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Abstract: A single-stage high frequency multi-bypass coaxial pulse tube cryocooler (PTC) has been developed for physical experiments. The performance characteristics are presented. At present, the cooler has reached the lowest temperature of 18.6K with an electric input power of 268W, which is the reported lowest temperature for single-stage high frequency PTC. The cooler typically provides 0.2W at 20.6K and 0.5W at 24.1K with the input power of 260W at 300K ambient temperature. The cooperation phase adjustment method of multi-bypass and double-inlet shows its advantages in experiments, they might be the best way to get temperature below 20K for single-stage high frequency PTC. The temperature stability of the developed PTC is also observed. [Copyright &y& Elsevier]

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]

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]