JPS5948487B2 - Composite superconducting wire - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a composite superconducting wire that can be wound around a superconducting coil or the like, and particularly relates to a copper-based ultrafine multicore twisted composite superconducting wire that is currently widely used. A superconducting coil (with poor heat dissipation) whose surface is thermally insulated from its surroundings and susceptible to external disturbances (
The object of the present invention is to provide a composite superconducting wire having an arrangement configuration that reduces the influence of local and transient temperature increases caused by these external disturbances even in resin-impregnated superconducting coils.

Superconducting coils are generally used after being cooled to near absolute zero using liquid helium (see 4.2), and the surface of the composite superconducting wire wound around the coil is cooled by liquid helium (see 4.2). Direct cooling type coils that are cooled by direct contact with helium, indirect cooling type coils such as insulated solenoid-wound or pancake-wound superconducting coils made by winding composite superconducting wire with an insulated surface, and even insulated composite coils. There are conductive cooling type superconducting coils in which a superconducting wire is wound into a solenoid shape and impregnated with an organic resin (for example, epoxy resin).

As the direct cooling method changes from the indirect cooling method to the conduction cooling method, the following advantages are expected to arise. In other words, (a) it becomes easier to work with;
(b) Fewer interlayer short circuit accidents, (c) The coil can be made more compact, (d) The coil average current density can be increased, (e) The coil structure itself can be made stronger, especially in conduction cooling coils, and the rotation and For example, the structure can be made to withstand vibrations. However, the only and very serious drawback that occurs when switching from direct cooling to conduction cooling is the phenomenon of degradation (current deterioration).
This means that coil instability, such as training phenomena, becomes more pronounced.

In other words, even in superconducting coils made by winding ultra-fine multicore twisted composite superconducting wires that were developed to be essentially stable, there is a degradation phenomenon in which the current cannot flow up to the rated value, and a large number of cycles are required to reach the rated current value. Quench (
Coil instabilities such as training phenomena occur that require superconductor breakdown). The cause of this coil instability is local disturbances caused by external disturbances such as heat generated by the movement of the superconducting wire (wire movement) and heat generated by cracking of the impregnating agent or adhesive fixing agent. A temporary and transient temperature rise has been pointed out. In particular, there are resin-impregnated superconducting coils in which an insulated round composite superconducting wire 1 is wound and then impregnated with an organic resin 2 as shown in FIG. Such as a solenoid-wound or pancake-wound superconducting coil in which a gap spacer 3 (made of organic resin, for example) is provided between the rectangular composite superconducting wires 1 so that liquid helium can flow.
In a superconducting coil in which the entire surface of the composite superconducting wire 1 in the wire direction or a portion over a certain section is thermally insulated from the surroundings, the coil instability is significant.

For this reason, despite having many of the excellent advantages mentioned above, the widespread use of superconducting coils, which have structures with poor heat dissipation such as conduction cooling or indirect cooling, is at risk. However, we will give due consideration to the copper-based ultrafine multicore twisted composite superconducting wire currently in use, and will focus on superconducting coils (for example, The present invention aims to provide a composite superconducting wire that can obtain relatively stable and reliable coil characteristics despite some disturbance even when wound around a resin-impregnated coil (resin-impregnated coil). Figures 3 and 4 are cross-sectional views of examples of conventional composite superconducting wires that are generally widely used in current superconducting coils, where 4 is a superconducting filament, 5 is a normal conductive metal base material, and 6 is an electrical insulator. It is.

When a conventional composite superconducting wire 1 as shown in FIGS. 3 and 4 is wound around a superconducting coil having a structure with poor heat dissipation as shown in FIGS. 1 and 2, problems such as wire movement and cracking occur. The coil is quenched (superconducting breakdown) due to a small, localized, and transient temperature rise of just a few degrees caused by external disturbances.

The relationship between this local and transient temperature rise ΔT degrees (based on the liquid helium temperature of 4.2 K) and the maximum current value of 1M (4.2+ΔT, B) amperes that can be passed without quenching is given by the following.

However, this equation is related to Nb-Ti superconductors that are currently commonly used, and B is the magnetic field (Tesla) and IO (
4.2, B) is the current value (ampere) that can flow when there is no temperature rise due to disturbances. According to this equation (1), for example, when B = 4 Tesla, a local and transient temperature rise of ΔT = 2.0 degrees causes I.

Although quenching (superconducting breakdown) occurs at a current value of 40% of (4.2, B), it can be seen that at a temperature increase of ΔT=1.0 degrees, it is possible to flow up to a current value of 70%. In this way, it can be seen how important a difference in temperature rise of just a few degrees has in superconducting coils. Therefore, in the following, we will discuss the transient temperature rise distribution inside the composite superconducting wire 1 after the heat generated by external disturbances such as wire movement and cracking increases the surface temperature of the composite superconducting wire 1 under adiabatic conditions. We will solve the problem below and investigate how to arrange the superconducting filament 4 in the normal conductive metal substrate 5 to reduce the influence of local and transient temperature increases from the outside.

In other words, a disturbance occurs due to some reason in the superconducting coil at the liquid helium temperature (4.2 K), and the width of the part of the round composite superconducting wire 1 closest to the disturbance is reduced to Dcm.
) increases in a rectangular manner by ΔTO degrees over time t seconds, under the condition that the surface of composite superconducting wire 1 is thermally insulated from the surroundings, the wire inside the composite superconducting wire Examine the temperature distribution in the direction (Z direction) and the radial direction (r direction). However, here it is assumed that the composite superconducting wire is made of only a normal conducting metal base material (high thermal conductivity copper, e.g. 0FHC-Cu), the center point of the initial surface temperature rise is Z=O, and the radius of the composite superconducting wire 1 is Rcm. . To see the temperature rise TK inside the composite superconducting wire under these conditions, the following equation can be solved. Here, DH is the specific heat C (J/Gr-K) of copper at 4.2K, and the thermal conductivity λ (W/Cm-
K), and the thermal diffusion coefficient (Cm=/Sec) given by the following equation using the density ρ (Gr/-).

This solution is when IO<t When IitO<t however becomes.

Here, JO and J are zero-order and first-order Bessel functions, respectively, and βn is a zero point of the zero-order Bessel function.

Using equations (3) to (7) above, ΔT is calculated for the round composite superconducting wire 1 with R=1 mm when DH=7.3×10°cm3/SeC. =1k,d=0.4mm and ΔTO=1k,d
= 0.2 mm, when two types of temperature disturbances are added, o < at the point Z = 0 where the degree of temperature rise is the most severe.
Calculating the temperature rise at t'-re, the fifth
The result will be as shown in Fig. 6. That is, FIG. 5 shows ΔT. =I
When K, d = 0.2 mm, Fig. 6 shows ΔT. =IK,d
Radial transient temperature at point Z=0 when = 0.4 mm,
It shows the rising distribution. Note that the duration is T. After seconds, the temperature within the composite superconducting wire decreases rapidly, so when discussing stability, it is sufficient to examine only the temperature characteristics at 0<t≦TO. As shown in these calculation examples, by using equations (3) to (7), you can understand the transient temperature rise distribution within each composite superconducting wire when there is an external temperature disturbance, and with regard to coil instability, Together with equation (1) showing the relationship between the temperature rise value and the quench current value, quantitative discussion and investigation from the standpoint of composite superconducting wires becomes possible, but in general the following can be said. That is, the temperature rises to the center of the composite superconducting wire even if there is a disturbance having a duration of about μSec. (2) A cooling effect can be expected due to linear thermal diffusion against temperature rise disturbances from the outside. Based on these general considerations, the following important information regarding coil instability can be obtained.

That is, in the conventional ultrafine multicore composite superconducting wire 1 as shown in FIGS. 3 and 4, the superconducting filaments 4 are arranged uniformly over the entire normal conductive metal base material or only in the base material around the wire. Therefore, when localized and transient temperature rise disturbances are applied to these ultrafine multicore composite superconducting wires 1 from the outside, they are highly susceptible to the effects of the disturbances, and as a result, However, when a conventional composite superconducting wire is wound around a superconducting coil with poor heat dissipation, such as a resin-impregnated coil, it has the disadvantage that coil instability such as a training phenomenon or a degradation phenomenon tends to occur. The purpose of this invention is to eliminate the drawbacks of the conventional composite superconducting wire, and by arranging the superconducting filament 4 at the center within 60% of the diameter of the composite superconducting wire 1, there is no temperature rise from the outside. Even if disturbances occur, the cooling effect of linear heat diffusion can alleviate the effects of such disturbances and provide relatively stable and reliable coil characteristics. The figure shows a cross-sectional view of a composite superconducting wire having the arrangement according to the present invention.

The temperature rise characteristic in the round composite superconducting wire 1 having the arrangement shown in FIG. 7 and having a wire radius of R=1 mm is, for example, a temperature rise of ΔTO=1K, d=0.2n]In As for external disturbances, as is clear from Figure 5, even if this temperature continues for 2.0 x 10-8 seconds, 0
．． No temperature rise occurs in the center within 6 mm (i.e. 60% of the diameter), but within 0.9 mm (i.e. 90% of the diameter)
At this point, a temperature increase of 0.4 K, or 40%, occurs. Furthermore, even for severe external disturbances that last for 1.0 x 10-6 seconds or more at ΔTO = 1K, d = 0.2 mm, at 0.6 mm, 0.2
K, that is, the temperature rises only by about 20%, but at 0.9 m
It can be seen that there is a temperature increase of 0.5K, that is, more than 50% at m. Although the difference in temperature rise is only a few tenths of a degree, considering the effect that this temperature difference has on the stability of the coil as mentioned earlier, it is difficult to prevent the superconducting filament 4 from increasing in temperature from the outside. Composite superconducting wire with remarkable cooling effect due to linear thermal diffusion against disturbances 1
It is important from the standpoint of coil stability that the coil should be placed only in the center within 60% of the diameter of the coil. Also the 6th
It can be seen from the figure that the above holds true even for more severe disturbances with large disturbance widths. Although only a round superconducting wire has been described here, the same applies to a rectangular superconducting wire, and this embodiment is shown in FIG. 8.

That is, in the case of the square wire 2, if the composite superconducting wire 1 is such that the superconducting filament 4 is arranged in the central normal-conducting metal base material 5 within 60% of the length and width of the cross section, good. In the above explanation, regarding the superconducting filaments 4 and 5, only Nb-Ti superconductors were described;
This may be a Nb3Sn superconductor or a V3Ga superconductor, and the normal conductive metal base material 5 is also Cu.
Although it is made of A1, it may be made of A1.

Furthermore, in the composite superconducting wire 1, the superconducting filament 4 is sufficiently thinned and twisted (transposed in some cases) to prevent instability due to internal causes of the composite superconducting wire itself, such as librax jumps. It is said that it will not occur. The effects of this invention can be more effectively utilized by adopting the so-called essential stabilization method. As has been detailed so far, the present invention provides a composite superconducting wire in which a plurality of superconducting filaments 4 are disposed in a normal conducting metal base material 5, and when the composite superconducting wire is a round wire, the superconducting filaments 4 are It is arranged at the center within 60% of the diameter of the composite superconducting wire, and in the case of a square wire, at the center within 60% of each side of the composite superconducting wire,
A superconducting coil formed by winding a composite wire according to the present invention has stable coil characteristics despite some degree of poor heat dissipation, and has great practical effects.

[Brief explanation of the drawing]

Fig. 1 is a partially cutaway perspective view of an organic resin-impregnated superconducting coil, Fig. 2 is a partially cutaway perspective view of a solenoid-wound or pancake-wound superconducting coil having a liquid helium flow path with a gap spacer, Figs. Figure 4 is a cross-sectional view of a conventional composite superconducting wire, and Figures 5 and 6 are ΔTO = 1K, respectively.
d=0.02cm and ΔTO=1K, d=0.04cm
Radial transient temperature distribution characteristic diagram for temperature rise disturbance,
7 and 8 are cross-sectional views of an embodiment of a composite superconducting wire having an arrangement according to the present invention. In the figure, 1 is a composite superconducting wire, 2 is an impregnating agent such as an organic resin, 3 is a gap spacer, 4 is a superconducting filament, and 5 is a superconducting filament.
is a normal conductive metal base material, and 6 is an electrical insulator.

Claims (1)

[Scope of Claims] 1. A composite superconducting wire in which a plurality of superconducting filaments are disposed in a normal-conducting metal base material, wherein the composite superconducting wire has a circular cross section, and the superconducting filament has a diameter smaller than the diameter of the composite superconducting wire. A composite superconducting wire characterized in that it is arranged in the center within 60%. 2. In a composite superconducting wire in which a plurality of superconducting filaments are disposed in a normal conducting metal base material, the composite superconducting wire has a rectangular cross section, and the superconducting filament has a rectangular cross section within 60% of the length of each side of the composite superconducting wire. A composite superconducting wire characterized by being placed in the center.