JPS6155275B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a persistent current switch when a superconducting magnet is used in persistent current mode, and its purpose is to provide a permanent current switch that does not impair the function of the persistent current switch and that is different from conventional persistent current switches. The aim is to eliminate the irreversibility of the heater current and the superconducting transition, and to ensure stable operation of the persistent current switch.

Generally, superconducting magnets are excited by a DC power source, but depending on the application of the superconducting magnet, it may be more effective to disconnect the power source and use it in persistent current mode. Examples include superconducting magnets for magnetic levitation high-speed trains and superconducting magnets for nuclear magnetic resonance.The former eliminates the need to load a power source onto the train by using it in persistent current mode, while the latter eliminates ripples from the power source by disconnecting the power source. It is possible to avoid the mixing of noise such as
In particular, operation of superconducting magnets in persistent current mode is effective for those that require high uniformity, such as superconducting magnets for nuclear magnetic resonance. In addition, the operation of superconducting magnets in persistent current mode is used for various purposes, such as because it reduces electric power costs.

A persistent current switch is required to enter persistent current mode, and initially the persistent current switch is
The permanent current switch is turned OFF and then excited by a DC power source to achieve the desired state, and then the persistent current switch is turned ON to create a persistent current circuit.

A superconducting persistent current circuit with a conventional persistent current switch is shown in FIG. A superconducting magnet 1 is excited by a DC power source 2. Further, there is a superconducting wire 3 in parallel with the superconducting magnet 1, and a composite superconducting wire embedded in a base material having high electrical resistance such as a CuNi alloy is usually used. Further, in this circuit, the area closer to the superconducting magnet 1 than the connection points 7 and 8 is entirely composed of superconducting wires. A heater 4 is wound around the superconducting wire 3, and the material of the heater is a high resistance metal such as manganin wire or nichrome wire. The heater 4 is connected to a power source 6 and is opened and closed by a switch 5. A persistent current switch 10 is constituted by the superconducting wire 3 and the heater 4, and the persistent current switch 10 and the superconducting magnet 1 are immersed in a cryogenic coolant such as liquid helium.
(Inside the dotted line) Next, the process of changing to persistent current mode is shown.

First, close the switch 5 and connect the power supply 6 to the heater 4.
A current is passed through. A heater 4 is wound around the superconducting wire 3. Heat generation by the heater 4 causes a temperature rise, and when the temperature rises above the critical temperature of the superconducting wire 3, the superconducting wire 3 transitions to a normal conductive state, and the superconducting wire Since the base material of superconducting wire 3 has a high resistance, superconducting wire 3 has a much higher electrical resistance than superconducting magnet 1. This state is the persistent current switch 10.
is in the OFF state. Persistent current switch 10
When current is applied from the DC power source 2 in the OFF state, most of the current flows into the superconducting magnet 1 and no current flows into the superconducting wire 3. After bringing the superconducting magnet 1 into the desired state using the DC power supply 2,
When the switch 5 is opened to reduce the current of the heater 4 to zero and the generation of heat from the heater 4 is reduced, the superconducting wire 3 returns from the normal conducting state to the superconducting state. This is the ON state of the persistent current switch. When the persistent current switch is turned ON and DC power supply 2 is turned off, the current becomes 1.
Persistent current continues to flow in the loop of -8-3-7, resulting in persistent current mode.

By the way, as mentioned above, manganin, nichrome wire, etc. have been conventionally used as materials for the heater 4. However, these materials have high electrical resistance but low thermal conductivity. Heat escape is poor, and the heater current and return to the superconducting state are not reversible.

This was demonstrated in experiments using a CuNi alloy with 80 cores of NbTi alloy embedded in the superconducting wire for the persistent current switch, and a 4.5Ω manganin wire for the heater. The configuration of the persistent current switch at that time is shown in FIG. A shows a heater 4 and a superconducting wire 3 stacked on a Teflon winding frame 9 and winds them, and a shows a superconducting wire 3 with a heater 4 spirally wound around a Teflon winding frame 9. be. Superconducting wire 3 is immersed in liquid helium.
A current of 1A is applied to the heater 4, and the current of the heater 4 is changed to change the superconducting wire 3 from superconductivity to normal conduction, and from normal conduction to normal conduction.
FIG. 3 shows the state of superconductivity transition investigated by measuring the voltage across the superconducting wire 3. In the figure, a and b correspond to cases a and b in FIG. 2, respectively. Further, the arrows indicate the direction of the loop due to increase/decrease in heater current. In case b, when the heater current is increased from the superconducting state, resistance appears from point A, and the state becomes completely normal conducting at point C. Conversely, as the heater current is decreased, a superconducting state begins to appear from point D and becomes completely superconducting at point E, creating a kind of large hysteresis. This is heater 4
This is thought to be because the heat generated by the heater does not escape easily due to the low thermal conductivity of the manganin used in the heater. Furthermore, even if the heater current is reduced from point G, which is halfway through the transition from the superconducting state to the normal conducting state, a similarly large hysteresis will occur and the state will return to point E. In the case of a, there is a sudden transition to the normal conduction state from point B,
Even if the heater current is reduced to zero, it will not return to the superconducting state. This is thought to be because, as can be seen from Figure 2, the structure allows more heat to be trapped compared to case b.

As described above, conventional persistent current switches lack reproducibility, are not reliable, and may lead to instability. In particular, the structure a in FIG. 2 is hardly usable as a persistent current switch, as is clear from the data in FIG. This phenomenon is thought to be due to the use of materials with high electrical resistivity, such as manganin wire or nichrome wire, for the heater of the persistent current switch. That is,
Since metals and alloys with high electrical resistivity generally have low thermal conductivity, it is thought that the above-mentioned hysteresis phenomenon appears.

Based on these study results, the present inventors used copper, aluminum, silver, or alloys mainly composed of these metals, which are metals with high thermal conductivity, as materials for the heater of a permanent current switch. This device succeeded in eliminating the above-mentioned drawbacks of current switches.

That is, the present invention is a persistent current switch in which a heater is wound around a superconducting wire, and is characterized in that the heater is made of copper, aluminum, silver, or an alloy mainly composed of these metals.

Thermal conductivity of these metals or alloys at liquid helium temperature (4.2K) is about 3 orders of magnitude higher than that of manganin, etc., and the heat generated by the heater is immediately removed when the heater current is reduced, making it difficult to maintain the thermal conductivity of conventional permanent heat. The hysteresis that current switches have is eliminated.

〔Example〕

An example using a persistent current switch according to the present invention will be shown. The structure was the same as shown in Figure 2, and both a and b were used. Formal coating as heater 4
The resistance value using 6m of 0.10mmφ copper wire was 0.12Ω in liquid helium. FIG. 4 shows the results of examining the characteristics in the same manner as in the case of FIG. 3. In the case of b, the transition from the superconducting state to the normal conducting state begins at point A, and the state becomes completely normal conducting at point C. Conversely, as the heater current is reduced, there is a slight hysteresis, but the superconducting state roughly follows the increasing line. The condition has returned to normal. In the case of a, it similarly transitions to the normal conduction state from point B and follows almost the same line even if the heater current is lowered.

In addition, experiments were also conducted using aluminum wire, silver wire, copper alloy wire, aluminum alloy wire, and silver alloy wire as heaters, and similarly good results were obtained.

As is clear from the above explanation, according to the present invention, the hysteresis characteristic seen in conventional persistent current switches can be eliminated and the operation of persistent current switches can be stabilized, which is an extremely important effect in practice. .

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a superconducting persistent current circuit having a persistent current switch, Figures 2a and b are sectional and perspective views showing the structure of different persistent current switches, and Figure 3 is a diagram of a conventional persistent current switch. Characteristic diagram,
FIG. 4 is a characteristic diagram of the persistent current switch of the present invention. 3...Superconducting wire, 4...Heater, 10...Persistent current switch.

Claims (1)

[Claims] 1. A persistent current switch in which a heater is wound around a superconducting wire, wherein the heater is made of copper, aluminum, silver, or an alloy mainly composed of these metals.