JPH0582045B2 - - Google Patents

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 housed in an inner tank 3 filled with a cooling medium, such as liquid helium 2 at an extremely low temperature (for example, 4.2 K). The cold container 4 that holds the superconducting magnet 1 in a superconducting state is composed 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. A shield tube 8 through which liquid nitrogen 7 flows is provided on the plate 6 to enhance the heat 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. At this time, power lead 1
0. Heat enters the extremely low temperature liquid helium 2 from the outside (for example, 300K) through thermal conduction or radiation through the low temperature pipe 11, liquid helium transfer pipe 12, outer tank 5, radiation shield plate 6, and inner tank 3. . 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 cylinder 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 cryogenic liquid helium 2. This liquid helium 2 is transferred to the inner tank 3 via a liquid helium transfer pipe 12.
to go into. In this way, the helium evaporated due to the intrusion heat cools the power lead 10, is liquefied by the helium liquefier 15, and is returned to the inner tank 3, where the circulation is repeated to maintain the superconducting magnet 1 in a superconducting state.

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 superconducting magnets used in, for example, single-crystal pulling devices (e.g., excitation current of about 300 to 500 A, at extremely low temperatures). (helium evaporation rate of about 1 to 2/h). 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 rate of about 1 to 2/h). Therefore, if a normal helium liquefier 15 is applied to a small superconducting magnet, the size or occupied area of the helium liquefier 15 will be excessive compared to the superconducting magnet 1, and furthermore, from a cost perspective, the helium liquefier 15 will be larger than the superconducting magnet 1. Liquefaction machine 1
5 becomes excessive, and the superconducting magnet device becomes even more 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 shields 6 and 8, and the outer tank 5, and the amount of evaporative gas cooling of the power lead 10 is not enough.

Therefore, normally a persistent current switch is attached to the superconducting magnet and the power lead is removable. After the superconducting magnet is excited, the power lead is removed and the heat intrusion from the power lead is cut off to prevent the permanent current mode. I am driving at. In this way, heat intrusion from the outside is limited to radiant heat and heat conduction from various low-temperature pipes, and it is possible to maintain the superconducting magnet in a superconducting state with just the cooling capacity of a conventional small refrigerator.

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, considering a small superconducting magnet device used in a single crystal pulling device,
There is a need to control the impurity concentration in a single crystal by varying or controlling the magnetic field strength during single crystal pulling. Therefore, it is necessary to change the magnetic field strength, that is, the excitation current value. In general, in devices using superconducting magnet devices, there are many demands to change or control the applied magnetic field strength, that is, the excitation current value.

[Purpose of the invention]

The present invention was made in consideration of the above circumstances, and its purpose is to provide a compact and low-cost superconducting magnet device that can control the excitation current value by combining a small refrigerator and a small superconducting magnet. It is about providing.

[Summary of the invention]

In order to achieve the above object, a superconducting magnet device of the present invention includes 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 a superconducting magnet device that includes a cooling container. A small refrigerator is attached to the cooling container and has a plurality of freezing stages inside the cooling container, a radiation shield plate is provided between an inner tank and an outer tank of the cooling container, and a radiation shield plate is installed in the cooling container, and and a power lead for supplying exciting current to the superconducting magnet from a superconducting magnet power source, and the conductor cross-sectional area and conductor length between each freezing stage are set so that the amount of heat intruding into the cooling medium is minimized. The structure shall be equipped with the following.

[Embodiments of the invention]

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

The second small refrigerator 30 also includes a compression unit 32 that compresses the refrigerant medium 31 circulating within the refrigerator, an expander 33 that expands and cools the compressed refrigerant medium 31 discharged from the compressor unit 32, and It consists of a second freezing stage 34 that is cooled to, for example, 80K by the freezing medium 31 cooled by the machine 33, and a third freezing stage 35 that is cooled to, for example, 20K by the freezing medium 31.

On the other hand, a power lead 10 that supplies an exciting current to the superconducting magnet 1 exits from the liquid helium 2 and enters the inner tank 3.
After passing through the radiation shield 6 and taking the required conductor length and conductor cross-sectional area, it is connected 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
Finally, after obtaining the required conductor length and conductor cross-sectional area, the conductor is passed through the outer tank 5 and connected to the superconducting magnet power source 9.

Here, power lead 10 and each freezing stage 3
4 and 35 are electrically insulated. In addition, a helium recondenser 2 passing through the inner tank 3
The penetrating portions of the helium 5 and the power lead 10 have an airtight structure, so that the evaporated 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 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 third freezing stage 35.
The cross-sectional area of the conductor between the superconducting magnets 1 is successively reduced in this order.

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

Generally, as for heat intrusion from the power lead, increasing the cross-sectional area of the power lead will reduce the Joule heat, but the intrusion heat due to thermal conduction will increase. vice versa,
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 temperature/refrigeration capacity of the second and third refrigeration stages 34 and 35, and the conductor length of the power lead. Therefore, in order to match the refrigerating capacity of the second and third refrigerating stages 34 and 35 of the second small refrigerator 30, between the liquid helium 3 and the third refrigerating stage 35 (for example, 20K), the third refrigerating Between the stage 35 and the second freezing stage 34 (e.g. 80K) and between the third freezing stage 35 and the outer tank 5 (e.g.
By appropriately selecting the conductor length and cross-sectional area of the power lead 10 between
The heat entering from the power lead 10 can be minimized.

This optimum condition can be determined, for example, by the following equation, which is generally well known. In other words, Q=I・√・T _h −T _c cos(τl/a)/sin(τl
/a) When /a=1/τ cos ^-1 T _c /T _h , the amount of heat penetration becomes the minimum value Qmin Qmin=I·√( _h ² − _c ² ). Here, Q: Heat penetration amount I: Current value λ: Thermal conductivity α: Constant (ρ=αT, ρ: Power lead resistivity,
T: Temperature) K: Thermal conductivity τ: τ=I・√ T _h : High temperature part temperature T _c : Low temperature part temperature a: Power lead cross-sectional area l: Power lead length In this way, the power lead 10 Heat intrusion can be reduced, and furthermore, since the power lead 10 does not need to be cooled with gas, the amount of evaporated helium is 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 is transferred to the inner tank 3.
The latent heat is removed by a helium recondenser 25 installed inside the helium tank, whereupon the latent heat is recondensed, and the droplets are returned as liquid helium 2 inside the sealed inner tank 3. Further, the radiation shield plate 6 is directly connected to the first freezing stage 24 (for example, 80K) of the first small refrigerator 20, and is directly cooled by heat conduction from the first freezing stage 24. This results in a compact and easily constructed shield.

As described above, the superconducting magnet device of this embodiment provides the following effects.

(a) Since the power lead 10 can be directly cooled by a small refrigerator and the inner tank 3 of the cold storage container 4 can be sealed, evaporation of liquid helium can be suppressed. Since the amount of helium evaporation is reduced, the helium gas in the inner tank 3 can be recondensed using a conventional small refrigerator.

(b) Since the power lead 10 is directly cooled,
Even while the superconducting magnet device is in operation, the magnetic field strength, that is, the magnetic field current value can be changed arbitrarily without destroying the superconducting state. As a result, when this device is applied to a superconducting magnet device for a single crystal pulling device, for example, it becomes possible to suppress the impurity concentration in the single crystal by controlling the magnetic 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 small refrigerators 20 and 30, which are equivalent to conventional ones and have a capacity corresponding to the capacity of the small superconducting magnet 1, are 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 example,
A second small refrigerator 3 that cools the power lead 10
The temperature of the second freezing stage 34 of 0 is made the same as the temperature of the first freezing stage 24 of the first small refrigerator 20, and the radiation shield plate 6 is connected to each freezing stage 3.
4 and 24 directly. With this configuration, the cooling capacity of the radiation shield is increased, the heat shield effect is improved, and the liquid helium 2 and the second freezing stage 3
The power leads 10 between the four are thermally shielded by the radiation shield plate 6 at the relevant temperature (for example, 80K),
The amount of heat entering from the power lead 10 is further reduced.

FIG. 4 also shows an example of the configuration of a 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 temperature of the cold storage container 4 is sequentially lowered (for example, 80K, 20K,
4.2K) Three freezing stages 104, 105, 1
A small refrigerator 100 having 06 is provided to cool the power lead 10 and recondense the evaporated helium in the inner tank 3 at the same time. With this configuration, a more compact superconducting magnet device can be obtained.

〔Effect of the invention〕

As explained above, according to the present invention, a small refrigerator and a small superconducting magnet are combined, and the conductor cross-sectional area and conductor length between each freezing stage of the power lead that supplies excitation current from the superconducting magnet power source are Since the structure is configured such that the amount of heat that enters the cooling medium inside the magnet is minimized, a compact and low-cost superconducting magnet device that can control the excitation current value can be provided.

[Brief explanation of the drawing]

Figure 1 is a configuration 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 ... Cooling 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 piping, 12 ... Liquid helium transfer pipe, 13 ... Gas helium, 14 ... Outer tank,
15... Helium liquefaction machine, 16... Magnetic field application equipment, 20, 30, 100... Small refrigerator, 21,
31,101...refrigeration medium, 22,32,102
... Compressor unit, 23, 33, 103 ... Expander, 24, 34, 35, 104, 105, 10
6...Freezing stage, 25...Helium recondenser.

Claims (1)

[Claims] 1. 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 multi-stage refrigeration system attached to the cold storage container and inside the cold storage container. A small refrigerator having a stage, a radiation shield plate provided between an inner tank and an outer tank of the cold storage container, and thermally coupled to the freezing stage, so that the amount of heat intruding into the cooling medium is minimized. A superconducting magnet device characterized in that a conductor cross-sectional area and a conductor length between each freezing stage are set as follows, and a power lead is provided for supplying excitation current to the superconducting magnet from a superconducting magnet power source.