JPH0432615B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a protection device for a superconducting coil, and particularly to a device for protecting a persistent current switch from superconductor breakdown during persistent current operation.

[Conventional technology]

Figure 3 shows, for example, page 97 of the proceedings of the 31st Low Temperature Engineering Research Conference (held May 7-9, 1984 at Kikai Shinko Kaikan, Tokyo), lecture number.
FIG. 3 is a circuit diagram showing a conventional superconducting magnet and its protection device shown in A3-20. In the figure, 10
is a cryogenic container filled with liquid helium 3, which is a refrigerant, and houses a superconducting coil 1, a persistent current switch 2, and a diode circuit 9 inside. These superconducting coils 1, persistent current switches 2
The diode circuits 9 and 9 are connected in parallel, and their lead wires are led out of the cryogenic container 10 and connected to the excitation power source 7. The persistent current switch 2 also includes a persistent current switch superconductor 4, a heater 5 for heating the persistent current switch superconductor 4, and a heat insulator for insulating the persistent current switch superconductor 4 and the heater 5 from the liquid helium 3. It is made of an insulator 6. The heater 5 has its lead wire led out to the outside of the cryogenic container 10 and connected to the heater power source 8.
It's starting to heat up. Further, Is indicates the output current of the excitation power source 7, and Ic indicates the excitation current of the superconducting coil 1.

Next, the operation will be explained. When the superconducting coil 1 is excited, first, the persistent current switch superconductor 4 is heated by the heater 5, thereby causing superconductivity breakdown in the persistent current switch superconductor 4 and bringing it into a normal conducting state. The equivalent circuit of the superconducting magnet in this normal conducting state is shown in FIG. R _N is the resistance value of the persistent current switch superconductor 4 in the normal conduction state. Diode circuit 9
As long as a voltage lower than the turn-on voltage of the diode is applied across the diode, its impedance becomes infinite and can be removed from the equivalent circuit. This point will be discussed in detail later.

In this state, the output current Is from the excitation power supply 7
The current is increased at a constant rate over time, and when the operating current Iop is reached, the current increase is stopped. Then, after waiting for a time sufficiently longer than the time constant determined by the inductance and resistance value R _N of the superconducting coil 1,
The current of the heater 5 of the persistent current switch 2 is cut off. Then, the persistent current switch superconductor 4 is cooled by the liquid helium 3 and eventually reaches a superconducting state. In this superconducting state, an operating current Iop flows through the superconducting coil 1, and the superconducting coil 1
Both ends of the switch are short-circuited by the persistent current switch superconductor 4 which is in a superconducting state. Therefore, if the output current of the excitation power source 7 is reduced here, the superconducting coil 1 will be operated with a persistent current at the operating current Iop. Moreover, if this process is followed in reverse, the superconducting coil 1 will be demagnetized.

By the way, in the above-mentioned superconducting magnet, the persistent current switch superconductor 4 is thermally insulated from the liquid helium 3 by the thermal insulator 6, so that it is difficult to cool down. In addition, in order to increase the resistance value R _N in the normal conduction state, the low resistance stabilizing copper that is normally clad in the persistent current switch superconductor 4 is removed from the persistent current switch superconductor 4. Since the persistent current switch superconductor 4 is superconductingly unstable, it must be protected from superconducting breakdown. Therefore, in the above-mentioned superconducting magnet, the diode circuit 9, which is a protection device, has a protective function.

Next, a diode circuit 9, which is a protection device and includes diodes connected in antiparallel, will be explained. First, to explain the characteristics of the individual diodes, the voltage-current characteristics of the diodes at room temperature are as shown by the broken lines in FIG. The diode used in the experiment was Mitsubishi Electric's FD200B (with an average forward current of 112
Specification of 240A at °C). The transition to a state in which a voltage higher than a certain voltage is applied across the diode and the impedance of the diode becomes sufficiently small is herein referred to as turn-on. As shown in FIG. 5, the turn-on voltage V _t1 , which is the forward voltage at which the diode turns on, is usually about 1V. The excitation voltage (or demagnetization voltage) Ve of the superconducting coil 1 is often 1V or more. Therefore,
If the diode circuit 9 connected as shown in FIG. 3 is used at room temperature, the diode will be turned on by the excitation voltage Ve, making it impossible to excite the superconducting coil 1. However, when the diode is cooled to an extremely low temperature, its current-voltage characteristics become as shown by the solid line in FIG. The solid curve is liquid helium (LHe)
These are the experimental results inside. That is, the turn-on voltage V _t2 of the diode at extremely low temperatures is sufficiently larger than the turn-on voltage V _t1 , reaching 8V in this experimental result. When the forward voltage of the diode exceeds the turn-on voltage V _t2 , diode current begins to flow, and as the current increases, the forward voltage drop becomes smaller. As is clear from these characteristics, if the turn-on voltage of the diode circuit 9 is set higher than the excitation voltage Ve of the superconducting coil 1,
The impedance of a diode during energization or demagnetization is nearly infinite. Therefore, there is almost no shunt current to the diode during excitation, and the superconducting coil 1
can be excited quickly. When superconducting breakdown occurs in the persistent current switch 2, a voltage Iop·R _N is applied to the diode circuit 9, but this voltage is usually sufficiently large compared to the turn-on voltage of a diode in cryogenic temperatures, so the diode circuit 9 is immediately turned on. Ru. As a result, most of the current flowing through the persistent current switch 2 is bypassed to the diode circuit 9, and the persistent current switch 2 is protected from burning out.

In this way, a very large current flows into the diode circuit 9. For example, the superconducting coil 1 that is operated with a current of about 1000 A is not particularly unusual, and can be said to be an extremely common superconducting coil today.
When superconducting breakdown occurs in the persistent current switch 2, a current of almost 1000 A flows into the diode circuit 9, which is a protection device. However, 1000A class diodes are quite special in the field of power electronics, are significantly more expensive than diodes with small current capacity, and are difficult to thermally design.
Diode cooling fins and the like become large, and their volume and weight increase significantly.

Furthermore, in very recent large-scale superconducting experiments, the operating current can reach 10,000A. At present, it is nearly impossible to pass such a large current through one diode, and it is also completely impractical in terms of cost, volume, etc.

[Problem that the invention seeks to solve]

Conventional protection devices configured as described above require a fairly large space for mounting the diode inside the cryocontainer, which increases the surface area of the cryocontainer and increases the radiation heat intrusion from the container surface. As a result, the amount of evaporation of the refrigerant liquid helium increased, creating a problem that forced the long-term continuous operation time of the superconducting coils to be shortened. Also,
In initial cooling of superconducting magnets from room temperature, there is a problem in that the amount of liquid helium used for initial cooling increases due to an increase in the weight to be cooled due to an increase in the weight of the diode. Furthermore, 10000A
In the case of ultra-high current operation of the superconducting coil, the development of the diode itself required a great deal of effort, time, and cost, and there were also problems that hindered the development of the superconducting coil itself, which was originally important.

This invention was made to solve the above-mentioned problems. It can make the diode circuit smaller, lighter, and lower in price. It also reduces the consumption of liquid helium, which is the most expensive maintenance cost for superconducting magnets. The purpose is to obtain a protective device that can reduce the

[Means for solving problems]

In the protection device according to the present invention, a diode circuit is installed in the guide pipe of the cryogenic container and near the second portion of the cryogenic container where liquid helium is vaporized and becomes a helium gas flow.

[Effect]

In the protection device of this invention, the diode circuit is forcibly cooled by a high-speed, cryogenic helium gas flow.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, numerals 1, 2, 3, 9, and 10 are the superconducting coil, persistent current switch, liquid helium, diode circuit, and cryogenic container, respectively, which were described above with respect to FIG. Furthermore, 31 is a helium gas flow evaporated from the liquid helium 3, 32 is a guide pipe into which a detachable power lead (not shown) is inserted, and 33 is an exhaust port. Cryogenic container 10
Usually has a multi-vessel structure consisting of a vacuum chamber and a liquid nitrogen chamber, but this is simply shown in the figure as it has no direct relation to this invention. In addition, the diode circuit 9
is located near the lower end of the guide pipe 32. 34 is a lead wire connecting the persistent current switch 2 and the diode circuit 9. The superconducting coil 1, persistent current switch 2, and diode circuit 9 are connected in parallel, and a persistent current flows through the superconducting coil 1.

If the persistent current switch 2 breaks down its superconductivity for some reason, the diode circuit 9 is immediately turned on and most of the current is shunted to the diode. However, since the forward voltage drop of about 1V of the diode is added to the normal conduction resistance of the persistent current switch superconductor 4, a Joule heat generation of about 1W or less occurs in the persistent current switch 2, and the amount of evaporation of the liquid helium 3 is reduced. Increase. When the liquid helium 3 is vaporized, it is discharged from the cryogenic container 10 through the guide pipe 32 and the exhaust port 33 . Since the diameter of the cryogenic container 10 is narrow near the lower end of the guide pipe 32,
The helium gas flow 31 becomes significantly faster. This helium gas stream 31 is located within the cryogenic container 10, so its temperature is extremely low. Since the diode circuit 9 installed near the lower end of the guide pipe 32 is exposed to the high-speed, cryogenic helium gas flow 31, the helium gas flow 31 efficiently absorbs the Joule heat generated by the current flowing through the diode. Cools well. When the diode circuit 9 is installed at a location other than the lower end of the guide pipe 32, for example, when it is installed directly above the superconducting coil 1, the liquid helium 3 is immediately vaporized by the large amount of heat generated by the diode, and then the diode circuit 9 is placed in a large space. Due to this, only a slow helium gas flow 31 exists.
Further, if the diode circuit 9 is installed further above the guide pipe 32, the temperature of the helium gas flow 31 will increase and the cooling effect will be reduced because it is close to the normal temperature part outside the cryogenic container 10.

FIG. 2 shows the effect when the flow velocity of the helium gas flow 31 increases. The source of this figure 2 is, 84
Mitsubishi Electric Semiconductor Data Book High Power Semiconductor Stack Edition (Seibundo Shinkosha, 1981). In FIG. 2, the horizontal axis represents the average wind speed of air hitting the cooling fins, and the vertical axis represents the thermal resistance corresponding to each wind speed. It is clear that as the average wind speed increases, the thermal resistance rapidly decreases, and the current value that can flow through the same diode rapidly increases. Fig. 2 shows a diode within the normal operating temperature range of -40°C to 80°C, but when a cryogenic helium gas flow 31 is used for cooling, the overall thermal resistance is further reduced. The current that can flow through the diode further increases.

〔Effect of the invention〕

As described above, according to the present invention, the diode circuit is installed in the part of the cryogenic container that is exposed to the high-speed cryogenic helium gas flow, so the diode can be efficiently cooled. Therefore, even when the operating current of the superconducting coil is large, the diode can be made small, which has the effect of making the device significantly smaller and cheaper. In addition, the helium gas flow is not specifically prepared for cooling the diode, and gas generated from other sources is effectively used, so there is no need for any special equipment for forced cooling of the diode. It has an effect you don't need.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a superconducting magnet equipped with a protection device according to an embodiment of the present invention, FIG. 2 is a characteristic curve diagram of a forced cooling diode, and FIG. 3 is a circuit diagram showing a superconducting magnet and a conventional protection device. FIG. 4 is an equivalent circuit diagram of the superconducting magnet shown in FIG. 3 during excitation (or demagnetization), and FIG. 5 is a voltage-current characteristic curve diagram of a conventional diode. 1... Superconducting coil, 2... Persistent current switch, 3... Liquid helium, 9... Diode circuit, 10... Cryogenic container, 31... Helium gas flow, 32... Guide pipe, 33... Exhaust port It is. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A cryogenic container that houses a superconducting coil, a persistent current switch, and a diode circuit connected in parallel, as well as liquid helium, and an excitation power source located outside the cryogenic container and connected in parallel with the superconducting coil. In the superconducting magnet, the cryogenic vessel is integral with a first portion filled with the liquid helium, a second portion in which the liquid helium is vaporized into a helium gas flow, and the cryogenic container is integral with the second portion. The guide pipe is narrower in diameter than the second part and has an exhaust port for the helium gas flow at the tip thereof, and the diode circuit is installed in the guide pipe near the second part. A protection device for superconducting magnets.