GB2592264A

GB2592264A - Method for detecting closure of a superconducting switch

Info

Publication number: GB2592264A
Application number: GB2002537.5A
Authority: GB
Inventors: Bampton Adrian; Chorley Simon
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2020-02-24
Filing date: 2020-02-24
Publication date: 2021-08-25
Anticipated expiration: 2040-02-24
Also published as: GB202002537D0; GB2592264B

Abstract

The status of a superconducting switch 20 is detected by directly measuring an electrical characteristic of the switch circuit. The transition of the switch from the resistive state 24 to the superconducting state 22 is detected by applying an AC current through the switch, monitoring a characteristic of the resulting AC voltage developed across the switch, and detecting a change in the monitored characteristic. The characteristic may be a reduction in magnitude or a reduction in the phase difference between the AC current and the AC voltage. The AC current may be a ripple current that is naturally present in the DC supply 12, or may be applied intermittently. The switch may be part of a magnet system in which a superconducting magnet 10 may be ramped up or down by heating the switch and monitoring its state using the disclosed method. Methods of ramping up or down the magnet are also disclosed.

Description

METHOD FOR DETECTING CLOSURE OF A SUPERCONDUCTING SWITCH

The present invention relates to superconducting switches, and methods and arrangements for detecting their closed or open 5 status, and for timing other operations which depend on the status of a superconducting switch.

A superconducting switch is a device operable between, typically, two alternative statuses. In one status, the switch 10 provides a superconducting path of essentially zero resistance. That is its "closed" status. In the other status, the switch provides a resistive path. That is its "open" status. That resistive path may not have a particularly large resistance. It may be in the order of tens of ohms, but in the context of a superconducting circuit, such a path is considered "open".

Fig. 1 illustrates an example superconducting circuit, representing a persistent superconducting magnet. Magnet 10 is represented as a single inductor, but such a magnet, schematically represented, will typically comprise a number of coils of superconducting wire, connected electrically in series. Such an arrangement has a practically zero resistance when operating in superconducting mode, but typically has a large inductance.

A power supply unit PSU 12 provides voltages V+ and V-at respective positive and negative terminals. Voltage taps 14, 16 are conventionally provided to measure a ramp voltage or 30 other diagnostic information.

Superconducting switch 20 is schematically represented. It can provide a "closed" current path 22 of essentially zero resistance, or an "open" resistive current path 24. A control input 26 causes the switch to change from one status to the other. Typically, the control input 26 provides current to a small heater (not illustrated) which heats a superconductive wire forming the current path 22/24 to a temperature in excess of its superconducting transition temperature. When current is applied to the small heater by control input 26, the superconducting wire of the switch becomes resistive, and the switch becomes "open". The superconducting switch 20 is typically arranged to be cooled to a cryogenic temperature, typically by the same cooling arrangement as is used to cool superconducting magnet 10. If current from control input 26 ceases, the superconductive wire will cool back to below its superconducting transition temperature and become superconducting again, offering a superconducting path of essentially zero resistance on "closed" status.

A rundown load 28 is provided, and may be connected in place of the power supply unit 12. The power supply unit 12 is used 15 to introduce current into the magnet 10, while run down load 28 is provided to remove current from the magnet 10.

In the circuit illustrated in Fig. 1, current may be introduced into the superconducting magnet 10 in the following manner. A 20 current is applied to the heater of switch 20 by control input 26, keeping the switch in its "open" status represented by resistive current path 24. The power supply unit PSU 12 applies a voltage to terminals of the magnet 10. Although the magnet 10 may be in a superconducting state and have 25 essentially zero electrical resistance, it has a high inductance. This inductance provides an Impedance, meaning that a change in applied voltage causes a relatively gradual change in current flowing in the magnet 10 over time. A certain amount of current may flow through the superconducting switch 20 at this time due to the voltage appearing across magnet 10. However, as the superconducting switch 20 provides a resistive path 24, some heat will be dissipated in the superconducting wire, helping to maintain the switch in its "open" status.

Once a sufficient amount of current has been introduced into the magnet (which may be measured as current passing through the power supply unit PSU), the voltages V+ V-should be removed and the switch 20 closed so that the magnet may operate in "persistent" mode, the magnet current flowing in a complete superconducting circuit.

Since the transition between "open" and "closed" status is a change in resistance of a single wire, and is not truly represented by the mechanical switch analogy of Fig. 1, the transition happens relatively gradually, and the present invention addresses the difficulty of determining which status a superconducting switch is in.

In superconducting magnets cooled by a bath of liquid cryogen, a bath of liquid cryogen is maintained in liquid form by a cooling system comprising a cryogenic refrigerator. The switch sits in the bath of liquid cryogen, and can be cooled from an "open" state to a "closed" state very rapidly, for example in a few seconds. In such a system, simply waiting a few seconds or minutes is enough to be confident that the switch has entered its "closed", superconducting state, and is ready to carry the magnet current.

On the other hand, such liquid-cooled systems have significant thermal inertia. The power applied to the switch heater to overcome the cooling power of the cryogen can be significant, 25 and much higher than the cooling power available from the cryogenic refrigerator. The only practical consequence of this is a small loss of cryogen, since the bath of liquid cryogen will very quickly re-cool the switch to its "closed" state once the heating of the switch ceases.

Recently, superconducting magnets have been produced which do not employ a bath of liquid cryogen for cooling. This is partly due to increasing cost and scarcity of helium.

In such magnets, cooling tends to be either by cooling loop (thermosiphon) or solid conduction to a cryogenic refrigerator. Such dry-or low-cryogen-systems cannot allow the dissipation during ramping (that is, introduction of current into the superconducting coils by application of a voltage) to exceed the cooling power of the refrigerator, otherwise the system tempera-tare will rise and may lead to a quench of the superconducting magnet coils. It has become conventional that, in such systems, the superconducting switch is only weakly coupled to the cooling system, so that it may be retained in its "open" state, with the superconducting wire of the switch at an elevated temperature, with a minimal heating power applied to the switch. This is intended to reduce the transmission of heat from the switch to other parts of the system, and to allow the switch to maintain an elevated temperature in presence of colder parts of the system. However, an unhelpful effect of this weak coupling to the cooling system is that cooling the switch back to its "closed" state once heating of the switch has ceased can take a significant time, for example, tens of minutes. It is important to be able to decide with confidence that the superconducting switch has closed, and has cooled sufficiently to carry the magnet current, as only then should the power supply unit PSU 12 be disconnected and the magnet current passed through the superconducting current path 22 of the switch 20. In arrangements such as discussed in U55093645, a temperature sensor may be thermally linked to the superconducting switch to give an indication of the temperature of the switch. Such measurement may be used to allow for variations such as in cooling power of cryogenic refrigerators, thermal conductivity of materials involved and of various thermal interfaces involved. These effects may mean that although a fixed delay could be established to have confidence that even the slowest superconducting switch has closed, such delay would be unnecessarily long for most systems using faster switches.

Furthermore, such method of using temperature sensors introduces cost and complexity. A calibrated temperature sensor is required, and a calibrated backup may also be required, for use in case of failure of the primary temperature sensor. These sensors require associated wiring looms to be installed, along with feed-throughs to provide connections outside of a vacuum vessel containing a superconducting magnet device; and measurement electronics also have to be provided.

When ramping up a magnet, the Power Supply Unit PSU 12 must maintain the desired current flowing through the leads and into the device until the switch goes fully persistent. At this point, the power supply current can be reduced to zero, such that the magnet current must flow through the superconducting switch. At this point, the Power Supply Unit PSU 12 can be removed.

The present invention aims to provide a simpler, cheaper method of detecting the superconducting transition of a 15 superconducting switch attached to a superconducting magnet.

The present invention accordingly provides structures and methods as defined in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, with reference to the accompanying drawings, wherein: Fig. 1 shows a schematic representation of a circuit, 25 conventional in itself, for introducing current into, and removing current from, a superconducting magnet; Fig. 2 shows example time curves for magnet voltage, PSU current and switch temperature for a method of introducing current into a superconducting magnet, according to a method 30 of the present invention; and Fig. 3 shows example time curves for magnet voltage, PSU current and switch temperature for a method of removing current from a superconducting magnet, according to a method of the present invention.

The present invention provides a method of detecting when a superconducting switch has closed. The method enables a reliable, cost-effective detection. It may be found useful in the context of conduction-cooled superconducting magnets. In such magnets, the rapid switch cooling conventionally provided by immersion of a superconducting switch in liquid cryogen is not available and the time taken to close the superconducting switch can be significant.

The methods and apparatus of the present invention do not require sensors to be deployed to monitor the temperature of the superconducting switch, but rather detect the status of the switch by an electrical circuit characteristic which may be observed.

The voltage applied by the power supply unit PSU 12 is a DC voltage but also provides an amount of AC ripple current. This may be a feature of an AC power supply used to generate the DC voltage, or it may be an AC ripple current especially generated for the purpose. A DC voltage should not be applied for the purpose of detecting the status of the superconducting switch, since the application of a DC voltage will introduce further DC current into the magnet coils 10 and ramp the magnet away from the desired current. The AC ripple current should be small enough that ohmic heating in the switch, in its "open" status, does not prevent or significantly delay the switch from closing. To reduce the heating effect of the AC ripple current in delaying switch closure, the AC ripple current may be applied intermittently, for example for short pulses, sufficient to effect reliable measurement, at intervals of several seconds.

The simplest arrangement may be possible where the power supply unit PSU produces a DC voltage with an AC ripple which can be used to detect switch status according to the present invention. In such an arrangement, no current is supplied to the switch or the magnet other than that which would be provided in the absence of the present invention. The amplitude or phase of a corresponding AC voltage across the switch 20, detected at terminals 14, 16 may be used to provide an indication of the status of the switch. In such embodiments, no additional hardware is required to provide the method of the present invention.

Considering the circuit representation of Fig. 1, when the switch 20 is "open", and the current passes through resistive current path 24, the magnet system as illustrated appears electrically to be a resistor (resistive current path 24) in parallel with an inductor (magnet 10), giving an apparent impedance between terminals 14, 16 of: Zu = 1/ (1/R + 1/(R, + 21-1fL")) = 1/(1/R" + 1/(0 + 2nfL")) > 0, where ZL is total impedance, Rs" is the resistance of the switch (typically either zero or some tens of ohms depending on the status of the switch), Rm is the resistance of the magnet (zero if the magnet is in superconducting state), f is the frequency of the applied signal and Lm is the inductance of the magnet (typically tens of Henrys).

When the switch 20 is "closed", the magnet system appears as an approximately zero resistance (the switch 20 using superconductive current path 22) in parallel with an inductor (magnet 10), giving an apparent impedance between terminals 14, 16 of: Zt = 1/(1/0 + 1/(0 + 2nfLm)) 0.

Therefore, when the superconducting switch 20 reverts to its 30 "closed" status, as superconducting current path 22, the apparent impedance between terminals 14, 16 would fall to approximately zero.

The terminals 14, 16 are typically provided across the magnet 35 to measure the ramp voltage and other diagnostic information. Measurement of an impedance between these terminals provides a simple method for detecting the status of the superconducting switch 20 without providing any additional equipment within the superconducting magnet system.

In one embodiment of the present invention, impedance between 5 terminals 14, 16 is measured by measuring the magnitude of the AC voltage across superconducting switch 20. In another embodiment of the present invention, impedance between terminals 14, 16 is measured by measuring a phase difference between the AC voltage across superconducting switch 20, and the AC current through it.

The methods of the present invention may be performed by conventional equipment, suitably programmed.

Methods of the present invention will involve the steps of detecting a magnitude and/or phase of an AC voltage across superconducting switch 20 caused by an AC ripple current provided to the superconducting switch. A larger voltage, or a larger phase difference, indicating an appreciable impedance Z_, will indicate that the superconducting switch is in its resistive "open" status, and a smaller or even undetectably small, voltage or phase difference will indicate that the superconducting switch is in its superconductive "closed" status.

Even once the superconducting switch 20 is determined to be in "closed" status, the full current through the power supply unit PSU 12 should not be switched immediately to flowing through the switch. The detected transition to the switch being in "closed" status takes place with essentially zero current flowing in it. If the full magnet current were immediately to flow through The superconducting switch, the combination of current, temperature and local magnetic field strength may be sufficient to drive the superconducting switch back into its resistive "open" status by exceeding the capability of the superconductor. A further-delay must be provided between detection of the switch 20 becoming "closed", and the time at which the superconducting switch is deemed ready to pass the full magnet current. It is not Possible to measure the correct timing for this transition, but a type test can be performed on any particular type of switch, to give a fixed further-delay time. The time from transition to "closed" status to readiness for application of the full current to the switch should be repeatable for all further switches of the same type.

The present invention may be found particularly useful in determining a transition time for the superconducting switch into "closed" status in instances where the switch heats to an unknown temperature. Rather than allowing a lengthy transition time, sufficient for re-cooling the switch from maximum temperature excursions, the present invention allows for simple detection of the actual transition time, to which a predefined further-waiting time is added, to define a time at which the magnet current may be passed through the superconducting switch: an event known as "persisting", that is, placing the magnet in its persistent state where the magnet current flows in a closed superconducting circuit, using superconducting current path 22.

The present invention accordingly allows the transition of a superconducting switch from "open", resistive, status to "closed", superconducting status to be detected electrically. In preferred embodiments, this method may be performed using only hardware which is already provided within a superconducting magnet system. The arrangement is inexpensive, as preferably no additional hardware is required. 30 The method is also reliable, as it does not rely on temperature sensors. The method is reliable as it detects the actual electrical transition of superconducting switch 20 from resistive to superconducting. By adding a predetermined further-delay time, the total waiting time before persisting the magnet can be minimised; and reduced as compared to the conventional method of simply waiting long enough to be confident that the superconducting switch will be ready to receive the full magnet current, under all circumstances.

Figs. 2 and 3 illustrate measurements which may be made in methods according to respective embodiments of the present invention. Example times are indicated in the format hh:mm.

Fig. 2 illustrates operation of the present invention in respect of a method for ramping the magnet to field -that is, introduction of a desired DC current into the magnet 10. In Fig. 2, the EMS magnet voltage is plotted against a logarithmic scale, while the magnet current and switch temperature are plotted against linear scales.

As illustrated, in this example, a DC voltage of a few volts is applied to the magnet 10. The magnet current begins to increase. Because of the large inductance of the superconducting magnet 10, the current through the magnet increases only gradually. As illustrated, the applied magnet voltage decreases over time, from 08:48 to 09:25, while the magnet current increases. During this time, the temperature of the switch increases to a peak value, and begins to decrease as the applied voltage decreases. At 09:25, the magnet current reaches a predetermined target value, and the magnet is deemed to be "at field". At that point, the power supply unit PSU 12 ceases to supply voltage to the magnet coil 10. The AC voltage across terminals 14, 16, represented in Fig. 2, falls to a noise voltage value of about 7mV. In the illustrated example, the superconducting switch 20 at that point is at a temperature of about 10.8K.

As the voltage applied by the Power Supply Unit 12 drops to a small noise value of 7mV, the magnet current stabilises at the predetermined value which represents the magnet "at field", and the temperature of the switch begins to decrease. At 09:40, the switch temperature has fallen to a value of about 8.6K. At that point, the superconducting switch reverts to its superconducting "closed" status. According to an aspect of this embodiment of the invention, this transition to the "closed" status can be observed in the magnet voltage as

II

observed at terminals 14, 16. There is a relatively sudden drop in the magnet voltage at this time, falling from approximately 7mV to approximately 3mV. This change may be detected, according to an embodiment of the present invention, 5 and represents a change in the status of the superconducting switch from "open" to "closed". A further-delay is then imposed, to 10:04, to allow the temperature of the switch to decrease further so that the superconducting switch is ready to carry the magnet current. At 10:04, the switch temperature 10 has reduced to about 5.2K, and the magnet current at this point begins to flow through the superconducting current path 22 of superconducting switch 20. The magnet thereby enters its persistent mode. The magnet current illustrated in Fig. 2 is measured as the current flowing though power supply unit PSU 12. At 10:04, this falls to zero as the magnet current transfers to the switch 20 through superconducting current path 22. This may be achieved by increasing an internal resistance within the power supply unit, or by simply disconnecting it.

According to the method provided by the present invention, the magnitude of the noise voltage is monitored, and when it changes at 09:40, this indicates that the switch has entered its "closed", superconducting mode. A predetermined further-delay then commences, of a duration determined by type testing.

In this illustrated embodiment, 09:40 to 10:04. At the end of sure that the switch is ready to and so the full magnet current the further-delay lasts from the further-delay, one can be carry the full magnet current, is transferred from the power using supply unit 12 to the superconducting switch 20 superconducting current path 22.

Fig. 3 illustrates operation of the present invention in respect of a method for partial ramp down of the magnet from field -that is, removal of DC current from the magnet 10 to a lower level. In Fig. 3, the RMS magnet voltage is plotted against a logarithmic scale, while the magnet current and switch temperature are plotted against linear scales.

Run-down of a magnet 10 is typically achieved by placing resistive run-down load 28 across the terminals 14, 16 of the magnet coil, illustrated in Fig. 1. The PSU 12 provides a DC current equal to the lower level. As described above, the PSU also provides the AC voltage.

Once the run-down load 28 is connected, the run-down procedure is commenced by heating the superconducting switch 20 to place it in its resistive "open" status. As illustrated in Fig. 3, this causes the voltage across the magnet to increase, in this example to a few volts, as the magnet current passes through the run down load 28. The resultant ohmic heating in the switch ensures that the superconducting switch remains in its 15 "open" status, and the run down load begins to reduce the magnitude of current flowing through the magnet 10. By approximately 11:00, the energy previously stored in the superconducting magnet coil 10 has been dissipated as heat in the run-down load. The current through the magnet has dropped to the lower level, as provided by the PSU 12. No further change in current occurs in the circuit, and so the voltage across the switch drops to a noise voltage of about 7mV or so. The switch temperature had attained a relatively high value, about 15.5K in this example, and begins to fall once the voltage across the superconducting switch is reduced. As very little current is now flowing through the switch 20, any resulting ohmic heating is insufficient to keep the switch in its "open" status. The temperature of the switch declines, as shown in Fig. 3, as the switch is cooled by the cooling system provided for cooling the superconducting magnet 10. At about 11:39, the superconducting switch has again cooled to approximately 8.6K, at which temperature the switch again returns to its "closed", superconducting state. Similarly to the representation in Fig. 2, the magnitude of the noise voltage drops, in this example to about 3-4mV. This transition is detected according to a feature of the present invention as an indication that the superconducting switch 20 has entered its "closed", superconducting state using superconducting current path 22. As before, a further-delay time is added, which in this example expires at about 12:04. By this time, the switch has cooled to about 5.2K. The remaining magnet current can then be carried by the superconducting current 5 path 22 in the switch 20. As in the case with ramping to field, discussed with reference to Fig. 2, the further-delay between the superconducting switch entering its "closed" superconducting state and all of the remaining magnet current being directed through the superconducting current path 22 may 10 be determined by type testing, and a same further-delay time period employed for all switches of the same type.

In the illustrated examples, the further-delay represents the time between the superconducting switch 20 reverting to its 15 "closed", superconducting state, and the temperature of the superconducting switch reaching 5.2K, at which point it is ready to carry the magnet current. The further-delay may accordingly be considered as the transition-to-ready time. In the ramp to field example of Fig. 2 above, this further-delay is set to 24 minutes, and in the ramp down example of Fig. 3, this further-delay is set to 23 minutes. These durations are believed to be similar as the conditions are similar in the two cases: the same switch cooling from superconducting transition (which takes place at about 8.8K in this example) and the switch reaching a temperature of 5.2K. On the other hand, cooling to 8.8K takes longer in the case of ramping down (Fig. 3) than it did in the case of ramping up (Fig. 2) because the switch is cooling from a higher temperature, in the case of ramping down, than in the case of ramping up. In the method of the present invention, there is no need to determine, or measure, the peak temperature reached by the switch. A detection is made when the switch has cooled back to its superconducting state, and a further-delay is added, to allow the switch to cool to a "ready" temperature, in this example, about 5.2K. There is no need to measure or estimate the temperature reached by the switch.

Claims

CLAIMS1.A method for detecting the transition of a superconducting switch from a resistive state to a superconducting state by an electrical circuit characteristic which may be observed.
2.A method for detecting the transition of a superconducting switch from a resistive state to a superconducting state by measuring an inductance change of the circuit.
3.A method for detecting the transition of a superconducting switch from a resistive state to a superconducting state, comprising the steps of: -applying an AC current through the switch; - monitoring a characteristic of an AC voltage across the switch, said voltage resulting from the passage of the AC current through the switch; - detecting a change in the monitored characteristic, which indicates the transition of the superconducting path to a superconducting state.
4.A method according to claim 1, wherein the monitored characteristic is voltage, and the detected change in the monitored characteristic is a reduction in magnitude of an AC voltage across the superconducting switch.
5.A method according to claim 1, wherein the monitored characteristic is phase, and the detected change in the monitored characteristic is a reduction in phase differ-ence between the AC current and of an AC voltage across the superconducting switch.
6.A method according to any preceding claim, wherein the AC current is applied intermittently.
7.A method according to any preceding claim, wherein the AC current is a feature of a DC voltage supplied by a power supply unit.
8.A method for ramping up a superconducting magnet, comprising the steps of: -connecting a power supply unit (12) across a parallel combination of a superconducting magnet (10) and a superconductive switch (20); - opening the superconductive switch by heating a superconductive wire which forms a current path within the superconductive switch; - providing a DC current from the power supply unit to the superconducting magnet, thereby developing a DC voltage across the superconducting switch; - once a current of a predetermined magnitude has de-veloped in the superconducting magnet, ceasing supply of the DC voltage from the power supply unit; - ceasing heating to the superconductive wire; - detecting the transition of a superconducting switch from a resistive state to a superconducting state, by a method according to any preceding claim; - waiting for a further-delay time; and then - disconnecting the power supply unit.
9.A method for ramping down a current through a supercon-ducting magnet to a lower level, comprising the steps of: - connecting a run down load (28) across a parallel combination of a superconducting magnet (10), a power supply unit (12) and a superconductive switch (20); - providing a DC current equal to the lower level through the power supply unit; - opening the superconductive switch by heating a superconductive wire which forms a current path within the superconductive switch; - passing a DC current from magnet through the run-down load, thereby developing a DC voltage across the superconducting switch and dissipating power in the run down load; -once a current in the superconducting magnet has di-minished to the lower level, and DC current has ceased to flow in the superconducting switch, ceasing heating to the superconductive wire; - detecting the transition of a superconducting switch from a resistive state to a superconducting state, by a method according to any one of claims 1-9; - waiting for a further-delay time; and then - disconnecting the run down load.