JPS6028211A - Superconductive magnet - Google Patents

Abstract

PURPOSE:To control an exciting current by combining small freezers and a small superconductive magnet and reduce the size of the apparatus by a method wherein the superconductive magnet is housed in an internal tank filled with liquid helium and the 1st and the 2nd small freezers are directly attached to a heat retaining container in which the internal tank is contained. CONSTITUTION:A superconductive magnet 1 is housed in an internal tank 3 filled with liquid helium 2 and the 1st and the 2nd small freezers 20, 30 are directly attached to an external tank 5 of a heat retaining container 4. A freezing stage 24 of the freezer 20 is connected to a radiation shielding plate 6 and a helium recondensor 25 is provided immediately above the helium 2. The 1st and the 2nd freezing stages 34, 35 of the freezer 30 are inserted into the container 4 and a power lead 10, which supplies an exciting current to the superconductive magnet 1 is connected to the stage 35. The lead 10 is directly cooled by the stages 34, 35 and the shielding plate 6 is directly cooled with heat transmission by the stage 24 to control the exciting of current the magnet 1.

Description

[Detailed Description of the Invention] Technical Field of the Invention The present invention relates to a superconducting magnet device comprising a combination of a small refrigerator and a superconducting magnet.

[Technical background of the invention and its problems] FIG. 1 shows the configuration of a conventional superconducting magnet device. In the figure, a superconducting magnet 1 is filled with a cooling medium such as extremely low temperature (for example 4.2K) liquid helium 2.
It is stored in 3 and C. A cold storage container 4 that keeps the superconducting magnet 1 in a superconducting state for 1.5 hours consists of an inner tank 3 and an outer tank 5, and a radiation shield plate 6 is provided between the inner tank 3 and the outer tank 5. Furthermore, a shield tube 8 through which liquid nitrogen 7 flows is provided in the radiation shield plate 6 to enhance the shielding effect.

In addition, the superconducting magnet 1 maintained in a superconducting state is connected to an externally installed superconducting magnet power source 9 via a power lead 10, thereby supplying an exciting current and applying a required magnetic field to the magnetic field. is applied to equipment 16. On this occasion,
Power lead 10. Via the low temperature pipe 11, the liquid helium transfer pipe 12, the outside 15, the radiation shield plate 6, and the inner tank 3, heat is transferred from the outside (for example, 300K) to the extremely low temperature (for example, 4L 2K) and the liquid helium 2 by heat conduction and radiation. invade.

Then, the liquid helium 2 evaporates due to this penetrating heat, and gas helium 13 is generated. This gas helium 13 flows inside the outer tube 14 through which the power lead 10 passes, cools the power lead 10 (gas cooling), and enters the low-temperature pipe 11. The amount of heat entering from the power lead 10 is reduced by this gas cooling. Gaseous helium 13 enters a helium liquefier 15 and is converted into liquid helium 2 at an extremely low temperature (for example, 4.2K). This liquid helium 2 enters the inner tank 3 via the liquid helium transfer pipe 12. This JZ sea urchin,
The helium evaporated by the intruding heat cools the power lead 10, is then liquefied in the helium liquefier 15, and returns to the inner tank 3, where the superconducting magnet 1 is kept in a superconducting state while repeating the circulation.

By the way, the superconducting magnet device configured in this way is suitable for large-sized superconducting magnets, but it is suitable for relatively small-sized superconducting magnets (for example, with an excitation current of 30
Helium evaporation m1 at extremely low temperatures, about 0 to 500 A
24/h) is not suitable. This is because the conventional helium liquefier 15 has been developed for use with large superconducting magnets, and there is no one suitable for small refrigerating capacity (for example, helium evaporation amount of about 1 to 2 t7 hours). Therefore, if a normal helium liquefaction machine 15゛ is applied to a small superconducting magnet, the size or occupied area of the helium liquefaction machine 15 will be excessive compared to 1 part of the superconducting magnet, and from a cost perspective, it will be compared to the superconducting magnet 1. Therefore, the price of the helium liquefier 15 becomes excessive, and the superconducting magnet device becomes extremely expensive. On the other hand, if you want to make the superconducting magnet device compact and low-cost by using a conventional small refrigerator that matches the capacity of this small superconducting magnet, the freezing capacity of the conventional small refrigerator is 4 cold containers, 3 inner tanks, , the shield 6.8, and the outer tank 5 to supply radiant heat, and furthermore, it is not enough to supply evaporative gas cooling m of the power lead 10.

Therefore, normally a persistent current switch is attached to the superconducting magnet, and the power lead is detachable. After the superconducting magnet is excited, the power lead is removed, and the persistent current is cut off by removing the heat from the power lead. I am driving in mode. In this way, the only heat intrusion from the outside will be radiant heat and heat conduction from various low-temperature pipes, making it possible to keep the superconducting magnet in a superconducting state with just the freezing function horn of a conventional small refrigerator. It is.

However, in this method, once the persistent current mode is entered, the excitation current is always constant and the current value cannot be changed. For example, when considering a small superconducting magnet device used in a single crystal pulling device, it is necessary to control the temperature of impurities in the single crystal by changing or controlling the magnetic field strength during single crystal pulling. Therefore, it is necessary to control the excitation current value as well as the magnetic field strength. In general, in devices using superconducting magnet devices, there are many demands to vary or control the intensity of the applied magnetic field, that is, the excitation current.

[Object of the invention] The present invention was made in consideration of the above circumstances, and
The purpose is to provide a low-cost superconducting magnet device that is capable of controlling the excitation current value by combining a small refrigerator and a small superconducting magnet.

[Summary of the Invention] In order to achieve the above object, the present invention includes a cold storage container consisting of an inner tank and an outer tank filled with a cooling medium, a superconducting magnet housed in the inner tank of the cold storage container, and A power lead that supplies excitation current to the superconducting magnet from a power source for the superconducting magnet, a shield body provided between the inner tank and the outer tank of the cold storage container, and a plurality of power leads that are directly attached to the cold storage container. A first small refrigerator that performs direct cooling by heat conduction from the cooling stage, and a refrigerant medium that has evaporated in the inner tank of the cold storage container is liquefied by a recondenser provided in the layer, and the shield body is and a second small refrigerator that directly cools the refrigerator by heat conduction from the cooling stage.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention shown in FIG. 2 will be described. FIG. 2 shows an example of the configuration of a superconducting magnet device according to the present invention, and the same parts as in FIG. 1 are denoted by the same reference numerals. In the figure, a superconducting magnet 1 is housed in an inner tank 3 filled with liquid helium 2. The first and second small refrigerators (20 and 30) are directly attached to the cold container 4.

This first small refrigerator IN 20 includes a compressor unit 22 that compresses a refrigerating medium (for example, helium) 21 circulating in the refrigerator, and a compressor unit 22 that compresses a refrigerating medium 21 that is compressed by the refrigerating medium 21.
an expansion state 23 that adiabatically expands and freezes the expansion machine 2;
The first refrigeration stage 24 is cooled to a radiation shield temperature of, for example, 80K by the refrigeration medium 21 cooled by 3.
and the helium liquefaction temperature by the freezing medium 21, for example 4.2
and a helium recondenser 25 cooled to K. Here, the first freezing stage 24 is directly connected to a radiation shield plate 6 installed between the outer tank 5 and the inner tank 3 of the cold storage container 4, and the helium recondenser 25 is connected inside the inner tank 3. It is provided at a position directly above the liquid level of the liquid helium 2.

The second small refrigerator 30 also includes a compression unit [-32] that compresses the refrigerating medium 31 circulating within the refrigerator.
An expander 33 expands and cools the compressed refrigeration medium 31 discharged from this, a second refrigeration stage 34 cooled to 80K, for example, by the refrigeration medium 31 cooled by the expansion machine 33; and a third freezing stage 35 cooled to, for example, 20K.

On the other hand, the power lead 10 that supplies excitation current to the superconducting magnet 1 is filled with liquid helium 2, penetrates the inner tank 3 and the radiation shield 6, and after taking the required conductor length and conductor cross-sectional area, is transferred to the third freezing stage. 35, and after taking the required conductor length and conductor cross-sectional area, it is connected to the second freezing stage 34, and finally after taking the required conductor length and conductor cross-sectional area, it penetrates the outer tank 5. The superconducting magnet power supply 9 is then connected to the superconducting magnet power supply 9.

Here, the power lead 10 and each freezing stage 34.35
The connection points are electrically insulated. Furthermore, the penetrating portions of the helium recondenser 25 and the power lead 10 that penetrate the inner tank 3 are constructed to be airtight so that the vaporized gas of the liquid helium 2 in the inner tank 3 does not leak to the outside. Furthermore, the conductor cross-sectional area of the power lead 10 is determined between the outer tank 4 and the second freezing stage 34, between the second freezing stage 34 and the third freezing stage 35, and between the third freezing stage 35 and the superconducting magnet 1. The conductor area is successively reduced in this order.

Next, the operation of the superconducting magnet device configured as described above will be explained. First, in order to apply a magnetic field to the magnetic field applying device 16 (for example, a single crystal pulling device), the power lead 1
When the superconducting magnet 1 is excited by the superconducting magnet power supply 9 via the power lead 10, Joule heat due to the electrical resistance of the power lead 10, liquid helium temperature (for example, 4.2), and atmospheric temperature (
For example, the liquid helium 2 begins to evaporate due to heat intrusion due to thermal conduction through the power lead 10 due to the temperature difference between the helium 2 and the helium 300, and heat intrusion due to radiation through the outer tank 5, the radiation shield plate 6, and the inner tank 3. Joule heat generated from the power lead 10 and intrusion heat due to thermal conduction are removed by the second and third freezing stages 34 and 35 of the second refrigerator 30.

Generally speaking, Joule heat will be reduced by increasing the cross-sectional area of the power lead, but the heat entering by heat conduction will increase. Conversely, if the cross-sectional area of the power lead is made smaller, the Joule heat will increase and the amount of heat introduced by thermal conduction will decrease. Therefore, there is an optimal cross-sectional area of the power lead that minimizes heat intrusion. This optimum cross-sectional area is determined by the excitation current value, the second . It is determined by the temperature and refrigeration capacity of the third refrigeration stage 34, 35 and the conductor length of the power lead.

Therefore, liquid helium 3 (for example 4.2) to third freezing stage 35 (for example 20 ), between the third freezing stage 35 and the second freezing stage 34 (for example, at 80), and between the third freezing stage 35 and the outer tank 5 (for example, at 300), the conductor length and disconnection of the power lead 10. By appropriately selecting the area, the heat entering the liquid helium 2 from the power lead 10 can be minimized.

This optimal condition can be determined, for example, by the generally well-known equation below. That is, when the odor is, the amount of heat penetration becomes the minimum value Q min . Here, Q: Heat penetration M I: Current ground λ: Thermal conductivity α: Constant (ρ = αT1ρ: Power lead resistivity, T: Temperature) To: Thermal conductivity τ: τ-■・fα/TC: Low temperature part temperature a: Power lead cross-sectional area t: Power lead length In this way, the amount of heat entering from the power lead 10 can be reduced, and furthermore, since the power lead 10 does not need to be cooled with gas, the amount of evaporated helium can be significantly reduced. . Therefore, all the helium gas in the sealed inner tank 3 that has been evaporated by radiant heat or intrusion heat due to heat conduction from various low-temperature pipes can be reliquefied only by the freezing capacity of the conventional small refrigerator.

That is, the evaporated liquid helium 2 has its latent heat removed by the helium recondenser 25 installed in the inner tank 3, is recondensed, and is returned as liquid helium 2 in the inner tank 3 having a sealed structure in the form of droplets. . Furthermore, the convergence=J shield plate 6 is directly connected to the first freezing stage 24 (for example, to 8o) of the first small refrigerator 2o, and is directly cooled by heat conduction from the first freezing stage 24. be done. This results in a compact and easily constructed shield.

As described above, the present superconducting magnet device provides the following effects.

(a) Since the power lead 10 can be directly cooled by a conventional small refrigerator and the inner tank 3 of the cold storage container 4 can be made airtight, the increase in evaporated helium that accompanies the usual gas cooling method can be avoided. Since the helium evaporation m is reduced, the evaporated helium in the inner tank 3 can be recondensed by the amount evaporated due to the intrusion heat using a conventional small refrigerator.

(b) Since the power lead 10 is directly cooled 11 , the magnetic field strength, that is, the excitation current value, can be arbitrarily changed without destroying the superconducting state even during superconducting magnet necking operation. As a result, when this device is applied to a superconducting magnet device for a single crystal pulling device, for example, Il
It becomes possible to control the impurity concentration in the single crystal by controlling the field strength.

(C) Since the radiation shield 6 is directly cooled by heat conduction by the freezing stage 24 of the small refrigerator 20, the apparatus becomes compact.

(d) Since the structure is such that the conventional small refrigerator 20, 30 that matches the size and capacity of the small superconducting magnet 1 is directly attached to the cold storage container 4, the device can be made compact and inexpensive.

Next, other embodiments of the present invention will be described.

FIG. 3 shows another example of the structure of the superconducting magnet device according to the present invention, and the same parts as in FIG. 2 are given the same reference numerals and the explanation thereof will be omitted. In this embodiment, the second freezing stage 34 of the second small refrigerator 30 cools the power lead 10.
The temperature is the same as that of the first freezing stage 24 of the first small refrigerator 20, and the radiation shield plate 6 is directly attached to each freezing stage 34 and 24. With this configuration, the cooling capacity of the radiation shield is increased and the shielding effect is improved, and the power lead 10 between the liquid helium 2 and the second freezing stage 34 is heated to the temperature (for example, 80°C) by the radiation shield.
), and the amount of heat intrusion from the power lead 10 is further reduced.

FIG. 4 also shows a (111) example of the superconducting magnet device according to the present invention, and the same parts as in FIG. (For example, to 80, then to 20.)

4.2) Three freezing stages 104, 105° 106
The structure is such that the power reed 10 is cooled and the evaporated helium in the interior 3 is recondensed at the same time by a small refrigerator 100 having a small refrigerator 100.With this structure, an even more compact superconducting magnet device can be obtained. I can do it.

[Effects of the Invention] As explained above, according to the present invention, a low-cost superconducting magnet device is provided in which the cold magnetic current value can be controlled by combining a small refrigerator and a small superconducting magnet. can.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional superconducting magnet device, FIG. 2 is a block diagram showing one embodiment of the present invention, and FIGS. 3 and 4 are block diagrams showing other embodiments of the present invention. . 1...Superconducting magnet, 2...Liquid helium, 3...
Inner tank, 4... Cold storage container, 5... Outer tank, 6... Radiation shield plate, 7... Liquid nitrogen, 8... Shield tube,
9... Power supply for superconducting magnet, 10... Power lead,
11... Low temperature pipe, 12... Liquid helium transfer pipe,
13...Gas helium, 14...Outer cylinder, 15...
For helium liquefaction, 16...Magnetic field application equipment, 20.3
0,100...Small refrigerator, 21.31,101...
- Refrigerating medium, 22, 32, 102... Compressor unit, 23.33.103... Expander, 24, 34, 35
, 104, 105, 106... freezing stage, 25.
...Helium recondenser. Applicant's agent Patent attorney Takehiko Suzue

Claims (1)

[Claims] (1) A cold container consisting of an inner tank and an outer tank filled with a cooling medium, a superconducting magnet housed in the inner tank of the cold container, and the power to supply exciting current to the superconducting magnet from a power supply for the superconducting magnet. A shield body provided between the lead, an inner tank and an outer tank of the cold storage container, and a first shield body that is directly attached to the cold storage container and that directly cools the power lead by heat conduction from a plurality of cooling stages. a small refrigerator; and a second small refrigerator that liquefies the evaporated refrigerant medium in the inner tank of the cold storage container by a recondenser provided in the tank and directly cools the shield body by heat conduction from the cooling stage. A superconducting magnet device comprising: (a) The superconducting magnet device according to claim (1), wherein the conductor cross-sectional area and conductor length of the power lead are set so that the amount of heat intrusion is minimized between each freezing stage.