In the wave of global clean energy transformation, nuclear fusion is the ultimate clean energy with great potential for landing. It is the core solution to solve the global energy and environmental crisis.Fusion engineering has very contrasting technical characteristics: the inside of the device is an ultra-high temperature of hundreds of millions of degrees, but the core superconducting magnet requires an extremely low temperature environment close to absolute zero.This kind of extreme temperature difference working condition is the key technical difficulty in the landing of nuclear fusion engineering.The current mainstream large-scale fusion devices use 4.5K (-268.65℃) as the standard operating temperature of superconducting magnets.This article will deeply disassemble and analyze the core technology system of nuclear fusion cryogenic engineering in an all-round way.
Superconducting technology is the core support of magnetic confinement nuclear fusion, and cryogenic refrigeration is a necessary condition for maintaining the superconducting state.When the temperature of the superconducting material is lower than the critical temperature (Tc), the resistance will completely disappear, and the current will be transmitted lossless. This is the phenomenon of superconductivity.
The superconducting coil can carry tens of thousands of amperes of high current and produce a high-strength and high-stability magnetic field.Its magnetic field performance far exceeds that of traditional resistive electromagnets, which is the key to stabilizing and restraining high-temperature plasma.Once the low-temperature cooling conditions are lost, the material will instantly exit the superconducting state, which can easily cause equipment failures and safety accidents.At present, large-scale fusion devices use low-temperature superconducting materials (LTS). The mainstream is divided into two types: niobium-titanium alloy (NbTi) and niobium-tin alloy (NbSnSN). The critical temperature of the two is extremely low, and they must rely on a cryogenic environment to work.
Cryogenic refrigeration is not only an auxiliary process, but also the core guarantee for the stable operation of fusion magnets. The core role is reflected in three points.The continuous ultra-low temperature environment can stably maintain the zero resistance characteristics of the material and completely eliminate the current transmission loss.
Deep low temperature can effectively improve the critical current density and ultimate magnetic field strength of superconducting materials, and improve the working performance of magnets as a whole.At the same time, the cooling system can take away the waste heat of the equipment in real time, and avoid the superconducting failure caused by excessive temperature from the root cause.
The 4.5K low temperature operating condition first provides sufficient safe temperature margin for the superconducting magnet.Whether it is an NbTi material with a critical temperature of 9–10K or an NBSnSN material with a critical temperature of 18K, the operating temperature of 4.5K is much lower than its critical threshold.
Secondly, the 4.5K temperature perfectly matches the physical properties of the supercritical helium cooling medium.Fusion superconducting magnets generally adopt the structure of an in-tube cable conductor (CICC), and the internal channel of the coil is fine and compact, which requires extremely high fluidity and heat transfer efficiency of the cooling medium.At this time, helium will be converted into supercritical helium.The helium medium in this state is a single-phase dense fluid with no boiling phase transition, high density and moderate viscosity. It can evenly penetrate the tiny pores of the CICC coil to achieve efficient global heat transfer and is perfectly adapted to the forced convection cooling needs of large fusion magnets.
From the perspective of engineering landing, 4.5K is the most mature and cost-effective working condition for large-scale cryogenic refrigeration technology.Compared with the lower 1.9K superfluid helium temperature zone, the 4.5K refrigeration system does not require complex low-pressure superfluid equipment, and can be applied on a large scale by relying on mature high-temperature circulating refrigeration technology.
At the same time, compared with the 20K temperature zone of high-temperature superconductivity, the 4.5K low-temperature superconducting technology has undergone decades of iteration, with a high degree of equipment standardization and strong operating stability. It is fully adapted to large-scale 10,000-ton fusion magnet systems such as ITER, which can balance engineering complexity, construction cost and operating performance.
The low temperature environment of the 4.5K superconducting magnet of the fusion reactor is accurately guaranteed by a complete closed-loop helium refrigeration system.The whole system has a precise structure and rigorous logic.
The core of the helium refrigeration system adopts an improved helium circulation process to gradually cool the room temperature helium to a supercritical state of 4.5K. First, room temperature helium is pressurized to a high pressure state of 10–20bar through an oil-free screw or centrifugal compressor, laying the foundation for subsequent cooling and heat transfer.
The high-pressure helium will first undergo liquid nitrogen-assisted pre-cooling, and the temperature will quickly drop to about 80K, which will initially reduce the temperature of the medium and reduce the energy consumption of the subsequent cryogenic process.The pre-cooled high-pressure helium will enter the multi-stage countercurrent heat exchanger, and cooperate with the turbine expander for isentropic expansion and cooling, which releases heat energy through work and greatly reduces the temperature of the medium.
Finally, after Joule-Thomson throttling expansion, helium is accurately converted into a supercritical state of 4.5K and 4–6bar to meet the standard operating conditions for magnet cooling.The entire refrigeration process is designed in a closed loop, with no dielectric loss, and can operate stably for a long time.
After the supercritical helium is generated, it is evenly transported to the CICC internal channel of each superconducting coil through the special cryogenic pipeline, valve box and cryogenic circulating pump of the cryogenic conveying system.The low-temperature helium medium continues to flow inside the coil, taking away all kinds of heat such as nuclear radiation heating, current loss heating, and environmental heat leakage in real time.
After the heat transfer and heating up, the helium medium will be returned to the refrigeration system, the cooling volume will be recovered through the heat exchanger, the temperature will be re-warmed to room temperature, and the compression will enter the refrigeration cycle again to form an uninterrupted closed-loop cooling system.In order to reduce the cooling pressure of the 4.5K core system, the outer layer of the equipment is equipped with an 80K low temperature heat shielding layer, which can intercept most of the heat leakage in the room temperature environment.
In view of the dynamic heat load characteristics of the pulsed operation of the fusion reactor, the system is equipped with a heat buffer tank, a flow regulating valve, a cryogenic compressor and other voltage regulating equipment.At the same time, the cooling process of large magnets adopts a gradient temperature control mode, which takes several days to several weeks, which can effectively avoid the thermal stress deformation caused by rapid cooling and protect the safety of the superconducting coil structure.In the event of a superconducting failure, the system can quickly relieve pressure and discharge helium to ensure the overall safety of the equipment.
At present, the cryogenic technology of superconducting magnets is mainly divided into three temperature zones: 2K superfluid helium, 4.5K supercritical helium, and 20K high temperature superconducting. The media characteristics, equipment performance, cost and application scenarios of the three types of technologies are significantly different, and they are adapted to the needs of superconducting equipment in different fields.
The 2K (1.8–1.9K) superfluid helium temperature zone is the extremely low temperature operating condition, which is lower than the 2.17K Λ phase transition point of helium, which can form superfluid helium (He-II).Superfluid helium has the special physical properties of ultra-high thermal conductivity and very low viscosity. Its heat transfer capacity far exceeds that of ordinary helium media. It can penetrate the tiny pores of the coil and greatly improve the thermal stability of the equipment.
This temperature zone is mainly used in large particle accelerators (such as the LHC Large Hadron Collider) and a few high-end Tokamak devices, which can support superconducting magnets to achieve higher magnetic field strength.However, this technology requires negative pressure superfluid refrigeration equipment, the system complexity is extremely high, energy consumption and cost are expensive, and the performance is excessive and the cost performance is extremely low for mainstream fusion devices, and it is not popular.
The 4.5K supercritical helium temperature zone is the mainstream standard solution in the field of fusion.It is suitable for two types of mature low-temperature superconducting materials, NbTi and NbnSn, and relies on the efficient heat transfer characteristics of supercritical helium to meet the forced cooling needs of large fusion magnets.
After decades of engineering iteration, the technology has mature technology and stable operation. The refrigeration cost and system complexity are in a controllable range. It can perfectly balance the performance requirements of the fusion device with the difficulty of engineering landing. It is the core low temperature solution for large fusion reactors such as ITER.The only shortcoming is that Kano has low efficiency and high energy consumption in long-term operation.
20K is an emerging high temperature superconducting (HTS) temperature zone, mainly suitable for REBCO rare earth barium copper oxygen high temperature superconducting strip.The critical temperature of this type of material far exceeds 77K, and it can still maintain ultra-high current density and high magnetic field performance under 20K operating conditions, and can support super-strong magnetic field output above 20T.
The efficiency of refrigeration Kano in the 20K temperature zone is much higher than that of 4.5K, the refrigeration energy consumption and equipment operation and maintenance costs are greatly reduced, and there is no need to rely on scarce liquid helium resources, and the system structure is more concise.At present, the technology is still in the iterative stage. The high-temperature superconducting strip has a high cost and the high-current cable winding process is immature. It is mainly used in the prototype of a new generation of compact fusion devices, which is the core development direction of fusion low-temperature technology in the future.
In the fusion cryogenic engineering system, helium refrigerators and helium liquefiers are two types of core equipment. The core components and thermodynamic cycle principles of the two are similar, but the functional positioning, operating mode and application scenarios are completely different. It is the core equipment that must be clearly distinguished in engineering design.
The helium refrigeratortion equipment is the core special equipment for the cooling of fusion superconducting magnets, and adopts a closed-loop cycle mode.Its core function is not to produce liquid helium, but to continuously output a stable low-temperature cooling capacity to ensure the constant temperature operation of the superconducting coil.
The helium medium in the entire system is closed throughout the cycle, without leakage and loss, continuously absorbs all kinds of heat loads generated by the operation of the magnet, and maintains a constant temperature of 4.5K.Equipment performance is based on “cooling power at a specific temperature (kW@4.5K )”As the core index, it is adapted to the long-term continuous operation requirements of the fusion device.
A large-scale low-temperature helium liquefier device uses a helium turbine reverse Breton refrigeration cycle to obtain the corresponding refrigeration capacity. First, high-pressure helium is obtained through a helium compressor. Part of the high-pressure helium is expanded and cooled in a turbine expander, and the rest of the high-pressure helium is cooled step by step in a multi-stage heat exchanger. Finally, liquid nitrogen is obtained after throttling through the coke-soup throttle valve, and the liquid helium obtained is directly output as a product. Such refrigeration equipment is called a helium liquefier.
The helium liquefier system adopts an open cycle mode. The core function is to continuously liquefy gaseous helium at room temperature to produce 4.2K liquid helium products. The liquid helium produced will be directly exported to the system for equipment filling, storage tank storage or experimental replenishment.
Due to the continuous output of the medium, the system needs to continuously replenish room temperature helium, which consumes higher cooling energy and lower efficiency. The cryogenic equipment is mainly used in laboratory liquid helium replenishment, MRI equipment filling and other scenarios, and is not suitable for long-term steady-state cooling of fusion magnets.
In large-scale fusion projects, the two types of equipment can be integrated and designed to realize steady-state refrigeration and liquid helium reserve functions at the same time according to project requirements, and adapt to complex engineering conditions.
Compared with conventional cryogenic equipment, the 4.5K cryogenic refrigeration system of the fusion reactor needs to be adapted to extremely complex nuclear operating conditions, large equipment scale and dynamic heat load, and faces multiple engineering and technical problems. It is also a key bottleneck restricting the commercialization of fusion energy.
The fusion superconducting magnet system can weigh up to 10,000 tons and requires low temperature cooling with a large coverage range.According to Kano’s law, the energy consumption of ultra-low temperature refrigeration itself is extremely high. The ITER device requires 75kW of cooling capacity in only the 4.5K temperature zone. The input energy consumption of the whole machine is huge. How to optimize the cooling efficiency and reduce the operating energy consumption is the core problem.
The second is the stability control of pulsed dynamic heat load.The fusion reactor adopts pulsed plasma operation mode, and the heat load of the equipment will fluctuate violently with the pulse period, and the peak instantaneous heat loss is extremely high.The helium refrigerator is more suitable for steady-state operating conditions, and severe heat load fluctuations can easily lead to system temperature and pressure imbalances.It is necessary to rely on thermal buffer devices and intelligent flow control systems to accurately suppress dynamic thermal fluctuations and ensure global temperature stability.
The nuclear radiation environment has strict requirements for the durability of cryogenic systems.The neutron radiation generated by fusion operation will continue to act on superconducting coils, insulating materials and low-temperature structural components, causing material performance attenuation and aging damage.The system needs to take into account the functions of radiation shielding, low temperature sealing and continuous heat dissipation to ensure the long-term service stability of the equipment.
Finally, the long-term reliability of equipment and the difficulty of failure protection are higher.The fusion device needs to operate continuously for several years, and the helium reserves of the cryogenic system can reach tens of tons.In the event of superconducting failure, massive amounts of energy storage will be released instantly, requiring high-precision failure detection, rapid pressure relief and helium recovery systems to avoid the risk of equipment damage.
In recent years, Fullcryo has successfully implemented its comprehensive cryogenic refrigeration solutions to meet the long-term, stable cooling requirements of 4.5K superconducting magnets in large-scale tokamak fusion devices. In November 2020, a 400W@4.5K refrigerator supplied by Fullcryo to the China Institute of Atomic Energy (under the China National Nuclear Corporation) passed acceptance testing and was deployed for use in linear superconducting accelerator experiments. In June 2022, a 500W@4.5K helium refrigerator passed acceptance testing; it will provide comprehensive support for the cryogenic system of the HL-2M (China HL-2M) tokamak device at the Southwestern Institute of Physics.
Standardized helium refrigeration systems in the 60–1400 W @ 4.5 K range are utilized for superconducting accelerators, nuclear fusion devices, and superconducting high-field magnets, while custom-engineered systems exceeding 1500 W @ 4.5 K serve superconducting power equipment and superconducting maglev applications.
Standardized helium refrigerators and liquefiers in the 500–3000 W @ 20 K range, along with custom-engineered liquefiers exceeding 3000 W @ 20 K, are employed in superconducting power equipment, superconducting maglev systems, aerospace liquid hydrogen fuel reactors, cold neutron sources (CNS), spallation neutron sources (SNS), and large-scale cryogenic pumps for space environment simulation chambers.
Although the lower 2K superfluid helium temperature zone has stronger heat transfer performance and higher thermal stability, the complexity, construction cost and operating energy consumption of refrigeration equipment will be greatly increased. For the heat load and magnetic field requirements of mainstream fusion devices, 4.5K operating conditions have been able to provide sufficient safety margin and heat transfer performance, which is more cost-effective and more engineering landing, which is the optimal balance between performance and cost.
4.5K is a supercritical single-phase helium fluid, relying on forced convection to achieve efficient heat transfer, mature technology, controllable cost, suitable for large fusion magnets; 2K is superfluid helium, with extreme thermal conductivity characteristics and stronger heat transfer performance, but the equipment process is complex and the energy consumption is extremely high. It is only used in special scenarios such as high-end accelerators and is not suitable for modular fusion projects.
It cannot be completely replaced at this stage.20K high temperature superconducting technology has lower energy consumption and simpler cooling, which is the development direction of compact fusion devices in the future.However, at present, 4.5K low temperature superconducting technology has mature technology and stronger stability, which is still the current mainstream solution.
can.After decades of engineering iteration, the closed-loop helium refrigeration system adapted to fusion operating conditions has mature technology, which can achieve uninterrupted steady-state operation for many years and meet the needs of continuous experimental and commercial operation of fusion devices.
The core risks include local temperature rise caused by dynamic thermal load imbalance, superconducting coil failure, helium medium leakage, blockage of low temperature channels, and material aging caused by nuclear radiation.
Nuclear fusion engineering is the ultimate temperature difference technology, constrained by hundreds of millions of degrees of high-temperature plasma, and completely dependent on the stable work of 4.5K ultra-low temperature superconducting magnets.Judging from the trend of technological iteration, 4.5K low-temperature superconducting technology will still dominate large-scale Tokamak fusion projects for a long time, and continue to support global fusion energy experiments and engineering verification.In the future, with the continuous upgrading of high-temperature superconducting materials, high-efficiency refrigeration technology and intelligent control technology, fusion cryogenic systems will iterate in the direction of lower energy consumption, higher stability, and lighter weight. Contact Fullcryo, we supply helium cryogenic technology and equipment to help mankind realize the commercial application of controllable nuclear fusion energy at an early date.
