JPS6353682B2 - - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a protection device for a superconducting device, and more particularly to a device for protecting a persistent current switch from superconductor breakdown during persistent current operation.

Prior Art A conventional superconducting device of this type is shown in FIG. In the figure, 1 is a superconducting coil, 2
3 is a persistent current switch, 3 is a protective resistor corresponding to a protection device, 4 is a persistent current switch superconductor, 5 is a heater, 6 is a thermal insulator, 7 is an excitation power source, and 8 is a heater power source. In addition, I _S is the output current of the excitation power supply 7,
I _cpil indicates the exciting current of the superconducting coil 1.
A persistent current switch 2 and a protective resistor 3 are connected in parallel to the superconducting coil 1, and the superconducting coil 1 is excited by an excitation power source 7.
The persistent current switch 2 is made up of a persistent current switch superconductor 4, a heater 5 for heating it, and a thermal insulator 6 for insulating both from a coolant (usually liquid helium). , the heater 5 is heated by a heater power source 8. It goes without saying that the superconducting coil 1 and the persistent current switch 2 are entirely cooled by the refrigerant.

When the superconducting coil 1 is excited, the persistent current switch superconductor 4 is first heated by the heater 5 to break down the superconductivity and bring it into a normal conducting state. The equivalent circuit of the superconducting device in this state is shown in FIG. In the figure, γ _P is the resistance value of protective resistor 3,
R _N represents the resistance value of the persistent current switch superconductor 4 in the normal conduction state, and L represents the self-inductance value of the superconducting coil 1. Further, _IS indicates the output current of the excitation power source 7, and I _cpil indicates the excitation current of the superconducting coil 1. In this state, when the output current I _S from the excitation current 7 is increased at a constant speed, the output current I _S of the excitation power supply 7 and the superconducting coil 1
The excitation current I _cpil changes as shown in FIG.
_IOP in FIG. 3 indicates the operating current of the superconducting coil 1.
At this time, the time constant τ that determines the transient phenomenon of the exciting current I _cpil is calculated from the self-inductance L of the superconducting coil 1, the protective resistance γ _P , and the normal conductivity resistance R _N of the persistent current switch superconductor 4: τ=L/γ _P・R _N /γ _P +R _N ...(1) is obtained as, and the output current I _S and the excitation current when the excitation of the superconducting coil 1 reaches a steady state
The difference in I _cpil is I _S −I _cpil = L・dI _cpil /dt／γ _P +L・dI _cpil /dt／R _N ……(
2), where the first term on the right side is the protective resistance 3
The second term represents the current being shunted to the persistent current switch superconductor 4. When the output current _IS reaches the operating current _IOP , the current increase is stopped, and after a time sufficiently longer than the time constant τ, the current of the heater 5 for heating the persistent current switch 2 is cut off. The persistent current switch superconductor 4 is cooled by the refrigerant and eventually reaches a superconducting state. In this state, the operating current is applied to superconducting coil 1.
I _OP is flowing, and both ends of the superconducting coil 1 are short-circuited to the persistent current switch superconductor 4 in the 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 .
Also, if we follow this process in reverse, superconducting coil 1
will be demagnetized.

By the way, in the above-mentioned superconducting device, the superconductor 4 is thermally insulated from the refrigerant by the thermal insulator 6, so it is difficult to be cooled, and the normal conductive resistance value _RN is greatly increased. Because the low-resistance stabilizing copper that is normally clad in the superconductor 4 is removed from the superconductor 4 in order to must be protected against. Therefore, in the above-mentioned superconducting device, the protective resistor 3 serving as a protective device has a protective function. Then, if the allowable current at the time of superconducting breakdown of the superconductor 4 is i _O , then the protective resistance value γ _P
is determined by γ _P ≦i _O /I _OP −i _O・R _N ……(3), but since I _OP ≫i _O , it suffices if γ _P ≪R _N ……(4) . Applying this relationship to equation (1), the time constant of the exciting current I _cpil of the superconducting coil 1 becomes τL/γ _P ...(5), and the time _constant (L /R _N ). Also, from equation (2), the current shunted to the protective resistor 3 is sufficiently larger than the current shunted to the superconductor 4 during excitation, so that the output current _IS of the excitation power source 7 and the excitation current of the superconducting coil 1
The difference between I _cpil becomes large. This situation is the same even during demagnetization.

As can be easily understood from the above explanation, in conventional protection devices for persistent current switches in superconducting equipment, the time constant of the transient phenomenon of the excitation current of the superconducting coil becomes long, and the time required to excite the superconducting coil increases. In addition, the difference between the output current of the excitation power source and the excitation current of the superconducting coil becomes large, causing problems such as difficulty in controlling the superconducting coil current.

Summary of the Invention The present invention provides a protection device using a novel method that solves the problems of the above-described conventional protection device. That is, according to the present invention, a diode circuit is connected in parallel to the persistent current switch instead of a conventional protective resistor, and this diode circuit is installed in an extremely low temperature region, thereby preventing damage to the persistent current switch due to superconducting breakdown. At the same time, the time constant of the transient phenomenon of the excitation current of the superconducting coil can be shortened, so the time required to excite the superconducting coil can be reduced, and furthermore, the difference between the output current of the excitation power supply and the excitation current of the superconducting coil is also small. Therefore, ease of control can be ensured.

EXAMPLES OF THE INVENTION The present invention will be described in detail below with reference to Examples. Fourth
The figure is a circuit diagram of a superconducting device using the protection device according to the present invention, in which a diode circuit 9 is connected in parallel to the persistent current switch 2 instead of the protective resistor 3 of the conventional device shown in FIG. It is. Other parts are the same as in FIG. As shown in the figure, this diode circuit 9 has two diodes D forming an anti-parallel pair, and is installed together with a superconducting coil 1 and a persistent current switch 2 in an extremely low temperature region.

To make it easier to understand the function of these diodes, we will first explain the characteristics of diodes.
The current-voltage characteristics of the diode at room temperature are as shown in FIG. The turn-on voltage Vt ₁ , which is the forward voltage for turning on, is usually 1V or less.
Excitation voltage (or demagnetization voltage) Ve of superconducting coil 1
is often over 1V. Therefore, if the diode circuit 9 connected as shown in FIG. 4 is used at room temperature, it 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 change as shown in FIG. In other words, the diode turn-on voltage at cryogenic temperatures
Vt ₂ can be as high as several volts. For example, a diode for 100A showed a turn-on voltage Vt ₂ of about 4V. When the forward voltage of the diode exceeds Vt ₂ , diode current begins to flow, and as the current increases, the forward voltage drop decreases. When such diodes are arranged in an anti-parallel pair, the current-voltage characteristics are as shown in FIG. As is clear from these characteristics, if the turn-off voltage Vt of the diode circuit 9 (in this case, Vt=Vt ₂ ) is made larger than the excitation voltage Ve of the superconducting coil 1, the condition of Vt>Ve...(6) is satisfied. If this is done, the electrical resistance of the diode circuit 9 will be almost infinite regardless of the direction of the excitation current I _S of the superconducting coil 1. Therefore, when a transient phenomenon of the excitation current I _S of the superconducting coil 1 occurs, constant τ
is determined only by the inductance L of the superconducting coil 1 and the normal conduction resistance R _N of the superconductor 4 of the persistent current switch 2, so that τ=L/R _N (7). This value is sufficiently smaller than the time constant in conventional protection devices (see equations (4) and (5)).

The turn-on voltage Vt of the diode circuit 9 is several V
Therefore, it is generally sufficient to excite the superconducting coil 1, but if it is desired to further increase the excitation voltage Ve, a diode circuit 9 is created by connecting a group of diodes in which a plurality of diodes D are connected in series in antiparallel. , and equivalently increase the turn-on voltage.

In addition, the output current I _S of the excitation power source 7 and the superconducting coil 1 when the excitation of the superconducting coil 1 reaches a steady state
The difference in the excitation current I _cpil can be found by setting γ _P to infinity in equation (2), and becomes I _S −I _cpil = L·dI _cpil /dt/R _N (8). Since this value is sufficiently smaller than the current difference in a conventional protection device using a protection resistor (see equations (2) and (4)), the superconducting coil current can be easily controlled.

Regarding the protection function, the turn-on voltage Vt of the diode circuit 9 is selected so that even if superconducting breakdown occurs in the persistent current switch 2, the condition Vt<I _OP・R _N (9) is satisfied. If this is done, damage to the persistent current switch 2 will be prevented. In other words, when superconducting breakdown occurs in persistent current switch 2, the voltage
I _OP・R _N is the turn-on voltage Vt of the diode circuit 9
is exceeded and the diode circuit 9 is turned on, so that the current flowing through the persistent current switch 2 is bypassed to the diode circuit 9, and damage to the persistent current switch 2 is prevented. At this time, even if the decay time of the superconducting coil current is sufficiently long, as long as the condition of Vti _O・R _N (10) is satisfied, the persistent current switch 2 will not be damaged.

In the above embodiment, the diode circuit 9 uses diodes D connected in antiparallel, but if the current direction of the superconducting coil 1 is always determined to be one direction, the diode circuit 9 is connected in the opposite direction. Of course, it is not necessary to use a parallel pair of diodes; in that case, if one diode is connected in the forward direction to the superconducting coil current, that is, the diode is connected to the anode terminal side when the superconducting coil 1 is excited. The cathode side may be connected to the cathode terminal side, and the anode side may be connected to the cathode terminal side.

Effects of the Invention As detailed above, according to the present invention, a diode circuit is connected in parallel to the persistent current switch, and the turn-on voltage of the diode circuit is selected to be higher than the excitation voltage of the superconducting coil, so that the superconducting coil current The time constant of the transient phenomenon can be shortened, the time required to excite the superconducting coil can be reduced, and the difference between the output current of the excitation power source and the excitation current of the superconducting coil can be reduced. It is possible to ensure the ease of controlling the superconducting coil current, and for the protection function, the turn-on voltage Vt of the diode circuit is set by the product of the normal conduction resistance R _N of the persistent current switch superconductor and the normal conduction permissible current i _O. Since it is selected to be small, the persistent current switch can be protected from damage due to superconducting breakdown.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a conventional superconducting device, Fig. 2 is an equivalent circuit diagram of the circuit shown in Fig. 1 in a normal conducting state, and Fig. 3 is an excitation power source for the superconducting device shown in Fig. 1. Figure 4 is a diagram showing changes in the output current and the excitation current of the superconducting coil.
The circuit diagram of the superconducting device according to this invention, FIG.
FIG. 6 is a diagram showing the current-voltage characteristics at room temperature of the diode used in the protection device according to the present invention, and FIG.
This figure shows the current-voltage characteristics of the antiparallel pair of diodes shown in FIG. 4 at extremely low temperatures. DESCRIPTION OF SYMBOLS 1... Superconducting coil, 2... Persistent current switch, 4... Persistent current switch superconductor, 5... Heater, 6... Thermal insulator, 7... Excitation power supply, 8...
Heater power supply, 9...Diode circuit, D...Diode. In each figure, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Scope of Claims] 1. In a superconducting device in which a persistent current switch is connected in parallel to a superconducting coil, a diode circuit is connected in parallel to the superconducting coil and the persistent current switch, and this diode circuit is connected in parallel with the superconducting coil and the persistent current switch. Installed in a low temperature range, the turn-on voltage Vt of the diode circuit
and the excitation voltage or demagnetization voltage of the superconducting coil.
Ve, and the voltage generated by the persistent current switch superconductor of the persistent current switch I _OP・R _N and i _O・R _N
Between Vt＞Ve Vt＜I _OP・R _N Vti _O・R _N However, R _N : Persistent current switch superconductor's normal conduction resistance, I _OP : Operating current of superconducting coil, i _O : Permanent 1. A protection device for a superconducting device, characterized in that the following relationships are simultaneously satisfied: a persistent current switch of a current switch, an allowable energizing current at the time of superconducting breakdown of a superconductor; 2. The protection device for a superconducting device according to claim 1, wherein the diode circuit comprises one diode connected in the forward direction with respect to the superconducting coil current. 3. The protection device for a superconducting device according to claim 1, wherein the diode circuit comprises a pair of diodes connected in antiparallel. 4. A protection device for a superconducting device according to claim 1, wherein the diode circuit comprises a desired number of diodes connected in series in a forward direction relative to the superconducting coil current. 5. The diode circuit according to claim 1, wherein the diode circuit has a desired number of anti-parallel pairs consisting of a pair of diodes connected in anti-parallel, and the desired number of anti-parallel pairs are connected in series. Protection device for superconducting equipment. 6. The superconducting device according to claim 1, wherein the diode circuit has two diode groups in which a desired number of diodes are connected in series in the same direction, and the diode groups are connected in antiparallel to each other. protection device.