CN111928519A

CN111928519A - Superconducting magnet and composite magnetic refrigerator

Info

Publication number: CN111928519A
Application number: CN202010692707.9A
Authority: CN
Inventors: 朱绍伟
Original assignee: Tongji University
Current assignee: Tongji University
Priority date: 2020-07-17
Filing date: 2020-07-17
Publication date: 2020-11-13
Anticipated expiration: 2040-07-17
Also published as: CN114151989A; CN114151989B; CN111928519B

Abstract

The invention relates to a superconducting magnet and a compound magnetic refrigerator, wherein the superconducting magnet is cooled by a residual cold head, the compound magnetic refrigerator is formed by coupling a regenerative refrigerator and a magnetic refrigerator, at least one residual cold head is added at a tail end regenerator of the regenerative refrigerator, so that the coldest tail end cold head is used as the ambient temperature of the magnetic refrigerator, and other required cold energy is provided by the residual cold head. Compared with the prior art, the invention has the advantages of low refrigeration temperature, large refrigeration capacity and the like.

Description

Superconducting magnet and composite magnetic refrigerator

Technical Field

The invention relates to the technical field of refrigeration, in particular to a superconducting magnet and a composite magnetic refrigerator.

Background

The lowest temperature of a traditional gas refrigerator such as a 4K GM refrigerator or a 4K pulse tube refrigerator can reach 2.2K, and the lowest temperature of a special design can be lower than 2K, but the lowest temperature is difficult to be lower than 1.9K due to the limitation of the over-flow of helium. Therefore, the refrigerating temperature lower than 2K can be compounded by adopting a 4K gas refrigerating machine and a magnetic refrigerating machine, and the 4K gas refrigerating machine is adopted to create a liquid helium temperature for precooling the magnetic refrigerating machine, so that the magnetic refrigerating machine takes the liquid helium temperature as the ambient temperature, and the temperature of about 100-1mK can be obtained as far as possible. We call millik class magnetic refrigerator.

The magnetocaloric effect is a thermal effect in which the temperature of a magnetocaloric material increases during the entry into a magnetic field and decreases during the exit from the magnetic field. Materials with such thermal effects are called magnetocaloric materials, and usually magnetocaloric materials have the strongest magnetocaloric effect around their curie temperature. In the composite magnetic refrigerator, when a magnetic field is applied to the magnetocaloric material, the magnetocaloric material is increased by the magnetization temperature, heat is released, the magnetocaloric material is moved out of the magnetic field, and the temperature of the magnetocaloric material in the demagnetization process is reduced.

The milli-K grade magnetic refrigerator generally adopts paramagnetic salt and the like as magnetocaloric materials, and utilizes the change of the temperature of the magnetocaloric materials along with the change of the magnetic field intensity in a superconducting magnetic field to refrigerate. The refrigeration is characterized by very large temperature ratio, for example, the temperature ratio from 4.2K to 1mK is 4200: 1, compared with the refrigerating temperature of a refrigerator, the difference is thousand times. Therefore, in order to obtain a low temperature, a superconducting magnet is generally used to obtain a strong magnetic field and a plurality of stages (i.e., several magnetic refrigerators are connected in series).

If the gas refrigerator can be operated at 2.2K, the temperature ratio can be reduced by a factor of about 1, a lower temperature can be achieved in a single stage, or one stage can be reduced, or the refrigeration capacity can be increased. However, the characteristic of helium gas at low temperature is that the refrigerating capacity of the gas refrigerating machine is very small at 2.2K, the refrigerating capacity can be still at 4.2K, and the refrigerating capacity is very large at 6K. A magnetic refrigerator comprises a magnetocaloric material, a superconducting magnet, a thermal switch and the like, the thermal load is large, and the gas refrigerator basically works at 4.2K and is difficult to work at 2.2K.

Disclosure of Invention

The present invention is directed to overcoming the above-mentioned drawbacks of the prior art and providing a superconducting magnet and a compound magnetic refrigerator that can use a lower precooling temperature, thereby reducing the temperature of a single stage or reducing the number of stages.

One of the objects of the present invention is to provide: a superconducting magnet cooled by a regenerative refrigerator, the regenerative refrigerator comprises an end regenerator and an end cold head, at least one residual cold head is added at the end regenerator, and the residual cold head cools the superconducting magnet. The superconducting magnet may be one used in various existing apparatuses, such as a magnetic refrigerator, a physical property measuring device under a magnetic field, and the like.

The second purpose of the invention is to provide a compound magnetic refrigerator, which is formed by coupling a regenerative refrigerator and a magnetic refrigerator.

The combination of a magnetic refrigerator and a regenerative refrigerator (e.g., a 4K GM refrigerator or a 4K pulse tube refrigerator) is an effective way to obtain the mk temperature, where the regenerative refrigerator provides the ambient temperature required by the magnetic refrigerator (the regenerative refrigerator cools to 4K) and then the magnetic refrigerator cools to the mk temperature.

At least one residual cold head is added at the tail end heat regenerator of the regenerative refrigerator, so that the coldest tail end cold head is used as the ambient temperature of the magnetic refrigerator, and other required cold energy is provided by the residual cold head.

Preferably, the magnetic refrigerator comprises a magnetic refrigeration material (including various commercial refrigeration materials such as paramagnetic salt and the like), a thermal switch, a superconducting magnet and auxiliary components (the auxiliary components comprise a lead, a structural member and the like), and the residual cold head is connected with the magnet. The magnetic refrigerator mainly has two parts of superconducting magnet and refrigerating material which need to consume larger cooling capacity, the two parts are separately cooled, the superconducting magnet and the related auxiliary components are cooled by the residual cold head of the regenerative refrigerator, and the refrigerating material is cooled by the final cold head of the regenerative refrigerator. Since the superconducting magnet and its associated auxiliary components do not need to be cooled to a very low temperature (e.g. 2.2K), it is sufficient to be cooled to a certain extent below its critical temperature (e.g. 4.2K or even 6K) by the residual coldhead. Thus, the paramagnetic salt can be cooled to a low temperature (about 2.2K) which can be reached by the gas refrigerator, that is, the ambient temperature of the magnetic refrigerator is very low (about 2.2K), so that the magnetic refrigerator can obtain a lower temperature, or the magnetic refrigerator can reduce one or more stages at the same temperature (the milliK temperature can also be reached by using a multi-stage magnetic refrigerator, but the cost is high, and the stages of the magnetic refrigerator can be reduced by adopting the method of the invention, so that the cost is reduced.

Further preferably, a second-stage radiation screen is arranged outside the magnetic refrigerator, and the second-stage radiation screen is connected with the residual cold head and is cooled by the residual cold head. The radiation screen can be connected with the first stage cold head (except the last stage cold head) to cool down the temperature of the radiation screen, and the radiation heat leakage of the environment temperature is borne by the first stage, thereby reducing the radiation heat load of the 4K temperature zone.

Furthermore, the thermal switch and the magnetic refrigeration material are covered with a final radiation screen, so that the influence of radiation on the lowest temperature of the magnetic refrigerator is further reduced.

Further preferably, the thermal switch may be a gas thermal switch, or may be a superconducting type, or a mechanical type, or may be in other forms, and the mechanical type is to open and close by using solid contact and non-contact.

When the thermal switch is a gas thermal switch, the adsorber of the gas thermal switch is connected with the residual cold head, and the residual cold head cools the adsorber; when the thermal switch is a superconducting thermal switch, the superconducting thermal switch is cooled by the remaining cold head.

Further preferably, the magnetic refrigerator has a multi-stage structure.

Further preferably, the regenerative refrigerator is a pulse tube refrigerator or a GM refrigerator.

Further preferably, the entire compound magnetic refrigerator may be placed in a vacuum chamber connected to the regenerative refrigerator by a bellows, which serves to further isolate the refrigerator from the system to reduce vibration.

Compared with the prior art, the invention has the following advantages:

1. a conventional regenerative refrigerator, such as a 4KGM refrigerator (a 4K refrigerator refers to a refrigerator capable of reaching the temperature of liquid helium, and its nominal refrigerating temperature is 4.2K), or when a 4K pulse tube refrigerator is used for precooling, its environment temperature is already about 4K, if a lower temperature, such as a temperature lower than 1K, needs to be prepared, since the regenerative refrigerator is limited by the superflow property of helium at a low temperature, it is difficult to obtain a low temperature lower than 2.2K (a special design can obtain a low temperature of about 1.9K). Therefore, the invention combines the regenerative refrigerator and the magnetic refrigerator as an effective means for obtaining lower temperature.

2. The magnetic refrigerator mainly comprises refrigerating salt (which can be conventional commercial refrigerating salt, such as paramagnetic salt and the like), a magnet (which can be conventional commercial superconducting magnet), and a thermal switch, wherein the refrigerating salt, the magnet and the thermal switch are required to consume refrigerating capacity, and the lead and other auxiliary components are also required to conduct heat to the low temperature and also need refrigerating capacity. The lowest temperature of a common regenerative refrigerator (such as a 4K pulse tube refrigerator or a 4KGM refrigerator) can reach 2.2K, and if the regenerative refrigerator is operated at about 2.2K, the ambient temperature is greatly reduced compared with 4.2K, so that the magnetic refrigerator can obtain lower temperature. However, the general 4K pulse tube refrigerator or 4KGM refrigerator is characterized in that the higher the temperature is, the larger the refrigerating capacity is, the refrigerating capacity at 2.2K is very small, and only the refrigerating capacity required by precooling paramagnetic salt can be provided, otherwise, the deficiency is not enough, and if all the cold energy is cooled by a tail end cold head, the cold energy can be basically maintained near 4.2K. Therefore, the method of adding the residual cold heads to the tail end heat regenerator of the regenerative refrigerator is adopted, so that one or more cold heads are added at the tail end, different cold heads provide different cold quantities, and other components except the refrigerating salt are cooled to proper temperature (such as 6K, 4K and the like), so that the coldest tail end cold head is used as the environment temperature of the magnetic refrigerator, and other needed cold quantities are provided by the residual cold heads with higher temperature. Thus, a compound magnetic refrigerator which can provide ultralow temperature (2K, even 1K) and has enough cold is obtained.

Drawings

FIG. 1 is a schematic structural diagram of a first two-stage pulse tube refrigerator;

FIG. 2 is a schematic diagram of a second two-stage pulse tube refrigerator;

FIG. 3 is a two-stage GM refrigerator;

FIG. 4 is a schematic structural diagram of a coupling of a two-stage 4K pulse tube refrigerator and a single-stage magnetic refrigerator;

FIG. 4a is a schematic diagram of a coupling structure of a two-stage 4K pulse tube refrigerator and a single-stage magnetic refrigerator in a vacuum cavity;

FIG. 5 is a schematic structural diagram of a second two-stage 4K pulse tube refrigerator coupled with a single-stage magnetic refrigerator;

FIG. 6 is a schematic structural diagram of a coupling of a two-stage 4K pulse tube refrigerator and a two-stage magnetic refrigerator;

FIG. 7 is a schematic structural diagram of a third two-stage 4K pulse tube refrigerator coupled with a single-stage magnetic refrigerator;

FIG. 8 is a schematic diagram of a GM refrigerator coupled to a single-stage magnetic refrigerator;

FIG. 9 is a schematic structural view of a first composite thermal switch cylinder;

fig. 10 is a schematic structural view of a second composite thermal switch cylinder.

The labels in the figure are: 1. a regenerative refrigerator, 11, a first stage pulse tube subunit, 111, a first stage pulse tube, 111a, a first push piston, 112, a first stage regenerator, 113, a first stage cold head, 113a, a first stage cold head thermal bridge, 12, a last stage pulse tube subunit, 121, a last stage pulse tube, 121a, a second push piston, 122, a last stage regenerator, 123, a last stage cold head, 123a, a last stage cold head thermal bridge, 1240, a residual cold head, 1241, a first residual cold head, 1241a, a first residual cold head thermal bridge, 1242, a second residual cold head, 1242, a first stage magnetic refrigerator, 211, a first stage thermal switch, 2111, a thermal switch upper flange, 2112, an inner cylinder, 2113a, a first intermediate cylinder, 2113b, a second intermediate cylinder, 2113c, a third intermediate cylinder, 2114, an outer cylinder 2115, a thermal switch lower flange, 211a, a first stage pulse tube, 211b, a first stage thermal switch magnet, 212. the device comprises a first-stage refrigeration salt 213, a first-stage superconducting magnet 221, a second-stage thermal switch 221a, a second-stage adsorber 222, a second-stage refrigeration salt 223, a second-stage superconducting magnet 31, a first-stage radiation screen 32, a second-stage radiation screen 33, a vacuum chamber 331, a corrugated pipe and a final-stage radiation screen 34.

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments.

The working principle is as follows:

when a traditional regenerative refrigerator (4K GM refrigerator or 4K pulse tube refrigerator is generally referred to as a double-stage refrigerator) is used for cooling a magnetic refrigerator, a superconducting magnet and paramagnetic salt are cooled by a final-stage cold head (a secondary cold head) together. Since the superconducting magnet and its related components need to consume a large part of cold, the ambient temperature of magnetic refrigeration is generally 4K. This construction is characterized by simplicity.

The magnetic refrigeration is characterized in that the lower the ambient temperature is, the lower the refrigeration temperature is, so some magnetic refrigerators even adopt 1.8K of super-flow helium as the ambient temperature, and the structure is extremely complex (the super-flow helium is difficult to prepare). Generally, 4K GM refrigerator can obtain low temperature of about 2K (special design can be slightly lower than 2K) at the lowest theoretically, can obtain very small cold quantity at 2.2K, and can obtain enough cold quantity at 4.2K. Thus it is difficult to cool the superconducting magnet together with the paramagnetic salt to 2.2K, but if only the paramagnetic salt is cooled, it can be cooled to 2.2K due to the reduced thermal load.

The 4K GM refrigerator with the residual cold head can obtain large cold quantity on the residual cold head with slightly higher temperature besides the cold quantity obtained on the last stage cold head. Thus, the paramagnetic salt can be easily cooled to a low temperature of 2.2K by using the separate cooling method.

4K GM refrigerator with residual cold head

On a 4K GM refrigerator with a cold surplus head, the cold capacity of the cold surplus head comes from the theoretical loss of the regenerator due to the physical properties of helium at low temperature. If no cold head remains, the cold energy of the cold head remains is converted into a part of the cold energy of the first stage. The temperature of the remaining coldhead is about 4.2-6K, while the temperature of the first stage coldhead is about 60K, so the irreversible loss is large. With the residual cold head, the residual cold quantity is obtained at about 4.2-6K, and a large use field can be allocated.

The 4K pulse tube refrigerator also has essentially the same characteristics, but it vibrates less because there are no moving parts at low temperatures.

The regenerative refrigerator can be a two-stage refrigerator, or a three-stage or higher refrigerator, and the magnetic refrigerator can be a one-stage refrigerator, or a two-stage or higher refrigerator.

The residual cold heads are arranged on a final regenerator of the regenerative refrigerator, and the number of the residual cold heads can be one or more.

As shown in fig. 1, it is a schematic structural diagram of a first two-stage pulse tube refrigerator, which is composed of a top flange 13, a first-stage pulse tube unit 11 and a last-stage pulse tube subunit 12, wherein the first-stage pulse tube unit 11 includes a first-stage pulse tube 111, a first-stage heat regenerator 112 and a first-stage cold head 113, the last-stage pulse tube subunit 12 includes a last-stage pulse tube 121, a last-stage heat regenerator 122 and a last-stage cold head 123, a residual cold head 1240 is installed on the last-stage heat regenerator 122, and the residual cold head 1240 includes a residual cold head, i.e. a first residual cold head 1241.

When the refrigerator works, gas enters and exits the refrigerator from the top flange, and expands to do work and refrigerate at the bottom of the pulse tube through the heat regenerator, and cold energy is output at the first-stage cold head 113, the last-stage cold head 123 and the first residual cold head 1241. The first stage coldhead 113 is typically at a temperature of about 60K, the last stage coldhead 123 is at a minimum temperature of about 2.2K, and the first remaining coldhead 1241 is at a temperature of about 4.2K.

As shown in fig. 2, the second two-stage pulse tube refrigerator has a structure substantially the same as that of the two-stage pulse tube refrigerator shown in fig. 1, except that the residual cold head 1240 has a second residual cold head 1242 in addition to the first residual cold head 1241. The second remaining coldhead may output cold at a temperature of about 6K.

In the same way, more residual cold heads can be added to meet different cold quantity requirements.

Due to the characteristics of helium at low temperature, there is a large theoretical regenerator loss in the last-stage regenerator 122, that is, when an ideal regenerator is used (no resistance, infinite heat exchange area, infinite filler heat capacity, no dead volume, no axial heat conduction), due to the physical properties of helium at low temperature, the regenerator still has a large heat leakage loss from high temperature to low temperature, which is an enthalpy flow loss caused by the difference of enthalpies of high-pressure gas and low-pressure gas at the same temperature. The residual cold head is a heat exchanger added in the tail end heat regenerator, when the residual cold head is added in the tail end heat regenerator, the residual cold head can exchange heat with gas in the heat regenerator, so that cold energy is intercepted and used for output, the cold energy is obtained additionally, the residual cold head is called residual cold energy, and the cold head is called residual cold head.

When the gas has high pressure of 2.1MPa, low pressure of 0.7MPa and flow rate of 1g/s, if the gas reversibly expands at the bottom of the final pulse tube 121 at the temperature of 2.2K, the expansion work is 8.47W, the enthalpy flow loss of the regenerator is 8.29W, and the refrigerating capacity is 0.18W. The regenerator enthalpy flow loss can produce 1.06W at 4.2K and 2.54W of residual cooling at 6K. The rest flows to the first-stage pulse tube subunit to become the cold energy of the first-stage pulse tube subunit. Because the first-stage pulse tube subunit has high temperature and high loss, more residual cold heads can be added to capture more cold at low temperature for other purposes. Due to the existence of the residual cold head, the two-stage 4K pulse tube refrigerator in FIG. 2 can be considered to become a virtual four-stage pulse tube refrigerator.

The temperature of each cold head may not be 60K, 6K, 4.2K, 2.2K, but other temperatures. The temperature values are for illustrative convenience.

As shown in fig. 3, a two-stage GM refrigerator with a first stage displacement piston 111a and a second stage displacement piston 121a instead of the pulse tube of fig. 2 can achieve the same effect. GM refrigerators vibrate more than pulse tube refrigerators, but the refrigeration efficiency is higher. The rest is the same as fig. 2. A first residual cold head 1241 and a second residual cold head 1242 are mounted on the final stage regenerator 122.

The regenerative refrigerator shown in fig. 1-3 is two-stage or multi-stage, but it is generally rarely used more than two stages because the two stages can reach a low temperature of about 2.2K and are mature commercial products. Due to the existence of the residual cold head, multiple stages are not needed.

The coupling of various regenerative refrigerators and various magnetic refrigerators will be described in detail by the following embodiments:

example 1

The two-stage 4K pulse tube refrigerator is coupled with the single-stage magnetic refrigerator, the structure of the two-stage 4K pulse tube refrigerator is shown in figure 4, the two-stage 4K pulse tube refrigerator and the first-stage magnetic refrigerator 2 are included, the first-stage magnetic refrigerator 2 comprises a first-stage thermal switch 211, a first-stage adsorber 211a, first-stage refrigerating salt 212 and a first-stage superconducting magnet 213, one end of the first-stage thermal switch 211 is connected with the first-stage adsorber 211a, the other end of the first-stage thermal switch is connected with the first-stage refrigerating salt 212, and the first-stage refrigerating salt 212 is arranged in the first-stage superconducting magnet.

A first radiation screen 31 is arranged outside a last pulse tube unit of the two-stage 4K pulse tube refrigerator, a second radiation screen 32 is arranged outside the first-stage magnetic refrigerator 2, and the second radiation screen 32 is connected with a first residual cold head 1241. Likewise, a radiation screen may be provided on each cold head. The final stage cold head 123 of the two-stage 4K pulse tube refrigerator is connected with the first stage thermal switch 211 through a final stage thermal bridge 123a, the first residual cold head 1241 is connected with the first stage superconducting magnet 213 through a first residual cold head thermal bridge 1241a, and the first stage cold head 113 is connected with the first radiation screen 31 through a first stage cold head thermal bridge 113 a. The adsorber 211a of the first thermal switch is connected to the first aftercooler 1241 via a first aftercooler thermal bridge 1241 a.

The refrigerator of fig. 4 is to be placed in a vacuum chamber, as shown in fig. 4 a. 33 is a vacuum chamber and 331 is a bellows connecting the refrigerator 1 and the vacuum chamber, which serves to further isolate the refrigerator from the system to reduce vibration. The corrugated pipe is not needed, so that the structure is simple. To further reduce the effect of radiation on the minimum temperature of the magnetic refrigerator, a thermal switch 211 and a cooling salt 212 may be housed in the end cold head plus final radiation shield 34. The final radiation shield 34 and the final cold head 123 are connected through a final cold head thermal bridge 123a, pass through the superconducting magnet 213, and cover the refrigerant salt 212 and the thermal switch 211.

The superconducting magnet 213 is a coil wound by a superconducting wire, and is cooled to a temperature lower than the critical temperature of the superconducting wire, and a magnetic field is obtained by electrifying the superconducting magnet. For situations where a controlled magnetic field is required, power is supplied from an external power source. For the case of a steady magnetic field, the superconducting collar has a closed loop and an external power supply may not be required.

In operation, the first and second radiation shields 31 and 32 are cooled to about 60K and 4.2K, and the ambient temperature of the first stage magnetic refrigerator is about 2.2K. The first stage refrigerant salt 212 is a paramagnetic salt that releases heat or increases in temperature when the magnetic field strength increases and absorbs heat or decreases in temperature when the magnetic field strength decreases. The refrigeration salt is called as magnetic refrigeration material, and the magnetic refrigeration material at different temperatures can be paramagnetic salt, metal, alloy, and the like. Ultralow temperatures are generally paramagnetic salts.

The advantage of this is that the ambient temperature of the first magnetic refrigerator can be the temperature of the end cold head 123, the refrigerating capacity of the refrigerator at this temperature is small, and the magnetic refrigerator itself does not need large precooling capacity. The superconducting magnet, with its current leads, radiation shield and adsorbers, then require large cold cooling, which may be provided by first cold surplus head 1241.

Meanwhile, various lead wires of the magnetic refrigerator may be attached to the first surplus cold head 1241, which is conducted to the first surplus cold head 1241 from heat conduction and leakage heat of high temperature, thereby reducing a heat load of the final stage cold head 123. A portion of the lead wires that need to be connected to the refrigerant salt are further attached to the final cold head 123, which further reduces heat leakage to the refrigerant salt 212.

The thermal switch can be a gas thermal switch, a superconducting type thermal switch, a mechanical type thermal switch or other forms. The mechanical type is to realize opening and closing by adopting solid contact and non-contact. The thermal switch structure used in this embodiment is shown in fig. 9, in which the first-stage thermal switch 211 is a gas thermal switch, and the upper and lower ends of the gas thermal switch are provided with metal plates (an upper flange 2111 and a lower flange 2115) having a close distance therebetween, and heat is conducted by helium gas sandwiched therebetween. The specific structure comprises an upper flange 2111, a lower flange 2115 and a composite thermal switch cylinder positioned between the upper flange and the lower flange, wherein the composite thermal switch cylinder comprises an inner cylinder 2112, an outer cylinder 2114 and a first middle cylinder 2113a, the top of the inner cylinder 2112 is fixed on the upper flange 2111, the bottom of the outer cylinder 2114 is fixed on the lower flange 2115, the first middle cylinder 2113a is positioned between the inner cylinder 2112 and the outer cylinder 2114, the bottom of the first middle cylinder 2113a is connected with the inner cylinder 2112, and the top of the first middle cylinder is connected with the outer cylinder 2114, so that a closed cavity with a folded section is formed. Fins are distributed on the upper flange 2111 and the lower flange 2115, and the distance between the fins is very short; as shown in fig. 9, the upper flange 2111 is provided with a downward extending fin a, the lower flange 2115 is provided with an upward extending fin b, the fins a and the fins b are arranged in a staggered manner, and the lengths of the fins a and the fins b are both smaller than the distance between the upper flange and the lower flange. The upper and lower flanges and the fins thereof are not in direct contact.

The inner cavity of the composite thermal switch cylinder is communicated with the first-stage adsorber 211a, and the adsorber is filled with active carbon or other adsorbing materials. The upper and lower ends of the first stage thermal switch 211 are almost adiabatic when turned off, the thermal conductivity is large when turned on, and the temperature difference between the upper and lower ends is small. When the first-stage adsorber 211a is heated, the activated carbon in the first-stage adsorber 211a releases helium, the cavity in the first thermal switch 211 is filled with helium, and the metal fins in the first-stage adsorber are conducted with heat through gas heat conduction; when the first-stage adsorber 211a is not heated, the activated carbon therein is cooled to absorb helium, the helium in the first thermal switch 211 is pumped away, and the fins therein are almost evacuated and do not conduct heat.

Here, the names of the upper and lower positions of the upper flange, the lower flange, and the like are only names, and there is no large correlation with the actual positions for the convenience of expression.

The cycle of the composite magnetic refrigerator is as follows:

(1) the first stage thermal switch 211 remains open (i.e., not conducting heat), the first stage superconducting magnet 213 increases the current plus the magnetic field strength, and the first stage refrigeration salt 212 increases in temperature;

(2) after the temperature of the first-stage refrigeration salt 212 reaches the temperature of the tail end cold head 123, the first-stage thermal switch 211 is opened and is in an open state (namely heat conduction), the current is continuously increased, the magnetic field intensity of the first-stage superconducting magnet 213 is increased, and the heat generated by the first-stage refrigeration salt 212 releases heat to the tail end cold head 123 through the tail end thermal bridge 123 a;

(3) the first stage thermal switch 211 is turned off, reducing the current, reducing the magnetic field strength of the first stage superconducting magnet 213, reducing the temperature of the first stage refrigeration salt 212,

(4) after the temperature of the first-stage refrigeration salt 212 reaches the refrigeration temperature, the first-stage thermal switch 211 is continuously switched off, the current is continuously reduced, the magnetic field intensity of the first-stage superconducting magnet 213 is reduced, and the first-stage refrigeration salt 212 absorbs heat to generate refrigeration capacity.

The different refrigeration salts, and the different magnetic field strengths, determine the refrigeration temperature of the first stage refrigeration salt 212, which can typically be below 1-0.001K.

The above-mentioned working conditions are intermittent, i.e. cold is generated in step (4) and can be utilized.

Example 2

The two-stage 4K pulse tube refrigerator shown in fig. 2 is coupled with a single-stage magnetic refrigerator, and the structure of the refrigerator is shown in fig. 5, and the residual cold heads comprise a first residual cold head 1241 and a second residual cold head 1242. The structure of the single-stage magnetic refrigerator is the same as that of embodiment 1, wherein the end radiation shield 32 and the first thermal switch absorber 211a are both connected with the second residual cold head 1242 through the second residual cold head thermal bridge 1242a, and the first superconducting magnet 213 is connected with the first residual cold head 1241 through the first residual cold head thermal bridge 1241a, so that the first superconducting magnet 213 solely uses the cold energy of the first residual cold head 1241 to sufficiently cool.

The current lead of the first superconducting magnet 213 may be attached to the second coldhead 1242 first, so that the heat conducted from the high temperature is transferred to the second coldhead 1242, thereby reducing the heat load of the first coldhead. The rest is the same as example 1.

Each cold head can be provided with a radiation screen, thereby reducing radiation heat leakage to the low-temperature area.

Example 3

The two-stage 4K pulse tube refrigerator and the two-stage magnetic refrigerator shown in fig. 2 are coupled, and the structure of the two-stage magnetic refrigerator is shown in fig. 6, wherein the two-stage magnetic refrigerator comprises a first-stage magnetic refrigerator and a second-stage magnetic refrigerator, the first-stage magnetic refrigerator comprises a first-stage thermal switch 211, a first-stage adsorber 211a, a first-stage refrigeration salt 212 and a first-stage superconducting magnet 213; the second-stage magnetic refrigerator comprises a second-stage thermal switch 221, a second-stage adsorber 221a, second-stage refrigerating salt 222 and a second-stage superconducting magnet 223, one end of the first-stage thermal switch 211 is connected with the first-stage adsorber 211a, the other end of the first-stage thermal switch is connected with the first-stage refrigerating salt 212, the first-stage refrigerating salt 212 is arranged in the first-stage superconducting magnet 213, the second-stage thermal switch 221 is respectively connected with the second-stage adsorber 221a and the second-stage refrigerating salt 222, and the second-stage refrigerating salt 222 is arranged in the second-stage superconducting magnet 223.

The second stage thermal switch 221 connects the second stage refrigerant salt 222 with the first stage refrigerant salt 212, and the second stage superconducting magnet 223 is connected with the first residual cold head 2141 through the first residual cold head thermal bridge 1241 a. The second-stage adsorber 221a is connected with the end radiation screen 32 together with a second residual cold head 1242 via a second residual cold head thermal bridge 1242 a.

The cycle of the first stage magnetic refrigerator is as follows:

(1) the first stage thermal switch 211 remains open (i.e., not conducting heat), the second stage thermal switch 222 is open (i.e., not conducting heat), the first stage superconducting magnet 213 increases the current plus the magnetic field strength, and the first stage refrigeration salt 212 increases in temperature;

(4) after the temperature of the first-stage refrigeration salt 212 reaches the refrigeration temperature, the second-stage thermal switch 222 is turned on (i.e., conducts heat) to continuously reduce the current, the magnetic field intensity of the first-stage superconducting magnet 213 is reduced, and the first-stage refrigeration salt 212 absorbs heat to generate refrigeration capacity.

The circulation of the second stage magnetic refrigerator is the same as that of the first stage magnetic refrigerator, but is in reverse phase, so that when the first stage magnetic refrigerator absorbs heat, the second stage magnetic refrigerator releases heat, the refrigerating temperature of the first stage magnetic refrigerator is the ambient temperature, the temperature is lower, and the temperature of the second stage refrigerating salt can reach 0.1-0.001K or lower.

The second stage magnetic refrigerator is cycled as follows:

(1) the second stage thermal switch 221 remains off (i.e., not conducting heat), the second stage superconducting magnet 223 reduces current to reduce magnetic field strength, and the temperature of the second stage refrigeration salt 212 increases and decreases;

(2) after the temperature of the second-stage refrigeration salt 212 reaches the refrigeration temperature, the current of the second-stage superconducting magnet 221 is continuously reduced to reduce the magnetic field intensity, and the second-stage refrigeration salt 212 absorbs heat to generate refrigeration capacity.

(3) Increasing the current to the second stage superconducting magnet 223 increases its magnetic field strength and the second stage refrigeration salt 222 increases in temperature.

(4) After the temperature of the second-stage refrigeration salt 222 reaches the temperature of the first-stage refrigeration salt 212, the second-stage thermal switch 221 is turned on and is in an on state (namely heat conduction), the current of the second-stage superconducting magnet 223 is continuously increased to increase the magnetic field intensity of the second-stage superconducting magnet 223, and the heat generated by the second-stage refrigeration salt 222 is released to the first-stage refrigeration salt 212 through the second-stage thermal switch 221.

Likewise, more stages can be made, resulting in lower temperatures.

Compared with the conventional magnetic refrigerator precooled by the regenerative refrigerator, the refrigeration salt is separated from other heat loads, so that the cold head at the tail end of the precooled refrigerator can work at a very low temperature. The refrigeration temperature is lower due to the great reduction of the precooling temperature.

The output refrigerating temperature of the magnetic refrigerators (fig. 4-6) in the above embodiments 1-3 is intermittent, that is, the refrigerating salt is refrigerated only in the heat absorption process, and the temperature of the refrigerating salt fluctuates at other times. In order to output stable cooling capacity, the following method can be adopted:

the second stage salt 222 may be a material with a large heat capacity to form a heat reservoir, so that the magnetic refrigerator outputs cold at a stable temperature, and the second stage superconducting magnet 223 may be omitted. The second thermal switch 222 is only open when the first refrigerant salt 212 absorbs heat and is closed at other times. In this case, the two-stage magnetic refrigerator in example 3 is actually a single-stage magnetic refrigerator with a heat reservoir. This mode requires a large thermal reservoir to achieve a smooth cooling temperature due to the limited thermal capacity of the thermal reservoir.

The second stage magnetic refrigerator may be operated in a semi-thermal reservoir mode: the second thermal switch 222 is only open when the first cooling salt 212 absorbs heat, and at this time, the second stage superconducting magnet 223 increases the current to increase the magnetic field, so that the second stage cooling salt 222 releases heat to the first cooling salt 212 at the cooling temperature of the first stage magnetic refrigerator, and the temperature is kept constant. When the second thermal switch 221 is turned off, the second-stage superconducting magnet 223 reduces the current and reduces the magnetic field, so that the second-stage refrigeration salt 222 absorbs heat from the thermal load, and the magnetic refrigerator outputs refrigeration at a stable temperature. In this case, fig. 6 is a single-stage magnetic refrigerator. Compared with a pure heat reservoir, the output temperature is more stable.

Example 4

The two-stage 4K pulse tube refrigerator and the single-stage magnetic refrigerator are coupled, wherein the thermal switch is a superconducting thermal switch, the structure of the thermal switch is shown in fig. 7, the first-stage thermal switch 211 is a superconducting thermal switch which is a superconductor, the first-stage thermal switch 211 is connected with the tail end cold head 123 through a final-stage cold head thermal bridge 123a, the first-stage superconducting magnet 213 is connected with the first residual cold head 1241 through a first residual cold head thermal bridge 1241a, the first-stage thermal switch magnet 211b is also connected with the first residual cold head thermal bridge 1241a, and the switching characteristics are as follows: when the magnetic field of the first-stage thermal switch magnet 211b is lower than the critical magnetic field strength of the first-stage thermal switch 211, the first-stage thermal switch magnet 211b is a superconductor and has a very low thermal conductivity, and when the magnetic field of the first-stage thermal switch magnet 211b is higher than the critical magnetic field strength of the first-stage thermal switch magnet 211b, the first-stage thermal switch magnet 211b is quenched and becomes a constant conductor, and the thermal conductivity is very. The rest is the same as example 1.

Example 5

The same as example 2, but the two-stage 4K pulse tube refrigerator is replaced by a two-stage 4K GM refrigerator, and the magnetic refrigerator is precooled by the GM refrigerator, and the structure is shown in fig. 8.

Example 6

As in example 1, only the thermal switch structure of the thermal switch structure is the thermal switch structure shown in fig. 10, and the thermal switch structure is composed of an upper flange, a lower flange and a composite thermal switch cylinder. The upper flange 2111 is thermally connected with a final-stage cold head, the lower flange 2115 is thermally connected with a refrigerant salt, and the composite thermal switch cylinder is formed into a folding structure by sequentially connecting an inner cylinder 2112, a first middle cylinder 2113a, a second middle cylinder 2113b, a third middle cylinder 2113c and an outer cylinder 2114.

When helium in the space 2116 in the thermal switch is sucked away by the adsorber and is vacuum, the upper flange and the lower flange are in a thermal insulation state and are in a thermal off state. When the adsorber is heated to release helium into the space 2116 in the thermal switch, the upper flange and the lower flange are thermally conductive through the helium and are in a hot on state. The upper flange and the lower flange are fully distributed with fins, and the fins are close to the fins.

The gas thermal switch can also adopt a simple thermal switch cylinder, namely one cylinder, instead of adopting a composite thermal switch cylinder, thereby simplifying the structure, but the thermal conductivity is higher when the gas thermal switch is closed.

The thermal switch has the requirements that the thermal conductivity between the upper flange and the lower flange is large enough when the thermal switch is opened, the thermal conductivity between the upper flange and the lower flange is small enough when the thermal switch is closed, but the thermal conductivity of the thermal switch cylinder made of a common circular tube is large. The thermal switch cylinder of fig. 9 of embodiment 1 is a composite thermal switch cylinder composed of an inner cylinder plus a first intermediate cylinder plus an outer cylinder, and has a thermal conductivity length about three times that of a conventional cylinder, so that the thermal conductivity is about 1/3 as compared with a simple one cylinder, thereby reducing the thermal conductivity in a thermally-off state. In this embodiment, a second intermediate barrel 2113b and a third intermediate barrel 2113c are added, which have longer thermal conduction length and lower thermal conductivity. The number of the intermediate barrels can be increased or decreased according to actual needs.

The refrigeration also has other auxiliary parts such as structural parts and the like for connecting components with high temperature to low temperature, and the heat leakage exists, and the residual cold head can be used for cooling so as to intercept a part of the heat leakage from the high temperature.

Example 7 is a superconducting magnet cooled by a regenerative refrigerator. The structure of the regenerative refrigerator is the same as that of the embodiment 1, and the superconducting magnet is cooled by the residual cold head. The final cold head can be used for cooling a sample to measure the physical property of the sample at low temperature, and the physical property in a wide temperature range can be obtained due to the low temperature of the final cold head.

In other situations where a superconducting magnet similar to a compound magnetic refrigerator is combined with other components, the superconducting magnet can be cooled by the residual cold head, and the end cold head provides lower refrigeration temperature for cooling other components.

The above description is only a preferred embodiment of the present invention, and not intended to limit the present invention, the scope of the present invention is defined by the appended claims, and all structural changes that can be made by using the contents of the description and the drawings of the present invention are intended to be embraced therein.

Claims

1. The superconducting magnet cooled by the regenerative refrigerator is characterized in that the regenerative refrigerator comprises a tail end regenerator and a tail end cold head, at least one residual cold head is added at the tail end regenerator, and the residual cold head cools the superconducting magnet.

2. A composite magnetic refrigerator is formed by coupling a regenerative refrigerator and a magnetic refrigerator, wherein the magnetic refrigerator comprises a magnetic refrigeration material and a superconducting magnet, and the regenerative refrigerator comprises a tail end regenerator and a tail end cold head cooler.

3. The compound magnetic refrigerator according to claim 1, wherein the magnetic refrigerant is cooled by the end coldhead by connecting the magnetic refrigerant to the end coldhead through a thermal switch.

4. A compound magnetic refrigerator according to claim 3 in which the thermal switch is a gas thermal switch cooled by the cold residual head.

5. A compound magnetic refrigerator according to claim 3 in which the thermal switch is a super conducting thermal switch cooled by the remaining cold head.

6. A compound magnetic refrigerator according to claim 1 further including an auxiliary component which is cooled by the remaining cold head.

7. The compound magnetic refrigerator according to claim 1, wherein a first stage radiation screen is provided outside a terminal regenerator of the regenerative refrigerator;

and a second-stage radiation screen is arranged outside the magnetic refrigerator and is connected with the residual cold head and is cooled by the residual cold head.

8. A compound magnetic refrigerator as claimed in claim 4 in which the thermal switch and the magnetic refrigeration material are covered by a final radiation shield.

9. A compound magnetic refrigerator according to claim 1, characterized in that the magnetic refrigerator is a multistage structure;

the regenerative refrigerator is a pulse tube refrigerator or a GM refrigerator.

10. A compound magnetic refrigerator according to claim 1 wherein the compound magnetic refrigerator is disposed in a vacuum chamber, the vacuum chamber being connected to the regenerative refrigerator by a bellows.