A He pulse tube cooler operating down to 1.3 K

Regenerative cryocoolers that employ ⁴He as working fluid can only reach a lowest temperature of about 2 K. This limitation can be overcome by the use of ³He as working fluid. Here we report on the performance of a two-stage pulse tube cooler that consists of two parallel stages with independent gas circuits. The pressure oscillation in each stage is generated by means of a separate compressor in combination with a rotary valve. With ⁴He in both stages, the minimum no-load temperature of the 2nd stage was 2.23 K, and cooling powers of 50 W at 53 K and 380 mW at 4.2 K were simultaneously available at electrical input powers of 4.54 and 1.45 kW to the 1st and 2nd stage, respectively. Using ³He as working fluid in the 2nd stage, a minimum stationary temperature of 1.27 K has been achieved, which is, up to now, the lowest temperature obtained by regenerative cryocoolers. At an electrical input power of 1.3 kW, the 2nd stage provides a cooling power of 42 mW at 2.0 K and 518 mW at 4.2 K. With ³He, at the same operating condition, the cooling power at 4.2 K was found to be larger than with ⁴He.     

Low-power cryocooler survey

A cryocooler survey has been performed on data of 235 cryocoolers, with cooling powers below some tens of watts and operating between 4 and 120 K. The state-of-the-art is discussed and trends are investigated on cooling performance, mass and size, cost, lifetime and reliability. The data were compared with earlier surveys to explore historical trends. Improvements in cooling performance were mainly in 80 K cryocoolers, an increase in efficiency by about a factor of 5. Coolers did not reduce in size significantly. Main reduction was in 80 K coolers because of the higher efficiency. In the survey, cost is related to input power, cooling power and operating temperature. Cost development is considered and related to learning curves. Since the mid-90s, the lifetime of Stirling-type cryocoolers has increased by one order of magnitude from typically 0.5 to 5 years or more. The confusion that exists on the term “reliability” is discussed.     

A pulse tube cryocooler with a cold reservoir

Phase difference between pressure wave and mass flow is decisive to the cooling capacity of regenerative cryocoolers. Unlike the direct phase shifting using a piston or displacer in conventional Stirling or GM cryocoolers, the pulse tube cyocooler (PTC) indirectly adjusts the cold phase due to the absence of moving parts at the cold end. The present paper proposed and validated theoretically and experimentally a novel configuration of PTC, termed cold reservoir PTC, in which a reservoir together with an adjustable orifice is connected to the cold end of the pulse tube. The impedance from the additional orifice to the cold end helps to increase the mass flow in phase with the pressure wave at the cold end. Theoretical analyses with the linear model for the orifice and double-inlet PTCs indicate that the cooling performance can be improved by introducing the cold reservoir. The preliminary experiments with a home-made single-stage GM PTC further validated the results on the premise of minor opening of the cold-end orifice.     

Numerical analysis of different valve effects on the cooling performance of a two-stage GM type pulse tube cryocooler

The cooling performance of GM type pulse tube cryocoolers (PTCs) is strongly affected by the valve setting for phase shifting. In this research, the effects of different valve openings on the no-load temperatures in a two-stage GM PTC have been investigated by numerical simulation. Results show that the no-load temperature at the 1st stage mainly depends on the openings of 1st stage orifice valve, and both 1st and 2nd stage double-inlet valves. While the no-load temperature at the 2nd stage below 10 K sensitively depends on the openings of both 1st and 2nd stage orifice valves, and 2nd stage double-inlet valves. There exist several local minimum no-load temperatures at the 2nd stage under different valve openings below 7 K. The best valve setting corresponds to minimizing the cold end phase angle, making the cold end acoustic power compensate the real gas enthalpy flow with maximum degree.     

GM型脉冲管制冷机热力学损失分析

对GM型脉冲管制冷机及其旋转阀系统进行热力学分析，针对脉冲管内压力波为正弦波形的情况，推导出了制冷系数（COP）的理论公式，给出了COP的定量结果，并与斯特林型脉冲管制冷机进行了对比，结果发现，由于气体在旋转阀中的不可逆损失，使得压缩机功耗增大，降低了GM型脉冲管制冷机的COP，双向进气的效果取决了温降比，在较低的制冷温度（较大的TH/TL)时作用更加明显。     

液氦温区脉管制冷机的频率特性

着重研究了操作频率对液氦温区脉管制冷机性能的影响,在实验和分析的基础上,明确了制冷温度、制冷量、制冷效率与工作效率之间的关系,并与4K G－M制冷机的情况进行比较,得出了一些有益的结论。通过频率优化,脉管制冷性能得以较大提高。在初步试验中,分别在1．2Hz和1．1Hz获得了30W@70K,500mW@4.2K以及20W@65K,590mW@4.2K的制冷量。同时还给出了脉管制冷湿度稳定性的测试结果。试验结果表明,研制的脉管制冷机温度波动均小于同类商品型4K GM制冷机及脉管制冷机。     

Regenerator performance improvement of a single-stage pulse tube cooler reached 11.1 K

In order to improve the cooling performance of pulse tube cooler (PTC) at 20-40 K, hybrid regenerators are often employed. In this paper a three-layer regenerator, which consists of woven wire screen, lead sphere and Er₃Ni is optimized to enhance the cooling performance and explore the lowest attainable refrigeration temperature for a single-stage PTC. The efforts focus on the temperature range of 80-300 K, where woven wire screens are used. Theoretical and experimental studies are carried out to study the metal material and the mesh size effect of woven wire screens on the performance of the single-stage G-M type PTC. A lowest no-load refrigeration temperature of 11.1 K was obtained with an input power of 6 kW. The PTC can supply 17.8 W at 20 K and 39.4 W at 30 K, respectively.     

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。     

Maximizing production rates of the Linde-Hampson machine

In contrast to the ideal case of unlimited size recuperator, any real Linde-Hampson machine of finite size recuperator can be optimized to reach the extreme rates of performance. The group of cryocoolers sharing the same size recuperator is optimized in a closed form by determining the corresponding flow rate which maximizes its rate of cold production. For a similar group of liquefiers an optimal flow rate is derived to maximize the rate of production of liquid cryogen. The group of cryocoolers sharing a constant and given flow rate is optimized by shortening the recuperator for reaching a maximum compactness measured by the cooling power per unit size of the recuperator. The optimum conditions are developed for nitrogen and argon. The relevance of this analysis is discussed in the context of practice of fast cooldown Joule-Thomson cryocooling.     

Numerical analysis of stirling type pulse tube cryocoolers

A one-dimensional finite volume discretization method is proposed and is implemented as a computer program for the modeling of a family of stirling type Pulse Tube Cryocoolers (PTC). The set of unsteady, one-dimensional, viscous compressible flow equations are written in a general form such that all, porous and non-porous, sections of the PTC can be modeled with these governing equations. In present work, temperature dependency of thermo-physical properties are taken into account as well as the heat transfer between the working fluid and the solid parts, and heat conductions of the gas and solid. The simulation tool can be used to model both the inertance tube type and the orifice type cryocoolers equipped with regenerators made up of different matrix constructions. The PTC might have an arbitrary orientation with respect to the gravitational field. By using the computer program, an orifice type and an inertance tube type pulse tube cryocooler are simulated. Diameter of the orifice and length of the inertance tube are optimized in order to maximize the coefficient of performance. Furthermore, the cooling power of the two types is obtained as a function of the cooling temperature. The behavior of thermodynamic parameters of the inertance tube PTC is investigated. Mean cyclic values of the parameters are presented.     

A single-stage pulse tube cooler reached 12.6 K

A single-stage G-M type pulse tube cooler (PTC) was designed and tested to explore the lowest attainable refrigeration temperature and to further improve the cooling performance in the temperature range of 15-40 K. The magnetic material Er₃Ni was used as part of the regenerative material besides the phosphor-bronze and the lead so as to improve the efficiency of the regenerator. With an input power of 6 kW, a lowest no-load refrigeration temperature of 12.6 K was obtained, which is a new record for the single-stage PTC. The cooling capacity at 15-40 K was also significantly improved, which may extend the application of the single-stage PTC for the cooling of superconductors and cryopumps.     

Heat exchanger versus regenerator: A fundamental comparison

Irreversible processes in regenerators and heat exchangers limit the performance of cryocoolers. In this paper we compare the performance of cryocoolers, operating with regenerators and heat exchangers from a fundamental point of view. The losses in the two systems are calculated from the entropy productions due to the various irreversible processes. Whether an optimized regenerator or heat exchanger performs better depends on the system parameters (molar flux, temperature, and pressure). At temperatures below 200 K the losses due to heat conduction in the axial direction are dominant.     

空间用4 K温区预冷型JT制冷机研究进展及关键技术分析

综述了空间用4 K温区预冷型JT制冷机方案及其相关的空间项目,研究表明该类型制冷机一般由斯特林或斯特林型脉管制冷机和JT制冷机组成,前者为后者提供预冷,后者通过节流效应降温获得4 K温区的制冷量。总结了制冷机的性能参数和系统结构,对比分析了该类型制冷机与GM/JT制冷机间存在的性能差距和关键技术差别,阐明了预冷温度和压力等参数对制冷机效率的影响,指出了当前该类型制冷机的改进空间和方案。     

Ultra-low-vibration pulse-tube cryocooler system – cooling capacity and vibration

This report describes the development of low-vibration cooling systems with pulse-tube (PT) cryocoolers. Generally, PT cryocoolers have the advantage of lower vibrations in comparison to those of GM cryocoolers. However, cooling systems for the cryogenic laser interferometer observatory (CLIO), which is a gravitational wave detector, require an operational vibration that is sufficiently lower than that of a commercial PT cryocooler. The required specification for the vibration amplitude in cold stages is less than ±1 μm. Therefore, during the development of low-vibration cooling systems for the CLIO, we introduced advanced countermeasures for commercial PT cryocoolers. The cooling performance and the vibration amplitude were evaluated. The results revealed that 4 K and 80 K PT cooling systems with a vibration amplitude of less than ±1 μm and cooling performance of 4.5 K and 70 K at heat loads of 0.5 W and 50 W, respectively, were developed successfully.     

Elevated-pressure mixed-coolants Joule-Thomson cryocooling

This paper explores the potential of mixed coolants at elevated pressures for Joule-Thomson cryocooling. A numerical model of a Joule-Thomson cryocooler is developed that is capable of simulating operation with mixtures of up to 9 components consisting of hydrocarbons, non-flammable halogenated refrigerants, and inert gases. The numerical model is integrated with a genetic optimization algorithm, which has a high capability for convergence in an environment of discontinuities, constraints and local optima. The genetic optimization algorithm is used to select the optimal mixture compositions that separately maximizes following two objective functions at each elevated pressure for 80, 90 and 95 K cryocooling: the molar specific cooling capacity (the highest attainable is 3200 J/mol) and the produced cooling capacity per thermal conductance which is a measure of the compactness of the recuperator. The optimized cooling capacity for a non-flammable halogenated refrigerant mixture is smaller than for a hydrocarbon mixture; however, the cooling capacity of the two types of mixtures approach one another as pressure becomes higher. The coefficient of performance, the required heat transfer area and the effect of the number of components in the mixture is investigated as a function of the pressure. It is shown that mixtures with more components provide a higher cooling capacity but require larger recuperative heat exchangers. Optimized mixtures for 90 K cryocooling have similar cooling capacity as those for 80 K. Optimized compactness for 80 K is about 50% higher than can be achieved by pure nitrogen. For 90 K, no mixture provides a more compact recuperator than can be achieved using pure argon. The results are discussed in the context of potential applications for closed and open cycle cryocoolers.     

Outgas analysis of mechanical cryocoolers for long lifetime

Mechanical cryocoolers for space applications are required to have high reliability to achieve long-term operation in orbit. ASTRO-H (Hitomi), the 6th Japanese X-ray astronomy mission, has a major scientific instrument onboard—the Soft X-ray Spectrometer (SXS) with several 20K-class two-stage Stirling (2ST) coolers and a 4K-class Joule Thomson (JT) cooler, which must operate for 3 years to ensure the lifetime of liquid helium as a cryogen for cooling of its detectors [1], [2]. Other astronomical missions such as SPICA [3], [4], LiteBIRD [5], and Athena [6] also have top requirements for these mechanical cryocoolers, including a 1K-class JT cooler to be operated for more than 3–5 years with no cryogen system.The reliability and lifetime of mechanical cryocoolers are generally understood to depend on (1) mechanical wear of the piston seal and valve seal, and (2) He working gas contaminated by impurity outgases, mainly H₂O and CO₂ released from the materials in the components of the cryocoolers. The second factor could be critical relative to causing blockage in the JT heat exchanger plumbing and the JT orifice or resulting in blockage in the Stirling regenerator and thereby degrading its performance. Thus, reducing the potential for outgassing in the cryocooler design and fabrication process, and predicting the total amount of outgases in the cryocooler are very important to ensure cryocooler lifetime and cooling performance in orbit.This paper investigates the outgas analysis of the 2ST and the 1K/4K-JT coolers for achieving a long lifetime. First, gas analysis was conducted for the materials and components of the mechanical cryocoolers, focusing on non-metallic materials as impurity gas sources. Then gas analysis of the mechanical wear effect of the piston seal materials and linear ball bearings was investigated. Finally, outgassing from a fully assembled cryocooler was measured to evaluate whether the outgas reduction process works properly to meet the requirement levels.     

分离型二级脉管制冷机的实验研究--Ⅱ:液氦温区二级脉管制冷机

研制1台新型液氦温区分离型二级脉管制冷机,该制冷机由2台独立的脉管制冷机组成,一级回热器冷端和二级回热器中部通过热桥相连,从根本上弥补了传统直接耦合型多级脉管制冷机级间干扰的不足.采用双压缩机双旋转阀驱动该二级脉管制冷机,第二级最低温度达到了2.5 K,在4.2 K下有508 mW制冷量,同时一级在37.5 K有15 W制冷量.第二级充气压力由1.7 MPa增大到1.85 MPa,制冷机在4.2 K下的制冷量可以达到590 mW.为了能简化结构、扩大应用,提出采用单压缩机单旋转阀驱动该分离型脉管制冷机,达到了相同的制冷性能.     

Experimental investigation on mixed refrigerant Joule-Thomson cryocooler with flammable and non-flammable refrigerant mixtures

The mixed refrigerant Joule-Thomson (MR J-T) cryocoolers have a wide application area covering the temperature range from 80 K to 200 K. The significant advantages of the system are simplicity of its design and working reliability with high level performance. The present paper discusses the experimental results of MR J-T cooler with different flammable and non-flammable mixture compositions. The work highlights the use of pressure-enthalpy and temperature-enthalpy diagrams for these mixtures to support the experimental results. A record lowest temperature of 65 K and a cooling capacity of 6 W at 80 K are obtained for a single stage MR J-T system starting at 300 K. Further, using a mixture of minimum flammable refrigerants, temperatures below 100 K is achieved.     

High-capacity turbo-Brayton cryocoolers for space applications

Long-life, high-capacity cryocoolers may be needed for future space systems utilizing stored cryogens. The cooling requirements for planetary and extraterrestrial exploration missions, extended-life orbital transfer vehicles, and space depots may range from 10 W to 50 W at temperatures between 20 K and 120 K. Turbo-Brayton cryocoolers are ideal for these systems because they are lightweight, compact and very efficient at high cooling loads due to the high power density of rotary machines. These benefits are in addition to their inherent attributes of high reliability; negligible vibration; long, maintenance-free lifetimes; flexibility in integrating with spacecraft systems; and ability to directly cool remote and distributed loads. To date, space-borne turbo-Brayton technology has been developed for low cooling loads. The first space implementation of a turbo-Brayton cryocooler was in the NICMOS Cooling System (NCS). The NCS has been operational on the Hubble Space Telescope for over 3.5 years without any degradation. It provides 7 W of cooling at 70 K. The scaling of the technology to higher capacities is the subject of this paper.     

Development of mechanical cryocoolers for Astro-H/SXS

The Soft X-ray Spectrometer (SXS) is a high-resolution spectrometer with an X-ray micro-calorimeter array onboard the Japanese X-ray astronomy satellite Astro-H, planned for launch in 2013. The micro-calorimeter is operated at cryogenic temperature of 50 mK provided by the Adiabatic Demagnetization Refrigerator (ADR) with a heat sink of 1.3 K liquid helium stored in the SXS Dewar. To extend the liquid helium lifetime to over 3 years in orbit, two types of mechanical cryocoolers are installed: 20 K-class double-staged Stirling (2ST) coolers and a 1 K-class Joule-Thomson (JT) cooler. Improvement of mechanical cryocoolers has been investigated and verified for higher reliability and cooling performance. The engineering model (EM) of upgraded mechanical cryocoolers was fabricated for a long lifetime test. The required cooling power of 200 mW at 20 K for the 2ST cooler and 10 mW at 1.7 K for the JT cooler are achieved by EM test.