Cryogenic refrigeration is a term that may be applied to the process of cooling equipment and components to temperatures below 150 K. The net capacity of a cryogenic refrigeration system at a particular temperature is the amount of heat that can be applied to a “cold station” in the system without warming the station above that particular temperature. The cold station may be a bath of cryogenic fluid, or the cold station may be a conductive surface cooled to the bath temperature to which equipment may be fastened.
Cryogenic refrigeration systems are different from the refrigeration equipment we encounter in our everyday environment. The refrigerants used in cryogenic systems are often helium (He), hydrogen (H2), or nitrogen (N2). Insulation techniques used to minimize heat leaks into the cooled parts of the systems usually depend on the use of high-vacuum technology, radiation shields, and structural materials with low thermal conductivity. Systems that use stored cryogens such as liquid helium, liquid hydrogen, or liquid nitrogen in a container called a “dewar” are usually refilled on a periodic basis. Solidified gases (such as hydrogen or methane) can also be used for cooling purposes,
much as solid carbon dioxide (dry ice) is used to refrigerate perishable foods during shipment.
Cooling microwave components and LNAs to cryogenic temperatures enables significant reductions in the operating noise temperature (Top ) of receiving systems. The sensitivity of a receiving system is directly proportional to A /Top , where A is the receiving antenna’s effective collecting area. For example, when Top = 80 K , an array of four identical antennas and receivers is needed to equal the sensitivity of one such antenna and receiver with a Top of 20 K.
Use of cryogenic cooling by the Deep Space Network (DSN) includes both open-cycle refrigeration (OCR) and closed-cycle refrigeration (CCR) systems. The temperatures achieved by these systems range from 1.5 kelvins (K) to about 80 K, depending upon the type of system used. These cryogenic systems are used to cool low-noise preamplifiers and some of the antenna feed system components for the DSN’s receivers. Liquid nitrogen (LN2) was used to cool reference loads (resistive terminations) used for noise temperature measurements, and liquid helium (LHe) was used to cool reference loads and antenna-mounted ruby masers.
Cryogenic cooling for detectors can also enable many space applications. Cooled detectors allow the collection of photons at longer wavelengths, allowing vast improvements in identification and discrimination capability with a minimum of sensor aperture growth. Smaller aperture produces cheaper, lighter sensors, much easier to host in a space-based environment. Other space missions such as communications, remote sensing, and weather monitoring can benefit from subsystems using cryogenic technology including super conducting electronics, high data rate signal processors, and high speed/low power analog to digital converters.
Priorities for the Air Force Research Laboratory cryocooler effort are to develop and demonstrate space qualifiable cryogenic technologies required to meet future requirements for Air Force and Department of Defense (DoD) missions. Other objectives are to develop state-of the-art cryocooler technology, characterize and evaluate the performance of development hardware, pursue advanced concepts for future spacecraft missions, and work to enhance cryocooler to spacecraft integration.
Early development efforts were on comparatively large capacity machines to support cooling requirements for the Space Surveillance and Tracking System (SSTS). The protoflight Cryocooler program produced two three-stage 10K cryocoolers (Contractors: Air Research and
Arthur D. Little) for cooling of the long wave silicon focal plane arrays. An additional program aimed at 10K primarily developed by NASA’s Jet Propulsion Laboratory and Aerojet using sorption, culminated with the BESTCE Shuttle flight experiment in 1995.
The Standard Spacecraft Cryocooler program (SSC) initiated in 1990 marked a change in emphasis from relatively large machines to more compact and efficient cryocoolers aimed at meeting cooling needs in the range from 60K to 150K for MWIR applications. Using Oxford Stirling cycle technology developed primarily in the United Kingdom, these machines utilized linear drive motors and tight clearance seal non-contacting piston shafts. The pulse tube cryocooler, a variation of this technology, replaces the actively moving expander piston with a non-moving regenerator and pulse tube. AFRL has also pursued alternate cryocooler concepts including reverse Brayton cycle designs; and for extremely low temperature cooling (~10K), variant using Joule-Thomson combinations and improved Stirling and pulse tube designs are being considered. As user confidence in cryocooler reliability has improved, focus is also being placed on reduced mass and improved efficiency.
There have been some recent advances in solid state cooling. A solid state cryocooler is ideal because it is small, extremely low weight, highly reliable, and has zero vibration. However, this is innovative technology research and development (R&D), and use of these devices for microsatellites, although ideal, will not be ready for flight for a few more years. Two particular types of solid state cooling to be touched briefly on include laser cooling and peltier cooling.
Laser cooling occurs in a crystal lattice by means of absorption of a photon, and then emission of a more energetic photon, with the extra energy extracted from lattice phonons. The removal of these phonons cools the crystal. Several studies have indicated that ytterbium or thulium doped solids can potentially provide efficient cooling below 100 K.
Peltier cooling provides the same benefits as laser cooling, but through a different mechanism. Multistage thermoelectric coolers are stacked with superlattice materials, and the heat is rejected from one stage to the next. Temperatures as low as 10 K are predicted
Space qualified cryocoolers have been extensively developed for large military and commercial satellite electro-optical (EO) infrared (IR) missions. These cryocoolers and the associated electronics routinely cost anywhere from $6-10M and can take 3-5 years to manufacture, making them a long-lead item for any EO IR space mission. Although progress has been made to reach a range of temperatures and heat loads, from 95 K at 10W heat load to 10 K at 250 mW heat load, the input power required to operate is significant, sometimes up to 500W. These space cryocoolers usually weight 22-25 kg, and if they are required to be located on a gimbal, this creates an even greater issue with the need for larger counterweights.
Clearly, the current state-of-the-art traditional cryocooler technology available for space far exceeds the limits of a microsatellite. A trend has developed in recent years to invest in military satellites that are cheaper, more responsive and yet still perform the mission. This has increased the need for microsat technology mission enablers, such as cryocoolers. Unfortunately, due to the complex thermodynamic processes involved, cryocoolers do not scale down linearly. In fact, as the size is decreased, parasitic effects become more pronounced, increasing non-linearly.
There are a few options for microsatellite miniature cryocoolers today. Often, tactical (or ground-based) cryocoolers are chosen for missions because they are significantly cheaper than space qualified coolers. However, these tactical coolers have their own set of issues, including extensive vibration, and still do not provide the same level of cooling power as traditional space qualified cryocoolers, and therefore the capabilities of the microsat mission are reduced. Three notable miniature space cryocoolers are the Air Liquide mini pulse tube, the Raytheon dual use cryocooler, and the Northrop Grumman high frequency microcooler.
Air Liquide has developed a small pulse tube cryocooler suitable for long life space applications. It is a single-stage cooler that provides 1.5 W heat lift at 80 K for an input power of 35 W and a mass of 2.8 kg; its vibration level is 20 mN. Raytheon has developed a dual use pulse tube cryocooler thermo mechanical unit (TMU) with a modified for space tactical cooler electronics intended for low cost and long life operations. Shown in Figure 6, the dual use cryocooler provides 1.5W heat lift at 67 K, with 84 W input power and a mass estimated at 4.5 kg. This miniature cryocooler prototype is also applicable to responsive space needs, as it can be assembled in just weeks versus months for the larger, traditional space qualified cryocoolers. It already has drive electronics to match the TMU.
Northrop Grumman Space Technology has developed a high frequency coaxial pulse tube microcooler optimized for rapid cool down. It provides 1.3W of heat lift at 77 K, and 4.0 W of heat lift at 150 K, with input power of 35W. Temperatures below 77K can be achieved with reduced heat lift capacity. It weighs an impressive 0.86 kg. This cryocooler is compatible with both tactical and space qualified electronics, and is the lightest weight microcooler with 1.3 W heat lift.
These are excellent examples of some of the miniature cryocoolers that have the potential to meet microsatellite military needs. However, for microsatellites with masses less than 100 kg and total payload power of less than 100 W, there is still a lot of research to be done to reduce input power, increase heat lift, and lower temperature in order to have the benefits of an on-board cryocooler outweigh the disadvantages.
China’s large-scale cryogenic refrigeration technology makes breakthrough, reported in April 2021
China’s large-scale cryogenic refrigeration technology, which is fundamental for important industrial sectors such as aerospace and hydrogen energy, has made a major breakthrough, with the ability to cool down -271C with hundred-watt level power.
China’s major scientific research project for developing the large-scale cryogenic refrigeration system in liquid helium to superfluid helium temperature range has passed experts’ appraisal, Xinhua reported on Saturday. This project is supported by the Ministry of Finance and undertaken by the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (TIPC-CAS).
China can now develop large-scale cryogenic refrigeration equipment with a liquid helium temperature of 4.2K (-269 C) kilowatt level and superfluid helium temperature of 2K (-271C) hundred-watt level, a milestone which has broken technology monopoly by the developed countries, and made China’s large-scale cryogenic refrigeration technology reach the global advanced level, according to TPIC-CAS’s official website.
Large-scale cryogenic refrigeration equipment from liquid helium to superfluid helium temperature range is an indispensable core foundation for strategic fields, such as aerospace and hydrogen energy.
China’s large-scale cryogenic refrigeration equipment has relied on imports for many years, TPIC-CAS’s official website reported, noting that foreign countries have prohibited the key core component and refrigeration equipment used in special fields from exporting to China.
The hundred-watt large refrigerating machines have been successfully put into operation in several industries, such as accelerators and nuclear fusion, according to Xinhua, adding that the research project in developing a large-scale cryogenic refrigeration system has also driven the rapid development of relevant industries, including cryogenic heat exchangers and cryogenic valves. The low-temperature industry cluster with complete functions and clear division of labor has been initially formed now.