At a time when the global carbon neutral strategy is in full swing and traditional fossil energy sources are becoming increasingly depleted, nuclear fusion energy is recognized as the ultimate clean energy source for mankind. With the core advantages of abundant raw materials, zero carbon emission and extremely high energy density, it is expected to completely solve the global energy crisis and reshape the global energy landscape. Extreme cryogenic refrigeration technology is the supporting technology for controlled nuclear fusion. In this article, we will analyze the unique physical properties of liquid helium, its core working principle and application scenarios in nuclear fusion reactors.
Liquid helium is the liquid form of the element helium formed at extremely low temperatures, and is one of the lowest temperature liquid substances known to mankind. As a product of liquefaction of a rare noble gas, liquid helium has very special physical properties that distinguish it from most conventional industrial media.
The most common helium isotope in nature is helium-4, which liquefies at a temperature of only 4.2 K at standard atmospheric pressure, which translates to a temperature of -268.95 °C in Celsius. The rare isotope helium-3 has an even lower liquefaction temperature of about 3.2 K, which makes it even more suitable for low temperatures, but natural reserves are extremely scarce and it is not possible to apply it on a large industrial scale.
The critical temperature of liquid helium is about 5.2 K, which means that once the ambient temperature exceeds this value, helium cannot be maintained in a liquid state, no matter how much pressure is applied. At the same time, the density of liquid helium is only 0.125g/cm³, about one-eighth that of liquid water, making it ultra-lightweight and suitable for cooling large precision equipment.
Most unusually, at standard atmospheric pressure, liquid helium will not solidify even when it is infinitely close to absolute zero (0 K). This phenomenon is due to the zero-point energon effect of helium atoms. Lightweight helium atoms are not able to stabilize into a rigid lattice structure, and can only solidify at high pressures of 25 bar or more, which makes liquid helium’s cryogenic stability far superior to that of other cryogenic media.
The most valuable property of liquid helium, and the one that underpins fusion engineering, is its superfluidity at low temperatures. When the temperature of helium-4 drops below 2.17 K (the λ-point), it undergoes a phase transition into a superfluid (He II), which exhibits quantum properties that overturn conventional fluids.
Superfluid liquid helium has zero viscosity and no friction in the flow process, and it can penetrate tiny pores and climb along the tube wall of the equipment, covering the microstructure of superconducting magnets in all directions and realizing dead-angle-free cooling. This is perfectly suited to the precise and complex internal piping structure of nuclear fusion equipment.
At the same time, the thermal conductivity of superfluid helium is 30 times higher than that of copper, providing the ultimate in rapid thermal conductivity and temperature equalization. During the operation of nuclear fusion equipment, there will be instantaneous fluctuation of heat, liquid helium can quickly absorb and conduct the residual heat, to avoid equipment failure caused by excessive local temperature difference.
Based on the two-fluid model theory, superfluid liquid helium consists of conventional fluid and superfluid components, and is extremely stable under low-temperature operating conditions. Compared to conventional cryogenic media such as liquid nitrogen, the quantum properties of liquid helium make it the optimal cooling medium for advanced superconducting devices.
Currently, the world’s most mature controlled fusion technology, which is the closest to commercialization, is magnetic confinement fusion (MCM), and the mainstream equipment includes tokamaks and star-like devices. Fusion reaction requires high-temperature plasma at hundreds of millions of degrees Celsius as the raw material for the reaction.
There is no physical container that can carry plasma at hundreds of millions of degrees Celsius, so the industry’s core solution is to use a strong magnetic field to build a “virtual container”, which will stably confine the high-temperature plasma inside the cavity of the device and maintain a continuous fusion reaction.
Conventional copper conductive magnets are extremely resistive, generate huge energy consumption and serious heating problems under high-current conditions, and are unable to maintain a high-strength, high-stability magnetic field for a long period of time, which does not satisfy the needs of sustained operation of nuclear fusion.
Only superconducting magnets can realize zero resistance, low energy consumption, and stable operation of high magnetic field, and the extreme low-temperature operation environment of superconducting magnets must rely on liquid helium to build and maintain.
Currently, the main superconducting materials used in large-scale global fusion programs are two types of low-temperature superconducting materials: niobium-titanium (NbTi) and niobium-tin (Nb₃Sn). These materials are the core magnet materials for large-scale fusion devices, with mature technology and strong magnetic field carrying capacity.
However, these low-temperature superconducting materials have a strict temperature threshold, only in the extremely low temperature environment of 4K-10K to realize a completely zero resistance superconducting state. Once the temperature exceeds the threshold, the superconducting properties will instantly disappear, triggering the failure of the magnet.
Among the existing industrialized media, only liquid helium (especially supercritical helium) can stably maintain a constant low-temperature environment of about 4K. Liquid nitrogen, at 77K, is completely unsuitable for the operation of cryogenic superconducting magnets.
It is clear that the superconducting magnet systems of all large-scale mature fusion devices at this stage use supercritical liquid helium as the core cooling medium, which is the core prerequisite to ensure zero-resistance operation of the magnets and the output of a stable and strong magnetic field.
The role of liquid helium in fusion equipment is not simply to cool the magnets, but to ensure comprehensive temperature control and pressure stabilization throughout the entire process of equipment operation. The supercritical helium circulating circuit can continuously carry away the heat loads generated by magnet excitation, plasma radiation and eddy currents.
The liquid helium thermostatic bath configured in the equipment can be used as a thermal buffer layer to effectively offset transient temperature fluctuations during fusion operation, avoiding superconducting failures caused by sudden changes in the magnet temperature, and guaranteeing pulsed, long-cycle and stable operation of the equipment.
At the same time, the cryogenic pumping unit cooled by cryogenic liquid helium can build an ultra-high vacuum environment inside the fusion cavity. The vacuum environment can prevent air impurities from interfering with the fusion reaction, which is an important basis for maintaining the purity of the plasma and guaranteeing the fusion efficiency.
The International Thermonuclear Experimental Reactor ITER is the world’s largest and most technologically advanced fusion experiment, and its cryogenic system represents the highest engineering level of current liquid helium applications, and is the core benchmark of the industry.
ITER’s complete superconducting magnet system weighs 10,000 tons, and relies on supercritical liquid helium to cool the entire system to a low temperature of 4.5K. The overall helium storage capacity of the project is about 25 tons, with a total length of 5 kilometers of closed loop piping network.
The project is equipped with the world’s largest helium refrigeration station, which can achieve a stable refrigeration power of 75kW at 4.5K, while the accompanying liquid nitrogen system provides a low-temperature environment of 80K for the heat shielding layer, minimizing the heat loss of the cryogenic system.
The equipment adopts the exclusive cooling structure of catheter-in-conduit conductor (CICC), where supercritical helium is forced to circulate in the center channel of the superconducting wire bundle without phase change fluctuation, which can accurately meet the stable heat dissipation needs of large magnets and provide core guarantee for ITER’s long-term experimental operation.
In addition to ITER, all mainstream fusion experimental devices that have been put into operation in various countries around the world are equipped with liquid helium cryogenic cooling system as standard, and the technical logic is highly unified, which fully verifies the irreplaceability of liquid helium.
Japan’s JT-60SA device is equipped with a helium cryogenic system with a cooling power of 9-9.5kW under 4.5K operating conditions, which cools the ring and pole magnetic field coils with supercritical helium, and at the same time adopts 3.6K cryogenic helium to ensure the stable operation of cryogenic pumps.
South Korea’s KSTAR device relies on a liquid helium bath and a supercritical helium circulation system to realize precise temperature control of the superconducting coils, which can support the device’s long-time pulsed fusion operation and the accumulation of steady-state fusion experimental data.
China’s EAST device and Germany’s W7-X imitator device also adopt liquid helium cryogenic refrigeration solutions, respectively, to complete the constant temperature cooling of hundreds of tons of cryogenic components, which is the core hardware support for the iteration of controlled fusion technology in various countries.
Throughout the world, all mainstream nuclear fusion devices uniformly adopt the core program of “supercritical helium forced cooling + closed-loop recycling system”, forming a standardized engineering application system.
Supercritical helium eliminates the gas-liquid phase transition problem of traditional liquid helium, resulting in a more stable flow state and a more balanced heat dissipation efficiency, which is perfectly suited to the dynamically fluctuating heat load conditions of fusion devices.
The closed-loop recycling system can minimize the loss of liquid helium through evaporation and leakage, effectively controlling the operating costs and ensuring the long-term stable operation of the equipment in the context of the scarcity of helium resources in the industry.
The liquid helium cryogenic system for nuclear fusion equipment is the world’s most sophisticated and complex large-scale industrial refrigeration system. The core consists of four modules, each of which operates in tandem with the other.
The helium refrigeration and liquefaction unit is the core power unit of the system. Through multi-stage compression, heat exchange and expansion processes, it converts normal temperature helium into supercritical cryogenic helium to provide a continuous cooling source for the entire equipment, while guaranteeing the redundancy of refrigeration power.
The helium storage management module contains normal temperature storage tanks, cryogenic buffer tanks, and purification and recovery equipment, which can realize helium storage, purification, recycling and reuse, and solve the core problems such as helium loss and insufficient purity.
The cryogenic pipeline network consists of vacuum adiabatic pipelines, precision control valve box, and cryogenic circulating pump, which can precisely control the flow, pressure, and temperature of supercritical helium, and realize the uniform distribution of cold source.
The auxiliary cryogenic module consists of heat shielding layer, cryogenic vacuum pump, and vacuum cryostat, which reduces the intrusion of external heat through graded temperature control, maintains the ultra-high vacuum environment of the cavity, and assists the fusion reaction to proceed stably.
During the operation of nuclear fusion equipment, the heat load is not constant, but presents dynamic fluctuation characteristics. Magnet excitation rise and fall, plasma radiation, vortex effect will generate instantaneous high heat load.
Relying on the thermal buffer capacity of liquid helium thermostatic bath, the system can quickly absorb the instantaneous peak heat to avoid drastic fluctuations in equipment temperature. At the same time, the cryogenic circulation pump adjusts the helium flow in real time to dynamically match the heat dissipation demand.
The whole set of control system has a redundant design, which can effectively avoid the failure of magnet failure, equipment overheating and shutdown, and guarantee the fusion device to run for a long time, steady state, pulse operation.
The cryogenic mass of large-scale fusion devices can reach thousands of tons, and the cooling cycle of the devices can be as long as several weeks or even one month. The cooling process requires precise control of the rate, which is very prone to problems such as uneven temperature and stress deformation.
The whole cryogenic refrigeration system is a high-energy equipment, multi-stage compressor running power up to tens of megawatts, long-term operation of the energy cost is very high, restricting the project scale landing.
Helium atom particle size is extremely small, pipeline sealing difficulty, even with top sealing technology, there are still trace leakage loss. At the same time, the technical threshold and cost of purifying high-purity helium and maintaining the equipment remain high.
Liquid helium is a typical non-renewable resource of geological origin. Almost all of the commercialized helium is extracted from natural gas, and there is no path for large-scale synthetic technology.
Helium has a very low density and once released into the atmosphere, it will quickly escape into space and cannot be recycled. From the human time scale, helium resources are consumable and scarce, and global proven reserves are limited.
Large-scale nuclear fusion projects require dozens of tons of liquid helium for a single equipment charge, and the future construction of commercial fusion power plant clusters will bring massive helium resource demand, and the pressure on resource reserves continues to intensify.
Global helium production capacity is highly concentrated, mainly relying on a few production areas such as Qatar, the U.S., and Russia, with a high degree of production capacity concentration, which is highly susceptible to factors such as equipment maintenance, geopolitical situations, and supply chain disruptions.
In 2025, the global helium market will experience significant price fluctuations, with peak prices rising by more than 400%, and the market price per ton exceeding $100,000, a record high, significantly increasing the cost of scientific research and industrial projects.
At the same time, many high-end fields such as semiconductors, high-end medical care, quantum technology, aerospace and others are competing for helium resources, and fusion energy projects are facing fierce competition for resources, with no guarantee of supply chain stability.
The deuterium-tritium reaction in nuclear fusion generates helium-4 by-products, which seem to supplement helium resources, but the product is extremely high-temperature and insufficiently pure, and cannot be directly converted into high-purity liquid helium for refrigeration.
With the current technology system, the helium produced by the fusion reaction cannot offset the helium losses of equipment operation. A commercial-scale fusion power plant loses about 2 tons of helium per year, while its own helium production is only 0.6 tons, leaving a persistent resource gap.
The fusion cryogenic system requires extremely high helium purity, and trace impurities may cause clogging of pipelines and a decline in refrigeration efficiency. The cost of purifying high-purity helium, as well as the cost of operation and maintenance, has further raised the threshold for landing the project.
High-temperature superconductivity (HTS) technology is the core breakthrough in the current reliance on liquid helium. Different from traditional low-temperature superconducting materials, new high-temperature superconducting materials such as REBCO can realize superconducting properties in the temperature range of 20K-77K.
This temperature range can rely on liquid nitrogen, room temperature helium or mechanical cooler to realize the cooling, without the need for scarce and expensive liquid helium medium, can greatly simplify the structure of the cryogenic system, reduce the cost of equipment construction and operation.
Frontier projects such as SPARC of CFS in the United States and Demo4 of Tokamak Energy in the United Kingdom have successfully verified the performance of high-temperature superconducting magnets, and the highest magnetic field strength can reach 11.8T, which fully meets the demand of fusion reaction.
High-temperature superconductivity technology can reduce the scale of cryogenic system by more than 80%, significantly reduce energy and resource consumption, and is the core technology direction for future commercialized fusion power plants.
To address the loss of existing liquid helium systems, the industry has iterated a high-precision closed-loop recycling, purification, and storage integrated system that minimizes helium evaporation and leakage loss through intelligent regulation.
The application of modularized intelligent helium refrigeration unit and AI dynamic temperature control system can adjust the refrigeration power in real time according to the equipment operating load, effectively reducing the system energy consumption and improving the refrigeration efficiency.
The landing of 3D printing and new insulation materials can optimize the structure of cryogenic pipelines and heat exchangers, reduce heat intrusion and media loss, and further enhance the stability and economy of the whole system.
In order to alleviate the problem of resource shortage, the industry is actively exploring new helium extraction paths, such as LNG-BOG gas helium extraction to break the single natural gas helium supply pattern.
At the same time, the industry is reducing helium consumption at the source and improving the utilization efficiency of existing resources by optimizing the insulation structure of equipment, reducing the helium storage capacity of the system and upgrading the sealing process.
In the next ten years, ITER and other large-scale national experimental devices will continue to use low-temperature superconductivity + liquid helium cooling as the core technology program, and continue to accumulate steady-state fusion operation data to verify the feasibility of fusion engineering.
The technology system of existing fusion devices in operation and under construction has been finalized, and it is impossible to complete the replacement of technology iteration quickly. Liquid helium is still the core medium to support the global fusion basic research and technology iteration.
After 2030, with the large-scale mass production of high-temperature superconducting materials and the maturity of engineering application technology, the new generation of commercial fusion power plants will be fully switched to high-temperature superconducting technology.
By then, liquid nitrogen and mechanical refrigeration will gradually replace the liquid helium refrigeration program, and the fusion industry’s dependence on scarce liquid helium resources will be greatly reduced, completely breaking the bottleneck of industrialization brought about by the scarcity of resources.
In the future, only some precision auxiliary systems will retain a small amount of helium applications, no longer need large-scale liquid helium reserves, fusion energy economy, sustainability will realize a qualitative leap.
Liquid helium has become the core of current magnetic confinement fusion technology due to its irreplaceable physical advantages, ranging from its unique superfluid flux characteristics to its extreme low-temperature cooling capability.
Focusing on fusion engineering, superconductivity research and high-end cryogenic experiments, Fullcryo provides liquid helium/supercritical helium cryogenic refrigeration units, helium recovery and purification systems, cryogenic distribution valves and integrated cryogenic turnkey solutions.
For customized high performance liquid helium cryogenic systems and helium recovery solutions for nuclear fusion projects, please contact Fullcryo’s technical team for one-on-one cryogenic engineering solutions.
