CN-122015314-A - Supercritical helium cryogenic system and method for nuclear fusion strong field magnet

Abstract

The invention discloses a supercritical helium low-temperature system and a method for a nuclear fusion strong field magnet, and relates to the technical field of refrigeration systems and temperature control. The distribution valve box is arranged to support simultaneous testing of multiple samples, different flow resistance requirements are adapted through serial connection of the double low-temperature fans, and the built-in purifier ensures cleaning of a loop. The independent circulation loop for refrigeration and magnet testing is realized, the working condition of the high-pressure operation of the magnet circulation loop which cannot be achieved by the helium compressor is solved, and the working conditions of high-pressure operation, low circulation flow resistance, uniform temperature and small thermal stress of the strong-field magnet are realized. The temperature supply with adjustable temperature of 4.5K-30K can be realized according to different working temperatures of different magnets to be tested, the heat load of the magnets to be tested can be matched, the 0-100% elasticity adjustable cold quantity in the range of 0W-2000W can be realized, and the testing conditions that the temperature difference between an inlet and an outlet of the magnets to be tested is less than 1K and the working pressure is more than or equal to 20bara can be realized.

Inventors

• DENG BICAI
• SHENG JIE
• GUO YUNZE
• WEN WEN
• CHEN YANG
• GUO YUQING
• JIN ZHIJIAN

Assignees

• 上海翌曦科技发展有限公司

Dates

Publication Date

20260512

Application Date

20260413

Claims (15)

1. 1. A supercritical helium cryogenic system for a nuclear fusion strong field magnet is characterized by comprising a refrigeration circulation loop and a magnet circulation loop to be tested; The refrigerating circulation loop comprises a helium compressor, a high-pressure helium pipeline, a low-pressure helium pipeline, a loading and unloading pipeline and a bypass pipeline, wherein an outlet of the helium compressor is sequentially communicated with the high-pressure helium pipeline, the low-pressure helium pipeline and an inlet of the helium compressor to form a loop; the refrigeration circulation loop is communicated with the magnet circulation loop to be tested through an intermediate heat exchanger; The magnet circulation loop to be tested comprises an intermediate heat exchanger hot side outlet communicated with a distribution valve box inlet, an inlet main valve is arranged in the distribution valve box and communicated with an inlet first branch pipeline, a first regulating valve is arranged on the inlet first branch pipeline and communicated with a magnet inlet to be tested, a magnet outlet to be tested is communicated with a return opening first branch pipeline, a return opening first regulating valve is arranged on the return opening first branch pipeline and communicated with a return opening main valve arranged in the distribution valve box, the return opening main valve is communicated with a distribution valve box outlet, the distribution valve box outlet first branch pipeline is communicated with a first low-temperature fan inlet, the first low-temperature fan outlet is communicated with a second low-temperature fan inlet, and the second low-temperature fan outlet is communicated with the intermediate heat exchanger hot side inlet; The magnet circulation loop to be tested is provided with a distribution valve box, an inlet main valve and a return port main valve are arranged in the distribution valve box, the inlet main valve is communicated with a plurality of branches, and each branch is provided with a regulating valve; The magnet circulation loop to be tested further comprises an electronic pressure regulating valve connected in parallel with two sides of a second shutoff valve of the sample loop, and the electronic pressure regulating valve comprises a first fan bypass valve arranged in front of an inlet of the first low-temperature fan and a fourth fan bypass valve arranged in front of an inlet of the second low-temperature fan, and a third fan bypass valve connected in parallel with the front of the inlet of the first fan bypass valve and an outlet of the first low-temperature fan, and the electronic pressure regulating valve comprises a second fan bypass valve connected in parallel with the inlet of the fourth fan bypass valve and the outlet of the second low-temperature fan.
2. 2. A supercritical helium cryogenic system for a nuclear fusion strong field magnet according to claim 1, characterized in that the refrigeration cycle circuit comprises in particular: the water inlet and outlet of the water-oil heat exchanger in the helium compressor are communicated with the inlet and outlet of the water chiller; The high-pressure helium pipeline comprises a helium compressor outlet, an oil filter, a cold trap, a first heat exchanger hot side inlet and outlet, a second heat exchanger hot side inlet and outlet, a third heat exchanger hot side inlet and outlet and a turbine expander inlet; the low-pressure helium pipeline comprises an electric heater inlet and outlet, an intermediate heat exchanger cold side inlet and outlet, a third heat exchanger cold side inlet and outlet, a second heat exchanger cold side inlet and outlet, a first heat exchanger cold side inlet and outlet and a helium compressor inlet, wherein the helium compressor outlet is communicated with an oil filter inlet, the oil filter outlet is communicated with a cold trap inlet, the cold trap outlet is communicated with a first heat exchanger hot side inlet, the first heat exchanger hot side outlet is communicated with a second heat exchanger hot side inlet, the second heat exchanger hot side outlet is communicated with a third heat exchanger hot side inlet, the third heat exchanger hot side outlet is communicated with a turbine expander inlet, the turbine expander outlet is communicated with an electric heater inlet, the electric heater outlet is communicated with the intermediate heat exchanger cold side inlet, the intermediate heat exchanger cold side outlet is communicated with the third heat exchanger cold side inlet, the third heat exchanger cold side outlet is communicated with the second heat exchanger cold side inlet, the second heat exchanger cold side outlet is communicated with the first heat exchanger cold side inlet, and the first heat exchanger cold side outlet is communicated with the helium compressor inlet; the air conditioner also comprises a throttle expansion valve which is connected in parallel with the outlet of the turbine expander and the outlet of the electric heater.
3. 3. The supercritical helium cryogenic system for a nuclear fusion high field magnet according to claim 2, further comprising a first air compressor, a first electromagnetic directional valve, a second air compressor and a second electromagnetic directional valve, wherein the first air compressor is communicated with the first electromagnetic directional valve, a first outlet of the first electromagnetic directional valve is communicated with an unloading valve pneumatic actuator in an unloading and loading pipeline, a second outlet of the first electromagnetic directional valve is communicated with a loading valve pneumatic actuator in the unloading and loading pipeline, and a third outlet of the first electromagnetic directional valve is communicated with a bypass valve pneumatic actuator in a bypass pipeline; The second air compressor is communicated with the second electromagnetic directional valve, the first outlet of the second electromagnetic directional valve is communicated with the pneumatic actuating mechanism of the inlet main valve, the inlet main valve is communicated with the inlet first regulating valve, the second outlet of the second electromagnetic directional valve is communicated with the pneumatic actuating mechanism of the inlet first regulating valve, the third outlet of the second electromagnetic directional valve is communicated with the pneumatic actuating mechanism of the return main valve, the return main valve is communicated with the return first regulating valve, and the fourth outlet of the second electromagnetic directional valve is communicated with the pneumatic actuating mechanism of the return first regulating valve.
4. 4. A supercritical helium cryogenic system for a nuclear fusion strong field magnet according to claim 1, further comprising a purification system, The purification system comprises an internal purifier, a purity analyzer, a first shut-off valve of a purification pipeline and a second shut-off valve of the purification pipeline, wherein the internal purifier is connected in parallel to a high-pressure helium pipeline and a low-pressure helium pipeline, an inlet of the internal purifier is communicated with the high-pressure helium pipeline, an inlet and an outlet of the internal purifier are connected in parallel with the purity analyzer, an inlet of the purity analyzer is communicated with an inlet of the internal purifier, an outlet of the purity analyzer is communicated with an outlet of the internal purifier, a first shut-off valve of the purification pipeline is arranged on a front pipeline at the inlet of the internal purifier, and a second shut-off valve of the purification pipeline is arranged on a front pipeline at the inlet of the purity analyzer.
5. 5. A supercritical helium cryogenic system for a nuclear fusion strong field magnet according to claim 2, further comprising a hot gas bypass line comprising a hot gas bypass main line and a hot gas bypass loop line; The hot gas bypass main pipeline is divided into a hot gas bypass first branch pipeline and a hot gas bypass second branch pipeline, the hot gas bypass first branch pipeline is communicated with the low-pressure helium pipeline, the interface position of the hot gas bypass first branch pipeline and the low-pressure helium pipeline is located at the outlet of the middle heat exchanger, the hot gas bypass second branch pipeline is communicated with the hot side outlet of the middle heat exchanger, one end of the hot gas bypass loop pipeline is communicated with the second branch pipeline of the distribution valve box outlet, the other end of the hot gas bypass loop pipeline is communicated with the low-pressure helium pipeline, and the other end of the hot gas bypass loop pipeline is located at the inlet of the helium compressor.
6. 6. A supercritical helium cryogenic system for a nuclear fusion strong field magnet according to claim 1, further comprising a helium recovery storage system; The helium recovery storage system comprises a booster pump, a pressure reducing pump, a first stop valve, a second stop valve, a first check valve, a second check valve, an external purifier, an air temperature gasifier, a storage tank hand valve and a second helium storage tank, wherein the parallel inlet of the booster pump and the pressure reducing pump is communicated with a second branch pipeline of an outlet of a distribution valve box, the first stop valve is arranged at the inlet of the booster pump, the first check valve is arranged at the outlet of the pressure reducing pump, the second stop valve is arranged at the outlet of the pressure reducing pump, the parallel outlet of the booster pump and the pressure reducing pump are communicated with the first external purifier, the second inlet and the second outlet of the external purifier are communicated with the inlet of the air temperature gasifier, the outlet of the air temperature gasifier is communicated with the inlet of the storage tank hand valve, and the outlet of the storage tank hand valve is communicated with a second helium storage tank interface.
7. 7. The supercritical helium cryogenic system for a nuclear fusion strong field magnet according to claim 1, further comprising a pressure sensor and a temperature sensor; The pressure sensor and the temperature sensor comprise a first pressure sensor and a first temperature sensor which are arranged at an outlet of a helium compressor, a second temperature sensor which is arranged at an outlet of a hot side of a first heat exchanger, a third temperature sensor which is arranged at an outlet of a hot side of a second heat exchanger, a second pressure sensor and a fourth temperature sensor which are arranged at an outlet of a hot side of a third heat exchanger, a third pressure sensor and a fifth temperature sensor which are arranged at an inlet of an cold side of an intermediate heat exchanger, a fourth pressure sensor and a sixth temperature sensor which are arranged at an outlet of an cold side of the intermediate heat exchanger, a seventh temperature sensor which is arranged at an outlet of an cold side of the third heat exchanger, an eighth temperature sensor which is arranged at an outlet of an cold side of the second heat exchanger, a fifth pressure sensor and a ninth temperature sensor which are arranged at an inlet of the helium compressor, a sixth pressure sensor and a tenth pressure sensor which are arranged at an outlet of a hot side of the intermediate heat exchanger, a seventh pressure sensor and an eleventh temperature sensor which are arranged at an inlet of a first branch pipeline inside a distribution valve box, an eighth pressure sensor and a twelfth temperature sensor which are arranged at an inlet of the first branch pipeline of the distribution valve box, a fourth pressure sensor and a thirteenth temperature sensor which are arranged at an outlet of the air compressor, a ninth temperature sensor and a fourth temperature sensor which are arranged at an outlet of the air compressor.
8. 8. A supercritical helium cryogenic method for a nuclear fusion strong field magnet, suitable for use in a supercritical helium cryogenic system for a nuclear fusion strong field magnet according to any one of claims 1-7, characterized in that the refrigeration cycle flow comprises: In a refrigeration circulation loop, a helium compressor sucks air from a low-pressure helium pipeline, the air is pressurized to obtain high-temperature high-pressure gas, cooling water provided by circulation of a cold water machine is used for cooling, impurities in the cooled high-temperature high-pressure helium gas are removed through a cold trap, the helium gas enters a hot side inlet of a first heat exchanger after coming out of the cold trap, the cooled helium gas reflowed by the cold side is cooled, the cooled helium gas reflowed by the hot side inlet of a second heat exchanger is cooled, the helium gas reflowed by the hot side inlet of a third heat exchanger is cooled, the heat-insulating expansion is carried out through a turbine expander, a throttling expansion valve is closed, the helium gas enters an intermediate heat exchanger for heat exchange and temperature rise with helium gas for cooling a magnet to be tested, the cooled helium gas enters a cold side inlet of the third heat exchanger for heat exchange and temperature rise with the hot side helium gas, the cooled helium gas enters a cold side inlet of the first heat exchanger for heat exchange and temperature rise with the high-pressure side helium gas of the second heat exchanger, the helium gas is sucked and pressurized again by the helium gas compressor, and the helium gas enters the next circulation; In the refrigeration cycle loop, an electronic expansion valve is opened, low-temperature low-pressure helium gas from an outlet of a turbine expander directly enters a throttling expansion valve, the throttling expansion valve carries out adiabatic expansion on the helium gas to obtain helium gas with lower temperature, and the helium gas enters a cold side inlet of an intermediate heat exchanger to cool the helium gas of the magnetic circulation loop to be tested, so that a lower-temperature test environment is provided for a magnet to be tested; in a circulation loop of the magnet to be detected, the serially connected low-temperature fans sequentially suck helium gas subjected to heat exchange from the magnet to be detected, pressure is increased, the helium gas enters an intermediate heat exchanger to exchange heat with the low-temperature helium gas of the refrigeration cycle and is cooled, the helium gas enters an inlet of the magnet to be detected through a distribution valve box, after the magnet to be detected is cooled, the helium gas is sucked again by the serially connected low-temperature fans, and the helium gas enters the next cycle; in the cooling process, the maximum temperature difference of the two loops is kept smaller than a preset value, the temperature measured by a sixth temperature sensor arranged at the outlet of the cold side of the intermediate heat exchanger and a tenth temperature sensor arranged at the outlet of the hot side of the intermediate heat exchanger is converted into an electric signal and transmitted to a control system, and the difference between the sixth temperature sensor and the tenth temperature sensor is calculated to control the mass flow of helium gas of the two loops so that the temperature difference of the two loops at the intermediate heat exchanger is smaller than the preset value; Setting a set value for an eleventh temperature sensor at an inlet of the magnet to be detected, setting a set value for a twelfth temperature sensor at an outlet, setting a difference value between the set value and the set value to be a preset value, transmitting an electric signal of the eleventh temperature sensor to a control system when the measured temperature of helium at the outlet is higher than the set value, adjusting the rotation speeds of a helium compressor and a turbine expander, and controlling the temperature of the magnet to be detected.
9. 9. The supercritical helium cryogenic process for a nuclear fusion strong field magnet according to claim 8, wherein the operational control of the cryogenic fans in series comprises: The serial low-temperature fans comprise a first low-temperature fan and a second low-temperature fan, the first low-temperature fan and the second low-temperature fan drive helium in a magnet loop to be tested to flow, when the pressure drop inside the magnet to be tested is large, the serial low-temperature fans sequentially suck helium subjected to heat exchange from the magnet to be tested to boost pressure, the helium enters an intermediate heat exchanger to exchange heat with low-temperature helium of refrigeration circulation to cool, the magnet to be tested enters an inlet of the magnet to be tested through a distribution valve box, the magnet to be tested is cooled and is sucked again by the serial low-temperature fans to carry out the next circulation, particularly, when the pressure drop inside the magnet to be tested is small, the control system is switched to a single-fan operation mode to match the pressure drop inside the magnet to be tested, the low-temperature fans are switched in an optimal operation interval according to set operation time, the PLC control system converts the seventh pressure sensor, the eighth pressure sensor and a differential pressure transducer measuring point into an electric signal according to the inlet and outlet of the magnet to be tested, the electric signal is transmitted to the control system, the starting and stopping state and the rotating speed of the low-temperature fans are reversely adjusted, and the modes of the low-temperature fans and single-machine driving are controlled according to the internal pressure drop of a sample.
10. 10. A supercritical helium cryogenic process for a nuclear fusion strong field magnet according to claim 8, wherein helium recovery is performed by a helium recovery storage system comprising: The high-pressure helium in the second helium storage tank flows to the side of the magnet to be detected, the pressure setting value of the pressure reducing pump is set to be the working pressure in the magnet to be detected through the pressure reducing pump, the impact of the high-pressure helium on the magnet to be detected is avoided, the helium is continuously output in a stable pressure mode, and the second helium storage tank continuously supplements the helium with the low temperature of a magnet loop to be detected until the helium is stabilized within the range of the working pressure required by the magnet to be detected; After the test of the magnet to be tested is finished, the helium gas expands after being heated, at the moment, a helium gas recovery shutoff valve and a storage tank hand valve are kept open, a booster pump and a first shutoff valve are opened, the expanded helium gas enters the booster pump through a helium gas recovery pipeline, an outlet pressure set value of the booster pump is set to be the working pressure of a second helium gas storage tank, impurities of the pressurized helium gas are removed through an external purifier, and the helium gas returns to normal temperature through a normal temperature air temperature gasifier to enter the second helium gas storage tank for storage, so that low-temperature impact on the second helium gas storage tank is avoided; When the pressure in the magnet to be measured is recovered to normal pressure, the hand valve of the storage tank and the helium recovery shutoff valve are closed, the helium compressor is closed after the system is recovered to normal temperature, the inlet main valve and the return main valve which are arranged in the distribution valve box are closed, and the magnet to be measured is removed.
11. 11. The method for supercritical helium cryogenic temperature of a nuclear fusion strong field magnet according to claim 8, further comprising the steps of arranging a return inlet main valve and an inlet and outlet branch valve in a distribution valve box for simultaneously testing a plurality of samples, wherein the return inlet main valve and the inlet and outlet branch valve are in a pneumatic valve mode, a second air compressor provides an air source to drive the valve to adjust the opening degree so as to adjust the cold quantity distributed to each sample, and a cold screen branch pipeline led out from an outlet of a second heat exchanger is connected into an interlayer of the distribution valve box to form a cold screen, so that heat loss caused by heat conduction and heat radiation of low-temperature helium gas in the distribution valve box and ambient normal-temperature air is reduced.
12. 12. The supercritical helium cryogenic method for a nuclear fusion strong field magnet according to claim 8, further comprising performing refrigeration cycle loop high and low pressure control: The first air compressor provides a compressed air source for the unloading valve, the loading valve and the bypass valve to drive an actuating mechanism of the valve and adjust the high pressure and the low pressure of the system, the loading valve, the unloading valve, the bypass valve, the inlet main valve, the inlet first adjusting valve, the return first adjusting valve and the return main valve are all in the form of pneumatic valves, the pneumatic actuating mechanisms are all configured, an electric signal output by the control system is a control instruction, the on-off and the reversing of an air channel are carried out through the first electromagnetic reversing valve and the second electromagnetic reversing valve, the piston of an air cylinder of the pneumatic actuating mechanism is further pushed to move, and the valve rod of the piston linkage valve is used for completing the on-off action or the continuous adjustment of the opening degree; The unloading valve is connected with a high-pressure outlet of the helium compressor and the first helium storage tank, when the high-pressure is higher than the set pressure, the unloading valve is opened, helium in the high-pressure helium pipeline returns to the first helium storage tank, the loading valve is connected with a low-pressure inlet of the helium compressor and the first helium storage tank, when the outlet pressure of the helium compressor is lower than the set pressure, the loading valve is opened, helium in the first helium storage tank enters the low-pressure helium pipeline, the bypass valve is connected with the high-pressure helium pipeline and the low-pressure helium pipeline and is used for adjusting the pressure of the low-pressure helium pipeline, when the low pressure is lower than the set value, the opening of the bypass valve is increased, and when the low pressure is higher than the set value, the opening of the bypass valve is reduced.
13. 13. The supercritical helium cryogenic process for a nuclear fusion strong field magnet according to claim 8, further comprising rapid rewarming via a hot gas bypass line: The first hot gas bypass valve, the second hot gas bypass valve and the third hot gas bypass valve are opened, the electric heater is started, high-temperature and high-pressure helium gas sequentially enters the cold side of the third heat exchanger, the cold side of the second heat exchanger and the cold side of the first heat exchanger through a hot gas bypass first branch pipeline to quickly rewire the helium gas in the refrigeration cycle system, and the other part of helium gas enters the magnet circulation loop to be tested through the hot gas bypass second branch pipeline to heat the helium gas in the magnet circulation loop to be tested and the magnet to be tested.
14. 14. The supercritical helium cryogenic process for a nuclear fusion strong field magnet according to claim 8, further comprising, cleaning by a purification system: Before a magnet to be tested is tested, a helium compressor is started to drive helium, a main high-pressure valve and a main low-pressure valve are started, the helium circularly flows in a refrigeration circulation loop, a built-in purifier is connected in parallel with an inlet and an outlet of the helium compressor, a first shutoff valve of a purification pipeline is started to adsorb non-condensable gas and impurities in the system, and a second shutoff valve of the purification pipeline and a purity analyzer are started to detect the cleanliness of the helium in the system; when the magnet circuit to be tested is cleaned, the main circuit high-pressure valve and the main circuit low-pressure valve are closed, the first hot gas bypass valve is closed, the valve is opened, the second hot gas bypass valve and the third hot gas bypass valve are opened, and at the moment, the helium compressor is communicated with the magnet circuit to be tested to form a circulation circuit.
15. 15. The supercritical helium cryogenic method for a nuclear fusion strong field magnet according to claim 8, wherein the start-up procedure of the supercritical helium cryogenic system comprises: Before an experiment starts, purifying gas in a pipeline, opening a switch of a water chilling unit in the operation process of a compressor, checking whether the water chilling unit works normally or not, ensuring that the flow of cooling water reaches the requirement, opening an inlet and outlet valve of a built-in purifier and a first shutoff valve and a second shutoff valve of a purifying pipeline, starting a purity analyzer, controlling a helium compressor to start at the lowest pressure maintaining rotating speed, driving helium to circulate in a closed mode in a refrigeration circulation loop, adsorbing impurities through the built-in purifier until the purity analyzer detects that the purity of the helium reaches the requirement, closing a purifying pipeline valve, completing the prepurification of the system, temporarily stopping the helium compressor, opening the first air compressor and the second air compressor, ensuring that the pressure of gas required by the pneumatic valve reaches the requirement, continuously working the water, ensuring that the flow of the cooling water reaches the requirement, starting the compressor, starting an inlet and outlet valve of the helium compressor, a main circuit high-pressure valve, a main circuit low-pressure valve, a bypass valve and a loading valve, keeping the preset opening of the refrigeration circulation loop, performing communication of the first helium storage tank, entering the compressor for circulation, automatically controlling the opening the compressor, controlling the outlet pressure of the helium compressor, gradually setting the low-pressure compressor to run at the preset pressure, setting the preset pressure, and stabilizing the operation of the compressor at the preset pressure, and starting the pressure, and setting the pressure and stable operation until the turbine rotation speed is stable; Controlling a first sample loop shutoff valve, a second sample loop shutoff valve, a helium recovery shutoff valve, an inlet main valve, an inlet first regulating valve, a return port first regulating valve and a return port main valve of a magnet loop to be tested, starting a first low-temperature fan and running at a preset rotating speed, keeping a second low-temperature fan closed, starting a decompression pump and the first shutoff valve, communicating a magnet circulation loop to be tested with a helium filling recovery system, enabling high-pressure helium in a second helium storage tank to flow to the magnet circulation loop to be tested, and filling the circulation loop to be tested after stabilizing the pressure by the decompression pump; The method comprises the steps of gradually increasing the internal pressure of a magnet to be tested to a preset value according to a preset speed through opening adjustment of an inlet first adjusting valve and a return opening first adjusting valve arranged at an inlet and an outlet of the magnet to be tested, controlling and improving the working strength of a turboexpander to be increased by a preset step, cooling by a three-stage heat exchanger, performing adiabatic expansion on helium gas, reducing the temperature, enabling the cold side inlet temperature of an intermediate heat exchanger to be steadily reduced to a target zone according to the preset speed, acquiring temperature differences of two loops by a sixth temperature sensor and a tenth temperature sensor arranged at the cold side outlet of the intermediate heat exchanger, controlling the cooling speed of the two loops, keeping the temperature difference of the two loops to be smaller than the preset value, controlling the running mode and the rotating speed of a low-temperature fan according to the actual pressure drop of the magnet to be tested detected by a differential pressure transmitter, acquiring sample inlet and outlet temperature, pressure drop and flow data in real time by a sensor, adjusting the rotating speed of the turboexpander and opening of each adjusting valve in a linkage mode, enabling the sample temperature, the inlet temperature, the outlet temperature and the temperature drop of the magnet to be tested, working pressure drop and the pressure drop of the magnet to be tested to be stabilized at the preset values, and enabling the system to enter a full-automatic closed loop control mode after parameters are stabilized, and testing the magnet to be started.

Description

Supercritical helium cryogenic system and method for nuclear fusion strong field magnet Technical Field The invention relates to the technical field of refrigeration systems and temperature control, in particular to a supercritical helium low-temperature system and method for a nuclear fusion strong field magnet. Background Nuclear fusion energy is regarded as a clean, efficient and sustainable novel energy source and is one of the important directions for solving the global energy crisis. The TF strong-field magnet is a core key component of the nuclear fusion device, and is mainly used for restraining high-temperature plasma and guaranteeing the stable progress of nuclear fusion reaction. The TF strong field magnet approaches the limit of materials, the difficulty is extremely high, the cost is high, the normal operation of the strong field magnet depends on an extremely low temperature environment and usually needs to work under an ultralow temperature working condition of 4.5K-20K, and therefore strict requirements are put on the performance of a low-temperature refrigerating system. Because of the excellent thermophysical properties, supercritical helium becomes an ideal working medium for a low-temperature cooling system of a nuclear fusion strong field magnet, and a corresponding supercritical helium low-temperature system also becomes important matching equipment of a nuclear fusion device. At present, the existing supercritical helium low-temperature system mostly adopts a brayton refrigeration cycle based on a turbine expander, and low-temperature preparation is realized through processes of compression, cooling, expansion and the like of helium, so that a stable cooling environment is provided for a strong field magnet. However, the prior art still has a plurality of problems to be solved in practical application, namely, the refrigerating circulation loop of a plurality of low-temperature systems and the testing loop of a magnet sample to be tested are integrally designed, so that pressure and temperature fluctuation in the refrigerating circulation process are directly transmitted to the magnet sample to be tested, damage is easily caused to the precise strong-field magnet sample, accuracy of a test result and safety of the sample are affected, working pressure and pressure ratio of the existing large-scale low-temperature system compressor cannot meet high-pressure operation requirements of the magnet, the suitability of the existing system for the magnet sample to be tested is poor, testing requirements of a plurality of magnet samples with different specifications are difficult to meet at the same time, flow distribution accuracy is insufficient, accurate cold quantity matching cannot be achieved according to heat load requirements of different samples, the operation stability of the system is to be improved, loop pressure fluctuation is large in the switching process of loading, unloading and the like, equipment faults are easily caused, helium is used as a scarce and expensive working medium, recovery efficiency of the existing system is generally low, operation cost of the system is greatly increased, after the test of the sample is finished, the system and the heat exchanger is easy to accumulate, the efficiency of the heat exchanger is difficult to be reduced, the heat exchange efficiency is seriously, and the heat exchange efficiency of the system is difficult to be influenced by long-time, and the heat exchange efficiency is difficult to be long-time and the heat exchanger is difficult to be prolonged, and the heat exchanger is easy to be long to be operated. Therefore, how to provide a supercritical helium cryogenic system and method for a nuclear fusion strong field magnet, which overcomes the defects existing in the prior art is a problem that needs to be solved by those skilled in the art. Disclosure of Invention In view of the above, the invention provides a supercritical helium low-temperature system and method for a nuclear fusion strong field magnet, which have the functions of double-loop independent operation, multiple sample adaptation, high stability, high helium recovery rate, rapid rewarming and loop cleaning, and improve the safety, accuracy and high efficiency of nuclear fusion strong field magnet test, and in order to achieve the above purposes, the invention adopts the following technical scheme: A supercritical helium cryogenic system for a nuclear fusion strong field magnet comprises a refrigeration circulation loop and a magnet circulation loop to be tested; The refrigerating circulation loop comprises a helium compressor, a high-pressure helium pipeline, a low-pressure helium pipeline, a loading and unloading pipeline and a bypass pipeline, wherein an outlet of the helium compressor is sequentially communicated with the high-pressure helium pipeline, the low-pressure helium pipeline and an inlet of the helium compressor to form a loop; the refrigeration circulati