GB2422060A

GB2422060A - Superconducting switch

Info

Publication number: GB2422060A
Application number: GB0500212A
Authority: GB
Inventors: Shaohai Zhang; Alexander John Marshall; Antonis Chris Ioannides
Original assignee: Magnex Scientific Ltd
Current assignee: Magnex Scientific Ltd
Priority date: 2005-01-07
Filing date: 2005-01-07
Publication date: 2006-07-12
Also published as: GB0500212D0; WO2006072630A1

Abstract

A superconducting switch system comprises two connecting parts, a main superconducting switch 1 and an auxiliary superconducting switch 2. The main superconducting switch 1 conducts current between the connecting parts when in a closed state and provides a very low first resistance value 3 between the connecting parts. The auxiliary superconducting switch 2 is connected between the connecting parts and provides a low second resistance value 4 between the connecting parts when in a superconducting mode so that when the main superconducting switch is in an open state, the superconducting current flows through the auxiliary superconducting switch 2. The second resistance value 4 is greater than the first resistance value 3 so that the superconducting current will revert to flowing through the main superconducting switch 1 when the main superconducting switch returns to the closed state. A plurality of main and auxiliary switches could be connected in parallel. This system could be used in a magnet circuit to decrease the risk of a magnet running down in the event of high current operation. If the main switch quenches, the current will transfer to the auxiliary switch until the main switch has re-cooled and recovered to a superconducting state.

Description

Superconducting Switch Systems This invention relates to superconducting

switch systems and persistent mode superconducting magnet systems, and is concerned more particularly with the recovery of the quenching of a superconducting switch during superconducting magnet operation.

Persistent mode superconducting magnets, used in nuclear magnetic resonance spectroscopy (NMR), magnet resonance imaging (MRI), electronic paramagnetic resonance (EPR) and Fourier transform mass spectroscopy (FTMS), incorporate a persistent switch, also referred to as a "main superconducting switch", which when closed permits the superconducting current flow within the magnet circuit to persist without any further injection of current into the circuit being required. The use of such a persistent switch serves to increase the stability of the superconducting magnet over long periods of time, and to reduce the amount of cryogenic liquid boil-off relative to the amount of boil-off that would take place in the event that current needed to be continually supplied to the magnet circuit.

A main superconducting switch typically comprises a short section of superconducting wire connected across the input terminals of the magnet and an integral heater for driving the superconducting wire into resistive mode when the switch is to be opened. When the heater is turned on and the superconducting wire becomes resistive, a voltage is built up across the magnet and the magnet can be energised (or de- energised). After such energisation from an external power supply, the heater is turned off, and the resistive superconducting wire is cooled down by a cryogenic liquid bath so that it again becomes superconductive. In this persistent superconducting mode, the external power supply can be turned off to reduce the heat input to the cryogenic liquid bath and the current will continue to circulate through the magnet circuit and the main superconducting switch.

The main superconducting switch may be caused to quench after a period of time of magnet operation, for example due to interference or magnet wire movement. If the main superconducting switch opens or quenches in this manner, the superconducting magnet current flow will lose its persistence and run down, or even cause the magnet to be quenched.

It might be thought that multiple superconducting switches connected in parallel would overcome this problem in that one of the switches would always be available to conduct the persistent current. However, it turns out that such an arrangement does not work effectively as a result of the fact that recirculating currents are generated within the superconducting switches, and also it becomes impossible to share currents between the two superconducting switches equally.

It is an object of the invention to provide a superconducting switch system that substantially obviates this problem.

According to one aspect of the present invention there is provided a superconducting switch system comprising two connecting parts, a main superconducting switch for conducting superconducting current between the connecting parts when in a closed state and providing a very low resistance of a first resistance value between the connecting parts when in a superconducting mode, and an auxiliary superconducting switch connected between the connecting parts and providing a low resistance of a second resistance value between the connecting parts when in a superconducting mode so that, when the main superconducting switch is in an open state, the superconducting current flows through the auxiliary superconducting switch, the second resistance value being greater than the first resistance value so that the superconducting current will revert to flowing through the main superconducting switch when the main superconducting switch returns to the closed state.

According to another aspect of the present invention there is provided a persistent mode superconducting magnet system comprising a magnet circuit having two connecting parts, a main superconducting switch for conducting superconducting current between the connecting parts when in a closed state and providing a very low resistance of a first resistance value between the connecting parts when in a superconducting mode, and an auxiliary superconducting switch connected between the connecting parts and providing a low resistance of a second resistance value between the connecting parts when in a superconducting mode so that, when the main superconducting switch is in an open state, the superconducting current flows through the auxiliary superconducting switch, the second resistance value being greater than the first resistance value so that the superconducting current will revert to flowing through the main superconducting switch when the main superconducting switch returns to the closed state.

In this manner the main superconducting switch can be protected from external or internal events or interference by using one or more auxiliary superconducting switches, which are connected to the main superconducting switch with the use of low resistance connections rather than superconducting connections. If an event takes place whereby the main superconducting switch opens, then the magnet current can be captured by the auxiliary superconducting switch or switches. If more than one auxiliary superconducting switch is used, the current in these switches is equally shared (in the dynamic and static senses) assuming that the resistance and inductance associated with each auxiliary superconducting switch is of the same value.

By using this approach of auxiliary superconducting switch or switches, the magnet current will only flow through the main superconducting switch during normal magnet operation, the main superconducting switch presenting no resistive joints to the magnet.

In the event that the main superconducting switch opens due to external or internal disturbance, then the magnet current will be diverted through the or each auxiliary superconducting switch as the resistance of the main superconducting switch increases to the order of a few ohms.

The resistance value of the auxiliary superconducting switch needs to be low enough so that the voltage drop across the main superconducting switch is low and therefore the power dissipation in the main superconducting switch is also low. The resistance associated with the auxiliary superconducting switch should be very low, for example of the order of I O f. This allows the main superconducting switch to cool down and close, and the current to then transfer from the auxiliary superconducting switch to the main superconducting switch with a time constant equivalent to L/R seconds, where L is the overall inductance of the auxiliary superconducting switch and the main superconducting switch and R is the resistance of the auxiliary superconducting switch only.

When the main superconducting switch opens and the current is diverted to the auxiliary superconducting switch, before eventually returning to the main superconducting switch, the loss of energy during the process is minimal (typically approximately I Joule).

In order that the invention may be more fully understood, a number of embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram of a superconducting magnet circuit incorporating a superconducting switch system in accordance with the invention; and Figures 2 to 9 are diagrams of a number of alternative embodiments of the invention.

Each of the embodiments of superconducting switch system to be described below incorporates main superconducting switch having a very low resistance of a first resistance value, and an auxiliary superconducting switch having a low resistance of a second resistance value (this resistance including the resistance of the resistive solder joints described below), the second resistance value being greater than the first resistance value so that the superconducting current will revert to flowing through the main superconducting switch when the main superconducting switch returns to the closed state as described in more detail below. Typically the second resistance value is of the order of I 08 0, say 1 0 - 10 0, whereas the first resistance value is of the order of 1012 Referring to Figure 1, a typical superconducting magnet circuit 40 comprises a number of magnet coils 41 serially connected together by superconducting joints 42 and having protection resistors 43 or a combination of resistors and diodes 44, connected in parallel therewith to protect the coils 41 in the event of the magnets quenching. A main superconducting switch I is connected to the circuit 40 by superconducting joints, and an auxiliary superconducting switch 2 is connected to the circuit 40 by resistive solder joints 45 in parallel with the main superconducting switch 1. A diode protection circuitry 47 is connected by further resistive solder joints 46 in parallel with both the main superconducting switch I and the auxiliary superconducting switch 2 to safeguard the switches from damage, and power supply terminals 48 for connection to a power supply (not shown) are connected in parallel with the diode protection circuitry 47. In operation, once appropriate current has been supplied to the circuit 40 by the power supply with the main superconducting switch 1 and auxiliary switch 2 in the open position, the main superconducting switch I and auxiliary switch 2 are closed in order to cause the current to persist in the circuit 40 without further power input. In this circuit the resistance value of the resistive solder joints 45 is of the order of i0 - 10 0, the inductance of the switch assembly is of the order of 10.6 - I0 H, the time constant of the switch assembly is of the order of 10 - 1000 seconds, and the energy loss of the magnet may be of the order of 1 Joule.

Figure 2 shows a first embodiment of superconducting switch system comprising a main superconducting switch 1, which carries the magnet operating current during normal operation of the circuit, and the auxiliary superconducting switch 2 connected in parallel with the main superconducting switch I, as already described above with reference to Figure 1. The auxiliary superconducting switch 2 is resistively connected to the circuit through connecting parts 3 and 4 preferably constituted by solder joints as described above. If the main superconducting switch I quenches, the current will transfer to the auxiliary superconducting switch 2 through the connecting parts 3 and 4.

Subsequently, after the main superconducting switch I has been re-cooled and has recovered to the superconducting state, the voltage drop through the resistive connecting parts 3 and 4 will drive the current back to the main superconducting switch 1.

If the magnet operating current is high, it is not possible for a single auxiliary superconducting switch to handle the operating current when the main superconducting switch is opened, due to the high current rate of change from the main superconducting switch to the auxiliary superconducting switch. In this event then more than one auxiliary superconducting switch is required. Figure 3 shows a second embodiment of superconducting switch system in which two or more auxiliary superconducting switches 6, 7, 8 are connected to the magnet circuit in parallel with the main superconducting switch 5, which carries the magnet operating current during normal operation of the magnet circuit, by resistive solder joints 9, 10, 11, 12, 13, 14. If the main superconducting switch 5 quenches (or opens), the current will transfer to the auxiliary superconducting switches 6, 7, 8 through the solder joints 9, 10, 11, 12, 13, 14.

Subsequently, after the main superconducting switch 5 has recovered, the voltage across the solder joints 9, 10, 11, 12, 13, 14 will drive the current back to the main superconducting switch 5.

To reduce the current transfer rate from the main superconducting switch to the auxiliary superconducting switch, an inductor may be added to the magnet circuit.

Figure 4 shows a third embodiment in which an inductor 16 is serially connected between the main superconducting switch 15 and the auxiliary superconducting switch 17. The inductor 16 serves to reduce the current transfer rate from the main superconducting switch 15 to the auxiliary superconducting switch 17 which is resistively connected to the circuit through the connecting parts 18 and 19 generally constituted by solder joints. If the main superconducting switch 15 quenches, the current will transfer to the auxiliary superconducting switch 17 through the connecting parts 18 and 19. Subsequently, after the main superconducting switch 15 and the inductor 16 have recovered, the voltage across the connecting parts 18 and 19 will drive the current back to the main superconducting switch 15. The presence of the inductor 16 may increase the time constant to about 30 minutes, and the energy loss of the magnet may be of the order of 10 Joules.

To reduce the number of components an inductively wound main superconducting switch 20 may be connected to the circuit, as in the fourth embodiment of Figure 5, in place of the serially connected main superconducting switch 15 and the inductor 16 of Figure 3. As in the third embodiment the auxiliary superconducting switch 21 is connected to the magnet circuit through the connecting parts 22 and 23 so that, if the main superconducting switch 20 quenches, the current will transfer to the auxiliary superconducting switch 21 through the connecting parts 22 and 23.

To reduce the current transfer rate from the main superconducting switch 24 to the auxiliary superconducting switch 25, an inductor 26 is serially connected between the auxiliary superconducting switch 25 and the magnet circuit in a fifth embodiment as shown in Figure 6. As in the third embodiment the auxiliary superconducting switch 25 is connected to the magnet circuit through the connecting parts 27 and 28 so that, if the main superconducting switch 24 quenches, the current will transfer to the auxiliary superconducting switch 25 through the connecting parts 27 and 28.

In a sixth embodiment as shown in Figure 7, an inductively wound auxiliary superconducting switch 30 is provided in place of the serially connected main superconducting switch 15 and the inductor 16 of Figure 3. As in the third embodiment the auxiliary superconducting switch 30 is connected to the magnet circuit through the connecting parts 31 and 32 sO that, if the main superconducting switch 29 quenches, the current will transfer to the auxiliary superconducting switch 21 through the connecting parts 31 and 32. The inductively wound auxiliary superconducting switch 30 reduces the current transfer rate from the main superconducting switch 29 to the auxiliary superconducting switch 30.

In a seventh embodiment as shown in Figure 8, a multi-strand superconducting switch 33 is provided in place of both the main superconducting switch and the auxiliary superconducting switch. One portion of the strands of the switch 33 is connected by a superconducting joint to the magnet circuit to serve as the main superconducting switch, and the other portion of the strands of the switch 33 is resistively connected to the magnet circuit through connecting parts 34 and 35 to serve as the auxiliary superconducting switch so that, if the main superconducting switch quenches, the current will transfer to the auxiliary superconducting switch through the connecting parts 34 and 35. Subsequently, after the portion of the strands constituting the main superconducting switch has recovered, the voltage across the connecting parts 34 and 35 will drive the operating current back to the portion of the strands constituting the main superconducting switch.

In an eighth embodiment as shown in Figure 9, one or more main superconducting switches 36 are connected in series within the magnet circuit, and one or more series connected auxiliary superconducting switches 37 are connected to the magnet circuit in parallel with the main superconducting switch or switches 36 by means of resistive connecting parts 38 and 39. If the or each main superconducting switch 36 quenches, the current will transfer to the auxiliary superconducting switches 37 through the connecting parts 38 and 39. The provision of series connected auxiliary superconducting switches 37 has the advantage of increasing the total resistance in the normal state, thereby increasing the magnet ramp rate during energising or de- energising.

In variant embodiments of the invention that are not illustrated, two or more main superconducting switches may be connected in parallel. Furthermore, in any of the embodiments described, the or each main superconducting switch may be single-strand or multi-strand wound. Also the or each auxiliary superconducting switch may be single-strand or multi-strand wound.

Claims

Claims 1. A superconducting switch system comprising two connecting parts,

a main superconducting switch for conducting superconducting current between the connecting parts when in a closed state and providing a very low resistance of a first resistance value between the connecting parts when in a superconducting mode, and an auxiliary superconducting switch connected between the connecting parts and providing a low resistance of a second resistance value between the connecting parts when in a superconducting mode so that, when the main superconducting switch is in an open state, the superconducting current flows through the auxiliary superconducting switch, the second resistance value being greater than the first resistance value so that the superconducting current will revert to flowing through the main superconducting switch when the main superconducting switch returns to the closed state.
2. A system as claimed in claim 1, wherein a plurality of auxiliary superconducting switches are connected in parallel between the connecting parts, each of the auxiliary superconducting switches providing a low resistance of the second resistance value between the connecting parts when in a superconducting mode so that, when the main superconducting switch is in the open state, the superconducting current flows in parallel through the auxiliary superconducting switches, in order to decrease the risk of the magnet running down in the event of high current operation.
3. A system as claimed in claim I or 2, wherein a plurality of main superconducting switches are connected in parallel between the connecting parts, each of the main superconducting switches providing a very low resistance of the first resistance value between the connecting parts when in a superconducting mode so that the superconducting current flows in parallel through the main superconducting switches when in the closed state.
4. A system as claimed in claim 1, 2 or 3, wherein an inductance is connected in series with the or each main superconducting switch between the connecting parts, in order to slow down the current rate of change when the superconducting current flow changes to the or each auxiliary superconducting switch.
5. A system as claimed in claim 1, 2 or 3, wherein the or each main superconducting switch is an inductive wound superconducting switch in order to slow down the current rate of change when the superconducting current flow changes to the or each auxiliary superconducting switch.
6. A system as claimed in any one of claims I to 5, wherein an inductance is connected in series with the or each auxiliary superconducting switch between the connecting parts, in order to slow down the current rate of change when the superconducting current flow changes to the or each auxiliary superconducting switch and prevent the or each auxiliary superconducting switch from opening.
7. A system as claimed in any one of claims I to 5, wherein the or each auxiliary superconducting switch is an inductive wound superconducting switch, in order to slow down the current rate of change when the superconducting current flow changes to the or each auxiliary superconducting switch and prevent the or each auxiliary superconducting switch from opening.
8. A system as claimed in any preceding claim, wherein the or each main superconducting switch is single-strand or multi-strand wound.
9. A system as claimed in any preceding claim, wherein the or each auxiliary superconducting switch is single-strand or multi-strand wound.
10. A system as claimed in any preceding claim, wherein the or each main superconducting switch is combined with an associated auxiliary superconducting switch in a single multi-strand wound switch component having one strand portion serving as the main superconducting switch and another strand portion serving as the auxiliary superconducting switch.
11. A system as claimed in any preceding claim, wherein the or each auxiliary superconducting switch is connected between the connecting parts by means of connecting means providing the low resistance of the second resistance value.
12. A system as claimed in claim 11, wherein the connecting means comprise solder connections.
13. A system as claimed in any preceding claim, wherein one or more main superconducting switches are connected in series between the connecting parts, and a plurality of auxiliary superconducting switches are connected in series between the connecting parts.
14. A persistent mode superconducting magnet system comprising a magnet circuit having two connecting parts, a main superconducting switch for conducting superconducting current between the connecting parts when in a closed state and providing a very low resistance of a first resistance value between the connecting parts when in a superconducting mode, and an auxiliary superconducting switch connected between the connecting parts and providing a low resistance of a second resistance value between the connecting parts when in a superconducting mode so that, when the main superconducting switch is in an open state, the superconducting current flows through the auxiliary superconducting switch, the second resistance value being greater than the first resistance value so that the superconducting current will revert to flowing through the main superconducting switch when the main superconducting switch returns to the closed state.
15. A superconducting switch system substantially as hereinbefore described with reference to one of the figures of the accompanying drawings.
16. A persistent mode superconducting magnet system substantially as hereinbefore described with reference to one of the figures of the accompanying drawings.