Unlocking the Cold Frontier: The Marvels of Cryogenic Refrigeration Technology and Its Diverse Applications”

Introduction:

In the quest for scientific and technological advancements, researchers have delved into the subzero realms of cryogenic temperatures, unveiling a fascinating world of possibilities. At the heart of this exploration lies cryogenic refrigeration technology, with cryocoolers leading the charge as essential devices capable of reaching and maintaining extremely low temperatures. Cryogenic refrigeration, a term associated with cooling equipment and components to temperatures below 150 K, has emerged as a pivotal technology with applications spanning from space exploration to quantum computing. In this article, we’ll explore the fundamentals of cryogenic refrigeration, the mechanics of cryocoolers, and the diverse applications that benefit from this cutting-edge technology.

Understanding Cryogenic Refrigeration Technology:

Cryogenic refrigeration involves the cooling of materials to extremely low temperatures, typically below -150 degrees Celsius (-238 degrees Fahrenheit). Achieving and maintaining these frigid temperatures is crucial for various scientific, industrial, and medical applications. Cryogenic refrigeration technology relies on cryocoolers, specialized devices designed to extract heat and lower temperatures to cryogenic levels.

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.

The Mechanics of Cryocoolers:

Cryocoolers, often referred to as cryogenic coolers or refrigerators, come in various designs, each catering to specific temperature requirements and applications.

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).

The insulation techniques involve high-vacuum technology, radiation shields, and materials with low thermal conductivity.  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.

One notable example is the Stirling cryocooler, as exemplified by Sunpower’s free-piston cryocooler. The Stirling cryocooler utilizes a free-piston mechanism with gas bearings, eliminating the need for lubrication and maintenance. The motion of the piston is controlled electronically, allowing for friction-free operation. This design ensures hermetic sealing, resulting in a reliable and durable cryocooler. Additionally, recent innovations, such as the active vibration cancellation (AVC-GEN2), have significantly reduced vibrations, making Stirling cryocoolers suitable for quantum applications and other vibration-sensitive fields.

Applications of Cryogenic Refrigeration Technology:

Cryogenic cooling extends to cooling microwave components and low-noise preamplifiers (LNAs), allowing for substantial reductions in operating noise temperature. 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.

Quantum Technology:

Single-Photon Sources: Cryocoolers play a pivotal role in maintaining the cryogenic temperatures necessary for the operation of quantum dots and other quantum devices. The integration of cryocoolers, as demonstrated by TU Berlin’s quantum key distribution testbed, enables the creation of compact, rack-mounted quantum systems.

Quantum Computing: Quantum computers rely on superconducting circuits that require cryogenic temperatures for optimal performance. Cryocoolers contribute to creating the ideal conditions for quantum bits (qubits) to function, paving the way for advancements in quantum computing.

Space Exploration:

Cryogenic technology plays a vital role in space missions, particularly in the Deep Space Network (DSN). Both open-cycle refrigeration (OCR) and closed-cycle refrigeration (CCR) systems are employed to cool low-noise preamplifiers and antenna feed system components. Cooled detectors enable advancements in space applications, enhancing identification and discrimination capabilities for missions such as communications, remote sensing, and weather monitoring.

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.

Telescope Instruments: Cryogenic refrigeration is crucial for space-based telescopes and spectrometers. Cryocoolers extend the operational lifespan of instruments by effectively cooling detectors, reducing noise, and enhancing the sensitivity of observations.

Deep-Space Communications: Cryogenic technology is employed in deep-space communication systems to enhance signal detection in the vastness of space. The ability to minimize background noise allows for clearer and more reliable communication with spacecraft.

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.

Medical Applications:

MRI Machines: Superconducting magnets in magnetic resonance imaging (MRI) machines require cryogenic temperatures. Cryocoolers contribute to maintaining these low temperatures, ensuring the efficiency and accuracy of MRI scans.

Material Science and Research:

Superconductors and Quantum Devices: Cryogenic refrigeration is instrumental in the development and testing of superconductors, which exhibit unique properties at low temperatures. Researchers use cryocoolers to create the ideal environment for studying quantum phenomena and advancing materials science.

Solid-State Cooling

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

Microsatellite cryocoolers

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.

Air Force Research Laboratory’s Cryocooler Efforts:

The Air Force Research Laboratory (AFRL) focuses on developing space-qualifiable cryogenic technologies to meet the future requirements of Air Force and Department of Defense missions. This includes advancing cryocooler technology, evaluating performance, exploring advanced concepts for spacecraft missions, and enhancing cryocooler integration. The evolution of cryocooler technology, from large-capacity machines to more compact and efficient designs, reflects the progress made in terms of reduced mass and improved efficiency.

China’s Breakthrough in Cryogenic Refrigeration Technology:

China achieved a significant breakthrough in cryogenic refrigeration technology in April 2021, showcasing its capability to cool down to -271°C with a hundred-watt power level. The achievement holds crucial importance for vital industrial sectors like aerospace and hydrogen energy.

A major scientific research project in China, focusing on developing a large-scale cryogenic refrigeration system spanning from liquid helium to superfluid helium temperatures, successfully passed expert appraisal. Supported by the Ministry of Finance and executed by the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (TIPC-CAS), this project has propelled China to develop equipment with a liquid helium temperature of 4.2K (kilowatt level) and a superfluid helium temperature of 2K (hundred-watt level). This milestone breaks the technology monopoly previously held by developed countries, positioning China’s cryogenic refrigeration technology at a global advanced level.

Large-scale cryogenic refrigeration equipment covering the temperature range from liquid helium to superfluid helium is deemed indispensable for strategic sectors like aerospace and hydrogen energy. Historically dependent on imports, China’s cryogenic refrigeration technology faced restrictions from foreign countries, prohibiting key components and equipment for special fields. The breakthrough enables China to overcome this dependency and strengthen its position in critical industrial domains.

The successful operation of hundred-watt large refrigerating machines in industries such as accelerators and nuclear fusion highlights the practical applications of this breakthrough. The research project not only advances cryogenic refrigeration technology but also catalyzes the rapid development of related industries, including cryogenic heat exchangers and cryogenic valves. This progress has led to the formation of a low-temperature industry cluster with well-defined functions and a clear division of labor.

Recent Breakthroughs

Sunpower’s Free-Piston Cryocooler for Quantum Technologies: Sunpower, Inc., has developed a cutting-edge free-piston cryocooler with applications in quantum technologies. The latest iteration of this cryocooler is integral for emerging systems in quantum communications and cryptography, where the transmission of single photons with high fidelity is crucial. Typically, single-photon emitters based on quantum dots need cryogenic temperatures for optimal operation. However, researchers at TU Berlin have successfully integrated a Stirling cryocooler from AMETEK Sunpower into a 19-inch module, making it suitable for practical use in real-world communications networks.

Compact Stirling Cryocooler for Quantum-Dot Emitters: The Stirling cryocooler employed in this design features a free-piston mechanism that utilizes gas bearings for friction-free operation. This cryocooler, known for its reliability and robust cooling solutions, is compact and maintenance-free. It operates without lubrication, ensuring a long and dependable lifetime. The free-piston design not only delivers higher cooling powers but also exhibits better thermal efficiency compared to other cryocoolers available in the market. The Cryotel GT, one of Sunpower’s compact devices, stands out with its impressive cooling efficiency, removing heat at a rate of 16 W with 240 W of input power while maintaining a temperature of 77 K.

Applications in Quantum Technology: Sunpower’s cryocooler, known for its effectiveness in instruments requiring precise cooling for detecting faint signals, is finding new applications in quantum technology. The recently released CryoTel GTLT offers enhanced cooling performance at lower temperatures, expanding its range of applications. Sunpower has also focused on reducing vibrations exported by the cryocoolers, a critical factor for certain applications. The company’s Active Vibration Cancellation (AVC) technology, including the latest AVC-GEN2 balancer, significantly reduces vibrations, making it suitable for quantum applications where minimizing vibrations is essential.

Meeting Quantum Technology Demands: Sunpower acknowledges the growing importance of quantum applications and remains committed to advancing its cryocooler technology. Ongoing efforts include achieving colder temperatures, higher cooling capacities, and further reductions in exported vibrations. The TU Berlin-developed quantum key distribution (QKD) system and the collaboration with UK start-up Aegiq highlight the cryocooler’s role in optimizing quantum-dot emitters’ performance while meeting the demands of the rapidly evolving field of quantum technology.

Conclusion:

Cryogenic refrigeration technology, once confined to niche applications, has evolved into a critical enabler across various fields. From enhancing the capabilities of space missions to facilitating quantum computing and enabling microsatellite technology, cryogenic refrigeration continues to push the boundaries of what is possible. As research and development efforts persist, the chilling innovations in cryogenic technology promise a future of unprecedented advancements and applications.