JPS62593B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a persistent current switch for large currents.

Superconducting coils used in magnetic levitation trains, energy storage devices, etc. need to be continuously excited at a constant current value over a long period of time, so persistent current excitation is often performed. A persistent current switch is used.

Although there are mechanical contact types of persistent current switches, superconducting type switches are generally used because of their small size, light weight, and ease of operation. This utilizes the transition between the superconducting state (zero electrical resistance) and the normal conducting state (finite electrical resistance) of the superconducting wire to perform a switching operation.

The biggest problem when using such a superconducting persistent current switch is the instability of the superconducting wire. For example, when current is passed through a switch in a superconducting state, superconductor breakdown (quenching) often occurs at a current value far lower than the critical current of the superconducting wire. In order to eliminate this instability, the superconducting wire used for the winding of ordinary superconducting magnets is stabilized by integrating the superconductor with a normal conducting metal, such as copper or aluminum, which has low electrical resistance at low temperatures. It is planned.

However, in persistent current switches,
Electrical resistance in normal conduction state (resistance when switch is open)
Since it is necessary to make the superconducting wire as high as possible, the above-mentioned stabilizing wire cannot be used, and the superconducting wire must be used in a considerably unstable state. Furthermore, the instability described above tends to become more pronounced as the cross-sectional area of the superconducting wire is increased in order to obtain a large current capacity.

For example, Figure 1 shows a unit wire (approximately 60 μm diameter Nb-
This is a diagram showing the relationship between the number of unit wires and the current capacity per unit wire of a persistent current switch using a plurality of strands of 305 Ti wires (the unit wire is a strand of 305 Ti wires). When attempting to construct a superconducting wire for a persistent current switch by bundling several wires together, it can be seen that the current capacity per unit wire decreases as the number of wires increases. This means that as the required current value increases, a dramatically larger amount of superconducting wire must be used, making it difficult to realize a small and lightweight persistent current switch for large currents.

As a means to solve these difficulties, the second
As shown in the figure, a superconducting wire with a small cross-sectional area, that is, a switch 1 with a small number of strands in the example of FIG. It is very easy to think that it would be good to use it. That is, in parallel connection in this case, each unit switch 1
are completely separated magnetically, electrically, and spatially, and the current capacity of each unit switch 1 is extremely large. Taking a superconducting wire with the characteristics shown in Figure 1 as an example, if you try to realize a switch with a current capacity of 1300A with a single switch, six unit wires are required, but one unit wire is required. If a unit switch made of wire is made, it has an allowable current of about 470A, so it is sufficient to use three of them in parallel, and the number of wires is halved compared to a single switch.

If we adopt the method of using unit switches in parallel in this way, we can realize a small and lightweight switch with less superconducting wire usage and high normal conduction resistance. After that, all current values can be covered by simply increasing the number of devices, which has the advantage of significantly simplifying design and manufacturing.

However, the above-mentioned system using a unit switch also has major drawbacks as described below.

That is, when connecting a plurality of unit switches 1 in parallel, the individual superconducting wires are connected with uniform connection resistance at the connection part 4 between the superconducting wire 2 and the conductor 3 to be connected to it (for example, the lead wire of a superconducting coil). This is because it is extremely difficult to connect as shown in FIG.

Normally, at the joints of superconducting wires, direct connection of superconductors by welding or brazing is not adopted. This is because the current capacity of such connections is often lower than that of other parts.
Furthermore, in the case of ultra-fine multicore superconducting wires (FM wires), etc., which are commonly used these days, it is technically difficult to remove the normal conducting metal base material to expose the superconducting core wires and connect them to each other. There is a risk that the characteristics of the product may deteriorate significantly.

For this reason, the superconducting wires are usually connected by brazing with solder or the like while the superconducting wires remain covered with a normally conducting metal base material (often copper). The superconducting wire used in persistent current switches is made by removing the low-resistance base material in order to ensure high resistance in the normal conduction state. For this reason, it is common practice to leave the base material and connect it by brazing with solder.

FIG. 3 shows the connection state of such a connection part, and the current flowing through the superconducting core wires 5, 5' is connected to the normal conductive metal base materials 6, 6' and the solder 7 at the connection part 4.
Therefore, there is electrical resistance, so-called connection resistance, in this part.
The connection resistance is determined by the resistance of the normally conducting metal substrates 6, 6' and the solder 7, and most of it is the resistance of the solder 7. Comparing the resistivity at liquid helium temperature of oxygen-free copper, the most commonly used base material, and Pb-60%Sn solder, a common solder.
The former is about 1×10 ^-8 Ωcm, the latter is 5×10 ^-7 Ω.
Since the latter has a value of about 50 times the former, at around cm, the resistance of the solder layer becomes dominant even if the thickness of the solder layer is considerably thinner than that of the base material layer. Such a relative relationship does not change much even if aluminum is used as the base metal and other low melting point alloys are used as the solder material.

If the connection resistance is determined primarily by the resistance of the solder as described above, it is inevitable that the connection resistance value will vary considerably. This is because the thickness and width of the solder layer 7 may vary greatly depending on the heating temperature during brazing, the surface condition of the superconducting wires, the pressure bonding strength between the superconducting wires, and the like. Figure 4 shows 365 Nb-Ti superconducting core wires with a diameter of 60 μm placed in an oxygen-free copper base material with a rectangular cross section of 1.6 mm x 3.2 mm.
This figure shows the results of measuring the connection resistance of a 5 cm long connection where FM wires with books embedded in them were connected using Pb-60%Sn solder. Measurements were performed on five samples prepared under exactly the same conditions using the usual four-terminal method by applying an external magnetic field of 5K ^G and passing a current of 500A. As is clear from the graph in FIG. 4, although the samples were manufactured in exactly the same way, the connection resistance varied considerably, with a difference of about 4 times between the maximum and minimum values.

Next, when unit switches such as those described above are used in parallel connection, the rate at which current flows through the unit switches will be considered using the equivalent circuit shown in FIG. In Fig. 5, L ₁ , L ₂ ...
..., Ln is the self-induction coefficient of each unit switch, r ₁ , r ₂
. . . , rn and r' ₁ , r' ₂ , . . . r'n are connection resistances between each unit switch and the lead wire connected to it.

Now, when current I ₀ flows through the circuit, currents I ₁ , I ₂ , ..., In flowing through each unit switch are given by the following equation in a steady state.

I ₁ = Rt/R ₁・I ₀ I ₂ = Rt/R ₂・I ₀ ...(1) 〓 In=Rt/Rn・I _0Here , 1/Rt=1/R ₁ +1/R ₂ + ...+1/
Rn R ₁ = r ₁ + r' ₁ , R ₂ = r ₂ + r' ₂ , ...Rn = rn - r'n If the allowable current of each unit switch is Iq,
The largest value of the shunt current given by formula (1),
In other words, the total current value when the current value of the unit switch with the smallest connection resistance reaches Iq becomes the total allowable current. Now, if we consider as an example the case where four single switches with Iq = 500A are used in parallel, the connection resistances of the four unit switches R ₁ , R ₂ ,
If R ₃ and R ₄ are all equal, the currents I ₁ , I ₂ , I ₃ , I ₄
become equal, and when this value reaches Iq, the overall allowable current Iqt is determined. Therefore, Iqt=4Iq=
It becomes 2000A.

On the other hand, there are variations in connection resistance, e.g.
If R ₁ :R ₂ :R ₃ :R ₄ = 1:2:3:4, then I ₁ :I ₂ :I ₃ :I ₄ =1:1/2:1/3:1/4 becomes. Since the total allowable current is determined by I ₁ = Iq,
Iqt=25/12・Iq=1042A, which is about 1/2 of the case of equal resistance.

The above is about the steady state, but considering the transient state, if the self-induction coefficients L ₁ , L ₂ , ..., Ln are sufficiently large compared to the resistances R ₁ , R ₂ , ..., Rn, then the total When the current I ₀ is changing, each current I ₁ ,
I ₂ , ..., In are approximately self-induction coefficients L ₁ , L ₂ , ..., Ln
Take the current distribution determined by , and when I ₀ = constant, the resistance becomes
The current distribution gradually shifts to the one determined by R ₁ , R ₂ , ..., Rn. In this transition process, the current value of the unit switch with the smallest connection resistance is the allowable current Iq of the unit switch.
Quench when reached. That is, a quench phenomenon accompanied by a time delay appears. In an example of an experiment conducted by the inventors, when three unit switches each having an allowable current of 470 A were connected in parallel and a current of 1000 A was applied, the current was kept constant and quenched after 53 minutes.

In summary, when a persistent current switch with a large current capacity is constructed by connecting multiple unit switches in parallel, variations in connection resistance are inevitable with the conventional connection structure.
A drawback was that it was not easy to obtain a highly reliable switch that could continue to supply current for a long period of time.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a small-sized persistent current switch with a large current capacity that can continue to supply a stable current for a long period of time.

The persistent current switch according to the present invention is characterized in that a superconducting core wire is embedded in a base material of a normal conducting metal (for example, Cypronickel) that has high electrical resistance at low temperatures as the superconducting wire of the unit switch. The connection resistances at the connection portions are made approximately equal, and the current distribution of each unit switch is made uniform.

In other words, the resistivity of the base metal Cypronickel at liquid helium temperature is 4 × 10 ^-5 Ωcm,
Since it is about 80 times larger than Pb-60%Sn solder, it can be said that the connection resistance at the parallel connection is determined almost entirely by the Cupronickel base material.
Moreover, the resistance of the Cypronickel base material takes approximately the same value if the geometric shapes of the superconducting wires are the same. Therefore, the connection parts can be connected with substantially equal connection resistance.

Figure 6 shows an FM wire with 114 superconducting wires embedded in a 0.3 mm diameter Cypronickel base material and an FM wire with a 2.1 mm diameter copper base material connected over a length of 15 cm using Pb-60%Sn solder. This figure shows the results of measuring the resistance of the connected portion. Measurements were performed on 18 samples prepared under the same conditions with an external magnetic field of 5 KG and a current of 100 A. As a result, as is clear from the graph shown, the variation in connection resistance is within 12%, which is extremely small compared to that shown in FIG.

Figure 7 shows the relationship between the number of parallel units and the allowable current value when a unit switch is fabricated using six stranded 0.3 mm diameter Cypronickel-based FM wires with the same configuration as the sample above, and these are connected in parallel. It shows. 1
Each unit switch has an allowable current value of approximately 550A, but when these are connected in parallel, the allowable current increases approximately in proportion to the number of units in parallel, and there is no decrease in the allowable current per unit. . Further, even if a current of 1400 A is continued to flow for more than 6 hours through a device using three unit switches, no quenching occurs during that time, and there is no quenching phenomenon accompanied by the time delay described above.

In the above description, six stranded FM wires made of Cypronickel base material with a diameter of 0.3 mm were used for the unit switch, but the number, size, etc. of the wires are not limited to these values and can be selected as appropriate. Further, it is not necessary to cover the entire length of the superconducting wires constituting the switch with the Cypronickel base material, and it is sufficient that only the connecting portions are present. Furthermore, the base material is not limited to Cypronickel, but may also be metals that have a resistivity similar to Cypronickel at low temperatures, that is, a resistivity of 10 ^-5 Ωcm or more, such as manganin, constantan, monel, inconel, austenitic stainless steel, etc. A similar effect can be expected by using .

As described above, according to the present invention, a metal having a resistivity of 10 ^-5 Ωcm or more at low temperatures, such as Cypronickel, is used as the base material for at least the parallel connection portion of the superconducting wires constituting the unit switch, so that it is stable for a long time. It is possible to provide a small, lightweight, and large current capacity persistent current switch that can continue to supply current.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the number of strands of superconducting wire and the allowable current per strand in a conventional persistent current switch. Figure 2 is a block diagram showing the case where multiple unit switches are connected in parallel. Figure 3 is a cross-sectional view showing the connection state between superconducting wires, Figure 4 is a graph explaining the variation in connection resistance at the connection part, Figure 5 is an equivalent circuit diagram when multiple unit switches are connected in parallel, and Figure 6 is a graph showing the connection state of the superconducting wires. The figure is a graph for explaining the equalization of connection resistance at the connection part of the Cupronickel-based superconducting wire used in the persistent current switch according to the present invention, and Figure 7 shows the relationship between the number of unit switches and the allowable current in the present invention. It is a graph. 1... Unit switch, 2... Superconducting wire, 3...
Conductor to be connected to the switch, 4... connection part, 5,
5'...Superconducting core wire, 6,6'...Normal conductive metal base material, 7...Solder. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. In a persistent current switch in which a plurality of unit switches are connected in parallel, each of which obtains an on/off state by the transition between a superconducting state and a normal conducting state of a superconducting wire, the superconducting wire has a base of normal conducting metal at least at the connecting portion where the superconducting wire is connected in parallel. A persistent current switch having a structure in which a superconducting core wire is embedded in a material, and wherein the normal conducting metal has an electrical resistivity of 10 ^-5 Ωcm or more at a temperature at which the superconducting wire exhibits a superconducting state. .