periodically magnetized and demagnetized at the center of the conduction-cooled HTS magnet which can stably generate both strong and alternating magnetic field from 0 T to 3.0 T (0–130 A) with an average ramp rate of 0.24 T s⁻¹. The cooling capacities of the ADR system at 20 K which is the normal boiling point (NBP) of hydrogen, are 11.1 J cycle⁻¹, 6.3 J cycle⁻¹ and 1.9 J cycle⁻¹ when the temperature spans are 1 K, 2 K and 3 K, respectively. We describe the detailed construction of the ADR system and discuss the test results with the operational parameters (the entrained helium pressure, the mass flow rate of helium and the operating temperature span) in the 20 K region.     

300kW超导单极电机低温技术及试验研究

简述了３００ｋＷ超导单极电机试验的低温系统，介绍了超导单极电机的磁体低温系统及液氦、液氮冷却，输液及运行情况，分析比较了多次试验研究的结果。     

Superferric quadrupoles for FAIR Super FRS energy buncher

The quadrupole magnets for FAIR Super FRS energy buncher have large usable aperture, high magnetic pole-tip field and high gradient field quality. The iron-dominated magnets with superconducting coils have to be used in this application. The NbTi coil, laminated iron, and support structure of about 22 tons is immersed in liquid helium. The 4.5 K helium chamber is completely covered with a thermal shield cooled by helium at 50–80 K on its outer and inner surface. The helium chamber and thermal shield is enclosed in a vacuum shell.The paper presents design details of the long quadrupole. Coupled thermal, magnetic and structural analysis was carried out to design the magnet iron, magnet coil, helium vessel and support links and ensure the required gradient field quality is achieved. The paper also presents the design of support links and outer vacuum chamber.     

Cryogenic design and operation of liquid helium in an electron bubble chamber towards low energy solar neutrino detectors

We are developing a new cryogenic neutrino detector: electron bubble chamber, using liquid helium as the detecting medium, for the detection of low energy p-p reaction neutrinos (<420 keV), from the Sun. The program focuses in particular on the interactions of neutrinos scattering off atomic electrons in the detecting medium of liquid helium, resulting in recoil electrons which can be measured. We designed and constructed a small test chamber with 1.5 L active volume to start the detector R&D, and performed experimental proofs of the operation principle. The test chamber is a stainless steel cylinder equipped with five optical windows and ten high voltage cables. To shield the liquid helium chamber against the external heat loads, the chamber is made of double-walled jacket cooled by a pumped helium bath and is built into a LN₂/LHe cryostat, equipped with 80 K and 4 K radiation shields. A needle valve for vapor helium cooling was used to provide a 1.7-4.5 K low temperature environments. The cryogenic test chamber has been successfully operated to test the performance of Gas Electron Multipliers (GEMs) in He and He + H₂ at temperatures in the range of 3-293 K. This paper will give an introduction on the cryogenic solar neutrino detector using electron bubbles in liquid helium, then present the cryogenic design and operation of liquid helium in the small test chamber. The general principles of a full-scale electron bubble detector for the detection of low energy solar neutrinos are also proposed.     

An ammonia-water absorption refrigerator with a large temperature lift for combined heating and cooling

The COP (Coefficient of Performance) of an ammonia-water absorption refrigerator can be significantly improved by incorporating a mixing column and a second absorber which is combined with a two-stage refrigerant expansion. At an internal temperature lift of 85 K (−20°C to + 65°C), these modifications result in a 50% higher COP for cooling—assuming ideal components. With real components, a 40% improvement may be achieved. The proposed cycle was realised in a laboratory test plant with a refrigeration capacity of 10 kW at −20°C evaporator temperature. This machine was operated at a solution temperature up to 225°C and a condenser temperature of 65°C corresponding to a pressure up to 3.0 MPa. COP versus temperature lift and load behaviour was tested. A COP of 0.38 was achieved at a temperature lift of 85 K. An efficient high-temperature lift cycle like that described in this paper may find applications for deep freezing, as a topping cycle to achieve triple-effect performance, or as a device to produce simultaneous heating and cooling.     

Proposed designs for a “dry” dilution refrigerator with a 1 K condenser

Recent development of “dry” dilution refrigerators has used mechanical cryocoolers and Joule-Thomson expansion stages to cool and liquefy the circulating ³He. While this approach has been highly successful, we propose three alternative designs that use independently-cooled condensers. In the first, the circulating helium is precooled by a mechanical cooler, and liquified by self-contained ⁴He sorption coolers. In the second, the helium is liquefied by a closed-cycle, continuous flow ⁴He refrigerator operating from a room temperature pump. Finally, the third scheme uses a separate ⁴He Joule-Thomson stage to cool the ³He condenser. The condensers in all these schemes are analogous to the “1-K pot” in a conventional dilution refrigerator. Such an approach would be advantageous in certain applications, such as instrumentation for astronomy and particle physics experiment, where a thermal stage at approximately 1 K would allow an alternative heat sink to the still for electronics and radiation shielding, or quantum computer research where a large number of coaxial cables must be heat sunk in the cryostat. Furthermore, the behaviour of such a refrigerator is simplified due to the separation of the condenser stage from the dilution circuit, removing the complex interaction between the 4-K, Joule-Thomson, still and mixing chamber stages found in current dry DR designs.     

A closed cycle He-He dilution refrigerator insensitive to gravity

The cooling power and the lifetime of an open cycle dilution refrigerator as developed for the Planck mission (100 nW at 100 mK during 30 months) are limited by the quantity of the helium isotopes carried on the satellite at launch, because the helium mixture obtained after the dilution process is rejected into space. Future space missions require to increase the cooling power and lifetime significantly (1 μW at 50 mK during 5 years).Therefore we are extending the open cycle dilution refrigerator with a helium isotope separator operating at 1 K to close the cycle. A first prototype to demonstrate the principle of the closed cycle dilution refrigerator has been tested and a cooling power of 1 μW at temperatures below 60 mK has been obtained. We present the apparatus and the experimental results and give some elements for its integration in a complete cooling chain. The advantages (continuous operation, absence of magnetic field, less weight) of a closed cycle dilution refrigerator with respect to an adiabatic demagnetization refrigerator are also discussed.     

合肥超导托卡马克装置首次致冷

１９９４年７月１２－２２日期间合肥超导托卡马克装置ＴＨ－７首次低温调试。４Ｋ温区冷重高达１４Ｔ的超导环场磁体经９６ｈ致冷，用二相Ｈｅ保冷３昼夜。超导线圈内外侧设侧设置的热辐射屏重约８Ｔ，用液氦致冷，约６０ｈ冷至８０Ｋ。磁体共励磁二次，最大电流达４６５０Ａ，相应的环场为２．２Ｔ，超导体的最高场为３．７Ｔ。     

Liquid-helium stage in a scanning electron microscope

A liquid-helium stage to be inserted into a scanning electron microscope (SEM) is described. The sample chamber, which is filled with liquid helium, has a volume of about 8 × 7 × 5 cm³. Translation of the cooled sample in x, y, and z directions is possible during electron-optical observation. Since mounting the liquid-helium stage to the SEM can be performed in less than 30 min, switching the SEM from regular operation at room temperature to the low-temperature mode and back is relatively easy. After cooling the system to 4.2 K, a 3 litre helium reservoir allows experimentation near 4.2 K for about 5 h.     

Experimental investigation and optimization of small-scale helium liquefaction with multi-cryocoolers

Small-scale helium liquefiers using regenerative cryocoolers with cooling power up to 1.5 W at 4.2 K could be used to re-liquefy evaporated helium gas of small- and medium-sized cryogenic devices such as MEG and PPMS. A serial–parallel-path helium liquefier with a liquefaction rate of 83 Litres per day (L/d) using five 4 K G-M cryocoolers is developed, and has been applied to the Wuhan National High Magnetic Field Center (WHMFC) in China. Different from parallel-path helium liquefier, the helium gas is effectively, stepwise precooled by heat exchangers on multi-cold flanges, and thus the additional purifier and precooling coil heat exchangers on the thinner part of the cold head cylinder containing the 2nd stage displacer could be removed to simplify the construction. Through theoretical calculation and conclusive analysis, an optimum configuration is proposed and makes a reference to the design of serial–parallel-path helium liquefier with multi-cryocoolers.     

Development of a Superconducting Magnet System with Zero Liquid Helium Boil-off

A homogeneous magnetic field superconducting magnet with a cold bore of 250 mm and a central field of 4.3 T has been designed, manufactured, and tested with zero liquid helium boil-off. As a result of magnetic field homogeneity considerations, the magnet is composed of three coaxial coils: one main coil and two compensation coils. All coils are connected in series and can be charged with a single power supply. The magnetic field homogeneity is about ±3.0 % from ?200 mm to 200 mm in axial direction with 86 mm in diameter. The magnet can be operated in persistent mode with a superconducting switch. A two-stage GM cryocooler with a capacity of 1.5 W at 4.2 K was used to cool the superconducting magnet. The cryocooler prevents the liquid helium from boiling off and leads to zero helium loss during static operation. The magnet can be operated in liquid helium circumstance by cooling the gas helium with the cryocooler without additional supply of liquid helium. Under this condition, the magnet is successfully operated up to 4 T without quench. The magnet system can be generating 0.25 L/h liquid helium with the cryocooler by supplying the gas helium without loading the magnet. In this paper, the magnet design, manufacture, mechanical behavior analysis, and the performance test results of the magnet are presented.     

混合工质脉管制冷的热力学性能预测

提出了用于预测混合工质脉管制冷热力学性能的改进型Brayton制冷循环,建立了制冷系统制冷量和制冷系数COP的理论表达式。在对多种低温流体的预测计算的基础上,提出了具有应用潜力的混合工质对。计算结果表明,如果用10%氮与90%氦的混合工质对代替纯氦,脉管制冷机在80K温度下的制冷系数和制冷量分别可以提高9.5%和6.7%。此外还讨论了其它可能用于80K温区脉管制冷的混合工质对,如氢-氦、氩-氦、氖-氦混合物等。     

液氦温区脉管制冷机的优化实验

研制了一台用作德国国家标准局 (PTB)约瑟夫森效应 (JosephsonEf fect) 1V电压标准冷却系统的二级脉管制冷机。其设计要求在 4 2K提供 1 0 0mW左右制冷量 ,并同时冷却 70K左右的冷屏。采用额定功率为 1 8kW的氦压缩机驱动脉管制冷机 ,在不同制冷量负荷条件下分别对其进行了优化。初步实验结果表明 ,在输入功率 1 8kW的情况下 ,该制冷机最低制冷温度达 2 8K ,4 2K制冷量最大达 1 90mW ,制冷系数达 1 0 6× 1 0 4,火用效率最高达 1 1 3% ,可以充分满足冷却电压标准芯片的需要。此外 ,还与用 6kW压缩机驱动同一制冷机的实验结果进行了比较。     

A circulating cooling system based on a commercial G-3 helium liquifier

A circulating cooling system for a superconducting magnet is described, based on a commercial G-3 (G-45) helium liquifer. A flow of helium in the pressure range 2026-101 kNm^?2 (20 to 1 atm) with a temperature of 4.3 to 4.35 K can be obtained with the system for a maximum consumption of 3.6 Kg h^?1.     

Progress on a microgravity dilution refrigerator

We have developed a shallow, single-cycle, helium dilution refrigerator that contains rather coarse metal sponge to study the ability to control the location of the liquid helium for microgravity applications. We have tested the refrigerator on the ground while tilting to put the mixing chamber either somewhat above or somewhat below the still. We calculated that the system could be tilted between 5° and 10° in either direction without interrupting the cooling. The initial test of this refrigerator gave cooling to below 0.060 K and operation for tilts of ±16°. The insights gained from this refrigerator allow the design of a continuously operating version.     

基于液氮冷源超导磁体氦气循环预冷系统设计

为了提高超导磁体300-80 K预冷过程中的降温效率和安全性,开发了一种新的预冷方法.设计了一台以液氮为冷源、氦气为循环介质的可控温预冷装置,对其内部结构进行了优化设计,包括低温风机、板式换热器、气动调节阀、翅片换热器等主要组成部分,整个装置与磁体构成一个闭合循环系统.在预冷装置的作用下,该超导磁体从300 K到80 ...