Cold Facts asked our members in the field of cryocoolers and cryostats to weigh in on the technology’s most important developments, significant contributors and anticipated future advances. Here is a roundup of their replies.

Most important developments

Ray Radebaugh (ret. National Institute of Standards and Technology) ranks multilayer insulation (MLI) among the most important developments in cryocoolers. “MLI is now used everywhere in low loss dewars and in cryocoolers for minimizing heat leak,” he said. “It is also used for thermal control in spacecraft. Dewar heat leaks were reduced by about an order of magnitude compared with the powder insulations used before MLI came on the scene.”

Radebaugh also included Gifford- McMahon cryocoolers as an important development, as they have been “the workhorse cryocooler for a very wide range of applications ranging from cryopumping, MRI systems and many R&D applications. The development of rare earth materials helped them expand their applications to the 4K range.”

Alan Caughley (Callaghan Innovation) listed the Gifford-McMahon cryocooler as well. “This really was the first useful cryocooler and started [the] use of cryocoolers instead of liquid cryogens,” he said. “It opened up the possibility of a dial-up temperature and has been the workhorse of the industry for 50 years.”

Many of our respondents consider the pulse tube cryocooler an important development. “Pulse tube cryocoolers have been extremely popular for the last decade or so for both commercial and space applications,” said Radebaugh. “Research on them continues today as the most studied cryocooler because of the lack of cold moving parts and the wide range of improvements that are possible. 4K pulse tube cryocoolers are having a major impact on eliminating the use of scarce liquid helium on many 4K experiments and applications.”

“The development of pulse tube cryocoolers has been one of the major advances in the past three decades that has helped expand the use of cryocoolers in both ground and space applications,” said Ali Kashani (Atlas Scientific).

“In my own narrow specialty of linear-motor-driven pulse tube cryocoolers,” said Philip Spoor (Chart, Inc., a CSA CSM), “one could argue that the single most important development, which allows us to be more productive every day, was the development of computer codes that model these devices accurately enough to make reasonable predictions about what they’ll do. This applies especially to the widely used computer codes Sage and DeltaE, because they enable colleagues to share information and designs more readily than if everyone is using their own codes.”

Peter Kittel (ret. NASA Ames) cited the invention of the orifice pulse tube, explaining, “This cryocooler was vastly more efficient than the original, basic pulse tube. Its efficiency was high enough that it could [be] put to practical use and it has been in medical, industrial and space applications.”

“The pulse tube implementation of the Stirling thermodynamic cycle with no moving parts, as pioneered by Ray Radebaugh, enables smaller, long life space cryocoolers at higher temperatures,” said Dale Durand (Northrop Grumman Aerospace Systems). He considers the Oxford-type linear compressor to be equally important for higher temperature space cryocoolers. Durand also listed the adiabatic demagnetization refrigerator for low temperatures as another significant development. “The ability of the magnetic field to interact with the spin temperature without a thermal parasitic enables fantastically low temperatures in a relatively small device,” he explained.

Others also listed developments pertaining to space. “The development of a spaceflight cryocooler that lifts 20 W at 20K is an important development for cryocoolers in recent years,” said Shuvo Mustafi (NASA Goddard). He continued, “The development of advanced MLI that uses polymer spacers instead of netting used for conventional MLI could enable a number of cryogenic storage and transfer options for cryostats.”

Jason Hartwig (NASA Glenn Research Center) cited another recent development, the demonstration of zero boiloff (ZBO) for liquid oxygen at 91K for an indefinite time. “All future manned space applications (including large upper stages, cryogenic fuel depots, sample return missions and ascent stages) will benefit from zero (not reduced) boiloff of the cryogen. Having recently demonstrated that we can store a 90K cryogen indefinitely represents a huge impact in cryocooler technology development.”

“The most important advancement in cryocoolers from my perspective, that being the storage of cryogenic propellants,” said David Plachta (NASA Glenn Research Center), “is the NICMOS reverse turbo-Brayton cycle machine. This flight cryocooler and its derivatives offer the potential to eliminate propellant boiloff in space, enabling depot concepts and human missions architecture plans being considered by NASA.”

Important contributors

Radebaugh recognized a number of important contributors to cryocooler and cryostat technology. “Several people and institutions appear to have been responsible for the development of MLI,” he said. “An entire session of the 1959 CEC (Vol. 5) was devoted to MLI as it suddenly exploded on the scene in about 1958-1959. P. Peterson of the University of Lund in Sweden is credited with the concept of multilayers of aluminum foil and glass wool spacers that had remarkable insulating qualities. His work occurred in 1951 but was not noticed much until several years later. Some leaders in the 1958-1959 development period were R. H. Kropschot of NBS in Boulder, L. C. Matsch of Linde-Union Carbide Corporation and M. P. Hnilicka of NRC Equipment Corporation.

“As the name indicates,” Radebaugh said, “William Gifford and Howard McMahon were the important figures in the development of the Gifford-McMahon cryocooler. Gifford was the leader in the innovations and the technical input and McMahon was the driving force behind the commercialization of the cryocooler. The introduction of spherical rare earth powders by Toshiba and the pioneering 4K work by T. Kuriyama of Toshiba were instrumental in the development of 4K GM and GM-type pulse tube cryocoolers.

“William Gifford was also an important figure in the development of pulse tube cryocoolers,” Radebaugh added. “Even though the type he developed was not successful for cryogenic temperatures, he introduced the concept of the empty tube in the 1960s, the “pulse tube” that laid the groundwork for the later version to come in the early 1980s in which an orifice was introduced to the system by E. I. Mikulin. Many improvements have been made since then by many people. The field of thermoacoustics has greatly matured as a result of the pulse tube cryocooler and related thermoacoustic devices, due in large part to Greg Swift. 4K pulse tube cryocoolers have become very successful because of the pioneering work of G. Thummes and C. Wang.”

Other cryocooler experts cited Radebaugh himself as an important contributor to the field. “Dr. Ray Radebaugh has been one of the key scientists in the development of pulse tube cooler technology,” said Kashani. “He has been instrumental in advancing our understanding of the physics of pulse tube coolers as well as developing analytical and experimental tools to study these coolers. He has also helped in educating and training many of the scientists working on pulse tube coolers.”

“Ray Radebaugh deserves much credit for advancing the field and educating/guiding so many professionals making additional advances,” Durand agreed.

Kittel also cited E. I. Mikulin, as well as A. A. Tarasov and M. P. Shrebyonock, authors of the paper “Low-temperature expansion pulse tube,” Adv Cry Eng 29 (1984), p. 629. “This is the first paper describing the orifice pulse tube,” he explained. “It showed a significant advance over the basic pulse tube (1963) of Longsworth and Gifford. By 1983, almost all interest in pulse tubes had died because the basic pulse tube was very inefficient… [and] never found a practical application. This paper reinvigorated the field, showing an efficient pulse tube could be developed, which led to the rapid developments of the ’80s and ’90s.”

“Because of their special importance of common language to collaboration,” said Spoor, “I would single out David Gedeon (of Gedeon Associates), creator of the Sage software package, and Greg Swift and Bill Ward (of Los Alamos National Laboratory), creators of the DeltaEC software package. Their contributions include not just the software for modeling cryocoolers, but the documentation that gives excellent basic instruction in the relevant physics. Their codes are also useful for modeling many other mechanical, acoustic and thermal devices besides cryocoolers.”

Mustafi and Plachta both credited Creare, Inc., which is working to develop a spaceflight cryocooler that lifts 20 W at 20K. Plachta cited the company and “its lead technologist, Mark Zagarola, as being the driving force behind this advance.” Hartwig recognized Mark Zagarola and Dave Plachta.

Mustafi also named Quest Thermal Group, which he said is developing advanced MLI insulation under guidance from NASA.

Caughley’s picks were “Bill Gifford of course, then Peter Gifford for building the business. Chao Wang sits quietly behind Cryomech and has been the source of its recent technical success.”

Future developments

Our member panel hopes to see a variety of developments in cryocooler and cryostat technology in the future.

“MLI has one disadvantage in that it is difficult to apply,” said Radebaugh. “Something easier to apply and at a lower cost would be a great improvement.” He continued, “Variable speed drives for GM compressors are beginning to be introduced now. I also would like to see increased efficiencies and reduced noise from these compressors.

“A significant development in pulse tubes would be that of really miniature pulse tube cryocoolers employing much higher frequencies, warm displacers, and a regenerator packing with very small hydraulic diameter,” Radebaugh continued. “Another important need is 4K pulse tube cryocoolers with Carnot efficiencies considerably higher than 1 percent, which could be possible with a much better understanding of the losses at 4K in regenerators and pulse tubes.”

Kashani noted, “Advancing regenerator technologies to improve efficiency of pulse tube coolers is one area that would be beneficial for many applications requiring cryogenic cooling.” Kittel also hopes to see increased efficiency, especially in the 4-10K range, as it would reduce dependence on helium as well as reducing the power consumption of superconducting machines.

“Of course I see hope in my work for the future, the metal diaphragm pressure wave generator coupled to a pulse tube or Stirling cold head,” said Caughley. “It is a really cost-effective and robust cryocooler for industrial uses.”

Spoor acknowledged that in the codes he considers to be an important development, one of their biggest limitations is that they are “essentially one-dimensional; they assume uniform pressure and flow along the computation axis. In the future, it would be very exciting to see computational fluid dynamics reach a state of maturity and speed of execution to enable us to look at 2- and 3-D effects in cryocooler designs, including the sensitivity of designs to small manufacturing flaws.”

Advances in cryocooler manufacturing also interest Durand. “New 3-D printing-type manufacturing may enable us to break through the current limitations in the design trades between thermodynamic optimum solutions and structural requirements by allowing designs that are currently impossible to manufacture,” he said.

In the future, Plachta would like to see a high performing broad area cooled shield within the MLI that thermally insulates a LH₂ propellant tank, as well as improvements to the modeling and prediction of cryocooler parasitic losses and the reduction of those losses. Hartwig hopes to see “ZBO LH₂ demonstration, both on the ground and in flight. To those who have been working in cryogenics at NASA GRC for the past 30 years, this represents the top, key milestone in cryogenics: being able to store the highest efficiency propellant indefinitely.”

“The development of cryocoolers that lift 100-500 W at 90K, coupled with the 20 W at 20K cryocooler, will allow for long-term ZBO active storage of cryogenics propellants such as liquid hydrogen and liquid oxygen,” said Mustafi. “The demonstration of advanced MLI on space flight missions and on large flight tanks with appropriate penetrations will increase the technology readiness level of these types of insulation to be used for long-term storage and transfer of cryogens. This technology, coupled with advances in subcooling technology, may even enable the long-term ZBO passive storage of cryogenic propellants such as liquid hydrogen and liquid oxygen.”