JPS61173625A - Protector for superconductive magnet - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention is directed to the maintenance of superconducting coils. ! The present invention relates to a device for protecting a persistent current switch from superconducting breakdown during persistent current operation.

[Conventional technology]

Figure 3 shows, for example, the 31st Cryogenic Engineering Research Presentation (/9te
7th month of the year? 1 is a circuit diagram showing a conventional superconducting magnet and its protection device shown in page 11 of the proceedings, lecture number A3-CO, held at the Tokyo 1st Kikai Shinko Kaikan on April 1, 2013. In the figure, (10) is a cryogenic container filled with liquid helium (,7), which is a refrigerant, and a superconducting coil (1), a persistent current switch (c), and a diode circuit (91) are housed inside. .This is a superconducting coil (1
), persistent current switch -) and diode circuit (9)
are connected in parallel to each other, and their lead wires are led out of the cryogenic container (10) and connected to an excitation power source (RI). In addition, the persistent current switch (k) consists of a persistent current switch superconductor (ri), a heater (!) for heating this persistent current switch superconductor (≠), and these persistent current switch superconductors (river and heater (3)). ') from liquid helium (,?).The heater (5) has its lead wire led out to the outside of the cryogenic container (lO) and is connected to the heater power source (,?). ff) so that it is heated.

Is indicates the output current of the excitation power source (7), and Is indicates the excitation current of the superconducting coil (1).

Next, the operation will be explained. When exciting the superconducting coil (1), first, by heating the persistent current switch superconductor (Hiroshi) with a heater, superconductivity breakdown occurs in this persistent current switch superconductor (Hiro), and it becomes normal conductor. The equivalent circuit of the superconducting magnet in this normal conducting state is shown in Figure D. RN is the resistance value of the persistent current switch superconductor in the normal conducting state. The diode circuit (9) becomes infinite and can be removed from the equivalent circuit.

This point will be discussed in detail later.

In this state, the output current Ill from the excitation power supply (7)
is increased at a constant rate over time, and the operating current Io
When p 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 RN of the superconducting coil (1), the persistent current switch (
(2) Cut off the current of the heater (5). Then, the persistent current switch superconductor (formed by liquid helium (3)
The superconducting state is eventually reached. In this superconducting state, an operating current IOP flows through the superconducting coil (1), and both ends of the superconducting coil (1) are short-circuited by the persistent current switch superconductor (<c) in the superconducting state. Therefore, if the output current of the excitation power source (RI) is reduced here, the superconducting coil (1) will be operated with a persistent current at an operating current of 10F. Moreover, if this process is followed in reverse, the superconducting coil (/l) will be demagnetized.

By the way, in the above-mentioned superconducting magnet, the persistent current switch superconductor (q) is thermally insulated from the liquid helium (j) by the thermal insulator (6), so it is difficult to be cooled. In addition, the resistance value RN in the normal conduction state
In order to increase (4') is unstable in terms of superconductivity, so it must be protected from superconducting breakdown.Therefore, in the above-mentioned superconducting magnet, a diode circuit (ri) as a protection device acts as a protection device. It has become.

Next, a diode circuit (?) which is a protection device and is composed of 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 FD Co0.
OB (specification of average forward current of 4 IOA at ti °C). The transition to a state in which a voltage higher than the voltage across the diode is applied and the impedance of the diode becomes sufficiently small is herein referred to as turn-on.

As shown in Figure 3, the turn-on voltage Vi, which is the forward voltage at which the diode is turned on, is usually about /V. Excitation voltage (or demagnetization voltage) of superconducting coil (1)
Me is often equal to or higher than /V. Therefore, if the diode circuit connected as shown in Fig. 3 is kept at room temperature Cm,
The diode is turned on by the excitation voltage Vs, making it impossible to excite the superconducting coil (1). However, when the diode is cooled to an extremely low temperature, its current-voltage characteristics change to
The result will be as shown by the solid line in the figure. The solid curve is the experimental result in liquid helium (LHe). That is, the turn-on voltage vtJ of the diode at an extremely low temperature is sufficiently larger than the turn-on voltage Vti, and reaches tV in this experimental result. When the forward voltage of the diode exceeds the turn-on voltage Vt2, a diode current begins to flow, and as the current increases, the head 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 the diode during excitation or extinction becomes almost infinite. It is. 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 (k), the voltage IOP 'I R) f
is applied to the diode circuit (ri), but this voltage is usually sufficiently large compared to the turn-on voltage of a diode in cryogenic temperatures, so that the diode circuit (de) is turned on immediately. As a result, most of the current flowing through the persistent current switch (C) is bypassed by the diode circuit (?), and the persistent current switch (C) is protected from burning out.

In this way, a very large current flows into k, which is a diode circuit. For example, a superconducting coil (1) operated with a current of about 100A 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 (a), a current of nearly 1000 A flows into the diode circuit (?) that serves as a protection device. However, tooh-class diodes are quite special in the field of power electronics, and they are significantly more expensive than diodes with small current capacity, and the thermal design of the diode is difficult, so it is difficult to cool the diode. As the fins and the like become larger, their volume increases steadily and their weight also increases.

In addition, in very recent large-scale superconducting experiments, the operating current was
It may even reach 0*. 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 liquid helium, which is a refrigerant, increases, creating a problem in which the long-term continuous operation time of the superconducting coils has to be shortened.

Furthermore, with respect to the initial cooling of the superconducting magnet 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 the increase in the weight of the diode. Furthermore, in the case of ultra-high current operation on the order of 10,000 volts, it takes a lot of effort and effort to develop the diode itself.
There were also problems in that it required time and money and hindered the development of the superconducting coil itself, which is 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]

The protection device according to the present invention has a diode circuit 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.

[For production]

In the protection device of this invention, the diode circuit is forcedly 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. 1st
In the figure, (1) (a (J) (?) (t 0.)
are the superconducting coil, persistent current switch, liquid helium, diode circuit, and cryogenic container, respectively, as described above with respect to FIG. Furthermore, (31) is a helium gas flow evaporated from liquid helium (3), (3) is a guide pipe into which a focused power lead (not shown) is inserted, (,?
J) is an exhaust port. The cryogenic container (io) usually has a multi-vessel structure consisting of a vacuum tank and a liquid nitrogen tank, but since it has no direct relation to this invention, it is simply shown in the figure. Further, the diode circuit (9) is located near the lower end of the guide pipe (3). (3d) is a lead wire that connects the persistent current switch (-) and the diode circuit. The superconducting coil (1), persistent current switch -) and diode circuit (?) are connected in parallel, and the superconducting coil (1)
) has a persistent current flowing through it.

If the persistent current switch (k) breaks down its superconductivity for some reason, the diode circuit (?) immediately turns on and shunts most of the current to the diode. However, since the forward voltage drop /V of the diode is added to the normal conductivity resistance of the persistent current switch superconductor (R), Joule heat generation of about /W or less occurs in the persistent current switch (K), and liquid helium The amount of evaporation of (3) increases.Liquid helium (,7
When 1 is vaporized, it passes through the guide pipe (3-) to the exhaust port (3-).
3) to the outside of the cryogenic container (1O). Near the lower ends of the guide pipes (3), the diameter of the cryogenic container (lO) becomes narrower, so the helium gas flow (3/) becomes significantly faster.

This helium gas stream (31) has a very low temperature since it is in the cryogenic container (10). , guide pipe (3λ
Since the diode circuit (ri) installed near the lower end of the helium gas fi (J /') provides effective cooling. When the diode circuit (9) is installed at a location other than the upper end of the guide pipes (3).

For example, if it is installed directly above the superconducting coil (/l),
After the liquid helium (3) is immediately vaporized by the large Joule heat generated by the diode, the J
Only a hollow helium gas flow (31) exists.Also, if a diode circuit (9) is formed further above the guide pipes (3), the cryogenic vessel (10)
), the temperature of the helium gas flow (31) increases and the cooling effect becomes smaller.

Figure 2 shows the effect of increasing the flow velocity of helium gas (Jt).The source of this figure is flI Mitsubishi Electric,
Semiconductor Data Book High Power Semiconductor Stack wA (Seibundo Shinkosha, Showa!). 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. Figure 2 shows a diode within the normal operating temperature range of −UO℃ to 10℃, but when using a cryogenic helium gas flow (j/) for cooling, the overall thermal resistance is even higher. decreases,
The current that can flow through the diode further increases.

〔Effect of the invention〕

This allows efficient cooling of the diode.

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, but the gas generated from other causes is effectively used.
This has the effect of not requiring any special equipment to forcefully cool the diode.

[Brief explanation of the drawing]

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. 3 is an equivalent circuit diagram of the superconducting magnet shown in FIG. 3 during excitation (or deactivation), and FIG. 3 is a voltage-current characteristic curve diagram of a conventional diode. (/l...superconducting coil, -)...persistent current switch,
(3)...liquid helium, (de)...diode circuit,
(10) Cryogenic container, (31) Helium gas In the diagrams, the same reference numerals indicate the same or equivalent parts. Horse 2 Figure Hayaken Calm (m/s) Figure 4 Procedural Amendment (Voluntary) April 30, 1985 1. Indication of the Director-General 2. 1985 Patent Application No. 10760 Name of 1. Relationship with the 3 persons who provide protection devices for superconducting magnets. Patent applicant 3. Location: Chiyoda, Tokyo 2-2-3-6 Marunouchi, Tokyo (601) Mitsubishi Electric Corporation Representative Hitoshi Katayama 4th floor, Marunouchi Building, 2-4-1 Marunouchi, Chiyoda-ku, Tokyo Detailed explanation of the invention Column 6, the statement of contents of the amendment, is amended as follows.

Claims (1)

[Claims] A superconductor comprising: a cryogenic container that houses a superconducting coil, a persistent current switch, and a diode circuit connected in parallel, and also houses liquid helium; and an excitation power source outside the cryogenic container and connected in parallel with the superconducting coil. In the magnet, the cryogenic container is integral with a first part filled with the liquid helium, a second part in which the liquid helium is vaporized into a helium gas flow, and the cryogenic container is integral with the second part. and a guide pipe having a diameter narrower than the second part and having 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.