JP2853347B2 - Current lead of superconducting magnet device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current lead for supplying a DC exciting current from an external power supply to a superconducting coil housed in a vacuum insulated container, and more particularly to a current lead using an oxide superconducting conductor for a low-temperature side lead. Quench protection structure.

[0002]

2. Description of the Related Art Since a superconducting coil of a superconducting magnet device is cooled by a cryogenic refrigerant such as liquid helium to maintain a superconducting state, a liquid shield is provided in a vacuum shielded container having a radiation shield using liquid nitrogen or a multilayer heat insulating layer. It is stored in a immersed state. Further, the current lead is cooled by the low-temperature helium gas in which the liquid helium is vaporized, and is configured to prevent intrusion heat from the normal temperature side and Joule heat generated in the current lead from entering the cryogenic portion. Conventionally, electric current conductors such as copper were used as conductors for current leads.
Since copper is a good conductor at the same time as a good conductor, the heat penetrating into the cryogenic part increases, and the vaporization loss of expensive liquid helium increases. Therefore, a current lead that uses an oxide-based superconductor, which is a high-temperature superconductor, on the low-temperature side of the current lead, reduces the Joule heat to zero, and at the same time uses the low thermal conductivity to greatly reduce the heat that enters the cryogenic part. Has already been proposed by the applicant of the present application (for example, Japanese Patent Application No. 2-8425).
No. 2).

FIG. 4 is a partially broken sectional view schematically showing a conventional structure of a current lead of a superconducting magnet device, and FIG. 5 is an enlarged sectional view showing a main part of FIG. In the figure, a superconducting coil 1 is housed in a vacuum insulated container 2 in a state of being immersed in liquid helium He, and is electrically connected to a low-temperature terminal 5A of a current lead 3 by a lead wire 6. The current lead 3 is formed as a series connection of a high-temperature side lead 4 having a normal-temperature terminal 4A on the upper side and a low-temperature side lead 5 having a low-temperature terminal 5A, and low-temperature helium gas GHe passes through the lead and enters the normal-temperature terminal 4A.
Cools by pulling out to the side. Fig. 5 for low-temperature lead
As shown in FIG. 1, a round bar having high rigidity and low thermal conductivity, such as stainless steel, manganese steel, or nichrome steel, is used as a core material 7, and an oxide superconducting conductor 8 is spirally wound around the core material 7. It is housed in an outer tube 9 made of a conductive metal and forms a cooling passage with low-temperature helium gas between the outer tube 9 and the outer tube.
The temperature of the oxide-based superconducting conductor 8 is set to the temperature of liquid nitrogen (about 77
By cooling to below K), the oxide-based superconducting conductor is brought into a superconducting state, and a current lead of a superconducting magnet device is obtained in which less heat enters the low-temperature side and consumption of liquid helium is small.

[0004]

In the current lead constructed as described above, a stable superconducting state is rarely obtained from the beginning of energization of the oxide superconducting conductor 8;
In general, a stable superconducting state is reached by gradually increasing the current while repeating a quenching phenomenon in which the superconducting state is locally broken and a normal conducting transition occurs.
In addition, even when a stable superconducting state is reached, quench may occur at an unexpected time. By the way, when the current supply to the superconducting coil is stopped every time a quench occurs, an abnormal voltage is generated due to the potential oscillation of the superconducting coil 1 each time, which causes damage to the insulation of the superconducting coil and short-circuit of the coil. There is a problem that the risk of developing is increased. In addition, a situation in which the oxide-based superconducting conductor in the portion where the normal conduction transition occurs is locally heated and burns out occurs. Therefore, it is desired to develop a current lead that can continuously supply the exciting current to the superconducting coil even if a quench occurs in the low-temperature lead and that can prevent burnout of the oxidized superconducting conductor in the quenched portion. ing. As a countermeasure for this, it is known to provide an electrically good conductor such as silver as a stabilizer in parallel with the oxide-based superconducting conductor, but the problem of increased heat penetration through the stabilizer, which is also a thermally good conductor, is known. Newly occurs.

SUMMARY OF THE INVENTION It is an object of the present invention to supply current to a superconducting coil and prevent damage to the oxide superconducting conductor even if a quench occurs in the oxide superconducting conductor of the low-temperature lead. An object of the present invention is to obtain a current lead with less invasion heat.

[0006]

According to the present invention, there is provided a superconducting coil which is housed in a vacuum insulated container and maintained at a very low temperature, and a current which flows an exciting current from an external power supply to the superconducting coil. The lead is composed of a series connection of a high-temperature side lead made of a good conductive metal and a low-temperature side lead made of an oxide-based superconductor, and cooled by a low-temperature refrigerant gas, wherein the quench of the oxide-based superconductor is performed. A bypass circuit that detects an abnormal voltage generated in the low-temperature side lead and becomes conductive is provided in parallel with the low-temperature side lead.

[0007] It is also assumed that the bypass circuit is a power diode that senses an abnormal voltage of the low-temperature side lead by a potential barrier on the junction surface and passes a forward current as a bypass current.

Further, the bypass circuit is turned on by receiving the abnormal voltage of the low-temperature side lead as a trigger signal,
It is assumed that the switching semiconductor element is a switching semiconductor element that passes current in the same direction as the low-temperature side lead.

Further, it is assumed that the bypass circuit is a voltage non-linear resistance element.

[0010]

In the configuration of the present invention, a bypass circuit is provided in parallel with the low-temperature side lead, and an abnormal voltage generated in the low-temperature side lead due to the quench of the oxide superconductor is detected to make the bypass circuit conductive. As a result, the current of the low-temperature side lead is bypassed to the bypass circuit side at the same time as the occurrence of the quench, so that the exciting current is continuously supplied to the superconducting coil. And the damage of the superconducting coil due to this can be avoided, and the function of suppressing the heat generation of the oxide superconducting conductor in the quench portion and promoting the recovery of the quench can be obtained.

Further, if the bypass circuit is a power diode that allows a forward current as a bypass current to flow,
When the abnormal voltage of the low-temperature side lead due to the quench exceeds the potential barrier at the junction surface of the power diode, a forward current flows through the diode, and the exciting current to the superconducting coil can be supplied via the bypass circuit. Is obtained, and the diode itself senses an abnormal voltage rise to perform a protection operation. Therefore, a bypass circuit having a simple configuration without a control circuit can be obtained.

Further, the bypass circuit is brought into conduction by receiving an abnormal voltage of the low temperature side lead as a trigger signal,
By using a switching semiconductor element that bypasses the current in the same direction as the low-temperature side lead, the same operation as described above can be obtained using a power switching element such as a thyristor or power transistor, and the abnormal voltage detection level is adjusted. By doing so, a highly reliable protection operation that does not malfunction due to noise or the like can be performed.

Furthermore, even if a voltage non-linear resistance element such as a zinc oxide surge absorber or a silicon surge absorber is used in the bypass circuit, a quench protection operation for bypassing a current can be obtained by utilizing its voltage suppressing action and discharge capability. Can be

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments. FIG. 1 is a configuration diagram schematically showing a current lead of a superconducting magnet device according to an embodiment of the present invention. In the following, the same parts as those in the prior art are denoted by the same reference numerals, and redundant description will be omitted. In the figure, a current lead 10 composed of a series body of a high-temperature side lead 4 and a low-temperature side lead 5 includes a bypass circuit 11 mainly composed of a power diode 12 in parallel with the current lead 5, and the power diode 12 has a forward direction. The low-temperature terminal 5A and the high-temperature side lead are conductively connected to appropriate places so as to be in the same direction as the DC exciting current I flowing through the current lead.

In the current lead 10 of the superconducting magnet device having the above-described structure, the oxide superconductor of the low-temperature side lead 5 is kept in a superconducting state, and in a steady conduction state where the electric resistance is zero, the power is not increased. The voltage between the terminals of the diode 12 is lower than the potential barrier (also referred to as contact potential) at the junction surface, no current flows through the power diode, and the exciting current I is supplied to the superconducting coil through the oxide-based superconducting conductor. On the other hand, when quenching occurs in the oxide-based superconducting conductor and the electric resistance locally increases, current tends to flow to the superconducting coil through the core material 7 made of a low conductive metal of the low-temperature side lead and the outer tube 9. However, since the electrical resistance of the core material and the outer tube is high, the voltage (resistance drop) of the low-temperature side lead rises abnormally, and the voltage between both ends of the power diode 12 rises beyond the potential barrier of the junction surface. As a result, a forward current as a bypass current flows in the power diode 12 in the same direction as the current lead, and continuously supplies an exciting current to the superconducting coil. Therefore, it is possible to avoid the abnormal voltage of the superconducting coil caused by the sudden stop of the current supply and to protect the superconducting coil. At this time, since the power diode 12 keeps a low forward drop, most of the current flowing in the low-temperature side lead flows by-passing to the bypass circuit, so that overheating of the oxide-based superconducting conductor stops. Cooling by helium gas promotes quench recovery.

FIG. 2 is a block diagram showing a different embodiment of the present invention. The current lead 20 is provided with a bypass circuit 21 including a thyristor 22 and a drive circuit 23 in parallel with the low-temperature side lead 5. This is different from the embodiment.
In other words, the thyristor 22 is connected so that its forward direction is the same as the current I flowing through the low-temperature side lead 5, the drive circuit 23 detects an abnormal voltage of the low-temperature side lead due to quench, and converts it into a thyristor gate pulse. The thyristor 22 is turned on by the supply, and the current is continuously supplied to the superconducting coil. In this embodiment, since the voltage level at which the thyristor is turned on can be adjusted by the drive circuit, a bypass circuit that does not malfunction due to external noise can be obtained. The same function as described above can be obtained by using not only a thyristor but also a bipolar or unipolar power transistor as a switching element.

FIG. 3 is a block diagram showing another embodiment of the present invention. In each of the above embodiments, the bypass circuit 31 is constituted by a voltage non-linear resistance element 32 such as a zinc oxide surge absorber and a silicon surge absorber. Is different.
In the current lead configured as described above, when the quench occurs in the oxide-based superconductor of the low-temperature side lead 4 and the voltage increases, the electric resistance of the element 32 decreases nonlinearly and the current of the low-temperature side lead increases. Is bypassed to the bypass circuit 31 side, so that the same protection operation as in the above embodiment can be obtained. In particular, when an element having a negative resistance characteristic such as a silicon surge absorber is used, the terminal voltage of the element 32 can be maintained at a constant value and the current can be bypassed, so that the method of selecting the breakover voltage of the element is higher. The advantage that a bypass circuit having protection performance can be configured can be obtained.

[0018]

As described above, according to the present invention, the bypass circuit is provided in parallel with the low-temperature side lead, and an abnormal voltage generated in the low-temperature side lead due to the quench of the oxide superconductor is detected to make the bypass circuit conductive. With this configuration, the current of the low-temperature side lead can be bypassed to the bypass circuit side and the excitation current can be continuously supplied to the superconducting coil at the same time as the occurrence of quench. Abnormal voltage caused by sudden stop,
This can avoid damage to the superconducting coil,
It is possible to provide a superconducting magnet device including a current lead having a function of suppressing heat generation of the oxide-based superconducting conductor in the quench portion and promoting recovery of the quench.

Further, if the bypass circuit is a power diode that passes a forward current as a bypass current,
Since the power diode self-detects the occurrence of quench by the potential barrier at its junction surface and performs a protection operation, a control circuit is not required, and a superconducting magnet device equipped with a current lead having a bypass circuit with a simple configuration can be economically manufactured. Advantages that can be provided advantageously

Further, the bypass circuit is turned on by receiving an abnormal voltage of the low-temperature side lead as a trigger signal,
If it is composed of a switching semiconductor element that bypasses the current in the same direction as the low-temperature side lead, it malfunctions due to noise etc. by adjusting the detection level of the abnormal voltage using a power switching element such as a thyristor or a power transistor. It is possible to provide a current lead of a superconducting magnet device having a highly reliable bypass circuit without any problem.

Furthermore, a zinc oxide surge absorber,
Even if a voltage non-linear resistance element such as a silicon surge absorber is used, it is possible to provide a current lead having a simple configuration of a bypass circuit that performs protection operation by detecting an abnormal voltage of a low-temperature side lead due to quench.

[Brief description of the drawings]

FIG. 1 is a configuration diagram schematically showing a current lead of a superconducting magnet device according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a different embodiment of the present invention.

FIG. 3 is a configuration diagram showing another embodiment of the present invention.

FIG. 4 is a partially broken sectional view showing a simplified structure of a conventional current lead of a superconducting magnet device.

FIG. 5 is an enlarged sectional view showing a main part of FIG. 4;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Superconducting coil 2 Vacuum insulation container 3 Current lead 4 High temperature side lead 5 Low temperature side lead 7 Core material 8 Oxide superconducting conductor 9 Outer tube 10 Current lead 11 Bypass circuit 12 Power diode 21 Bypass circuit 22 Thyristor 23 Drive circuit 31 Bypass circuit 32 Voltage non-linear resistance element

Claims (4)

(57) [Claims] A current lead for passing an exciting current from an external power supply through a superconducting coil housed in a vacuum insulated container and kept at a very low temperature; a high-temperature side lead made of a good conductive metal;
It consists of a series connection with a low-temperature side lead made of an oxide-based superconducting conductor and is cooled by a low-temperature refrigerant gas, and detects an abnormal voltage of the low-temperature side lead caused by quenching of the oxide-based superconducting conductor. A current lead for a superconducting magnet device, wherein a bypass circuit that is in a conductive state is provided in parallel with the low-temperature side lead. 2. The power supply according to claim 1, wherein the bypass circuit is a power diode that senses an abnormal voltage of the low-temperature side lead by a potential barrier at the junction surface and passes a forward current as a bypass current. Current lead of superconducting magnet device. 3. The switching circuit according to claim 1, wherein the bypass circuit is a switching semiconductor element which is rendered conductive by an abnormal voltage of the low-temperature side lead as a trigger signal and bypasses the current in the same direction as the low-temperature side lead. Current lead of superconducting magnet device. 4. A current lead for a superconducting magnet device according to claim 1, wherein said bypass circuit is a voltage non-linear resistance element.