JPS59111381A - Thermoswitch for superconductive magnet - Google Patents

Abstract

PURPOSE:To shorten the insensitive band td improving the responsiveness of a thermoswitch by a method wherein a solenoid is provided on a part of superconductive member to impress there with a magnetic field more intensive than the marginal magnetic field of said member as well as with the current for impressing the magnetic field simultaneously with supplying a heater with current or earlier than that. CONSTITUTION:In order to turn a switch off, i.e. to release a shortcircuit between (a) and (a'), a coil 8 is supplied with current simultaneously with supplying heater current Ih from a control circuit 9 or earlier than that to impress superconductive member 6 with a magnetic field. A limited resistance value may be attained after t1 or td' since the magnetic field is so quick to rise up that the marginal magnetic field is attained at the time t3. For a while after transmitting to the normal conductive status of the super conductive member 6 by a magnetic field, the temperature of the member 6 is heated up to the temperature exceeding the marginal temperature. Therefore even if the impression of the magnetic field is stopped at this time, the temperature of said superconductive member 6 may be maintained at the temperature in the normal conductive status. Besides, the insensitive band td' may be extremely shortened down to around 1sec i.e. 1/10 compared with the conventional one.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermal switch for rapidly turning on and off the persistent current mode of a superconducting coil.

For example, in a nuclear magnetic resonance apparatus, a superconducting magnet that generates a strong magnetic field is used to obtain high resolution. In this superconducting magnet, it is necessary to bring the superconducting coils (main coil and shim coil) into a persistent current state and to release the persistent current, and a thermal switch has conventionally been used for this purpose.

FIG. 1 is a diagram schematically showing a conventional superconducting magnet, in which a dewar 12 is liquid helium injected into the dewar. A superconducting coil 3 is immersed in liquid helium 2 within this dewar, and the coil is cooled to an extremely low temperature and maintained in a superconducting state. The superconducting coil is connected to a DC power source (not shown) via a normal conducting wire 4, so that the coil 3 can be placed in a persistent current mode or released from that mode. Further, a thermal switch 5 is connected between both ends a and a' of the superconducting coil 3, so that the persistent current 'E-' can be turned on and off.

The thermal switch is made up of a superconducting member and a heater, and when a heating current Ih is supplied to the heater from the outside, a part of the superconducting member reaches a critical temperature or higher, breaking the superconducting state and changing to a normal conducting state. The short circuit between a and a' is released. When switching between these modes, it is preferable that the thermal switch 5 operates quickly. Especially in the case of shim coils, a delay in cutting time has a significant effect on correction accuracy, so a switch with high responsiveness is required. be.

However, the conventional heat switch as described above only changes the temperature of the superconducting member and does not supply heater power that would raise the temperature of the superconducting member above the critical value TO, so it is not fast enough. Switch action cannot be obtained. FIG. 2 shows the characteristics or operating state of a conventional thermal switch, in which the horizontal axis represents the temperature T, and the vertical axis ]-1 represents the applied magnetic field. The shaded area indicates the superconducting region,
The superconducting member at point A, which is in a superconducting state, passes through critical point B by heating with a heater and reaches, for example, the 0 point in the normal conduction region. In the figure, TC indicates the critical temperature of the superconducting member, and )lc indicates the critical magnetic field strength. It takes quite a long time to reach point B from point A, and the switch becomes a dead zone during that time. FIG. 3 is a diagram for explaining this, and (a) 4 shows the current supplied to the heating heater, and shows the case where supply is started at time t1. In response to the supplied current, the superconducting member's temperature rises slowly as shown in FIG.
Reach O. Therefore, as shown in Figure (0), the finite resistance R occurs at time t2, which is delayed by td from time t1. This delay time td usually reaches several to several tens of seconds, resulting in an extremely long dead zone.

Therefore, the present invention aims to shorten the dead zone td and improve the responsiveness of the thermal switch.

The feature of the structure of the present invention is that it has a superconducting member and a heater connected to both ends of the superconducting coil, and a heating current is supplied to the heater to release the persistent current mode of the superconducting coil. In the thermal switch, a solenoid coil for applying a strong magnetic field higher than the critical magnetic field of the superconducting member is provided in a part of the superconducting member, and a magnetic field is generated in the solenoid coil at the same time as or earlier than the current supply to the heating heater. The thermal switch for the Ic superconducting magnet device is designed to supply current for the Ic superconducting magnet device.

FIG. 4 is a diagram for explaining the present invention in detail, and the second
This corresponds to the figure. In the present invention, the switch is not turned off simply by a change in temperature, but is turned off by the combined use of a magnetic field. In the figure, point A indicates the superconducting operating point of the conventional superconducting member, and the path D-+E→F-)C is followed from point A to the normal conduction state.

That is, in the present invention, the transition from superconductivity to normal conduction is achieved not only by temperature but also by the application of an impulse magnetic field, thereby achieving a rapid transition and extremely shortening the dead zone of the switch. .

The present invention will be explained below based on an embodiment shown in FIG.

In the figure, 3 is a superconducting coil, and 5 is a thermal switch, which is connected between both ends a and a' of the superconducting coil. The thermal switch is composed of a superconducting member 6, a heater 7, and a solenoid coil 8 for generating a magnetic field, and the heater and coil are connected to a control circuit 9. Then, currents of lh and 1m are supplied, respectively. The heater current 1
h is a current similar to the conventional one shown in FIG. 3(a), and is shown in FIG. 6(b). In addition, the magnetic field generating current lll1 is a current that can generate a magnetic field greater than the critical magnetic field (approximately A voltage is applied to the superconducting member 6.

In such a configuration, switch off, i.e. a, a'
To release the short circuit between the two, the control circuit 9 supplies 1 h of heater current, or at the same time, the coil 8 current is supplied to apply a magnetic field as shown in FIG. 6(a) to the superconducting member 6. do.

As is clear from the figure, the magnetic field rises very quickly and reaches the critical magnetic field TC at time t3, so that it has a finite resistance value after td' from tl as shown in the diagram (d). After the superconducting member 6 transitions to the normal conductive state due to the magnetic field, the temperature of the superconducting member 6 rises due to heating by the heater and reaches a critical temperature or higher, as shown in FIG. Therefore, even if the application of the magnetic field is cut off at this point, as shown in Figure (a), the superconducting member will maintain a normal conduction state in terms of temperature. The above td' is extremely short, about 1 second, and can be reduced to about 1/10 of the conventional value.

Note that the above is an example of the present invention, and various modifications are possible in reality. For example, although the magnetic field is applied in an impulse manner in the above example, a constant magnetic field may be continued to be applied after the normal conduction state is reached. In this case, the operating point is not the 0 point in Figure 4 but E.
The point is between point and 0, for example, point F.

With the configuration described in detail above, the switch-off operation can be performed extremely quickly, and a dead zone that poses a problem when using the superconducting magnet device does not occur.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an outline of a conventional superconducting magnet device, FIGS. 2 and 3 are diagrams showing the operating state of a conventional thermal switch, FIG. 4 is a diagram explaining the present invention in detail, and FIG. 6 is a schematic diagram showing an embodiment of the present invention, and FIG. 6 is an explanatory diagram of the operation of the embodiment of FIG. 3: Superconducting coil 5: Thermal switch 6: Superconducting member 7: Heating heater 8: Coil for magnetic field generation 9: Control circuit Patent applicant JEOL Ltd. Representative Ikuo - Figure 1 k → Figure 5 ^ Tc-7→

Claims (1)

[Claims] A thermal switch connected to both ends of a superconducting coil, having a superconducting member and a heating heater, and configured to release a persistent current mode of the superconducting coil by supplying a heating current to the heater. A solenoid coil for applying a strong magnetic field higher than the critical magnetic field of the superconducting member is provided in a part of the superconducting member, and a current for generating a magnetic field is supplied to the solenoid coil at the same time as or earlier than the current supply to the heater. A thermal switch for a superconducting magnet device, which is characterized by: