JP2653451B2 - Method for determining insulation thickness of superconducting conductor - Google Patents

Description

The present invention relates to a method for determining the insulation thickness of a superconducting conductor for determining the thickness of an insulating material such as formal coated on the surface of a superconducting wire. is there.

(Prior art) Generally, if a wire is covered with an insulating material such as formal,
Since the heat transfer characteristics can be improved, attempts have been made to use this insulator as an insulating coating for a superconducting wire even in a superconducting conductor.

The thickness of the insulator coated on the ordinary wire is about 60 μm or more. However, if the superconducting wire is coated with the same thickness of the insulator, the following problem occurs.

That is, the heat generated by the superconducting conductor used in a high magnetic field q
_g is Indicated by Here, I is a conduction current, S is a cross-sectional area of the stabilizing material, P is a cooling circumference, and f is a coefficient representing a ratio of a superconducting state to a normal conducting state in the superconducting conductor at a certain temperature T.

When the above equation (1) is displayed on the heat transfer characteristic of the superconducting conductor, it becomes as shown by the broken line in FIG. As is clear from the characteristic shown by the broken line, if the temperature T of the superconducting conductor is equal to or lower than the shunt starting temperature Ts, f = 0 and no heat is generated and all the current flows through the superconducting wire, and if T is equal to or higher than the critical temperature Tc. If f = 1, the current flows through the stabilizing material and generates heat,
When T is between Ts and Tc, f = (T−Ts) / (Tc−Ts) indicates a state in which heat is gradually generated. However, the critical temperature Tc depends on the magnetic field B, and decreases as the magnetic field increases.

However, when the superconducting wire is covered with an insulator having a thickness similar to that of a normal wire as described above, the heat transfer characteristic becomes a curve as shown in FIG. 1, so that heat due to disturbance applied to the superconducting conductor is reduced. Even if the heat flux is less than the peak heat flux q _{u of} nucleate boiling, the superconducting state is broken and transitions to the normal conducting state, and in some cases, the normal conducting region propagates to reach quench.

(Problems to be Solved by the Invention) When the thickness of the insulator covering the superconducting wire is made the same as that of a normal wire, there is a problem that the superconducting conductor becomes unstable.

An object of the present invention is to provide a method for determining the insulation thickness of a superconducting conductor that can improve the stability in a superconducting state.

[Configuration of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention uses an insulating material to cover the surface of a unique superconducting wire in which a current to be passed, a cross-sectional area of a stabilizing material of the superconducting wire and a cooling circumference are determined in advance. In the method of determining the thickness of the insulating material of the superconducting conductor formed by coating, the critical temperature of the superconducting wire is T _c , the applied magnetic field is B, the heat transfer coefficient of the insulating material is k, the liquid helium temperature is T _b , Wire heating q _g = f
(T) I ² / SP (where, I usually current, S is the cross-sectional area of the stabilizing material of the superconducting wire, P is cooled circumference, f (T) start shunts the T _s temperature, the critical temperature T _c when T ≦ T _s is taken as, when _{f (T) = 0 T s} <T <T c, f (T) = (T-T s) / (T c -T s) T c <T Is a function of the temperature T such that f (T) = 1, and a coefficient representing the ratio between the superconducting state and the normal conducting state in the conductor at the temperature T of the superconducting conductor. As t, t ≦ k (T _c (B) −T _b ) / q _g (where T _c (B) is the critical temperature of the superconducting conductor uniquely determined by the applied magnetic field B) The thickness of the insulating material is determined.

(Operation) Therefore, in the method for determining the insulation coating thickness of a superconducting wire, the critical temperature T _{c is} determined by setting the thickness of the insulating material covering the surface of the superconducting wire to a value that satisfies the condition of the above equation. The superconducting conductor can be stabilized even if the value fluctuates according to the strength of the magnetic field.

Embodiment An embodiment of the present invention will be described below with reference to the drawings. In this embodiment, as shown in FIG. 2, the thickness of an insulator 4 such as formal covering the surface of a superconducting wire 3 composed of a superconducting material 1 made of NbTi or Nb ₃ Sn and a stabilizer 2 made of copper. The thickness is set to satisfy the following conditions.

That is, although the insulator 4 is a heat insulating material although having good heat transfer characteristics, a temperature difference occurs between the liquid helium and the superconducting wire 3. This temperature difference ΔT is between the heat flux q There is a relationship. Where k is about formal
0.045 W / mk, and q may be the heat generation q _{g of the} superconducting wire. At this time, if the temperature difference ΔT is satisfied a relationship ΔTTc (B) -Tb ...... (3 ), becomes stable to peak heat flux q _u less disturbance of nucleate boiling as described above. Where Tc
(B) is the critical temperature depending on the magnetic field B, and Tb is the liquid helium temperature.

Therefore, if the heat generation q _g of the superconducting wire 3 is 0.4 W / cm ² , the thickness t of the formal is t11.3 × 10 ⁻⁶ × (Tc (B) − Tb) (m) (4) is obtained.

The heat transfer characteristics of the insulator 4 coated on the superconducting wire 3 by changing the thickness thereof are shown in a curve in FIG. As is clear from this curve, the insulator 4
It can be seen that the heat transfer characteristic changes as shown by the curve of →→ as the coating thickness of the insulator 4 becomes thinner than the curve of 60 μm or more, which is the same as the thickness of a normal wire. In this case, the heat transfer characteristic when the coating thickness of the insulator 4 satisfies the equation (4) is as shown by the curve, and the heat transfer characteristic when there is no insulator is as shown by the curve.

By setting the thickness of the insulator 4 covering the surface of the superconducting wire 3 to a value that satisfies the condition of the above equation (4), the critical temperature Tc varies according to the strength of the magnetic field. Also, the superconducting conductor can be stabilized.

Here, the thickness of a superconducting wire 3 composed of a superconducting material 1 made of NbTi and a stabilizing material 2 coated with formal is calculated based on the above equation (4) for a magnetic field of 6T to 9T. 3 shows that the relationship shown in FIG. 3 is obtained, and it can be seen that the hatched portion in the drawing is in a stable state.
Therefore, when the magnetic field is 8 T, the formal coating thickness may be set to 17.6 μm or less.

In the above embodiment, the case where formal is used as an insulator has been described, but it is a matter of course that other insulators may be used as long as heat transfer characteristics can be improved. Further, in the above embodiment, the condition of the coating thickness of the insulator is described based on the expression (4) in which specific numerical values are incorporated.
As long as the condition of tk (Tc (B) -Tb) / q _g is satisfied from the formulas (2) and (3), even if the material of the insulator and the heat generation of the superconducting wire are different, This can be implemented in the same manner as described above.

〔The invention's effect〕

As described above, according to the present invention, it is possible to make the thickness of the insulating material covering the superconducting wire an optimum thickness according to the strength of the magnetic field, thereby improving the stability to the superconducting state. And a method for determining the insulation thickness of the superconducting conductor.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat transfer characteristic of a superconductor and a heat generation characteristic of a superconductor used in a high magnetic field. FIG. 2 is a sectional view showing an embodiment of the superconductor according to the present invention. It is a figure which shows the relationship between the coating thickness of formal and a magnetic field in the example. 1. Superconducting material made of NbTi or Nb ₃ Sn 2.
Stabilizer, 3 ... superconducting wire, 4 ... insulator.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Yoshikazu Takahashi 801 Nakahara, Naka-cho, Naka-machi, Naka-gun, Ibaraki Pref. Inside the Japan Atomic Energy Research Institute Naka Research Laboratory (72) Kiyoshi Okuno Nakahara, Naka-cho, Naka-cho, Naka-gun, Ibaraki 801-1 Inside the Japan Atomic Energy Research Institute Naka Research Laboratory (72) Inventor Kotaro Hamajima 2-4, Suehirocho, Tsurumi-ku, Yokohama-shi, Kanagawa Prefecture Inside the Toshiba Keihin Works Co., Ltd. (56) References JP-A-62-259307 (JP) JP-A-62-105317 (JP, A) JP-A-60-235309 (JP, A) JP-A-63-81708 (JP, A) JP-A-63-200414 (JP, A) 63-13208 (JP, A)

Claims (1)

(57) [Claims] 1. A thickness of a superconducting conductor in which a surface of a superconducting wire having a predetermined conducting current, a cross-sectional area of a stabilizing material of a superconducting wire, and a cooling circumference is determined in advance, is determined. In the method, the critical temperature of the superconducting wire is T _c , the applied magnetic field is B, the heat transfer coefficient of the insulating material is k, the liquid helium temperature is T _b , and the heat generation of the superconducting wire q _g = f (T) I ² / SP (Where I is the normal current, S is the cross-sectional area of the stabilizing material of the superconducting wire, P is the cooling circumference, and f (T) is
T _s the current sharing temperature, when T ≦ T _s when the T _c and the critical temperature, when the _{f (T) = 0 T s} <T <T c, f (T) = (T-T s) / (T _c −T _s ) T _c <T is a function of the temperature T such that f (T) = 1 and represents the ratio of the superconducting state to the normal conducting state in the superconducting conductor at the temperature T. And the coating thickness t of the insulator, t ≦ k (T _c (B) −T _b ) / q _g (where T _c (B) is uniquely determined by the applied magnetic field B) A method for determining the insulation thickness of a superconducting conductor, wherein the thickness of the insulation material is determined so as to satisfy a condition (critical temperature of the conductor).