Contents lists available at ScienceDirect Cryogenics journal homepage: www.elsevier.com/locate/cryogenics Technical note Progress on a novel VM-type pulse tube cryocooler for 4 K Changzhao Pan a, ⁎ , Jue Wang a,b , Kaiqi Luo a,b , Junjie Wang a,b , Yuan Zhou a,b, ⁎ a Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China ARTICLE INFO Keywords: VM Thermal-compressor Pulse tube Liquid helium temperature Multi-bypass ABSTRACT 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. 1. Introduction With the development of various physics fields, the demands for small scale and high-efficiency 4 K cryocooler are becoming urgent. The traditional small commercial 4K cryocooler is two-stage Gifford- McMahon (GM) cryocooler, such as Sumitomo SRDK-101D [1]. How- ever, the efficiency of this type cryocooler is relatively low due to the low efficiency of its GM compressor. The large bulk and oil filtering system of the GM compressor also limit its applications in space. The Joule-Thomson (J-T) cryocooler was another type of 4 K cryocooler, which was normally used in space [2,3]. This type of cryocooler needs another mechanical cryocooler to precool, so it showed the dis- advantage of complex. Besides, some impurities in helium may choke the J-T valve and making the whole cooling system failure. The multi-stage Stirling type pulse tube cryocooler (SPTC) is a new type 4 K cryocooler and develops rapidly because of its compactness, long life, and low vibration [4–6]. This type of cryocooler is driven by the linear compressor which normally works at the frequency above 20 Hz. However, it needs to use the rare earth material, such as Er 3 Ni and HoCu 2 , to increase the volume heat capacity of regenerator at the temperature below 10 K. Due to mechanical reasons, this type material can only be processed into the shape of sphere. The high-frequency oscillating flow would generate high pressure drop loss in regenerator, which makes it difficult to work with high efficiency at 4 K. Therefore, a feasible way is to develop a low-frequency SPTC to increase its effi- ciency. Vuilleumier (VM) cryocooler is one kind of low-frequency Stirling type cryocooler driven by a thermal compressor. Most recently, a no- load temperature below 8 K has been obtained by using a one-stage VM cryocooler [7,8]. For the two-stage VM cryocooler, a no-load tem- perature below 4 K has also been obtained [9,10]. Therefore, combining the advantages of VM and pulse tube cryocooler, it could develop a new type 4 K cryocooler, VM-PTC. Dai et al. [11] have introduced the con- cept of VM-PTC and built a this type cryocooler, which obtained the lowest temperature of 3.5 K by using 77 K and 20 K pre-coolers. This work also demonstrated its capacity of obtaining liquid helium tem- perature. Matsubara et al. [12] also presented the concept of thermally actuated He-3 PTC. Nowadays, Wang et al. [13] developed a 15 K single-stage VM-PTC by using an 80 K Stirling type PTC as pre-cooler. The single stage 4 K VM-PTC has not been researched until now. This paper presented the recently experimental developments of VM-PTC with multi-bypass in our laboratory. By optimizing the double- inlet and multi-bypass, a no-load temperature of 4.9 K has been ob- tained, which is the first time to obtain the liquid helium temperature by using this type cryocooler. 2. Experimental set up Fig. 1 shows the structure schematic and picture of VM-PTC. The overall cryocooler is oriented downward. It consists of the pre-cooler, thermal-compressor and pulse tube cryocooler. For convenience, the first stage of a commercial two-stage cryocooler (Sumitomo RDK-415D) was used as pre-cooler. In order to estimate the heat consumption in the thermal-compressor, the cooling power of this GM cryocooler was measured at first. Fig. 2 showed the cooling power of the first stage in this GM cryocooler. It can supply about 60 W cooling power at 77 K. http://dx.doi.org/10.1016/j.cryogenics.2017.10.013 Received 28 August 2017; Received in revised form 26 September 2017; Accepted 4 October 2017 ⁎ Corresponding authors at: Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China (C. Pan, Y. Zhou). E-mail addresses: panchangzhao@mail.ipc.ac.cn (C. Pan), zhouyuan@mail.ipc.ac.cn (Y. Zhou). Cryogenics 88 (2017) 66–69 Available online 06 October 2017 0011-2275/ © 2017 Elsevier Ltd. All rights reserved. MARK