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• A gas-coupled two-stage high-frequency pulse tube refrigerator was developed. • A no-load temperature of 5.7 K was achieved. • The prototype achieves a cooling capacity of 80 mW/8 K with 400 W electric power. • The prototype can output the cooling capacity of 0.1 W/9.3 K and 1000 mW/40 K simultaneously. • The pre-cooling stage can obtain a cooling capacity of 4.3 W/40 K@330 W electric power before gas-coupling. High-frequency pulse tube refrigerator (HPTR) has significant advantages in terms of compactness and high reliability, but it is difficult to obtain very low cooling temperatures. To obtain temperatures below 10 K, multi-stage structures driven by more than one compressor are currently used. This paper reports a two-stage gas-coupled HPTR driven by a single compressor that is extremely compact and even comparable to a single-stage structure. The pre-cooling temperature influences and gas distribution characteristics, which are two important issues in the research of gas-coupled structure refrigerator, were first numerically calculated and analyzed. Then the calculation results were verified by experiments and the cooling performance was tested. Before gas-coupling with the lower stage, the pre-cooling stage can obtain 4.3 W/40 K cooling capacity with 330 W electric input power, which belongs to a high-efficiency 40 K refrigerator that has been reported so far. With an electric power of 400 W, the gas-coupled HPTR taking the developed 40 K refrigerator as the pre-cooling stage achieves a no-load temperature as low as 5.7 K, which is the lowest record of the current two-stage gas-coupled structure in HPTCs. The developed two-stage HPTR can obtain a cooling capacity as high as 80 mW/8 K, and the two stages can also output the cooling capacity of 100 mW/9.3 K and 1000 mW/40 K simultaneously. [ABSTRACT FROM AUTHOR]

Highlights • The mechanism of multi-bypass was numerical studied. • Multi-bypass has the functions of orifice and double-inlet, making it like two-stage cryocooler. • A minimum no-load temperature 3.7 K was obtained, and the cooling power was 14 mW@4.2 K. • New mK cooling chain for HUBS satellite was proposed and analyzed. Abstract The Vuilleumier (VM) type pulse tube cryocooler (VPTC) is a new kind of 4 K cryocooler which had been experimentally verified. This paper presents the recent advances on a 4 K VPTC in our laboratory. First, the mechanism of multi-bypass was numerically studied using Sage software. The results showed that the function of the multi-bypass is not only similar to a double-inlet, but also to an orifice, which makes the VPTC work like a two-stage cryocooler. Based on the simulation results, the performance of VPTC was experimentally studied. Under its optimal operating conditions, a no-load temperature of 3.7 K has been obtained, which is the first demonstration of a single-stage VPTC obtaining temperatures below 4 K. It can supply about 14mW cooling power at 4.2 K and about 100 mW cooling power at 35 K simultaneously, which has potential application as the pre-cooler for an adiabatic demagnetization refrigerator (ADR). Finally, based on this VPTC, a new mK cooling chain for HUBS (Hot Universe Baryon Surveyor) satellite is proposed and its prospect has also been theoretically analyzed. [ABSTRACT FROM AUTHOR]

High-frequency pulse tube cryocooler (HPTC) has advantages of compact structure, low vibration, high reliability and long operation time. In this study, Theoretical analysis and experimental tests have been conducted in four aspects based on a developed 4 K HPTC. Firstly, a compressor with larger power output capability was employed and the impedance match between the cold head and the compressor was discussed. Secondly, simply using inertance tube configuration to replace the traditional inertance tube-gas reservoir structure. Then, the type and the size of the regenerator materials working at 4–20 K have been experimentally optimized. Finally, the performance of double-inlet working at as low as 20 K has also been tested for the first time for the HPTC. The present prototype achieved a no-load temperature of 3.6 K, which is the lowest temperature record ever reported for HPTC using helium-4 as working gas. A cooling power of 6 mW/4.2 K was also obtained with 250 W input power and a precooling power of 12.1 W/77 K. [ABSTRACT FROM AUTHOR]

VM type pulse tube cryocooler is a new type pulse tube cryocooler driven by the thermal-compressor. This paper presented the recent experimental results on a novel single-stage VM type pulse tube cryocooler with multi-bypass. The low temperature double-inlet, orifice and gas reservoir, and multi-bypass were used as phase shifters. With the optimal operating frequency of 1.6 Hz and optimal average pressure of 1.4 MPa, a no-load temperature of 4.9 K has been obtained and 30 mW@5.6 K cooling power has been achieved. It was the first time for the single-stage VM-PTC obtaining liquid helium temperature reported so far. Moreover, it was also the first time for the multi-bypass being used in the low-frequency Stirling type pulse tube cryocooler. [ABSTRACT FROM AUTHOR]

Vuilleumier coupling pulse tube (VM-PT) cryocooler is a novel kind of cryocooler capable of attaining liquid helium temperature which had been experimentally verified. Depending on different coupling modes and phase shifters, VM-PT cryocooler can be designed in several configurations. This paper presents a numerical investigation on three typical types of VM-PT cryocoolers, which are gas-coupling mode with room temperature phase shifter (GCRP), gas-coupling mode with cold phase shifter (GCCP) and thermal-coupling mode with cold phase shifter (TCCP). Firstly, three configurations are optimized on operating parameters to attain lower no-load temperature. Then, based on the simulation results, distributions of acoustic power, enthalpy flow, pressure wave, and volume flow rate are presented and discussed to better understand the energy flow characteristics and coupling mechanism. Meanwhile, analyses of phase relationship and exergy loss are also performed. Furthermore, a GCCP experimental system with optimal comprehensive performance among three configurations was built and tested. Experimental results showed good consistency with the simulations. Finally, a no-load temperature of 3.39 K and cooling power of 9.75 mW at 4.2 K were obtained with a pressure ratio of 1.7, operating frequency of 1.22 Hz and mean pressure of 1.5 MPa. [ABSTRACT FROM AUTHOR]

This paper presents experimental results on a novel two-stage gas-coupled VM-PT cryocooler, which is a one-stage VM cooler coupled a pulse tube cooler. In order to reach temperatures below the critical point of helium-4, a one-stage coaxial pulse tube cryocooler was gas-coupled on the cold end of the former VM cryocooler. The low temperature inertance tube and room temperature gas reservoir were used as phase shifters. The influence of room temperature double-inlet was first investigated, and the results showed that it added excessive heat loss. Then the inertance tube, regenerator and the length of the pulse tube were researched experimentally. Especially, the DC flow, whose function is similar to the double-orifice, was experimentally studied, and shown to contribute about 0.2 K for the no-load temperature. The minimum no-load temperature of 4.4 K was obtained with a pressure ratio near 1.5, working frequency of 2.2 Hz, and average pressure of 1.73 MPa. [ABSTRACT FROM AUTHOR]

A high frequency pulse tube cryocooler with precooling (HPTCP) has been developed and tested to meet the requirement of weak magnetic signals measurement, and the performance characteristics are presented in this article. The HPTCP is a two-stage pulse tube cryocooler with the precooling-stage replaced by liquid nitrogen. Two regenerators completely filled with stainless steel (SS) meshes are used in the cooler. Together with cold inertance tubes and cold gas reservoir, a cold double-inlet configuration is used to control the phase relationship of the HPTCP. The experimental result shows that the cold double-inlet configuration has improved the performance of the cooler obviously. The effects of operation parameters on the performance of the cooler are also studied. With a precooling temperature of 78.5 K, the maximum refrigeration capacity is 0.26 W at 15 K and 0.92 W at 20 K when the input electric power are 174 W and 248 W respectively, and the minimum no-load temperature obtained is 10.3 K, which is a new record on refrigeration temperature for high frequency pulse tube cryocooler reported with SS completely used as regenerative matrix. [ABSTRACT FROM AUTHOR]

The Vuilleumier (VM) refrigerator, known as heat driven refrigerator, is one kind of closed-cycle Stirling type regenerative refrigerator. The VM refrigerator with power being supplied by liquid nitrogen was proposed by Hogen and developed by Zhou, which shows great potential for development below 10 K. This paper describes the experimental development of a VM cryocooler operating below 8 K, which was achieved by using liquid nitrogen as a heat sink of middle cavity. The regenerator was optimized by using a part of metallic magnetic regenerator material Er 3 Ni to replace the lead sphere and a no-load temperature of 7.8 K was obtained. Then all the lead spheres were replaced by Er 0.6 Pr 0.4 material and a no-load temperature of 7.35 K was obtained, which is the lowest temperature for this kind of refrigerator reported so far. The cooling power at 10 K is about 500 mW with a pressure ratio near 1.6 and a charge pressure of 1.8 MPa. Especially, the magnetic material Er 0.6 Pr 0.4 was found to be a potential substitution for the conventional lead. [ABSTRACT FROM AUTHOR]

• A work-hardening layer with refined α grains was formed under cryogenic sliding. • Hindrance to dislocation motion caused great fatigue effects under cryogenic sliding. • Wear rate was decreased by up to 66% under cryogenic sliding compared to room temperature. • Brittle fracture in tribological behavior was derived from the fatigue process at cryogenic temperature. The cryogenic sliding induced surface deformation and tribological behavior of TA2 pure titanium were investigated in the present work. Friction and wear tests under dry sliding condition at room and cryogenic temperatures were conducted separately. An external cryogenic device was designed to produce a steady cryogenic environment through liquid nitrogen during the sliding process. Microstructure of subsurface under beneath the worn surface were detected by Optical Microscope (OM), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The results showed that sliding at room temperature induced a severe plastic deformation (up to 50 μm in depth), while the deformation layer was converted to a work-hardening layer (10 μm) and a gradient microstructure was formed under cryogenic sliding condition. Sliding at cryogenic temperature induced the formation of dislocation walls and α phase sub-grains, which hindered the dislocation motion during sliding process and was responsible for the decrease of plastic deformation. Furthermore, the refinement of α phase grains in this hardening layer could be attributed to the strain accumulation and severe local deformation at cryogenic temperature. With the effects of subsurface deformation, wear rate of TA2 samples was decreased by up to 66% under cryogenic sliding condition. The wear mechanism followed the general characteristics of oxidation and adhesive wear under dry sliding wear condition at room temperature. Under cryogenic sliding, a successive tribological behavior consisting of strain concentration, cracks generation and propagation, and fatigue fracture can be induced by the formation of work-hardening layer in the subsurface of the sample. [ABSTRACT FROM AUTHOR]

• Cryogenic thermal conductivity of Al 7050 after combined heat-cryogenic treatment was measured. • Cryogenic thermal conductivity was increased by aging while decreased by cryogenic treatment. • Al-Cu compound precipitates induced by cryogenic treatment were quite distinct from aging. • Mechanism of cryogenic treatment can be studied via thermal conductivity measurement. The cryogenic thermal conductivity of high-strength 7050 aluminum alloy subjected to the processes with different combinations of solution, aging and deep cryogenic treatment, was measured by a one-dimensional steady-state thermal conduction method. The microstructure evolution of this alloy after different treatments was also characterized to investigate the changes in thermal conductivity. The results showed that the thermal conductivity of aluminum alloy was approximately linearly reduced with the decrease of temperature in general. Under the effects of different treatments, it was found that the thermal conductivity at different temperatures was sharply decreased by 30% after solution. Deep cryogenic treatment further decreased the thermal conductivity when it was conducted after solution. However, the conduction of aging increased the thermal conductivity after solution or solution-deep cryogenic treatment. Intensive lattice distortion and abundant of point defects that enhanced the electro scattering were the main reasons for decreasing the thermal conductivity after solution. The relatively large size of Al-Cu precipitates and increased dislocation density contributed to the further decrease of thermal conductivity after deep cryogenic treatment. The precipitation of a large number of ultra-fine secondary phase particles after aging increased the thermal conductivity by sufficiently releasing the lattice distortion. This work also provided an efficient method for investigating the mechanisms of deep cryogenic treatment on aluminum alloy by studying the thermal conductivity at low temperatures. [ABSTRACT FROM AUTHOR]

• A cryogenic apparent thermal conductivity measurement apparatus was developed. • The apparent thermal conductivities of GOS, HoCu 2 , Er 3 Ni, and lead were tested. • The natural convection effect of helium-4 was measured. • The effects of temperatures (4–40 K) and pressures (0–2 MPa) were investigated. The axial heat leakage between the cold and hot ends of the regenerator caused by the huge temperature difference is an important part of the cryocooler loss, where the apparent thermal conductivity of regenerator materials is the key parameter that determines the heat leakage. At present, only the apparent thermal conductivity of stainless steel wire mesh, lead and copper sphere above 80 K can be obtained, while measurement results for lower temperatures or other materials are currently not available. A low-temperature apparent thermal conductivity measurement apparatus for regenerator materials has been developed, and the apparent thermal conductivities of sphere-type material (GOS, HoCu 2 , Er 3 Ni, and lead), mesh-type material (stainless steel wire mesh), and helium-4 under various pressures have been measured. When the regenerator is in a vacuum state (below 10-4 Pa), the experimental results show that the thermal conduction factors (the ratio of the apparent thermal conductivity of the filled regenerator to that of the material itself) are 0.02 for GOS in 4–10 K temperature range, 0.28 for HoCu 2 , 0.43 for Er 3 Ni in 4–20 K temperature range, 0.005 for lead sphere and 0.13 for stainless steel wire mesh in 10–40 K temperature range, respectively. However, when the regenerator is filled with helium-4, the thermal conduction factor values increase by one order of magnitude. [ABSTRACT FROM AUTHOR]

A high-frequency coaxial multi-bypass pulse tube refrigerator (MBPTR) was designed, manufactured, and tested. The cold finger is driven by a linear compressor through a connection tube to reduce the vibrations coming from the compressor. The pulse tube refrigerator adopts a coaxial configuration with a double-inlet and a single multi-bypass to improve the performance. With Er 3 Ni spheres at the cold end of the regenerator, the refrigerator can reach a no-load temperature of 13.9 K with 250 W electric input power, which is the lowest temperature for this kind of refrigerator reported so far. Especially, the interplay of double-inlet and multi-bypass was observed experimentally. An Er 3 Ni spheres regenerator was found to be a potential substitution for stainless steel wire mesh (SSWM) regenerator for this kind of refrigerator. [ABSTRACT FROM AUTHOR]