In September 2008, China independently mastered the key technology of extravehicular activities following the launch of the Shenzhou VII spacecraft, joining the ranks of the United States and Russia. While the nation celebrated this achievement, scientists at the Institute of Physics and Chemistry, Chinese Academy of Sciences (referred to as the Institute of Physics and Chemistry), were pondering a crucial issue for the continuous development of China's aerospace industry.
For the aerospace industry to advance, it relies on large-scale carrier rockets with greater thrust, higher efficiency, and environmentally friendly propellants. Liquid hydrogen and liquid oxygen, as environmentally friendly rocket propellants with higher thrust, can generate greater velocity increments under the same conditions, making rocket engines using liquid hydrogen and liquid oxygen more efficient. Hydrogen turns into a liquid at -253°C, requiring large-scale low-temperature refrigeration equipment to produce liquid hydrogen. However, at that time, the United States strictly prohibited the export of refrigeration equipment and core components below -250°C to China.
"Large-scale low-temperature technology is one of the indispensable key technologies for space security." In January 2009, scientists from the Institute of Physics and Chemistry voluntarily applied to the Chinese Academy of Sciences to tackle the "fortress" of large-scale hydrogen-helium low-temperature refrigeration technology.
Thus began a silent struggle for the localization of large-scale low-temperature refrigeration equipment.
Prelude: "Even if we have to sell the pot and the iron"
Creating a low-temperature environment relies on continuously extracting heat from the environment using low-temperature refrigeration equipment while preventing external heat from entering. Large-scale low-temperature refrigeration equipment is often referred to as the "super low-temperature factory." Similar to a giant refrigerator, it can lower temperatures to below -253°C or even -271°C, maintaining cooling capacities ranging from hundreds of watts to thousands of watts.
This "super low-temperature factory" is a crucial strategic support equipment for the country, playing an irreplaceable role in important areas such as aerospace low-temperature propellant assurance, special material extraction, hydrogen utilization and storage, strategic helium resource exploitation, and national major scientific infrastructure.
Due to this irreplaceability, some Western countries attempted to block the development of related technologies in China. Since the 1950s, senior scientists at the Institute of Physics and Chemistry, such as Academician Hong Chaosheng and Academician Zhou Yuan, had been researching low-temperature technology, attempting to break through barriers.
However, by 2009, the country had not officially approved a research and development project for large-scale low-temperature refrigeration equipment to meet new demands. Fragmented small projects could not support systematic breakthroughs. Concerned scientists urgently proposed project approval to the Chinese Academy of Sciences.
After a detailed understanding of the situation, the Academy's leadership decisively stated, "What are we waiting for? Even if we have to sell the pot and the iron, we must do it!" The Chinese Academy of Sciences granted special funds to the Institute of Physics and Chemistry and urgently deployed projects in critical directions.
Simultaneously, relevant departments of the Chinese Academy of Sciences began assessing whether large-scale low-temperature refrigeration equipment met the conditions to become a national major scientific research equipment development project.
At that time, the country had initiated pilot projects for the development of major scientific research equipment. The Ministry of Finance allocated special funds to support independent innovation in major scientific research equipment, with the Chinese Academy of Sciences as the pilot institution, exploring the support model of national finance for independent innovation in major scientific research equipment.
During the project approval process, Li Qing, a researcher at the Institute of Physics and Chemistry who had long been engaged in low-temperature refrigeration technology in the liquid hydrogen temperature range, repeatedly justified the project proposal with his team. Despite suffering from back pain and sweating profusely, he continued to strive for China to gain a foothold in large-scale low-temperature refrigeration technology internationally. At one point, he even lived in the office and worked almost every night until late.
Based on extensive expert surveys and rigorous justifications, in 2010, the Ministry of Finance and the Chinese Academy of Sciences jointly launched the "Development of Large-Scale Low-Temperature Refrigeration Equipment" project, led by the Institute of Physics and Chemistry, with a project budget of 173 million yuan, and Li Qing appointed as the chief scientist.
This project later became known to many as the "Phase One Project." With the support of this project, Chinese scientists independently developed a ten-thousand-watt level liquid hydrogen temperature refrigeration equipment, overcoming five key technologies: stable technology for high-speed helium gas bearing expander, design and manufacturing technology for ultra-low leakage plate-fin type low-temperature heat exchanger, high-precision oil separation technology, manufacturing technology for pneumatic low-temperature regulating valve, and system integration and control technology.
In the initial stages of development, due to the lack of domestic design and manufacturing of large-scale low-temperature refrigeration equipment, the sealing performance of domestically produced compressors, low-temperature heat exchangers, and other key equipment was two orders of magnitude lower than required. The project team set up a makeshift building next to Building 5 of the Institute of Physics and Chemistry. During the summer, sweat dripped down as they wrapped insulating materials. In winter, they relied on a small heater, working through the night with cold hands and feet.
Their hard work paid off. On April 29, 2015, thanks to the efforts of Li Qing and his team, the Phase One Project passed acceptance, enabling China to independently design and manufacture large-scale low-temperature refrigeration equipment for liquid hydrogen temperature levels, solving the problem of the absence of large-scale liquid hydrogen temperature zone low-temperature refrigeration equipment.
It was like a key that opened the door to the localization of large-scale low-temperature refrigeration equipment in China. For the scientists, their journey had only just begun.
Goal: "We must continue to descend further"
Starting from the "Twelfth Five-Year Plan" period, China deployed numerous large scientific equipment projects. While witnessing the launch of these projects, scientists at the Institute of Physics and Chemistry were thinking further ahead.
Many large scientific equipment projects require the use of superconducting devices, including superconducting magnets and superconducting radio frequency cavities. To achieve good performance, most of these superconducting devices must operate in the liquid helium temperature range (around -269°C) to the superfluid helium temperature range (around -271°C). Some say, "Without large-scale liquid helium or superfluid helium temperature zone refrigeration equipment, most large scientific equipment is nothing more than a pile of scrap metal." Therefore, low-temperature refrigeration machines for even lower temperature zones became essential.
At that time, there was no domestic capability to produce large-scale low-temperature refrigeration equipment for lower temperature zones. Some large scientific equipment projects faced passive situations in the international scientific and technological competition due to issues such as uncontrollable supply times for imported products.
"We cannot be satisfied with just reaching the liquid hydrogen temperature range; the temperature needs to continue to drop, covering the entire temperature range, dropping to the liquid helium temperature range or even the superfluid helium temperature range." This was discussed by Zhang Liping, the then director of the Institute of Physical and Chemical Research, and the former director and chief consultant of the Phase I project, Zhan Wenshan.
With the technology available at the time, achieving -253°C with a large cryogenic refrigerator was already quite challenging. To lower it to -269°C or even -271°C, the difficulty was evident. After much discussion, it was decided to allocate a small portion of the Phase I budget to demonstrate the feasibility of realizing large-scale cryogenic technology covering the entire temperature range.
Based on this, in 2014, the Institute of Physical and Chemical Research submitted a report to the Chinese Academy of Sciences, applying to continue the project.
This report was revised no less than 25 times, with the most debated issue being whether to proceed with a "step-by-step" approach or a "leapfrog" approach.
The "step-by-step" approach involved first tackling the equipment for cooling in the liquid helium temperature range, followed by tackling the equipment for cooling in the superfluid helium temperature range in separate projects. This method was steady but slow.
The "leapfrog" approach involved only one project to tackle the equipment for cooling in both the liquid helium and superfluid helium temperature ranges, developing a single device capable of providing temperatures of -269°C and -271°C simultaneously. This method was faster but carried higher risks.
Initially, most people supported the more cautious route. "We are confident in the device for the liquid helium temperature range, but many have never even seen a superfluid helium refrigerator, let alone manufactured one," said Gong Linghui, a researcher at the Institute of Physical and Chemical Research and the executive deputy commander of the Phase II project.
However, the practical demand for refrigeration in China and the intense international competition left no room for hesitation. The Chinese Academy of Sciences and the Institute of Physical and Chemical Research ultimately decided to take the "leapfrog" approach.
In 2015, the project proposal was approved by the state. With the support of a major national research equipment development project, the Phase II project "Development of a Large-scale Cryogenic System from Liquid Helium to Superfluid Helium Temperature Range" seamlessly continued, with a budget of 187 million yuan.
Gong Linghui felt immense pressure. The Phase II project included the development of three types of refrigerators: a hundred-watt liquid helium refrigerator with a cooling capacity of 250 watts at -269°C, a kilowatt-level liquid helium refrigerator with a cooling capacity of 2500 watts at -269°C, and a hundred-watt superfluid helium refrigerator with a cooling capacity of 500 watts at -271°C. Moreover, the latter two refrigerators needed to be integrated into a single device.
"With so much to do and such tight deadlines, I'm afraid we won't make it," Gong Linghui admitted.
Journey: Advancing to -269°C
As expected, the dual pressures of technology and time significantly increased the project's difficulty.
According to the plan, the research team first needed to develop a smaller liquid helium system. In September 2016, the 250-watt liquid helium system was integrated and began testing. "Unexpectedly, nothing went as planned. As soon as we turned it on, problems arose, and the temperature wouldn't drop," said Liu Xinjian, the then deputy director of the Institute of Physical and Chemical Research and the overall commander of the Phase II project.
Numerous discussions were held, and the reasons were investigated extensively, but the problem persisted without a solution, causing everyone's confidence to plummet. "We're stuck halfway; what should we do next?" Liu Xinjian pondered.
By the summer of 2017, it was clear that the conservative "band-aid" approach was no longer effective, so a bold decision was made: "Let's dismantle it! Reassemble it!"
The main equipment to be dismantled was the cryostat. From the outside, the cryostat looked like a large container with many internal components where helium was transformed into liquid helium or superfluid helium.
The dismantling and reassembly took place at the Institute of Physical and Chemical Research's Langfang campus. At that time, the campus was not yet completed, and the factory buildings had just been connected to utilities. To save time, Gong Linghui and a few young team members moved in with sleeping mats, blankets, a kettle, a water heater, and boxes of instant noodles.
Working day and night for over a month, they finally identified the issue. With the debugging progress now smooth, by October 2017, the 250-watt liquid helium refrigerator passed expert inspection, with a domestication rate of key components reaching 100%.
The confidence of the research team soared. Building on this success, they further developed and perfected the key core equipment - the high-speed helium turboexpander, integrating it into a 2500-watt liquid helium refrigerator. By September 2019, the performance of the large-scale liquid helium refrigeration system met the design specifications.
Acceptance testing of helium screw compressors for 2500-watt low-temperature systems.
However, the development of hundred-watt and kilowatt-level liquid helium refrigeration equipment, the project's task was only half completed, with bigger challenges awaiting them.
Breakthrough: Sprinting to -271°C
The other half of the project involves developing a hundred-watt-level superfluid helium refrigeration machine. This part of the task was almost simultaneously initiated with the liquid helium refrigeration machine development.
The key equipment for the superfluid helium refrigeration machine is the centrifugal cold compressor, which is also the most nerve-wracking equipment in the entire project.
The cold compressor can operate at low temperatures, using electromagnetic bearings, requiring the control system to respond accurately to any rotor deviations within 0.5 milliseconds. The Low-Temperature Technology Experimental Center (a predecessor of the Institute of Physics) has been developing various contact and non-contact bearing compressors since 1959 but has never developed an electromagnetic bearing compressor.
For safety, the project team prepared two approaches: one was to purchase the entire machine or components from abroad, and the other was to develop everything independently. Both approaches progressed simultaneously.
As a result, the first approach encountered numerous problems, causing everyone's emotions to fluctuate like a roller coaster.
In 2015, the research team found a foreign company with experience in producing cold compressors, and they had already negotiated the purchase. However, just before the bidding, the company sent an email saying, "To obtain an export license, we discussed and negotiated with government departments last week, but they hold a negative view on issuing export licenses due to your organization being on the government's restricted list." Later, the research team found another foreign company, but the cold compressor produced by this company failed during the first test run in Langfang, and the repair progress kept getting delayed.
Although the second approach was equally challenging, it provided a sense of security.
When the first approach proved to be unfeasible, the research team had accumulated years of technical experience and independently solved the technical challenges of the electromagnetic bearings. Subsequently, the research team focused all their efforts on the second approach. By 2019, the team had independently developed the cold compressor, which entered the technical testing phase.
Due to the need for a large amount of liquid helium for testing, they moved the testing site to the High Magnetic Field Science Center of the Hefei Institutes of Physical Science, Chinese Academy of Sciences, which had the necessary testing conditions. The cold compressor testing went smoothly overall. In November 2019, the cold compressor was moved back to Langfang and integrated into the superfluid helium system test bench.
The next goal for the researchers was to achieve a 500-watt superfluid helium system, combined with the 2500-watt liquid helium system.
Integration: Achieving dual functionality
In November 2019, the prototype cold compressor was installed in the superfluid helium system for the first joint test with the liquid helium system. In December, during the second joint test, the superfluid helium system reached -271°C for the first time... The tests seemed to progress smoothly. However, as they approached the higher goal of 500 watts and -271°C, problems arose.
In February 2020, the third joint test yielded both good and bad results. The good result was that the superfluid helium system achieved an instantaneous cooling capacity of 502.9 watts at -271°C; the bad result was a shutdown due to instrument system failure.
At that time, the tests frequently stopped due to motor overheating and instrument alarms. "With self-developed equipment, we can pinpoint where the problem lies and continuously iterate on the technology." Although anxious, Liu Liqiang, the chief scientist of the second-phase project and a researcher at the Institute of Physics, remained confident.
While continuing to improve various performance aspects of the cold compressor, the team worked hard to solve various issues, including motor overheating. The stable operation time of the superfluid helium system gradually increased: in early July 2020, during the ninth joint test, it ran stably for 1 hour; by the end of the month, during the tenth joint test, it ran stably for 5 hours; in August, during the eleventh joint test, it ran stably for 7 hours. However, the issue of motor overheating remained unresolved.
With determination, the team decided to perform a "major surgery" on the cold compressor, replacing all the coils in the motor and optimizing the cooling structure.
Every team member felt that even if progress was slower, the equipment entrusted to the country could not afford any risks. "Using a 'conservative treatment' may achieve a stability target of 72 hours, but the appearance of any accidental factor could render it unstable," Liu Liqiang said.
In October 2020, in the Langfang park, after the motor coils were replaced, the team prepared for the twelfth joint test. This time, they aimed to challenge the 72-hour mark.
1 hour, 5 hours, 7 hours, 10 hours... Liu Liqiang saw signs of success. In previous tests, there were always issues with values being too high or too low, but this time everything went smoothly, and the system ran steadily.
12 hours, 24 hours, 36 hours, 48 hours... The system remained stable. Until October 20, the equipment ran steadily for 72 hours.
"This task is finally completed!" Liu Liqiang felt the pressure lift off his shoulders.
On December 29, 2020, the liquid helium and superfluid helium cryogenic refrigeration system passed the expert group acceptance test.
Future Outlook: Building a World Leader in Low-Temperature Technology Industry
On April 15, 2021, the second phase of the project passed the project acceptance and scientific and technological achievements appraisal.
The acceptance opinion stated that the project "comprehensively broke through the core technology of large-scale helium cryogenic refrigeration equipment"; the appraisal opinion believed that the project "has developed the research capability of kilowatt-level large-scale helium cryogenic devices, breaking the technological monopoly of developed countries, and the overall technology of the project has reached international standards."
This is China's first large-scale cryogenic refrigeration device capable of reaching the superfluid helium temperature range. From that day on, China had its own "super low-temperature factory."
Following the second phase of the project, the Chinese Academy of Sciences established a strategic leading science and technology special project to continue supporting the Institute of Physics in developing a 5-ton/day large-scale hydrogen liquefaction system. On March 8, 2024, the system passed the test acceptance, running stably at full load for 8.5 hours, with a hydrogen liquefaction rate of approximately 5.17 tons/day.
To many witnesses, the success of the "super low-temperature factory" can be attributed to an effective major project management system and mechanism.
Wang Xuesong, director of the Institute of Physics, introduced that during the implementation of the low-temperature equipment project, the Institute of Physics explored a distinctive and effective management mechanism, creating an innovative research and development model of "researching, applying, and transforming simultaneously" and a complete development chain.
In terms of management, "we broke down the barriers between the original 'PI (principal investigator) system' research groups, physically integrated three research groups, established a research center focusing on major strategic goals, formed a scientific architecture to work together on major tasks, and laid a solid foundation for the overall management of the project," said Wang Xuesong.
Currently, the Institute of Physics has implemented 18 units of large-scale cryogenic refrigerators. Among them, the hundred-watt liquid helium refrigerator is not only used in the superconducting magnet test device of the Institute of High Energy Physics of the Chinese Academy of Sciences and the linear accelerator of the China Institute of Atomic Energy but has also been used abroad in the Korea Superconducting Tokamak Advanced Research (KSTAR) facility; liquid helium and superfluid helium refrigerators are used in the accelerator-driven transmutation experimental line of the Institute of Modern Physics of the Chinese Academy of Sciences.
China's first 200-watt helium cryocooler for international export has been developed.
Simultaneously, the Institute of Physics has collaborated with enterprises on the research and development of screw compressors, helium valves, high-speed motors, electromagnetic bearings, and other components. Over 20 companies in Shandong, Fujian, and other regions have achieved technological breakthroughs in a short period, with some technologies reaching international advanced levels.
"We undertake major national projects, not only to complete the projects but also to lay an industrial foundation for the country," said Gong Linghui.
From the initial project approval in 2010, through the second-phase acceptance in 2021, to the recent breakthroughs in key special projects, Chinese scientists have completed in just over 10 years what took Western countries decades. This has propelled China's large-scale cryogenic technology into a new development stage.
Today, these scientists once again see new national demands for cryogenic refrigeration equipment, leading to research on ultra-low temperature, high-capacity cryocoolers. This will help China become a global hub for low-temperature technology and industries, providing comprehensive support for national strategic resources, aerospace, scientific innovation, and other developments.
Large-scale cryogenic refrigeration team. Image provided by Institute of Physics and Chemistry.
(Intern Zhao Yutong also contributed to this article)
