This is a very inconspicuous looking thing. It is only the size of a palm, and it is a little heavy on the hand. Its shape is quite special. A thin metal tube is like a pole, carrying two thick metal cylinders, and one of the metal cylinders is followed by a half of a tube slightly thinner than the little finger.
One of the aerospace-grade pulse tube refrigerators
Zhao Miguang, a researcher at the Institute of Physics and Chemistry of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Physics and Chemistry of the Chinese Academy of Sciences), said that it would cost about one million yuan to make this thing now. However, before 2000, foreign countries would not sell it to China, no matter how much it cost.
It is the "cooling artifact" that has plagued China's optical remote sensing satellites for decades - the aerospace-grade pulse tube refrigerator, a high-tech product that can quickly reduce the local temperature to minus 200 degrees Celsius and will not be damaged for many years.
Its birth and development in China benefited from a "long run" by researchers from the Chinese Academy of Sciences.
Cool down to minus 200 degrees Celsius in a few minutes
On January 25, the ZY-1 02E satellite equipped with three domestically-made pulse tube refrigerators successfully acquired high-quality images. These three pulse tube refrigerators are the 26th, 27th, and 28th aerospace-grade pulse tube refrigerators developed by the Institute of Physics and Chemistry of the Chinese Academy of Sciences and successfully used in space projects.
The new crown epidemic has made many people aware of "infrared temperature measurement" technology. When the human body passes through the thermal imaging camera, the hot spots on the body will appear red. In order to measure the temperature of the earth and the atmosphere, many meteorological satellites are also equipped with infrared detectors. However, due to the distance from the target detection object, the infrared detector on the satellite needs to maintain a low temperature or even an extremely low temperature level in order to detect the slightest heat change on the distant target.
So, how to keep the infrared detector on the satellite in a low temperature state? Scientists have invented two types of refrigeration technologies, one is passive radiation refrigeration, which is based on the principle that high-temperature objects can radiate energy to low-temperature objects; the other is active mechanical refrigeration, the principle is to make gas through mechanical movement. expansion, and the cooling phenomenon occurs when the gas expands.
"The cooling temperature of radiation cooling is high and the cooling capacity is small. Although it can be used to cool small detectors, it cannot meet the higher requirements for low temperature of aerospace devices. In contrast, mechanical cooling technology is more feasible." Chinese Academy of Sciences Physics and Chemistry Chen Houlei, a researcher at the institute, told the China Science Journal.
Pulse tube refrigeration technology is a kind of mechanical refrigeration technology. In 1963, American scientists accidentally discovered that when there is an alternating pressure wave in a hollow tube, its closed end will heat up, and a large temperature gradient can be formed along the axial direction of the tube. Based on this, they developed a pulse tube refrigeration. machine. In 1984, Soviet scientists transformed the pulse tube refrigerator, which greatly improved its cooling efficiency, and the related research heat also increased.
Pointing to the pulse tube refrigerator in his hand, Chen Houlei introduced: "This part similar to the little finger is called the 'cold finger'. This small refrigerator can reduce the fingertip to minus 200 in a few minutes. Celsius. We put the probe on the fingertip, and when the temperature of the probe cools down, we can see the earth clearly.”
From theory to engineering
In the 1980s, the Cryogenic Technology Experiment Center of the Chinese Academy of Sciences (one of the units of the Institute of Physics and Chemistry of the Chinese Academy of Sciences) took the lead in starting the basic research on pulse tube refrigeration technology in China. By the 1990s, they had set the record for the lowest refrigeration temperature several times, achieved a series of original achievements, and won the "Kapicha Award" and "Linde Award" of the International Society of Refrigeration.
In 1998, the United States was the first to engineer pulse-tube refrigerators for use in space programs. Although my country has achieved world-leading results in the basic research of pulse tube refrigeration, optical remote sensing satellites have been suffering from lack of their own space refrigerators. "At that time, the research and development unit could only import from abroad, and the aerospace-grade refrigerator was embargoed to our country. The research and development unit could only try every way to buy the ground-level refrigerator." Zhao Miguang said.
The reason for saying "bitter" is that the lifespan of imported ground-level refrigerators is less than 1/10 of that of aerospace-grade refrigerators, and problems often occur, and after satellites are launched, they cannot be repaired at any time like they are on the ground. Once on-orbit failure, the country may pay a huge price.
In the late 1990s, with the support of the National Science Fund for Distinguished Young Scholars, the Chinese Academy of Sciences and other relevant departments, the research team of the Institute of Physics and Chemistry of the Chinese Academy of Sciences began to transform into applied research as a whole. Facing the major needs in the field of national space remote sensing and infrared detection, they stepped out of the pile of old papers and carried out research on the engineering of pulse tube refrigerators and space applications. Starting in 2000, the team received systematic support from the National Space Agency.
The road from theory to engineering is full of challenges. "What we want to do is aerospace-grade products. If the refrigerator will work in the sky in the future, it must be guaranteed that it will not break for 8 years, 10 years or even longer." Chen Houlei said.
The refrigerators are filled with helium gas, and they must ensure a very low leakage rate of helium molecules through reliable welding and sealing technology. The compression and expansion of the gas depends on the piston. They must make the piston and the cylinder wall both close and non-contact, so as to ensure that the piston does not wear when oil cannot be used. The movement of the piston is supported by a flat spring, and they must find ways to ensure that the spring is not crooked, let alone broken.
Every "must" means a pressure. "At that time, we encountered some problems, and we couldn't move forward for a long time, and everyone's morale was very low." Zhao Miguang said.
Summarize the progress at 8:00 in the morning, and leave work at 12:00 in the evening. In this way, 8 years have passed. In the past 8 years, Riyuexingchen has witnessed their road to tackling key problems: the pulse tube refrigerator they developed achieved full welding of titanium alloys; the distance between the piston and the cylinder reached the micron level; the leaf spring can move 100 in the anti-fatigue test. More than 100 million times, there is even a spring's anti-fatigue test "running" for more than 10 years, and there is no problem so far.
28 pulse tube refrigerators have been put into service
In 2008, under the leadership of researcher Liang Jingtao, the current director of the Key Laboratory of Space Work-Heat Conversion Technology, Institute of Physics and Chemistry, Chinese Academy of Sciences, the scientific research team developed my country's first long-life pulse tube refrigerator in space, and successfully carried out on-orbit verification. The refrigerator has achieved a historic leap from scratch, from tracking to entering the latest international development stage, and my country has thus become the second country after the United States to master the space pulse tube refrigeration technology.
Since then, the "stumbling block" in the development of optical satellites in the infrared spectrum has been kicked away, and the planning and demonstration of a series of satellite models in the country has been in full swing.
Technological innovation creates demand, which in turn drives technological development. In the following ten years, the development of my country's spaceborne infrared detectors has continuously put forward higher requirements for spaceborne pulse tube refrigerators.
On May 9, 2018, the Gaofen-5 satellite was successfully launched. It was equipped with a pulse tube refrigerator with large cooling capacity and long life developed by the Institute of Physics and Chemistry of the Chinese Academy of Sciences.
This satellite carries my country's first full-spectrum spectral imager covering the visible, near-infrared, short-wave, medium-wave, and long-wave infrared spectrum bands. Compared with previous optical detectors, it is larger and heavier. In order to adapt to the demand, the chiller project team boldly adopted a new solution. The chiller could only carry the cold finger of the cold platform with a size of more than ten millimeters and a weight of a few grams, and designed it into a detector assembly with a size of one hundred millimeters and a weight of one kilogram. .
In the first few years, the road to turning the design into reality was bumpy. However, the more failures you experience, the more unforgettable the joy of success becomes.
"This is a new international initiative!" Chen Houlei's eyes lit up when he said this.
At 11:11 on December 26, 2021, the "Ziyuan No. 1 02E" satellite was successfully launched at the Taiyuan Satellite Launch Center. The long-wave infrared detector carried on it must work in a low temperature environment of minus 193 degrees Celsius. The spectral imager on Gaofen-5 is larger and requires twice the cooling area of the pulse-tube refrigerator.
During the development process, researchers encountered unprecedented difficulties. "The design life of the 'Zuanyu No. 1 02E' satellite is 10 years, which is higher than the previous 8-year life requirement. In order to meet the project requirements, we proposed a plan to use the switching device to backup the compressor. In the case of increasing the limited weight, the large It greatly improves the life of the refrigerator." Liu Yanjie, the chief designer of this pulse tube refrigerator and an associate researcher at the Institute of Physics and Chemistry of the Chinese Academy of Sciences, told the China Science Journal.
"During the design of the cold finger, the large-area cold plate we used had a deformation problem due to thermal expansion and contraction, and the detector on it was easily broken." The cold finger designer of this pulse tube refrigerator, Quan Jia, an associate researcher at the Institute of Physics and Chemistry of the Chinese Academy of Sciences, recalled that after countless discussions and debugging, they made a new material match for the cold plate and finally solved the problem.
Today, they have developed aerospace-grade pulse tube refrigerators of various shapes and performances, of which 28 have been launched into orbit.
Every project is full of challenges, and these challenges are the driving force behind the pulse tube refrigerator development team.
"We are developing more advanced space low-temperature refrigeration technology with a cooling capacity of more than 100 watts, a temperature close to absolute zero, and a weight as low as a few hundred grams." Liang Jingtao said that the team will build on the successful development of the space pulse tube refrigerator. Continue to carry forward the spirit of "daring to challenge and fight hard", bravely assume "national responsibility", and forge ahead to meet the needs of low-temperature refrigeration in future spaces.
On the development road of aerospace-grade cryogenic refrigerator technology, this "long run" continues.
