The development of pulse tube cryocoolers has been a significant topic of research and development over the past 20 or so years. Pulse tube cryocoolers have a number of advantages over other types of cryocoolers and are now available commercially. In order to understand pulse tube cryocoolers, it’s best to review some common aspects of cryocoolers. Cryocoolers (See Cold Facts, Winter 2009) generally use oscillatory flows, provided by a warm compressor, regenerative heat exchangers and a cold displacer. The cold displacer puts the motion of the gas in proper phase with the pressure oscillations, thus separating the cooling and heating effects of the cryocooler cycle. In Gifford-McMahon (GM) and Stirling cycle cryocoolers, the cold displacer is a piston that moves at cryogenic temperatures. There is generally a very large temperature difference across the cold displacer but a relatively small pressure difference.
Cold displacers have some disadvantages. Since they are moving equipment at cryogenic temperatures, they reduce the system reliability and add cost. Cold displacers can also add vibrations to the cold end of the system and thus to the item being cooled. Such vibrations can be a problem, particularly in the application of sensor cooling. A pulse tube cryocooler is essentially the answer to the question, “Can we eliminate the cold displacer?”
In a pulse tube cryocooler, the displacer is replaced by a pulse tube in which a slug of helium gas essentially acts as the displacer and separates the cooling and heating aspects of the cycle. The pulse tube has to be carefully designed so that the helium gas doesn’t mix or set up convective heat transfer loops, reducing the efficiency of the cycle. With the elimination of the cold displacer, the cryocooler is now more reliable and lower in vibration, providing significant advantages for many applications. A downside of pulse tube cryocoolers is that they tend to be lower in capacity compared to GM or Stirling cycle cryocoolers at a given temperature, but ongoing work is improving this issue. The development of pulse tube cryocoolers doesn’t completely replace the need for Stirling or GM cryocoolers and there are many applications for which Stirling and GM coolers are a better choice.
Typical capacities for commercial pulse tube refrigerators range from 30-100 W at 77–80K, 4–22 W at 20K and 0.5-1.5 W at 4.2K. A listing of commercially available pulse tube cryocoolers can be found in the CSA Buyer’s Guide (http://2csa.us/bg). The low vibration and high reliability of pulse tube cryocoolers make them particularly attractive to space missions.
Optimized design of pulse tube cryocoolers requires a thorough understanding of pulse tube thermodynamics. This has been described in rigorous detail by P. Kittel in Cold Facts (Spring 2010-Summer 2013). Very good overviews of cryocoolers, including pulse tube cryocoolers, are given by R. Radebaugh, “Cryocoolers: the State of the Art and Recent Developments,” J. Phys. Condens. Matter 21 2009 and A.T.A.M. de Waele, “Basic Operation of Cryocoolers and Related Thermal Machines,” J. of Low Temp. Phys. 164:179-236, 2011.
Recent examples of pulse tube cryocooler development and application include the following:
“Research of Stirling-type multi-bypass pulse tube cryocoolers with temperatures below 20K,” Chen Liubiao et al.; “Investigation of gravitational effects in pulse tube cryocoolers using 3-D CFD,” T. I. Mulcahey et al. both in Adv. Cryo Engr. Vol 59 (AIP Conf Proc 1573) 2014; “Study of low vibration 4K pulse tube cryocoolers,” M. Xu et al., Adv. Cryo. Engr. Vol 57A (2012); “Improvement of single stage active buffer pulse-tube for HTS magnet of Maglev,” Y. Miyazaki et al., Proc ICEC 2012 (2013); and “Comparative numerical simulation of inline and coaxial pulse tube refrigerators,” C. Gu et al., Proc ICEC 2012 (2013).
