JPH08203726A - Superconducting coil device - Google Patents

Abstract

PURPOSE: To provide a superconducting magnet device which provide stable operating properties in an energizing process and a demagnetizing process. CONSTITUTION: During energizing and magnetizing time, a helium vessel 2 is decompressed by means of a pressure reducing pump 8 and the temperature of liquid helium 3 is lowered to below 4.2K, thereby lowering the temperature of a superconducting coil 1. This drop in the temperature increases a temperature margin of the superconducting coil 1 and reduces instability of the superconducting coil 1. In the steady permanent current mode, the pressure reduction is stopped and the pressure reducing pump 8 is removed, thereby operating the superconducting coil 1 at the temperature of 4.2K.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting coil device such as a superconducting magnet.

[0002]

2. Description of the Related Art FIG. 7 is a diagram showing a conventional superconducting magnet device used in, for example, a medical magnetic resonance imaging apparatus. In the figure, the superconducting coil 1 is immersed in liquid helium 3 stored in a helium container 2 which is a cryogenic container, cooled to a cryogenic temperature of about 4.2 K, and maintained at that temperature. In order to reduce the evaporation amount of the liquid helium 3, radiant heat shields 4 and 5 are provided between the helium container 2 and the vacuum container 6 covering the whole, and the helium container 2, the radiant heat shields 4 and 5, the vacuum container Each of 6 is thermally insulated. The radiant heat shields 4 and 5 are kept at a low temperature by the refrigerator 7 to reduce the evaporation amount of the liquid helium 3.

FIG. 8 is a diagram showing a circuit of a superconducting magnet device. Reference numeral 12 is a current supply lead, 13 is a permanent current switch (hereinafter referred to as PCS) for short-circuiting and opening the superconducting coil 1, and 14 is a resistance for the PCS 13. A heater for generation, 15 is a power source for a PCS heater, 16 is a power source for excitation / demagnetization, and 17 is a protection element.

Next, the operation will be described. When the superconducting magnet device is operated, first, the current supply lead 12 is attached, and then the heater 14 is energized from the PCS heater power source 15 to open the PCS 13. Next, a current is supplied from the excitation / demagnetization power supply 16 to the superconducting coil 1 to perform excitation, and when a predetermined magnetic field output is reached, the heater 14 is turned off and the PCS 13 is short-circuited (excitation process). As a result, the current is confined in the superconducting coil 1 (permanent current mode), and it is possible to obtain a stable and high magnetic field peculiar to the superconducting magnet device as a steady state. After the permanent current mode is set, the current supply lead 12 is removed to reduce heat intrusion, and the excitation is completed.

When performing degaussing, the current supply lead 12 is attached again and the current from the excitation / degaussing power supply 16 is made to rise. When the current from the excitation / demagnetization power supply 16 becomes the same as the current of the superconducting coil 1, the PCS heater power supply 15 energizes the heater 14 to open the PCS 13. Next, the current of the excitation / demagnetization power supply 16 is reduced to zero. After the current becomes zero, the heater 14 is turned off and the current supply lead 12 is removed to complete the demagnetization (degaussing process).

[0006]

Since the conventional superconducting magnet device is constructed and operated as described above, heat is generated by the heater 14 of the PCS 13 and heat from the current supply lead 12 is used during its excitation and demagnetization processes. The temperature of the liquid helium rises due to the penetration, and the temperature margin of the superconducting coil 1 becomes smaller than that in the persistent current mode. Further, the electromagnetic force generated changes with the operations of excitation and demagnetization, and slippage is likely to occur in the superconducting coil 1. Then, when slip occurs, heat is generated due to friction, and the temperature of the wire rod rises. In the worst case, a quench in which the superconducting state is destroyed occurs in the superconducting coil 1. As described above, the conventional superconducting magnet device has a problem that the instability is significantly increased during the excitation and demagnetization operations as compared with the steady operation.

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a superconducting magnet device which can obtain stable operation characteristics during excitation and demagnetization. Moreover, it is intended to realize the above object economically as much as possible.

[0008]

A superconducting coil device according to a first aspect of the present invention is configured so that the temperature of the superconducting coil in the excitation process and the demagnetization process is made lower than the temperature in the permanent current mode.

In the superconducting coil device according to the second aspect of the invention, the temperature of the refrigerant is lowered by depressurizing the inside of the cryogenic container during the excitation process and the demagnetization process.

Further, a superconducting coil device according to a third aspect of the present invention includes a heat conductor for guiding the heat of the superconducting coil to the outside of the cryogenic container, and the heat from the outside of the cryogenic container is used by the refrigerator in the exciting process and the degaussing process. The superconducting coil is cooled via a conductor.

A superconducting coil device according to a fourth aspect of the present invention is provided with a heat conductor for guiding the heat of the refrigerant to the outside of the cryogenic container, and the heat conduction from the outside of the cryogenic container by the refrigerator in the exciting process and the degaussing process. The refrigerant is cooled through the body.

[0012]

In the superconducting coil device according to the first aspect of the present invention, when the temperature of the superconducting coil is lowered during excitation / demagnetization, the temperature margin increases, and due to the peculiar heat penetration and the like during the same excitation / demagnetization. The instability of the superconducting coil is reduced.

Further, in the superconducting coil device according to the second aspect, the temperature of the refrigerant is lowered by depressurizing the inside of the cryogenic container at the time of demagnetization, and the temperature of the superconducting coil is accordingly reduced.

Further, in the superconducting coil device according to the third aspect of the present invention, at the time of excitation / demagnetization, the superconducting coil is directly cooled by the refrigerator via the heat conductor, and the temperature thereof is lowered.

Further, in the superconducting coil device according to the fourth aspect, the refrigerant is cooled by the refrigerator through the heat conductor and its temperature is lowered at the time of demagnetization, and accordingly, the temperature of the superconducting coil is lowered.

[0016]

【Example】

Example 1. 1 is a sectional view showing the structure of a superconducting magnet device according to a first embodiment of the present invention, which is used in a magnetic resonance imaging apparatus for medical use as in the prior art. In the figure, the superconducting coil 1, the helium container 2, the liquid helium 3, the radiant heat shields 4, 5, the vacuum container 6 and the refrigerator 7 are the same as those in the conventional device, and their explanations are omitted. A decompression pump 8 is structured to communicate with the inside of the helium container 2 via an exhaust pipe 9. Although illustration is omitted, the circuit configuration of the superconducting magnet device is the same as that of the conventional FIG.

Next, the operation of exciting the superconducting magnet device of FIG. 1 (excitation process) will be described with reference to the timing chart of FIG. In the initial state before the exciting operation (time t = t _{0 in} FIG. 2), the pressure P ₀ in the helium container 2 is approximately 1 atm (= 0.1 MPa), and the temperature of the superconducting coil 1 is equal to that of the liquid helium 3. The temperature T ₀ is kept at 4.2K. When the superconducting magnet device is to be excited, the decompression pump 8 first evacuates the exhaust pipe 9 to decompress the helium container 2. Along with this, the temperature of the liquid helium 3 drops, for example, according to the vapor pressure characteristic shown in FIG.

The pressure is reduced until the temperature of the superconducting coil 1 reaches a predetermined temperature T ₁ lower than the initial temperature T ₀ (time t =
t ₁₎ and, thereafter, vacuum pump 8 to maintain the pressure P ₁ at that time. In this state, the current supply lead 12 is attached (time t = t ₂ ), and when the heater 14 is energized by the PCS heater power source 15 to turn it on, the PCS 13 is opened (time t = t ₃ ).

Now that the preparation is completed, the excitation / demagnetization power supply 16 is operated to start the current from time t = t ₄ . A magnetic field is generated in the superconducting coil 1 by passing this current. When the current magnetic field reaches the desired values I ₀ and B ₀ (time t = t ₅ ), the excitation / demagnetization power supply 16 holds the output current value I ₀ . Next, turn off the power supply to the heater 14 and set P
After the CS 13 is short-circuited (time t = t ₆ ), the excitation / demagnetization power supply 16 is operated again to decrease the output current (time t
= T ₇ ), the current becomes zero at time t = t ₈ .

After that, when the current supply lead 12 is removed at time t = t ₉ , the so-called excitation process started at time t = t ₂ ends. After that, since the above-mentioned thermal instability factor disappears, the pressure inside the helium container 2 returns to about 1 atm (P ₀ ) when the evacuation operation by the decompression pump 8 is stopped from the time t = t ₁₀ , and the superconducting coil 1 Also becomes the original temperature T ₀ (about 4.2 K) (time t = t ₁₁ ). That is,
The superconducting magnet device enters the state of steady operation in the permanent current mode, and the excitation is completed.

Next, effects and the like when the temperature of the superconducting coil 1 is specially lowered in the exciting process will be described based on concrete materials and physical properties. FIG. 4 is an example of the critical current characteristics of a practically used NbTi superconducting wire with temperature as a parameter. Hereinafter, the case where the superconducting wire of FIG. 4 is used for the superconducting coil 1 will be described.

In FIG. 4, the characteristic A is the critical current (A) and the magnetic flux density (T = T = 2) when the temperature of the superconducting coil is 4.2K.
(Tesla). Now, assuming that the superconducting coil is operated at an operating point with a current of 250 (A) and a maximum magnetic flux density in the coil of 3.1 (T) (indicated by a circle in the figure), the critical current characteristic B including this operating point Corresponds to the characteristic at a temperature of 5.4K as shown in the figure. Therefore, the temperature margin of the superconducting coil is 1.2K.

In the excitation process, the instability of the superconducting coil is most increased during the period from time t = t ₅ to t _{6 in} FIG. 2, that is, when the current rises and reaches its maximum value (steady value) in the transient state. If the coil slips at this time, the frictional heat causes the temperature of the superconducting coil to rise by about 0.9K and the thermal margin to decrease to only 0.3K. However, in the first embodiment, the pressure inside the helium container 2 is reduced by the pressure reducing pump 8 during the period from time t = t ₁ to t _{10 in} FIG.
Since the temperature of the superconducting coil is lowered, the temperature margin increases correspondingly, and the instability of the superconducting coil can be reduced.

That is, the period from the time t = t ₁ to t ₁₀ ,
The pressure in the helium container 2 is, for example, 0.0240 (M
If the pressure is reduced to Pa), the temperature of liquid helium becomes about 3.0K (FIG. 3), and the temperature of superconducting coil 1 can also be lowered to about 3.0K. As a result, the critical current characteristic of the superconducting coil becomes the characteristic shown in C of FIG. 4, and the temperature margin of the superconducting coil increases from 1.2K to 2.4K.

The excitation process has been described above. However, even in the demagnetization process of decreasing the current from the permanent current mode,
As in the case of the excitation process, the pressure in the helium container 2 is lowered to lower the temperature of the superconducting coil 1 to a temperature lower than 4.2K, thereby increasing the temperature margin and obtaining a superconducting magnet device with less instability. You can

Except at the time of demagnetization, the exhaust pipe 9 and the decompression pump 8
It is also possible to remove the pressure reducing device such as. By removing this decompression device, heat intrusion can be reduced, and the required installation space of the superconducting magnet device can be reduced to operate in the same installation space as the conventional superconducting magnet device.

Further, as compared with the period in which the superconducting magnet device is operated in the permanent current mode, which is the original operating mode thereof, for several months or several years, the time required for excitation / demagnetization is about several hours. Therefore, when a plurality of superconducting magnet devices are installed, one set of decompressor can be used in common by shifting the excitation / demagnetization work of each device in terms of time. Countermeasures can be realized extremely economically.

Embodiment 2 FIG. FIG. 5 is a sectional view showing the structure of a superconducting magnet device according to a second embodiment of the present invention. 1
6 are the same as those in the first embodiment. Reference numeral 10 denotes a refrigerator, the cooling capacity of which is enhanced as compared with the refrigerator 7 of the first embodiment so that the superconducting coil 1 can be cooled at a temperature lower than 4.2K. Reference numeral 11 denotes a heat conductor whose one end is directly joined to the superconducting coil 1 to guide the heat of the superconducting coil 1 to the outside of the helium container 2.

Next, the operation, particularly the exciting operation of the superconducting magnet device will be described. First, the refrigerator 10 cools the superconducting coil 1 via the heat conductor 11 to lower the temperature of the superconducting coil 1 to a predetermined temperature lower than 4.2K. After that, as in the first embodiment, the current supply lead 1
2 is attached and the heater 14 is turned on to open the PCS 13. A current is supplied from the excitation / demagnetization power supply 16 to perform excitation, and when the predetermined magnetic field output is reached, the heater 14
Is turned off to bring the PCS 13 into a short circuit state. afterwards,
The current of the power supply 16 for excitation / demagnetization is lowered to lead 1 for supplying current.
When 2 is removed and the excitation process is completed, the output of the refrigerator 10 is adjusted to bring the temperature of the superconducting coil 1 back to 4.2K, and the permanent current mode is entered. In the demagnetization process as well, the output of the refrigerator 10 is adjusted to reduce the temperature of the superconducting coil 1 to a predetermined value.

As described above, in the second embodiment, since the heat conductor 11 is provided in the helium container 2, it is possible to directly cool the superconducting coil 1 by the refrigerator 10 and lower its temperature. As in Example 1, the critical current characteristics of the superconducting wire are improved, the temperature margin of the superconducting coil can be increased, and the instability of the superconducting coil can be reduced.

Since the refrigerator 10 directly cools the superconducting coil 1, it is necessary to have a higher cooling capacity than the conventional refrigerator 7. However, as described above, the periods of excitation / demagnetization are compared. It is a relatively short time, and it is sufficient to withstand overload operation of this time rating, and a relatively slight capacity enhancement measure is sufficient. Further, if only the conventional refrigerating machine 7 is provided with a refrigerating machine for capacity enhancement only during excitation / demagnetization, as described in the first embodiment, one refrigerating machine for capacity enhancement is used as a plurality of superconducting superconductors. Cost sharing can be realized by sharing the magnet device.

Example 3. 6 is a sectional view showing the structure of a superconducting magnet device according to a third embodiment of the present invention. The difference from the second embodiment is the heat conductor 21. That is, the heat conductor 21 draws heat from the liquid helium 3 by the refrigerator 10 and lowers the temperature of the liquid helium 3. Then, the superconducting coil 1 is cooled by the liquid helium 3 whose temperature has dropped, and the temperature thereof drops.

Although an indirect cooling system is adopted in that the temperature of the superconducting coil 1 is lowered, the heat conductor does not come into direct contact with the superconducting coil 1, so that there is an advantage that electrical insulation of this portion can be facilitated. .

Although the description will not be repeated, the instability of the superconducting coil at the time of excitation / demagnetization can be reduced in the third embodiment as in the above-described embodiments.

In the application of the present invention, the means for lowering the temperature of the superconducting coil only in the excitation / demagnetization process, the temperature value to be lowered, the superconducting wire and the refrigerant are limited to those described in the above embodiment. Needless to say, the present invention is not limited to the superconducting magnet device for the medical magnetic resonance imaging apparatus, but can be widely applied to the superconducting coil device and has the same effect.

[0036]

As described above, in the superconducting coil device according to claim 1 of the present invention, the temperature of the superconducting coil is set to be lower than that in the persistent current mode only during the demagnetization process, so that it is possible to reduce the cost. It is possible to reduce instability of the superconducting coil during the same demagnetization.

Further, in the superconducting coil device according to the second aspect of the present invention, the temperature of the refrigerant is lowered by depressurizing the inside of the cryogenic container at the time of demagnetization, so that the temperature of the superconducting coil is lowered. It is not necessary to change the refrigerating apparatus, and the instability of the superconducting coil can be reduced at low cost.

Further, in the superconducting coil device according to the third aspect of the present invention, at the time of demagnetization, the refrigerating machine directly cools the superconducting coil through the heat conductor to lower its temperature. Only increasing the capacity of the refrigerator is sufficient, and the instability of the superconducting coil can be reduced at low cost.

Further, in the superconducting coil device according to the fourth aspect, the temperature of the refrigerant is lowered by the refrigerator via the heat conductor at the time of demagnetization, so that the temperature of the superconducting coil is lowered. It is sufficient to increase the capacity of the refrigerator of this type, and the instability of the superconducting coil can be reduced at low cost.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration of a superconducting magnet device according to a first embodiment of the present invention.

FIG. 2 is a timing chart in the excitation process of the superconducting magnet device in the first embodiment.

FIG. 3 is a table showing the relationship between the saturated vapor pressure of helium and temperature.

FIG. 4 is a diagram showing a critical current characteristic of an NbTi superconducting wire.

FIG. 5 is a sectional view showing the structure of a superconducting magnet device according to a second embodiment of the present invention.

FIG. 6 is a sectional view showing a structure of a superconducting magnet device according to a third embodiment of the present invention.

FIG. 7 is a cross-sectional view showing a configuration of a conventional superconducting magnet device.

FIG. 8 is a diagram showing a circuit of a conventional superconducting magnet device.

[Explanation of symbols]

1 superconducting coil, 2 helium container, 3 liquid helium, 7, 10 refrigerator, 8 decompression pump, 11, 12
Heat conductor, 13 PCS, 16 Power supply for excitation / demagnetization.

Claims (4)

[Claims] 1. A superconducting coil, a refrigerant for cooling the superconducting coil, a cryogenic container for storing the superconducting coil and the refrigerant, a power supply for supplying a current to the superconducting coil, and a permanent current for short-circuiting and opening the superconducting coil. A switch is provided, the permanent current switch is opened, the current of the superconducting coil is increased by the power source, and the permanent current switch is short-circuited after the current reaches a predetermined value. Permanent current mode in which the persistent current switch is constantly short-circuited to confine the current in the superconducting coil, and following this permanent current mode, the permanent current switch is switched from the short-circuited state to the open state to lower the current in the superconducting coil by the power supply. In a superconducting coil device having an operating mode of a demagnetizing process, Superconducting coil apparatus the temperature of the superconducting coil in the demagnetization process called characterized in that as allowed to lower than the temperature in the persistent current mode. 2. The superconducting coil device according to claim 1, wherein the temperature of the superconducting coil is lowered by reducing the temperature of the refrigerant by depressurizing the cryogenic container during the exciting process and the degaussing process. . 3. A heat conductor for radiating the heat of the superconducting coil to the outside of the cryogenic container, and cooling the superconducting coil from outside the cryogenic container by a refrigerator through the heat conductor during the excitation process and the degaussing process. The superconducting coil device according to claim 1, wherein the temperature of the superconducting coil is lowered by doing so. 4. A heat conductor for radiating the heat of the refrigerant to the outside of the cryogenic container, wherein the refrigerant is cooled from outside the cryogenic container by a refrigerator via the heat conductor during the excitation process and the degaussing process. The superconducting coil device according to claim 1, wherein the temperature of the superconducting coil is lowered by the above.