JPH0640531B2 - Superconducting magnet protector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protective device for a superconducting magnet, and more particularly to a device for protecting a persistent current switch from a superconducting breakdown during a persistent current operation.

[Conventional technology]

Figure 6 shows, for example, the 31st Cryogenic Engineering Research Presentation (1984).
FIG. 9 is a circuit diagram showing a superconducting magnet and a conventional protection device shown in Lecture No. A3-20, Proceedings of 97 pages, held at Machinery Promotion Center, Tokyo, May 7-9. In the figure, (10) is a cryogenic container filled with liquid helium which is a refrigerant, in which a superconducting coil (1), a persistent current switch (2) and a diode circuit (9) are housed. The superconducting coil (1), the persistent current switch (2) and the diode circuit (9) are connected in parallel with each other, and the lead wires thereof are led out of the cryogenic container (10) to be connected to the superconducting coil (1). Is connected to the excitation power supply (7). The permanent current switch (2) is a permanent current switch superconductor (4), a heater (5) for heating the permanent current switch superconductor (4), and these permanent current switch superconductors (4) and heaters (4). 5) is composed of a thermal insulator (6) for insulating the cryogenic container (10) from the refrigerant filled therein. Heater (5)
The lead wire is led to the outside of the cryogenic container (10) and connected to the heater power supply (8) to be heated. Further, I _S shows the output current of the exciting power source (7), and I _C shows the exciting current of the superconducting coil (1).

Next, the operation will be described. In the case of exciting the superconducting coil (1), first, the permanent current switch superconductor (4) is heated by the heater (5) to cause the superconducting breakdown to be in the normal conducting state. The equivalent circuit of the superconducting magnet in this normal conducting state is as shown in FIG. Note that R _N
Is the resistance value of the persistent current switch superconductor (4) in the normal conducting state. Further, the diode circuit (9) can be excluded from the equivalent circuit because its impedance is infinite as long as the voltage at the end of the turn-on voltage of the diode is applied to both ends of the diode. This point will be described in detail later. In such a state, the output current I _S from the excitation power supply (7) is increased at a constant rate with time. When the output current I _S reaches the operating current I _op , the current increase is stopped, and after a time sufficiently longer than the time constant determined by the inductance of the superconducting coil (1) and the resistance value R _{N of the} permanent current switch superconductor (4). From the permanent current switch (2)
The current of the heater (5) is turned off. As a result, the persistent current switch superconductor (4) is cooled by the refrigerant and eventually becomes superconducting. In this superconducting state, the operating current I _op is flowing in the superconducting coil (1), and both ends of the superconducting coil (1) are short-circuited by the superconducting permanent current switch superconductor (4). There is. Therefore, if the output current I _S of the excitation power supply (7) is reduced here, the superconducting coil (1) will be operated with a permanent current at the operating current I _op . If this process is followed in reverse, the superconducting coil (1) will be demagnetized.

By the way, in the above-mentioned superconducting magnet, the permanent current switch superconductor (4) is in a heat-insulating state from the refrigerant due to the heat insulator (6), and thus is in a state of being difficult to be cooled. Further, in order to increase the normal conduction resistance value R _N , the low resistance stabilized copper, which is usually clad in the persistent current switch superconductor (4), is used as the persistent current switch superconductor (4).
Since the permanent current switch superconductor (4) is superconductingly unstable because it has been removed from the superconductor, it must be protected against superconducting breakdown. Therefore,
In the above-mentioned superconducting magnet, the diode circuit (9), which is a protective device, is adapted to perform its protective action.

Next, the diode circuit (9) which is a protection device will be described. This diode circuit (9) is composed of two diodes connected in anti-parallel as shown in the figure. First, the characteristics of the individual diodes will be explained. As shown. The diode used for the experiment is FD200B made by Mitsubishi Electric.
(Specification of average forward current of 240 A at 112 ° C.). A transition to a state in which a forward voltage equal to or higher than a certain voltage is applied to both ends of the diode so that the impedance of the diode becomes sufficiently small is referred to as turn-on here. As shown in the figure, the turn-on voltage V _t1 which is a forward voltage for turning on is usually about 1V. Superconducting excitation voltage (or demagnetization voltage) V _e of the coil (1) is often greater than or equal to 1V. Therefore, if the diode circuit (9) connected as shown in FIG. 6 is used at room temperature, the diode is turned on by the excitation voltage V _e , and the superconducting coil (1) is almost short-circuited. It becomes impossible to excite 1). However, when the diode is cooled to an extremely low temperature, its current-voltage characteristic becomes as shown by the solid curve in FIG. This solid curve is for liquid helium (L
It is an experimental result in He). That is, the turn-on voltage V _T2 of the diode at an extremely low temperature is sufficiently higher than the turn-on voltage V _T1 of the diode at room temperature, and reaches 8 V in this experimental result. When the forward voltage of the diode exceeds the turn-on voltage V _T2 , the diode current starts flowing, and the forward voltage drop decreases as the current increases. As is apparent from these characteristics, if a higher turn-on voltage of the diode circuit (9) than the excitation voltage V _e of the superconducting coil (1), the impedance of the diode in the excitation or during degaussing is almost infinite . Therefore, there is almost no shunt current to the diode during excitation,
The superconducting coil (1) can be rapidly excited. When the superconducting breakdown occurs in the persistent current switch (2), the voltage I _op · R _N is applied to the diode circuit (9), which is usually much larger than the turn-on voltage V _V2 of the diode at extremely low temperature, The diode circuit (9) is immediately turned on. As a result, most of the current flowing in the permanent current switch (2) is bypassed to the diode circuit (9), so that the permanent current switch (2) is prevented from being burnt out and is protected itself. It

Since such a very large current flows into the diode circuit (9), a method of satisfying the current capacity by connecting a plurality of diodes in parallel may be used. However, experiments conducted by the inventors of the present invention have revealed that the turn-on voltage of the diode in liquid helium varies considerably. For example, when an experiment is performed a plurality of times with the same voltage rise across the same diode, the turn-on voltage variation is about 5%, and another diode with the same specifications under the same experiment conditions is used. In the case of the above experiment, the variation of the turn-on voltage of the two diodes reaches about 10%. Therefore, FIG. 9 shows an experimental circuit for examining the variation of the diode. (15) and (16) are a diode I and a diode II respectively, (17) is a DC voltage source for applying a voltage to the diode, and (18) is a resistor for controlling a current value. The experimental results obtained by the circuit shown in FIG. 9 are shown in FIG. 11, and the voltage-current characteristics of each diode in FIG. 9 are shown in FIG. Now, let us consider a case where the rate of increase in the voltage across the diode is sufficiently large compared to the turn-on time of the diode (described later). As shown in FIG. 10, first, the voltage is diode I
When the turn-on voltage V ₁ of (15) is reached, the diode I (15) turns on. As a result, the current I _O flows through the diode I (15), and the voltage across the diode drops to V ₀ . Therefore, diode II (16) cannot be turned on at all. If diode I
(15) a current capacity I _0/2 about the case optimum design that causes shed connexion current I ₀ by the two diodes of the parallel circuit, the diode in the above situation I (15)
Will burn out. V ₂ is a diode II (16)
Is the turn-on voltage of. The above-mentioned turn-on time of the diode means the time until the diode turns on and its impedance becomes sufficiently small, and the current reaches the maximum value. The diode used in the experiment is a stack type FDS200BG (manufactured by Mitsubishi Electric). From FIG. 11, it can be seen that the turn-on time is 2 ms. Therefore, when the diodes are connected in parallel and used as a protection device, the turn-on voltage V ₂ is reduced within a very short time of 2 ms.
Need to reach more. That is, the applied voltage must be higher than the voltage at which all the diodes turn on before one diode turns on. The smaller the diodes used in this experiment, the shorter the turn-on time tends to be.

The voltage across the diode is I _op · R _N. I _op is a constant current value during the permanent current operation. But R _N
Reaches a final value R _N, which increases as a function of time after starting to energize the heater (5) of the permanent current switch (2). The permanent current switch (2) is often used for many years, and cracks often occur in its thermal insulator (6). In such a case, it takes a longer time to reach the resistance value R _N. Under the above-mentioned circumstances, the rate of rise of the voltage applied to the both ends of the diode may decrease significantly, and may exceed the turn-on time of the diode of several ms.

[Problems to be solved by the invention]

Since the conventional protection device is configured as described above, it can only be used in circuits where the rate of voltage rise is extremely high, and the rate of increase of the normal resistance of the superconductor of the persistent current switch is significantly limited. There was a point.

The present invention has been made to solve the above problems, and an object thereof is to obtain a protection device that can be used even when the rate of voltage rise is extremely low.

Another object of the present invention is, in addition to the above object, an object of the invention is to provide a protection device capable of turning on a plurality of diodes almost simultaneously.

[Means for solving problems]

In the protection device according to the present invention, a plurality of diodes are closely connected in parallel in one current direction, and the diodes are connected by a heat conductor.

Further, a protective device according to another invention of the present invention is the above one, in which a heat conductor is surrounded by a heat insulator.

[Action]

The protection device according to the present invention lowers the turn-on voltage of the latter diode by applying the heat generated by the current of the turned-on diode to the diode that does not turn on.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. First
In the figure, (1) to (2), (4) to (8) and (10) are exactly the same as those described in FIG. Unlike the diode circuit (9) of FIG. 6, the diode circuit (9A) is the same when the direction of the exciting current I _c is fixed, for example, when the exciting power source (7) is connected to the polarity (actual battle) shown in the figure. It includes a diode set (31a) in which a plurality of rated diodes, for example, two diodes are closely connected in parallel in the same direction (the forward direction of these diodes is tentatively referred to as A direction). When the polarity of the excitation power source (7) is reversed (dotted line), the diode group (31a) has exactly the same configuration, but the diode group (31b) in which the forward direction of the diode is opposite to the A direction is Connected. Furthermore, when two diode groups (31a) and (31b) connected in anti-parallel with each other are used, it is not necessary to care about the polarity of the excitation power source (7) and hence the direction of the excitation current. Further, (32) is a heat conductor, for example, a copper block, and this copper block (32) is installed so as to be in close contact with a plurality of diodes of each diode group and surround these diodes.

FIG. 2 is a side view of a diode circuit (9A) using a flava type diode, and FIG. 3 is a sectional view of a diode set (31a) and a copper block (32). In the figure, (33) is a diode group (31a),
(31b) cooling fin, (34) insulator, (35)
Is a lead wire. The arrow in the figure indicates the direction of forward current when the diode is turned on. Since the cooling fins (33) are made of a conductive metal such as aluminum or copper, the current to each diode is given through the cooling fins (33).

Assuming now, for example, that only one diode in the diode group (31a) is turned on (the cause of this situation has been described above), all the diodes connected in parallel turn on and the superconductivity increases. The design is such that the operating current of the coil (1) is shared and borne, and if the current is concentrated on one diode, that diode will be remarkably heat-juiled. As a result, the diode through which the current flows exhibits the characteristic indicated by the broken line in FIG. 8, while the diode through which the current does not flow still exhibits the characteristic indicated by the solid line in FIG. However, the heat generated by the juule heating is transmitted to the diode in which no current flows through the copper block (32). As a result, the two diodes of the diode group (31a) have the characteristics shown in FIG. In FIG. 4, the curve is the diode that is turned on first and a large current flows, and the curve is that the diode temperature rises due to the heat transfer from the copper through the copper block (32), and the diode is not turned on yet. The voltage-current characteristics appearing in the non-exposed diode, and the curve is the characteristic when the temperature of the diode in the curve is further increased.

The operating point is a point when all the diodes are turned on. However, here, one of the two diodes did not turn on, and the operating point of the diode through which the current was flowing was a point corresponding to a current value twice the point. The diode, which has a current twice as high as usual, has a remarkably large energizing current, so that the diode is heated to a high temperature. This heat is transferred to the diode which is not turned on through the copper block (32), and the diode which does not flow current has a characteristic that the turn-on voltage is remarkably lowered and the curve of the diode is flowing a large current. It is turned on by the descent. The curve turn-on voltage is higher than the curve turn-on voltage because the curved diode is cooler than the curved diode. This is because a certain amount of heat is dissipated while the heat is transmitted through the copper block (32). The operating points of the respective diodes immediately after the two diodes are all turned on are and. However, the curve tries to approach the curve due to the generation of juule heat due to the current flowing into the diode, and finally the curve becomes the state, and the operating points become points and points, respectively. Since these series of operations are completed in an extremely short time, the diode is hardly affected by the overcurrent.

FIG. 5 shows an embodiment of the second invention of the present invention. In the figure, (36) is a thermal insulator of the diode, which is attached around the copper block (32) in close contact with the copper block (32). Due to the provision of this thermal insulator (36), the heat generated by the diode of the turned-on diode is transmitted to the diode which is not turned-on with almost no heat radiation from the periphery of the copper block (32).
Therefore, the voltage-current characteristic of the diode whose turn-on is delayed becomes almost the same as that of the diode which turned on earlier, and the imbalance of the shunt currents of the respective diodes becomes extremely small. Therefore, since a diode having a small current capacity can be used, there is an effect that the price is low and the device is small and lightweight. Further, since the time required to raise the diode whose turn-on has been delayed to a certain temperature is short, the time required to reach the turn-on is shortened. Therefore, the time during which the diode that has been turned on earlier is overloaded is short, and the life of the diode can be remarkably extended.

〔The invention's effect〕

As described above, according to the present invention, since a plurality of diodes are thermally coupled by the copper block, even if all the diodes do not turn on due to variations in the turn-on voltage of the diodes, the burnout accident of the diodes will occur. Nothing happens and you can immediately turn on the diode that does not turn on. Therefore, there is an effect that a requisite highly reliable protective device can be obtained.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a protection device according to an embodiment of the present invention, FIG. 2 is a side view of the protection device shown in FIG. 1, and FIG. 3 is a protection device taken along line AA in FIG. Side sectional view of the part,
FIG. 4 is a voltage-current characteristic curve diagram of the protection device of the present invention, FIG. 5 is a side sectional view of a part of the second invention, FIG. 6 is a circuit diagram showing a superconducting magnet and a conventional protection device, and FIG. Fig. 8 is an equivalent circuit diagram of a superconducting magnet in which the persistent current switch is in the normal conducting state, Fig. 8 is a voltage-current characteristic curve diagram of the diode of the conventional device, and Fig. 9 is an operation test circuit diagram of the diode.
FIG. 10 is a voltage-current characteristic curve diagram of each diode in the circuit of FIG. 9, and FIG. 11 is a graph diagram showing an experimental result of the turn-on characteristic of the diode. (9A) ... Diode circuit, (31a) and (31b)
...... Diode set, (32) …… Copper block, (36)
...... The heat insulator of the diode. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (5)

[Claims] 1. A superconducting magnet in which a superconducting coil, a persistent current switch and a diode circuit are connected in parallel to each other and are housed in a cryogenic container, and an exciting power source for the superconducting coil is connected in parallel to the parallel connected body. The diode circuit is composed of a diode group in which a plurality of diodes are closely connected in parallel in the same current direction, and a heat conductor which is in close contact with and surrounds the plurality of diodes of the diode group. Characteristic superconducting magnet protector. 2. A protection device for a superconducting magnet according to claim 1, wherein the polarities of the excitation power supplies are reversed and the connecting directions of the diode groups are also reversed. 3. A superconducting magnet protector according to claim 1, wherein the diode circuit includes two diode sets which are connected in anti-parallel with each other. 4. A superconducting magnet protection device according to any one of claims 1 to 3, wherein the heat conductor is a copper block. 5. A superconducting magnet in which a superconducting coil, a permanent current switch and a diode circuit are connected in parallel with each other and are housed in a cryogenic container, and an exciting power source for the superconducting coil is connected in parallel to the parallel connected body. The diode circuit includes a diode group in which a plurality of diodes are closely connected in parallel in the same current direction, a heat conductor that closely surrounds the plurality of diodes of the diode group, and a heat conductor of the diode. A protective device for a superconducting magnet, which comprises a heat insulator closely surrounding the body and surrounding the heat conductor.