JPH06132572A - Permanent current switch - Google Patents

Abstract

PURPOSE:To obtain a permanent current switch, which has a high critical current value, is small in off-heater power consumption, is high in exciting or demagnetizing speed and can be stably operated, by a method wherein the switch is formed by a method of winding a superconductive wire on a bobbin obtained by molding integrally a matrix resin and a reinforcing fiber having a negative expansion coefficient. CONSTITUTION:A permanent current switch 7 is one formed by the method of winding a superconductive wire 9 on a bobbin obtained by molding integrally a matrix resin, such as an epoxy resin, and a reinforcing fiber having a negative expansion coefficient, that is, a negative expansion fiber. The negative expansion fiber is an organic fiber which is extended when a temperature is dropped from a room temperature, such as a polyethylene and a polyamide, and a carbon fiber and the like. Thereby, when the switch 7 is operated in a turn-ON state, the whole superconducting circuit can be stably operated and an increase of the resistance of the circuit can be made easily.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for operating a superconducting coil of a nuclear magnetic resonance diagnostic apparatus (MRI), a nuclear magnetic measurement apparatus (MRS), a magnetic line train, etc. in a permanent current mode.
The present invention relates to a permanent current switch (PCS) that short-circuits both ends of this superconducting coil in a state where the resistance value is substantially zero, and causes a current interruption state when necessary.

[0002]

2. Description of the Related Art A superconducting coil such as a nuclear magnetic resonance apparatus or a magnetic levitation train is disconnected from a DC power source during operation, short-circuited by a permanent current switch, current circulates in a closed loop, and continues to flow for a long time. It is generally used in a so-called permanent current mode. The superconducting wire forming the superconducting coil is in a superconducting state at an extremely low temperature and its electric resistance is zero, and theoretically, a current will continue to flow forever. FIG. 1 is a circuit diagram showing the configuration of a superconducting device using a persistent current switch. In the figure,
(1) is a superconducting coil which is housed in a cryostat (3) together with a refrigerant (2) such as liquid helium. Both ends of the superconducting coil (1) are connected via current leads (4).
It is led out of the cryostat (3) and is connected to an external power source (5) and a protective resistor (6). (7) is a persistent current switch connected to both ends of the superconducting coil (1) via a lead wire. A circuit diagram showing the configuration of this permanent current switch is shown in FIG.

In the figure, reference numeral (9) is a superconductor connected to the lead wire (8) and constitutes a line to be opened / closed as a permanent current switch. (10) and (11) are a heating wire as a heating device and a switch DC power supply for supplying a current to the heating wire (10).

Next, the operation of the superconducting device including the permanent current switch (7) will be described. First, superconducting coil (1)
In order to excite the current, the switch DC power supply (1) is turned on and a current is passed through the heating wire (10) to heat the superconductor (9) to a temperature above its TC to bring it into a normal conducting state. Turn off the switch. Here, a current is supplied to the superconducting coil (1) from an external power source (5). At this time, the resistance R1 of the superconductor (9) in the off state and the resistance R2 of the protective resistance (6) should be sufficiently larger than the inductance L of the superconducting coil (1) and the impedance that allows the rate of change of the current flowing therethrough. For example, most of the current supplied from the external power source (5) is consumed for exciting the superconducting coil (1). When quenching the coil, it is desirable that the resistance R2 of the protective resistor (6) be larger than the resistance R3 of the superconductor (1) from the viewpoint of energy recovery. Further, the resistance value R1 in the OFF state of the permanent current switch is larger than the resistance value R2 of the protective resistor (6), which can protect the permanent current switch and shorten the excitation time when exciting the superconducting coil (1). Is important for. Therefore, for this superconductor (9), a material having a high resistivity in the normal conducting state above the critical temperature TC is adopted. That is, in order to increase the resistivity in the switch-off state, only a superconductor from which stabilized copper is removed as a coating material, or a cupro nickel alloy Cu-10 having a resistivity higher than that of copper by two digits or more is used.
It is necessary to use wt% Ni or the like. In such a superconducting wire, a so-called quench, which suddenly changes from the superconducting state to the normal conducting state, is likely to occur for some reason. Therefore, in order to reduce the strength of the magnetic field in the vicinity of the superconducting wire, the permanent current switch (7) The superconducting winding (9) uses a special winding method called non-inductive winding. Further, it is known that the friction heat generated by a slight movement of the superconducting wire while the superconducting winding (9) maintains superconducting causes a quench to occur, and such movement of the superconducting wire is prevented. In order to prevent this, a. Measures are taken such as winding while applying an adhesive, and then molding with a resin. B. Another method is to apply a tensile stress to the superconducting wire during winding.

When the predetermined excitation operation is completed, the switch power supply is turned off and the external power supply (5) is disconnected. As a result, the superconductor is cooled by the refrigerant (2) to be in the superconductor state, the permanent current switch is turned on, and the current supplied from the excitation power source is the CDEF including the superconducting coil (9).
The circuit continues to flow, and the mode shifts to the permanent current mode. In order to maintain this persistent current mode, the critical current 1c of the superconductor (9) of the persistent current switch is equal to or higher than the maximum operating current Iop of the superconducting coil (1) and the superconductor (9) is equal to or lower than Iop. It is necessary that the operation is stable. By thus operating the superconducting coil (1) in the permanent current mode, it is possible to reduce the cost of long-term operation and generate a magnetic field that does not fluctuate with time.

[0006]

In order to stably operate the superconducting coil in the persistent current mode, the critical current 1c of the superconductor (9) of the persistent current switch is equal to or higher than the maximum operating current Iop of the superconducting coil (1). And the superconductor (9) must be stable in operation at a current of Iop or less. On the other hand, the resistance value R in the off state of the superconductor (9)
1 must be as high as possible, for which a. Increase off resistance by using Cu-Ni for the matrix of superconducting wire. B. Although it is possible to make the superconducting wire as long as possible, b. Has a limited cryostat size and a limited length. Therefore a. Inevitably, the matrix must be made to have a high resistance, which is a major factor causing instability of the superconducting coil (9).

On the other hand, there is a limit in selecting the type of matrix to increase the resistance R1 of the superconducting wire in the normal conducting state. In order to make R1 apparently larger than the protective resistance R2 and the resistance R3 of the superconducting coil (1) during quenching, it is necessary to increase the critical current density of the superconducting wire (9). This depends on the stability of the superconducting wire wound around the bobbin. The physical stability of the superconducting wire wound around the bobbin is closely related to the electrical stability of the superconducting coil itself. Usually, a metal such as aluminum or glass fiber reinforced plastic (GFRP) is used as the reel, but when these are wound at room temperature and cooled to liquid nitrogen (LNT) or liquid helium temperature (LHeT), both are wound. It contracts greatly in the circumferential direction of the frame. On the other hand, when the superconducting wire is excited,
The repulsive force resulting from the Lorentz force loosens the winding. These two movements are combined to cause microscopic mutual movement between the superconducting wires, causing disturbance due to frictional heat generation on the surface, and the superconducting coil is quenched. In addition, the aluminum frame is more unstable because it is conductive and heat is generated by Joule heat accompanying eddy current. Further, this has a drawback that the heat radiated from the heating wire easily escapes into the refrigerant due to its high thermal conductivity, resulting in large power consumption. The ease of quenching here is related to the critical current value 1c and Iop
As described above, it is desirable that Ic is as high as possible for stable operation. In order for the operation to be stable below Iop, it is a necessary condition that the coil itself on which the superconducting wire is wound is stable.

The present invention has been made in order to solve the above-mentioned problems, and a permanent current switch that enables stable operation of the entire superconducting circuit during the operation of turning on (closing) the permanent electric conduction switch and easily achieves high resistance. Aim to get.

[0009]

In the persistent current switch according to the present invention, a superconducting wire (9) is a reel formed by integrally molding a matrix resin and a reinforcing fiber having a negative expansion coefficient (hereinafter referred to as negative expansion fiber). It is wound around.

Here, polyethylene is used as the negative expansion fiber,
Organic fibers such as aramid, polyarylate, PBZ polymer (polybenzbisoxazole, polybenzbisthiazole, etc.), polyvinyl alcohol, polyethylene terephthalate, polyethylene naphthalate, polyimide, polyamideimide, polyetheretherketone, polyetherketoneketone, etc. , And carbon fiber. All of these are high-strength, high-modulus fibers, which have a lower specific gravity than glass and have a high specific strength and a high specific modulus and are lightweight.

Next, regarding the dimensional change, all of these fibers have a peculiar property that they have a negative expansion coefficient (elongate when the temperature is lowered from room temperature) by the preparation method.
On the other hand, many matrix resins exhibit positive expansion, so that in press molding, pultrusion molding, etc., the ratio is changed appropriately, or the orientation angle is selected appropriately so that the dimensional change is 0 when the temperature is lowered from room temperature. More control during extension is possible. Among the above fibers, the high-strength polyethylene fiber that exhibits the best performance in terms of negative expansibility, specific gravity, strength and elastic modulus is preferable. Such polyethylene fibers can be obtained by using the production method disclosed in, for example, JP-A-55-107506 and JP-A-56-15408. As the form of the fibers, various yarns such as short fiber filaments, twisted yarns, spun yarns, and woven fabrics of known forms such as plain weave, twill weave, satin weave, basket weave, and basket can be used. Further, at this time, the fibers having the negative expansion and the inorganic fibers having the positive expansion can be mixed and used. In that case, examples of the inorganic fibers include glass, alumina, silica, fibers made of ceramics such as titania, zirconia, silicon nitride and silicon carbide, and aluminum, single metals such as steel, and metal fibers made of alloys thereof. However, fibers such as glass, alumina, zirconia, and silica are particularly preferable because they have low thermal conductivity and excellent mechanical properties.

As a mixing method in this case, a method in which two or more kinds of negative expansion fibers or negative expansion fibers and filaments of the above-mentioned inorganic fibers are mixed with each other, and a filament of one fiber is used as a core and the periphery thereof is the other A method of producing a core-sheath structure yarn by coating with a filament, a method of producing a mixed filament yarn by using the produced filaments by superimposing and focusing each filament bundle of both fibers in an opened state, Examples include a method of laminating prepregs using expanded fibers or a prepreg using negative expansion fibers and a prepreg using inorganic fibers, but the inorganic fibers are significantly inferior to negative expansion fibers in terms of weight and expansion coefficient. Therefore, it is necessary to consider the blending amount.

As the matrix used here, an epoxy resin, an unsaturated polyester resin, a vinyl ester resin, a urethane resin, a urethane acrylate resin or the like can be used, but an epoxy resin is particularly preferable.

A filament winding method, a tape winding method or a sheet winding method, in which a prepreg is wound around a mandrel while impregnating a matrix resin into a thread or woven (cloth) tape or sheet of the present fiber, and prepregs are laminated. Known methods such as press molding method in which pressure is applied in a mold, bulging method in which fibers and matrix resin are integrally extruded from a die under pressure, vacuum impregnation method in which fibers and matrix resin are integrally impregnated after molding at midnight, and autoclave method. The method of is mentioned. The mixing ratio of the fibers and the matrix resin in the composite material is preferably 35 to 85%, more preferably 40 to 70% as the volume fraction (Vf) of the fibers. If the Vf of the fiber is less than 35%, the reinforcing effect of the fiber is not exhibited,
If it exceeds 85%, it is difficult to impregnate with the matrix resin and the mechanical properties of the composite material deteriorate, which is not preferable.

Various shapes of the reel can be used, such as a cylinder, an elliptic cylinder, various prisms, etc., which can be solid or hollow, but have weight, strength and winding workability. From the viewpoint of, a hollow pipe is most preferable.

[0016]

Examples As negative expansion fibers, polyethylene fibers (Toyobo Dyneema, SK-60), aramid fibers (Japanese aramid fibers, Twaron HM), polyacrylates (Kuraray,
Vectran), carbon fiber (Hercules, AS-
4), using glass fibers (Nittobo, T-glass), a total of 6 types of fiber reinforced plastics of Examples 1 to 5 and Comparative Example 6 were prepared by the filament winding method. The following epoxy resin was used as the matrix. Epicoat 827 (Oilized shell) 100 HN-2200 (Hitachi Chemical Co., Ltd.) 80 EMI-24 (Oilized shell) 2 These were uniformly mixed to form a resin dope. Next, various fibers were impregnated with epoxy resin and wound around a mandrel to form a cylindrical shape. Next, while keeping it on the mandrel, it was held at 110 ° C for 2 hours, then at 130 ° C for 2 hours.
Cured and molded, fiber volume content 65%, outer diameter 60 mmφ /
A molded body having an inner diameter of 50 mmφ and a length of 100 mm was obtained. The thermal expansion coefficient of RT to LHeT of this molded product and each reinforcing fiber was measured. The results are shown in Table 2. Next, the superconducting wire (9) shown in Table 1 can be folded into two around each of seven reels in which aluminum is added as a metal frame, and the superconducting wires (9) shown in Table 1 can be wound in a non-inductive state. Next, as the heating wire (10), a sheet-shaped one is used, which is a sandwich-like structure formed by depositing aluminum on the surface of an insulating film (PET) with the same film. The permanent current switch was created by further winding this on the previously wound super electric wire. Next, the characteristics of each persistent current switch were examined. Circuit diagrams of the permanent wire switch and the entire device are as shown in FIGS. The operation of exciting and demagnetizing the superconducting coil (1) is the same as described above, and will be omitted. The measured characteristic values are the critical current value of the superconducting wire (9) in the persistent current switch (7), the off resistance when the superconducting wire (9) becomes normal conducting when the heating wire (10) is turned on, and Table 2 shows the switching time (Ts) and the demagnetization speed when the power, the persistent current switch is off and on.

(Thermal expansion coefficient) The dimensional change rate from room temperature to the liquid He temperature of each sample was measured by the TMA method to calculate the thermal expansion coefficient. (Raising rate: 5 ° C / min)

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

According to the present invention, the critical current value is high, the off-heater power consumption is low, the switching time is short,
It has become possible to provide a persistent current switch having a high excitation / demagnetization speed and capable of stable operation.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a superconducting device using a persistent current switch.

FIG. 2 is a configuration diagram of a permanent current switch.

[Explanation of symbols]

1: Superconducting coil 2: Refrigerant 3: Cryostat 4: Current lead 6: Protective resistance 7: Permanent switch 9: Superconductor 10: Heating wire

Claims (1)

[Claims] 1. A winding frame, a superconducting winding formed by winding a superconducting wire around the winding frame, a cooling device for cooling the superconducting wire below a critical temperature, and heating for heating the superconducting wire above the critical temperature. A permanent current switch provided with a device, wherein the reel is made of a fiber reinforced plastic obtained by molding a matrix resin and a reinforcing fiber having a negative expansion coefficient as one body.