Nuclear fusion is recognized as one of the most promising clean energy sources in the future, which can provide nearly unlimited, low-carbon and stable power supply for mankind and completely change the existing energy structure.
Magnetic confinement fusion devices operate under an extraordinary temperature contrast. The plasma in the fusion core can reach temperatures of approximately 100–200 million °C, while the superconducting magnet systems surrounding the plasma must operate at cryogenic temperatures close to absolute zero.
This is where cryogenic helium refrigeration for fusion applications becomes essential. Superconducting magnets need to remain below their critical temperature to maintain superconductivity and generate the strong magnetic fields required for plasma confinement. Depending on the magnet technology, operating temperatures may range from approximately 20 K for some high-temperature superconducting systems to around 4.5 K for conventional low-temperature superconducting magnets.
For large fusion facilities using superconducting magnets, a reliable helium refrigeration system must continuously remove static and dynamic heat loads while maintaining stable temperature, pressure and helium flow.
Cryogenic technology also supports other important fusion subsystems. Cryogenic vacuum pumps can provide high pumping speeds by condensing or adsorbing gases on extremely cold surfaces, helping maintain the high-vacuum environment required inside the fusion vessel. In addition, cryogenic engineering can support thermal shielding and other systems that help reduce heat transfer to the superconducting magnet system.
Traditional low-temperature superconductors such as NbTi and Nb3Sn have been widely used in large superconducting magnet systems and generally require operation at temperatures around 4 K. At the same time, the development of high-temperature superconductors such as REBCO is creating new opportunities for higher-temperature cryogenic operation in next-generation fusion magnets.
Fusion superconducting magnet systems are among the largest cryogenic loads in modern scientific facilities, requiring high cooling capacity, stable temperature control and reliable helium circulation, which requires extremely high stability of temperature and pressure. Small temperature fluctuations may cause magnet quench and cause the whole device to stop. The cryogenic helium refrigeration system relies on the improved Claude cycle and Brayton cycle to achieve precise control of helium temperature and pressure.
Through continuous forced circulation cooling, the system provides stable low temperature conditions for core equipment such as magnet conduits, heat insulation shielding layers, and cryogenic pumps. At the same time, it relies on the buffer tank body, low-temperature compressor and distributed pipeline system to offset the temperature and pressure fluctuations during operation and fully avoid the risk of magnet quench.
The system can be fully adapted to the full life cycle conditions of the fusion device, covering all operation modes such as cooling, steady-state plasma operation, pulse condition switching, heating shutdown, etc., laying the foundation for the long-term reliable operation of the device.
A key requirement of cryogenic refrigeration for superconducting magnets is maintaining sufficient temperature margin. If the magnet temperature rises above its operating limit, part or all of the superconducting conductor may transition into the normal resistive state, a phenomenon known as a quench. Therefore, cryogenic stability, quench detection and protection systems are important parts of fusion magnet operation.
Stable strong magnetic field is the core premise of confining high temperature plasma and realizing continuous fusion reaction. Nuclear fusion devices require ultra-high magnetic field strength of 5T to 20T, and conventional conductors are completely unable to meet the energy consumption and heat dissipation requirements of this working condition.
The cryogenic helium refrigeration system stably maintains the superconducting magnet in a zero-resistance state, allowing a large current to pass through the magnet coil without loss, and continuously generating a high-intensity, uniform and stable confinement magnetic field. During the operation of the device, the system can take away the heat generated by various losses in real time, including AC loss, eddy current heat, neutron nuclear heat and other dynamic heat sources.
Through continuous temperature control, the superconducting magnet is always kept within the safety threshold of critical temperature, critical magnetic field and critical current, which effectively supports the stable operation of compact and high-performance fusion magnets.
From the perspective of energy consumption structure, if the conventional resistance magnet is used in the nuclear fusion device, the electric energy consumed to maintain the confinement magnetic field will occupy the vast majority of the fusion power generation output, and the net energy gain cannot be achieved at all.
The zero resistance characteristic of the superconducting magnet completely solves the energy consumption problem of magnetic field maintenance and greatly reduces the basic operation energy consumption of the device.
Large fusion cryogenic plants can themselves represent a significant auxiliary power load, particularly at very low operating temperatures. Therefore, improving refrigeration efficiency is an important engineering consideration when designing a large-scale fusion facility.
The modern fusion helium refrigeration system adopts a multi-temperature zone hierarchical refrigeration design, covering 4.5K magnet refrigeration, 50K medium temperature zone refrigeration, 80K heat insulation shielding layer refrigeration, and is matched with waste heat recovery technology to further optimize thermodynamic efficiency and enable the fusion device to achieve positive net energy balance.
The thermal load of the nuclear fusion device is not a constant state, but there are static and dynamic double loads, and the working conditions fluctuate greatly. The static load mainly comes from fixed heat sources such as heat radiation, structural support heat conduction, and residual gas heat conduction.
Dynamic pulse heat load is the core difficulty of system control, plasma pulse operation, magnetic field gradient switching, high-energy neutron radiation heat production and other conditions will produce severe heat fluctuations in a short time, which can easily cause magnet temperature imbalance.
The helium refrigeration system is equipped with large-capacity low-temperature buffer storage liquid, variable-speed low-temperature circulation unit, auxiliary low-temperature box and other structures, which can quickly absorb instantaneous high heat load and smooth the fluctuation of working conditions. Under the premise of ensuring the steady-state operation of the main refrigeration unit, the temperature of the magnet is precisely controlled to be stable, and the fusion pulse operation characteristics are perfectly adapted.
Cryogenic helium refrigeration also plays a role in the fusion fuel cycle. The system prepares solid deuterium and tritium fuel particles through low-temperature refrigeration, realizes deep plasma fuel injection, effectively improves fusion reaction efficiency, and alleviates operational failures such as plasma rupture.
The ultra-high vacuum environment of the device is completely dependent on the helium-cooled cryogenic pump to maintain the residual gas through low-temperature condensation to prevent impurities from interfering with the fusion reaction. At the same time, low-temperature distillation technology relies on helium refrigeration low-temperature conditions, can achieve the separation and recovery of hydrogen isotopes in the tail gas, efficient recovery of unburned fusion fuel, reduce fuel consumption, control device operating costs.
Among all low-temperature working materials, helium is the only refrigeration medium suitable for the extreme conditions of nuclear fusion, which has irreplaceable comprehensive advantages. The standard boiling point of helium is only 4.2K, which is the lowest boiling point of the working medium in nature, and can realize the cryogenic refrigeration close to absolute zero.
Modern helium refrigeration systems commonly use combinations of the Claude cycle, Brayton-cycle expansion and Joule-Thomson throttling to achieve cryogenic temperatures. Depending on the required operating temperature and cooling capacity, the system can be configured with helium compressors, turbine expanders, plate-fin heat exchangers and a vacuum-insulated cryogenic cold box.
The whole refrigeration cycle is divided into 5 core processes, which are logically closed-loop and efficient. The first is the high-pressure compression stage. The normal temperature helium is pressurized to 10 to 20bar by a multi-stage compressor, and the pressure and temperature rise simultaneously.
Subsequently, it enters the countercurrent heat exchange stage, and the high-temperature and high-pressure helium completes heat exchange with the low-temperature and low-pressure helium in the high-efficiency plate heat exchanger, so as to realize pre-cooling and greatly reduce the subsequent refrigeration energy consumption.
The pre-cooled helium is divided into two ways to complete the deep refrigeration, the mainstream helium through the high-speed turbine expander to achieve isentropic expansion, the external output work and a significant cooling, is the main source of cooling system. After the remaining helium continues to cool down, it is cooled by the Joule-Thomson throttle valve to achieve isenthalpy throttling refrigeration, and part of the helium is liquefied to form cryogenic liquid helium/supercritical helium working medium.
Cryogenic helium working fluid flows through loads such as magnets, heat insulation shielding layer and cryogenic pump, absorbs the heat of the equipment, heats up and vaporizes, returns through the heat exchanger again, and re-enters the compressor to complete the closed-loop cycle. The system can flexibly switch between pure refrigeration mode and liquefaction mode to adapt to the diverse load requirements of fusion devices.
A complete set of fusion-level helium refrigeration system is composed of a compressor unit, a cryogenic cold box, a control system and other core modules. Each module works together to ensure the stable operation of the system.
Take the Fullcryo 20K helium refrigerator as an example: the 20K helium refrigerator uses helium turbine reverse Brayton refrigeration cycle to obtain 20K refrigeration capacity. in the 20K helium refrigerator, the obtained cold helium below 20K returns to the cold box after cooling the user equipment, and recycles the cold energy after recycling.
The main equipment of 20K helium refrigerator includes: compressor, oil filter, 20K cold box, control system, helium buffer tank, etc. The 20K cold box is a vacuum insulation cold box, which contains multi-stage heat exchanger, turbine expander, internal adsorber, cryogenic valve, measuring element and other components.
The world’s mainstream large-scale magnetic confinement fusion devices, including ITER, EAST, JT-60SA and W7-X, all adopt helium refrigeration systems of 4.2K to 4.5K, which is currently the most mature technology and the most perfect refrigeration scheme for industrial landing.
The helium in this temperature zone is supercritical normal helium (He I), without gas-liquid stratification, and can be forced to flow through the conduit, which is perfectly adapted to the conductor cooling structure in the conduit of the fusion magnet. The whole system is based on the classic Claude cycle design, the technical system is mature, the operation stability is high, and the failure rate is low.
From the point of view of working condition adaptability, the 4.5K refrigeration system can effectively offset the static heat loss and pulse dynamic heat load of the fusion device, and fully meet the operating requirements of NbTi and NbLSn low-temperature superconducting magnets. At the same time, the thermodynamic energy consumption of the system is lower, no complex negative pressure cryogenic equipment is needed, and the overall construction and operation and maintenance costs are controllable.
The operating temperature of the 2K refrigeration system is lower than the Lambda phase transition point of helium (2.17K), which can generate super fluid helium (II He). Superfluid helium has the ultimate thermal conductivity, thermal conductivity efficiency is much higher than normal helium, can quickly export local heat with a very small temperature difference, thermal stability is extremely strong.
This technology can effectively improve the critical current and critical magnetic field of superconducting magnets, and help the equipment to achieve higher magnetic field strength and power density. It has been used in the high-field magnet test of Tore Supra(WEST) device.
However, the short board of 2K superfluid helium refrigeration is also prominent. The system needs to be equipped with complex equipment such as negative pressure vacuum pump and low temperature compressor, which greatly increases the structural complexity. At the same time, the thermodynamic energy efficiency is extremely low, the power consumption under the same cooling capacity is 3 to 4 times that of the 4.5K system, and the construction and operation and maintenance costs remain high.
The required operating temperature is one of the most important parameters when selecting a cryogenic refrigeration system for superconducting magnets.For large fusion devices using NbTi and NbLSn low-temperature superconducting magnets, 4.2K to 4.5K standard helium refrigeration system is preferred, taking into account stability and economy.
For the new fusion device of high temperature superconducting magnets such as REBCO, the medium temperature refrigeration scheme of 10K to 20K can be selected, which greatly reduces the energy consumption and complexity of the refrigeration system. Only high-field magnet scientific research test scenarios, can be targeted to configure 2K superfluid helium refrigeration system.
Modern fusion devices adopt multi-temperature zone integrated refrigeration design, covering 4.5K magnet refrigeration, 50K current lead refrigeration and 80K heat insulation shielding layer refrigeration at the same time, realizing low temperature protection at the whole equipment level.
When selecting a helium refrigerator for fusion applications, engineers should first determine the total cooling load at the required temperature.
System selection needs to complete the global thermal load fine measurement, covering all low-temperature load equipment. The static heat conduction and radiation heat load of superconducting magnets, as well as the dynamic heat load generated by plasma pulse operation and neutron irradiation are mainly calculated.
At the same time, the cooling demand of auxiliary equipment such as insulation shield, cryogenic pump, current lead, diagnostic equipment, fuel injection system, etc. shall be included, and the average load and peak pulse load shall be calculated comprehensively. The general selection criteria in the industry are: 1.5 times the redundant capacity is reserved on the basis of the average steady-state load, and the cooling start-up and extreme pulse conditions are adapted to the equipment.
Large-scale fusion helium refrigeration systems need to give priority to the mature Claude cycle architecture, with high-efficiency heat exchangers and energy-saving expansion units, to improve Carnot efficiency and reduce long-term operation energy consumption. At the same time, the equipment layout is optimized according to the site conditions, and the floor area and noise pollution of the unit are controlled.
Priority is given to integrated units with waste heat recovery and multi-stage load adaptation capabilities to reduce helium loss through process optimization, reduce later operation and maintenance costs, and adapt to the long-term operation needs of commercial poly-substation.
Fullcryo provides customized cryogenic engineering solutions covering the 2 K–20 K temperature range and other specialized cryogenic applications. Our capabilities include helium refrigeration, helium liquefaction, cryogenic cold boxes and integrated cryogenic systems for superconducting magnets and scientific research facilities.
The 4.5K helium refrigerator independently developed by Fullcryo is widely used in major scientific infrastructure at home and abroad. The first (set) 200W@4.5K helium refrigerator made in China is exported to KSTAR fusion device in South Korea. The 400W@4.5K model is equipped with the linear superconducting accelerator in Beijing, China. The 500W@4.5K model supports the operation of the HL-2M tokamak device. The 1kW and 4kW@4.5K helium refrigerator has landed on a superconducting testing platform in a laboratory in China, accumulating a large number of mature engineering cases in the fields of nuclear fusion, nuclear physics and superconducting technology.
Many conventional low-temperature superconducting magnets operate around 4.2–4.5 K, while some advanced superconducting systems can operate at higher temperatures.
Helium has a very low boiling point and remains suitable for cryogenic refrigeration at temperatures required by superconducting magnet systems.
A 4.5 K system is widely used for large superconducting magnet applications, while 2 K systems use superfluid helium and are generally intended for specialized high-performance or scientific applications.
Cooling capacity is determined by the total static and dynamic heat loads, required operating temperature, helium flow rate, thermal shielding requirements and operating conditions of the superconducting magnet and associated cryogenic systems.
Yes. Large fusion facilities typically require customized refrigeration capacity, temperature levels, helium flow rates, pressure conditions and interfaces according to the specific superconducting magnet and cryogenic load requirements.
The low-temperature helium refrigeration device determines whether the fusion device can operate stably, efficiently and for a long time. It perfectly balances the extreme engineering contradiction between the high temperature of hundreds of millions of degrees Celsius and the low temperature of near absolute zero, and provides all-round guarantee for the operation of superconducting magnets, plasma confinement, fuel circulation and the construction of ultra-high vacuum environment. Contact us for more helium refrigeration equipment parameters.
