JP4258397B2 - Nb-Ti superconducting wire - Google Patents

Description

  The present invention relates to a Nb—Ti superconducting wire that can be suitably used for a pulsed magnetic field generating superconducting magnet that generates a varying magnetic field.

  Since a normal superconducting magnet is used in a DC magnetic field (static magnetic field) as represented by medical MRI and analytical NMR (nuclear magnetic resonance apparatus), the loss generated from the superconducting wire is almost zero and can be ignored. What is required for such a DC superconducting magnet is a high current density for magnet miniaturization and the stability with which the magnet can be operated stably. Therefore, high-purity copper and aluminum are compounded as stabilizers in the superconducting wires used, and the ratio of the stabilized copper part to the superconducting filament part (copper ratio) depends on the required current density and stability. It is determined.

  On the other hand, SMES (superconducting power storage), which is expected to be put into practical use, repeatedly charges and discharges and generates a variable magnetic field. In such a superconducting magnet for generating a pulse magnetic field, an AC loss is generated from the superconducting wire, and the loss becomes heat and the liquid helium as a refrigerant evaporates. In order to liquefy the evaporated helium gas again, a liquefier is required. However, due to the structure of the liquefier, the liquefaction efficiency is very low at 0.2%. For example, the helium gas evaporated with 1W of heat is liquefied again. Therefore, 500 W is consumed by the liquefier, and the efficiency of the entire superconducting magnet system including the liquefier is greatly reduced. Therefore, it is necessary to take measures for reducing AC loss in the superconducting wire.

  In addition, if the AC loss is too large or if the cooling performance is poor due to epoxy impregnation or the like, the superconducting state may be broken (quenched) by the heat generated by the loss.

The AC loss generated from the superconducting wire is roughly divided into the following three types.
(1) Hysteresis loss due to magnetization of superconducting filament; Wh (W / m ³ )
(2) Coupling loss caused by coupling current flowing between superconducting filaments through a matrix having a finite resistance; Wc (W / m ³ )
(3) Eddy current loss generated from low resistance metals such as copper; We (w / m ³ )

  Among the above three losses, first, the hysteresis loss (Wh) in (1) increases in proportion to the magnetic field fluctuation rate (magnetic field frequency f) and the filament diameter (d) (Wh∝f · d). As a countermeasure, it is necessary to reduce the filament diameter.

The coupling loss Wc in (2) increases in proportion to the square of the magnetic field fluctuation rate (magnetic field frequency) and the twist pitch (Lp) of the superconducting wire, and the equivalent electrical resistivity of stabilized copper or the like compounded with the superconducting wire. It increases in inverse proportion to ρ (in proportion to the equivalent electric conductivity σ) (Wc∝ (f · Lp) ² / ρ). Therefore, the twist pitch is shortened as a reduction measure, and the loss is reduced by quickly attenuating the coupling current using a high resistance metal such as CuNi alloy in addition to the stabilized copper as the matrix metal to be combined.

The eddy current loss We in (3) increases in proportion to the square of the magnetic field fluctuation rate (magnetic field frequency) and the dimension (r) of the stabilized copper compounded to the superconducting wire, and the electric resistance of the stabilized copper or the like. It increases in inverse proportion to the rate ρ (We∝ (f · r) ² / ρ). Therefore, it is necessary to divide and reduce the dimension r of the stabilized copper by inserting CuNi or the like as a reduction measure.

  As described above, in the Nb-Ti superconducting wire for pulse use, the Nb-Ti filament diameter is reduced in order to reduce the AC loss, the twist pitch is shortened, and the CuNi alloy is used in addition to the stabilized copper as the composite metal. It is common. In the Nb—Ti superconducting wire with such AC loss reduction measures, the coupling loss of (2) generally occupies the highest ratio among the three AC losses. For this reason, it is often the case that ultimately reducing the coupling loss is an important issue.

  As described above, there are two points for reducing the coupling loss, that is, shortening the twist pitch and using a high-resistance metal such as CuNi, and a commonly used high-resistance metal is Cu-10 wt% Ni (for example, , See Patent Document 1). Its resistivity ρ is about 3 orders of magnitude higher in liquid helium (4.2 K) than ρ of stabilized copper.

FIG. 7A shows a cross-sectional configuration diagram of a general pulse Nb—Ti superconducting wire.
The Nb—Ti superconducting wire is formed on the outer periphery of the central portion 1 made of stabilized copper, in order, a first coating layer 3 made of Cu-10 wt% Ni, a filament region 5, and a second made of Cu-10 wt% Ni. A covering layer 7 and an outer skin 9 made of stabilized copper are covered. The filament region 5 is formed by coating the periphery of the filament 51 made of Nb—Ti with a third coating layer 53 made of stabilized copper, in a plurality of matrix layers 55 made of Cu-10 wt% Ni. It has a structure. As shown in FIG. 7B, the path of the coupling current causing the coupling loss is as follows: inside the filament region 5 (A), stabilized copper portion (B) in the central portion 1, and stabilized copper portion (C) in the outer skin 9 ).
JP 05-290647 A

  However, Cu-10 wt% Ni has a resistivity ρ in liquid helium (4.2 K) that is about three orders of magnitude higher than that of stabilized copper (high-purity copper) as described above. Disadvantageous. After all, there is a trade-off between reducing AC loss and improving stability, and it is required to balance the two according to operating conditions (magnetic field fluctuation rate, magnet load rate, etc.).

  In general, the operating current of a superconducting magnet is determined from the relationship between the critical current characteristic, which is the limit performance of the superconducting wire, and the load characteristic of the magnet. In general, a superconducting magnet is often used near a load factor of 50 to 70%, and the stability decreases as the load factor increases. Therefore, in the Nb—Ti superconducting wire shown in FIG. 7, the matrix ratio (stabilized copper: Cu—Ni: Nb—Ti) and the cross-sectional configuration of the wire are important factors for ensuring stability.

  In Nb-Ti superconducting wires for pulse applications, there are cases in which stability and magnet miniaturization (high current density) are more important than AC loss reduction depending on the operating conditions of the magnet used. For example, in the case of SMES for countermeasures against instantaneous voltage drop, the instantaneous voltage drop of the power system due to lightning strikes is several times a year, and the frequency of use is low in only a few seconds per charge / discharge. Normally, it is in a standby state where only a DC magnetic field is generated, and in the long term, the system efficiency decrease due to AC loss can be ignored, and the AC loss is so large that the superconductivity is broken (quenched) by the heat generated by AC loss Unless otherwise, some AC loss is acceptable. This is a case where it is more important to reduce the size of the magnet by operating at a higher load factor than reducing the loss.

If a high resistance metal such as Cu-10wt% Ni is used more than necessary for some of these Nb-Ti superconducting wires, the stability of the magnet is reduced due to excessive AC loss reduction measures. Magnets that require load factor operation require more attention to stability. For example, if a certain pulse for superconducting wires, to meet the required AC loss tolerance is thinner than 1μm thickness of Cu-10 wt% Ni layer disposed on the second of the covering layer 7 in FIG. 7 Even when a high-resistance layer is sufficient, the thickness of the layer may be 10 μm or more due to manufacturing restrictions. As a result, as compared with the case where the Nb—Ti superconducting wire having the same radius and length is formed, the proportion of the stabilized copper is relatively decreased by the amount of Cu-10 wt% Ni. Stability will be lowered.

  Accordingly, an object of the present invention is to provide an Nb—Ti superconducting wire suitable for pulse superconducting magnet applications, which can reduce AC loss and ensure stability.

In order to achieve the above object, the Nb—Ti superconducting wire of the present invention is formed in order from the first coating layer, the filament region, the second coating layer, and the stabilized copper on the outer periphery of the central portion made of stabilized copper. An Nb-Ti superconducting wire having a plurality of Nb-Ti filaments on which a third coating layer is formed and a matrix layer formed so as to cover the filament region. It is made of stabilized copper, and the third coating layer is made of copper or a copper alloy having a resistance in the range of 1 × 10 ^{−9 to} 3 × 10 ⁻⁸ Ω · m at a temperature of 10K or less. Features. Stabilized copper is copper that stabilizes the superconducting state, and indicates that it is used for the purpose of magnetic stability related to the essence of superconductivity and the temperature that contributes to the stability. Is a term commonly used in In the invention according to the present application, the stabilized copper refers to high-purity copper having an electrical resistivity at a temperature of 10K or less and 5 × 10 ⁻¹⁰ Ω · m or less.

The first and second coating layers are also preferably made of copper or a copper alloy having a resistance in the range of 1 × 10 ^{−9 to} 3 × 10 ⁻⁸ Ω · m at a temperature of 10K or less.

  The ratio (x / y) between the thickness x of the third coating layer and the radius y of the Nb—Ti filament is preferably 0.1 or less.

  The Nb—Ti filament diameter is preferably 30 μm or less.

  In the cross-sectional shape of the superconducting wire, the occupation ratio X of the third coating layer to the Nb-Ti filament is 0.1 to 0.8, and the occupation ratio Y of the stabilized copper is 1.5 to 3. Is preferred.

  Since the Nb-Ti superconducting wire of the present invention has an excellent balance between AC loss and stability, the superconducting magnet for generating a pulsed magnetic field using the Nb-Ti superconducting wire of the present invention is high in a state where the AC loss is kept low. Stability can be ensured, and high-load factor pulse operation is possible.

  Hereinafter, embodiments of the Nb—Ti superconducting wire according to the present invention will be described with reference to the drawings.

  FIG. 1 shows a cross-sectional configuration diagram of an Nb—Ti superconducting wire according to an embodiment of the present invention.

  This Nb—Ti superconducting wire is formed on the outer periphery of the central portion 1 made of stabilized copper, in order, a first coating layer 23 made of Cu-2 wt% Ni, a filament region 25, and a thickness of 20 μm made of Cu-2 wt% Ni. A second covering layer 27 having a thickness and an outer skin 9 made of stabilized copper are coated to form a diameter of 1 mm as a whole.

  The filament region 25 is a matrix layer composed of a plurality of stabilized copper layers, in which the periphery of the filament 51 composed of Nb—Ti having a diameter of 16 μm is coated with a third coating layer 63 composed of phosphorous deoxidized copper having a thickness of 0.5 μm. The structure is arranged in 65.

The electrical conductivity of Cu-2 wt% Ni constituting the first coating layer 23 and the second coating layer 27 corresponds to about 5 times that of Cu-10 wt% Ni conventionally used. The phosphorous deoxidized copper, which is a material for forming the third coating layer 63, has a resistance in the range of 1 × 10 ^{−9 to} 3 × 10 ⁻⁸ Ω · m at a temperature of 10K or lower. . The ratio of high purity stabilized copper to Nb—Ti (copper ratio) is 2.25.

The third coating layer 63 is made of phosphorus-free oxygen-free copper (P20-OFC), Cu-2 wt% Ni, Cu-Si alloy, Cu-Fe alloy, Cu-Zr alloy, etc. Copper or a copper alloy having a resistance in the range of 1 × 10 ^{−9 to} 3 × 10 ⁻⁸ Ω · m at a temperature of 10K or lower can be used. The reason for this will be described later.

Next, the ratio (x / y) of the thickness x of the third coating layer 63 disposed around the filament 51 made of Nb—Ti and the radius y of the Nb—Ti filament 51 is 0.1 or less. preferable. If this exceeds 0.1, the occupation ratio of the third covering layer 63 as the resistance layer reaches 22% with respect to the filament area, and the superconducting filament 51 and the third covering layer 63 with respect to the entire filament region 25 are reached. When the space factor of λ ₀ is λ ₀ , when λ ₀ = 0.6 (60%), the Nb—Ti space factor decreases to 38% and the current density decreases. Moreover, since the space factor of the stabilized copper around a filament reduces, stability falls.

Further, in the cross-sectional shape of the superconducting wire, the occupation ratio X of the third coating layer 63 to the filament 51 made of Nb—Ti is 0.1 to 0.8, and stabilized copper (in this specification, electricity at a temperature of 10 K or less). Occupancy ratio Y of a resistivity of 5 × 10 ⁻¹⁰ Ω · m or less is 1.5 to 3 (copper or copper alloy: stabilized copper (high purity copper) : Nb—Ti = X: Y: 1, X = 0.1 to 0.8, Y = 1.5 to 3). This is because the stabilized copper ratio of the superconducting wire varies greatly depending on the conditions used, but when used for a pulse magnet, a stabilized copper ratio of 1.5 or more is generally required in order to operate the magnet stably. . On the other hand, if the stabilized copper ratio is increased too much, the stability is improved, but the Nb—Ti occupancy is decreased, the current density is decreased, and the magnet is enlarged. In addition, since the AC loss also increases, the upper limit of the stabilized copper ratio is set to 3.0. Further, when the occupation ratio X of the third covering layer 63 used for reducing the AC loss is less than 0.1 with respect to Nb—Ti, the loss reduction effect is small, and the conventional Nb—Ti for DC use is small. Only a reduction effect of 20% or less can be expected with respect to the AC loss of the wire (a wire composed only of stabilized copper and Nb—Ti). On the other hand, when the occupancy exceeds 0.8, the AC loss can be expected to be greatly reduced by 90% or more compared to the DC wire, but the Nb—Ti occupancy decreases, so the current density decreases. End up. In addition, a decrease in stability cannot be ignored.

  Furthermore, the diameter of each filament 51 made of Nb—Ti is preferably 30 μm or less. This varies depending on the magnetic field conditions, but when the filament diameter exceeds 30 μm, the main component of AC loss changes from the coupling loss component to the hysteresis loss component, and the AC loss becomes so large that it cannot be ignored for pulses. is there.

Next, the reason why the above-described copper or copper alloy having a resistance of 1 × 10 ⁻⁹ Ω · m or more and 3 × 10 ⁻⁸ Ω · m or less is used as the third coating layer will be described in detail. To do.

The coupling loss Wc (J / m ³ ) of the superconducting wire of the cross-sectional model as shown in FIG.

Where, A ^* : coefficient resulting from the superconducting wire shape factor, r _s , radius of the superconducting wire, r _b ; radius of the filament region outer periphery, H _m ; magnetic field amplitude, ω; 2πf
Here, τ _eff is a coupling equivalent time constant and is expressed by the following equation.

Where L _{s is} the twist pitch of the line, σ _{eff is} the equivalent conductivity of the matrix, and is expressed by the following equation.

Here, σ _i ; equivalent conductivity of the filament region, σ _{o1 to} σ _c2 ; conductivity of each part in FIG. 2, m, n, o, p; constants depending on the geometric dimensions of the superconducting cross-sectional structure.
Here, σ _i is expressed by the following equation when the filament region has a two-layer structure of the metal b layer 152 and the c layer 154 having different conductivity around the Nb-Ti filament 151 as shown in FIG. The

However, lambda _0; comes to the equivalent radius of the c layer and c in the space factor of the superconducting filaments and the b layer portion and _{λ 0 = (b / c)} 2 to the entire filament region.
σ _m ; conductivity of the c layer part. σ ₂ ; conductivity of the b layer portion. αf; the coefficient when the thickness t of the b layer is expressed as a, t = b−a = αfa, and thus αf = (b−a) / a, and when t is smaller than the filament radius a, αf << 1.
Two cases where the conductivity of the b layer and the c layer are different will be described below.

i) When the b layer is a high conductivity layer made of stabilized copper , and when the c layer is a low conductivity layer such as Cu—Ni, σm in the equation (4) is the conductivity σ _CuNi of the CuNi alloy, and σ ₂ is stable The conductivity of copper _chloride is σ _Cu . Therefore, since (σ ₂ / α _f ) >> σ _m holds, equation (4) is approximately

ii) When the b layer is a low conductivity layer such as Cu-Ni, and the c layer is a high conductivity layer made of stabilized copper
(4) Conductivity σcu next σm stable copper in the formula, sigma ₂ is the conductivity sigma _CuNi of CuNi. Therefore, since (σ ₂ / α _f ) << σ _m holds, equation (4) is approximately

Further, in the case of being composed only of stabilized copper and Nb—Ti as in the case of a normal DC Nb—Ti wire, σ _m = σ ₂ = σ _Cu when applied to the equation (4), and λ ₀ is Nb− If the space factor of Ti is λ, it has the same form as equation (5). From αf << 1,

When comparing this case (6) and (7), in the case of _{λ 0 = λ = 0.6, (} 6) Equation The _{σ i = 0.25σ Cu, (7} ) next sigma _i = 4 [sigma] _Cu in formula Although most of the matrix is stabilized copper, it can be seen that σ _i differs 16 times as the denominator and numerator are reversed. Therefore, by disposing a thin high resistance layer around the Nb—Ti filament, the equivalent conductivity σ _i is reduced as shown in the equation (6), and as a result, the AC loss can be reduced. However, when a Cu-10 wt% Ni alloy is selected as the high resistance layer, σ _i can certainly be reduced. However, placing a metal three orders of magnitude higher than the stabilized copper directly on the outer periphery of the superconducting filament is more stable. It causes a decrease.

Therefore, by using a structure in which a metal or alloy having an intermediate electrical resistivity between stabilized copper and Cu-10 wt% Ni is thinly coated around the Nb-Ti filament, σ _i can be expressed by equations (6) and (7). Therefore, it is possible to realize an Nb—Ti wire material in which AC loss and stability are balanced.

In order to set σ _{i to} an intermediate value between the expressions (6) and (7), it is important to adjust σ ₂ and αf in the expression (4).
Now, λ ₀ = 0.6, σ _m = σ _Cu = 7 × 10 ⁹ (1 / Ω · m), σ ₂ = 10 ⁹ (1 / Ω · m) and αf from 0.01 to 0.2 When changed (case 1), when σ ₂ = 5 × 10 ⁸ (1 / Ω · m) and αf is changed from 0.01 to 0.2 (case 2), σ ₂ = 3.3 × FIG. 4 shows changes in σ _i when α f is changed from 0.01 to 0.2 (case 3) as 10 ⁷ (1 / Ω · m).

In FIG. 4, σ _i = 2.8 × 10 ¹⁰ (1 / Ω · m) in the case of all stabilized copper without a high resistance layer, but from case 1, σ ₂ = 10 ⁹ (1 / Ω · m ), Even when αf = 0.03, σ _i = 1.6 × 10 ¹⁰ (1 / Ω · m), and the resistance layer 7 times higher than the stabilized copper is 3% thick with respect to the radius of the Nb-Ti filament. Σ _i can be reduced by about 43% simply by arranging the outer periphery of the filament.

In Case 2, σ ₂ = 5 × 10 ⁸ (1 / Ω · m) is equivalent to the conductivity at 4.2K of industrial pure copper (phosphorus deoxidized copper) generally used for copper pipes for air conditioners. As compared with high-purity oxygen-free copper for a superconducting stabilizer, the conductivity is 1/10 or less at an extremely low temperature of 4.2K. Thus, it can be seen that σ _i can be reduced to less than half by providing a thin layer of phosphorous deoxidized copper around the Nb—Ti filament.

In case 3, σ ₂ = 3.3 × 10 ⁷ (1 / Ω · m) is a value corresponding to the conductivity of Cu-2.5 wt% Ni at 4.2 K, and the filament radius around the Nb-Ti filament is 1 It can be seen that σ _i can be reduced to about 1/6 by providing a thin Cu-2.5 wt% Ni layer of about / 100.

For the above reason, in order to control σ _i from the viewpoint of the balance between AC loss and stability, the conductivity σ of the metal arranged around the Nb-Ti filament is 10 ^{9 to} 3.3 × 10 ⁷ (1 / Ω · m) is a reasonable value, and when the conductivity σ is converted into the resistivity ρ, the range is ρ = 10 ^{−9 to} 3 × 10 ⁻⁸ Ω · m.

  An Nb—Ti superconducting wire having the structure shown in FIG. 1 was prepared as an example, and an Nb—Ti superconducting wire shown in FIGS. 5 and 6 was prepared as a comparative example. Table 1 shows the specifications of these three types of Nb—Ti superconducting wires.

  The wire diameter, filament diameter, twist pitch, and space factor of Nb-Ti were standardized in order to investigate the relationship between the high resistance metal compounded in each wire, AC loss, and stability.

As shown in FIG. 1, the superconducting wire of the example has, in order, a first covering layer 23 made of Cu-2 wt% Ni, a filament region 25, Cu-2 wt% on the outer periphery of the central portion 1 made of stabilized copper. A second coating layer 27 made of Ni and having a thickness of 20 μm and an outer skin 9 made of stabilized copper are coated. The filament region 25 is composed of 1394 filaments made of phosphorous deoxidized copper having a thickness of 0.5 μm and the periphery of the filament 51 made of Nb—Ti having a diameter of 16 μm, and stabilized copper. The structure is arranged in the matrix layer 65. The total ratio of high purity stabilized copper and phosphorus deoxidized copper to Nb—Ti is 2.25.

As shown in FIG. 5, the superconducting wire of Comparative Example 1 has, in order, a first covering layer 33 made of Cu-10 wt% Ni, a filament region 35, and Cu-10 wt on the outer periphery of the central portion 1 made of stabilized copper. A second coating layer 37 made of% Ni and having a thickness of 20 μm, and an outer skin 9 made of stabilized copper are coated. In addition, the filament region 35 is formed by covering the periphery of the filament 51 made of Nb—Ti having a diameter of 16 μm with a third covering layer 73 made of stabilized copper having a thickness of 0.5 μm, Cu-10 wt% Ni. The structure is arranged in a matrix layer 75 made of The specific rate of stabilization copper is formed on 2.1 for Nb-Ti.

The superconducting wire of Comparative Example 2 has a direct current Cu / Nb-Ti structure that does not contain CuNi and the like, and as shown in FIG. And an outer skin 9 made of stabilized copper. The filament region 45 has a structure in which 1394 filaments 51 made of Nb—Ti having a diameter of 16 μm are arranged in a matrix layer 85 made of stabilized copper. The specific rate of stabilization copper is formed on 2.5 for Nb-Ti.

  The critical currents in the external magnetic field 5T of the three types of wires were measured by the 4-terminal method. For measurement, a short sample of about 1 m in length is wound on a FRP cylindrical holder with tension, soldered at both ends to a current terminal, the distance between the voltage terminals is set to 400 mm, and the holder is set in a superconducting magnet. To 5 T and applying a magnetic field. The definition of the critical current was a voltage reference of 0.1 μV / cm.

  AC loss is a 55-turn / single-layer coiled sample with an inner diameter of 75 mm and a height of 70 mm, and the coiled sample is set in a magnetization measurement pickup coil. A 2 ± 0.5T triangular wave AC magnetic field (DC bias magnetic field 2T, 3 wave AC magnetic field amplitude 0.5T, f = 1 Hz) is applied to measure the magnetization of the coiled sample, and an AC is obtained from the obtained magnetization curve. Loss was sought.

  As a test for quantitatively evaluating the stability, MQE (minimum quench energy; the minimum amount of heat energy necessary for quenching the superconducting wire, the higher the MQE, the higher the thermal stability) was measured. A manganin wire heater with a 0.2mm diameter insulation film is wound around the center of a 1m long superconducting wire over a length of 10mm, and an epoxy resin is applied on top of it to transfer most of the heater's heat to the superconducting wire. I did it. A superconducting wire is wound around the above-mentioned cylindrical holder for measuring critical current, and a constant current (350A) corresponding to about 55% of the critical current value is energized and held in a state where a magnetic field of 5 T is applied from the outside, and a manganin wire heater The wire was heated by passing a pulse current of 20 ms in width. The pulse current peak value was gradually increased until the wire was quenched, the amount of heat input was calculated from the current value and the heater resistance value when the quench occurred, and the value was defined as MQE (minimum quench energy).

  Table 2 shows the critical current, AC loss, and MQE values of the above-mentioned three types (Examples, Comparative Examples 1 and 2) of Nb-Ti wires.

The critical current value was almost the same for all three samples.
The AC loss was smaller in the order of the samples of Comparative Example 1, Example, and Comparative Example 2, and the wire obtained by combining Cu-10 wt% Ni, which has a three-digit electrical resistivity higher than that of stabilized copper, was the lowest. The sample of Comparative Example 2 for direct current that does not include a high-resistance layer has an extremely large AC loss compared to the other two samples. 90% or more of the large AC loss of the sample of Comparative Example 2 is a coupling loss component. The MQE value is large in the order of the samples of Comparative Example 2, Example, and Comparative Example 1, Comparative Example 2 having the Cu / Nb-Ti configuration is the highest, and the sample of the Example has stability close to that of the sample of Comparative Example 2. Indicated.

  From the above results, when it is desired to reduce the AC loss as much as possible, the sample of Comparative Example 1 is optimal. However, the AC loss is low but the stability is low (the MQE value is low), and the magnet operates at a high load factor. It turns out that it is unsuitable for use. On the other hand, although the AC loss of the sample of Example is as high as 1.94 times that of the sample of Comparative Example 1, the MQE value is close to the sample of Comparative Example 2 having a Cu / Nb—Ti structure. It can be seen that it is 2.35 times as stable. From the above, it was confirmed that the sample of the example maintained stability while reducing AC loss.

It is sectional drawing of the Nb-Ti superconducting wire which concerns on one Embodiment of this invention. It is sectional drawing which shows the structure and dimension of a general Nb-Ti superconducting wire for pulses. It is sectional drawing in case the Nb-Ti filament periphery has a two-layer structure (b layer and c layer). It is a graph which shows the relationship between equivalent electrical conductivity (sigma) _i and (alpha) f (thickness of a high resistance layer / Nb-Ti filament radius). 3 is a cross-sectional view of an Nb—Ti superconducting wire of Comparative Example 1. FIG. 6 is a cross-sectional view of an Nb—Ti superconducting wire of Comparative Example 2. FIG. It is sectional drawing of the general Nb-Ti superconducting wire for pulses.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Center part 3,23,33 1st coating layer 5,25,35,45 Filament area | region 7,27,37 2nd coating layer 9 Outer skin 51 Filament 53,63,73 3rd coating layer 55,65, 75,85 matrix layer

Claims (5)

The outer periphery of the central portion consisting of stabilized copper, in turn, a first coating layer made of a copper alloy, a filament area, the second coating layer made of a copper alloy, and the outer skin consisting of stabilized copper coated, the filament region Is a Nb-Ti superconducting wire having a plurality of Nb-Ti filaments on which a third coating layer is formed and a matrix layer formed so as to cover them, the matrix layer is made of stabilized copper, and the third The Nb—Ti superconducting wire is characterized in that the coating layer of the electrode is made of copper or a copper alloy having a resistance in the range of 1 × 10 ^{−9 to} 3 × 10 ⁻⁸ Ω · m at a temperature of 10K or less. The Nb-Ti superconducting wire according to claim 1, wherein the first and second coating layers are made of Cu-2 wt% Ni .   3. The Nb—Ti superconductivity according to claim 1, wherein a ratio (x / y) between a thickness x of the third coating layer and a radius y of the Nb—Ti filament is 0.1 or less. line.   The Nb-Ti superconducting wire according to any one of claims 1 to 3, wherein the Nb-Ti filament diameter is 30 m or less.   In the cross-sectional shape of the superconducting wire, the occupation ratio X of the third coating layer to the Nb-Ti filament is 0.1 to 0.8, and the occupation ratio Y of the stabilized copper is 1.5 to 3. The Nb-Ti superconducting wire according to any one of claims 1 to 4, wherein

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