SUPERCONDUCTOR MAGNET SYSTEMS AND METHODS FOR GENERATING MAGNETIC FIELDS

Abstract

A superconductor magnet system including a superconductor magnet including a plurality of field coils connected in series, each field coil having a plurality of turns including superconductor material. The system also includes a primary electric current source connected across the plurality of the field coils for supplying a DC electric current to the field coils to generate a magnetic field. The system further includes a secondary electric current source connected in parallel with the primary electric current source across a subset of the field coils for supplying an additional DC electric current to the or each field coil in the subset to modify or correct the magnetic field.

Claims

1. A high temperature superconductor (HTS) magnet system comprising: a superconductor magnet comprising a plurality of field coils connected in series, each field coil having a plurality of turns comprising HTS material; a primary electric current source connected across the plurality of the field coils for supplying a DC electric current to the field coils to generate a magnetic field; and a secondary electric current source connected in parallel with the primary electric current source across a subset of the field coils for supplying an additional DC electric current to the or each field coil in the subset to modify or correct the magnetic field.

2. The HTS magnet system according to claim 1, further comprising a control system for adjusting the additional DC electric current supplied by the secondary electric current source to modify or correct the magnetic field by increasing the homogeneity of the magnetic field in a target region of space.

3. (canceled)

4. The HTS magnet system according to claim 2, further comprising a magnetic field sensor for measuring one or more parameters of the magnetic field generated by the superconductor magnet.

5. The HTS magnet system according to claim 4, wherein the control system is configured to adjust the additional DC electric current supplied by the secondary electric current source to increase the homogeneity of the magnetic field in the target region of space based on the one or more measured parameters.

6. The HTS magnet system according to claim 1, wherein the system is configured such that the HTS material in the or each field coil in the subset has a higher critical current than the HTS material in the field coils not in the subset when the DC electric current from the primary electric current source is supplied to the field coils.

7. The HTS magnet system according to claim 1, wherein the primary and secondary electric current sources are configured such that the additional DC electric current supplied by the secondary electric current source is less than the DC electric current supplied by the primary electric current source.

8. The HTS magnet system according to claim 1, wherein the field coils comprise a stack of planar coils and the subset of field coils comprise one or more individual adjacent field coils in the stack.

9. The HTS magnet system according to claim 8, wherein the subset of the field coils excludes one or both of the field coils at either end of the stack.

10. The HTS magnet system according to claim 1, wherein the turns in each of the field coils are connected by an electrically conductive material such that electric current can be shared between the turns in the field coil; and/or wherein each field coil has an alternative current path across it, the alternative current path comprising electrically conductive material and having a low inductance compared to the respective coil such that a changing current across the field coil preferentially flows through the alternative current path.

11. (canceled)

12. The HTS magnet system according to claim 10, wherein the secondary electric current source is configurable to cause an additional AC electric current to flow via the electrically conductive material of the or each field coil in the subset, whereby resistive heating of the electrically conductive material heats the HTS material of the or each field coil in the subset.

13. The HTS magnet system according to claim 1, further comprising a cryostat housing the magnet, the cryostat being configured to maintain the HTS material at temperatures below a critical temperature of the HTS material during operation of the magnet, the primary electric current source and the secondary electric current source being housed within the cryostat, the cryostat comprising feedthroughs for supplying electrical power to the primary electric current source and the secondary electric current source, the primary electric current source and the secondary electric current source being configured to receive electrical power from different feedthroughs.

14. The HTS magnet system according to claim 1, further comprising a further secondary electric current source connected across a further subset of the field coils for supplying an additional DC and/or AC electric current to the field coils in the further subset.

15. The HTS magnet system according to claim 14, wherein the secondary electric current source and the further secondary electric current source are connected in parallel across the further subset of the field coils.

16. A method of generating a magnetic field using a high temperature superconductor (HTS) magnet comprising a plurality of field coils connected in series, each field coil having a plurality of turns comprising HTS material, the method comprising: using a primary electric current source connected across the plurality of the field coils to supply a DC electric current to the field coils to generate a magnetic field; and using a secondary electric current source connected in parallel with the primary electric current source across a subset of the field coils to supply an additional DC electric current to the or each field coil in the subset to modify or correct the magnetic field.

17. The method according to claim 16, wherein modifying or correcting the magnetic field comprises increasing the homogeneity of the magnetic field in a target region of space.

18. The method according to claim 17, further comprising obtaining measurements of one or more parameters of the magnetic field generated by the superconductor magnet, and modifying or correcting the magnetic field based on the measurements.

19. The method according to claim 16, wherein the additional DC electric current supplied by the secondary electric current source is less than the DC electric current supplied by the primary electric current source.

20. The method according to claim 19, wherein the additional DC electric current supplied by the secondary electric current source is adjusted such that a maximum transport current to critical current ratio of the HTS material in each of the field coils differs by less than 20%.

21. The method according to claim 16, wherein the turns in each of the field coils are connected by an electrically conductive material such that electric current can be shared between the turns in the field coil and/or each field coil has an alternative current path across it, the alternative current path comprising electrically conductive material and having a low inductance compared to the respective coil such that a changing current across the field coil preferentially flows through the alternative current path, the method further comprising using the secondary electric current source to supply an additional AC electric current that flows in the electrically conductive material of the or each field coil in the subset, whereby resistive heating of the electrically conductive material heats the HTS material in the or each field coil in the subset.

22. The method according to claim 16, wherein the or each field coil in the subset has a time constant defined by a ratio of the inductance of the field coil to a radial resistance of the field coil and the additional DC electric current is maintained over multiple time constants.

23. (canceled)

24. (canceled)

25. (canceled)

26. (canceled)

27. (canceled)

28. (canceled)

29. (canceled)

30. (canceled)

31. (canceled)

32. (canceled)

33. A nuclear magnetic resonance (NMR) device comprising an HTS magnet system according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 is a schematic representation of an HTS tape;

[0055] FIG. 2 is a schematic representation of a wound HTS coil;

[0056] FIG. 3 is a schematic representation of a sectional HTS coil;

[0057] FIG. 4 is a cross section of a pancake coil;

[0058] FIG. 5 is a cross section of a double pancake coil;

[0059] FIG. 6 is a circuit diagram of a superconductor magnet system;

[0060] FIG. 7 is a circuit diagram of another superconductor magnet system; and

[0061] FIG. 8 is a schematic vertical cross section view of a superconductor magnet system.

DETAILED DESCRIPTION

[0062] The present disclosure provides systems and methods for generating a desired magnetic field from a superconductor dipole magnet comprising a plurality of field coils connected in series. Several such dipole magnets may be arranged to form a quadrupole magnet, a sextupole magnet or other magnet configurations.

[0063] In the below description, the coils are assumed to be planar, spirally wound coils (i.e., pancake coils) with partial insulation provided by a conductive layer separating the turns. This is purely for ease of illustration. It will be recognised that the techniques described below can be applied to many coil constructions, including those discussed in the background introduction, and that the below is just one, non-limiting example.

[0064] FIG. 6 shows a circuit diagram of a superconductor magnet system 600 comprising a superconductor magnet 602 that has first, second and third field coils 604A-C connected in series with one another. Each of the field coils 604A-C is represented in the figure by a resistor 606A-C connected in parallel with an inductor 608A-C. In the present example, each of the field coils 604A-C are partially insulated, with a conductive layer interposed between the turns. The conductive layer allows electric current to flow radially between the turns, thereby bypassing the spiral path provided by the HTS material in the turns around which the current circulates in order to generate a magnetic field. However, in some cases, fully insulated field coils (in which radial turn-to-turn conduction is prevented by an insulating layer between the turns) may be used instead or in addition.

[0065] In the present example, each field coil 604A-C is a pancake coil comprising turns of HTS tape, e.g. as described above with reference to FIG. 1, although tape comprising LTS material may alternatively be used in some implementations. The field coils 604A-C are arranged face-to-face, one atop the other to form a stack. Current is supplied to the superconductor magnet 602 from a primary electric current source 610 connected across a pair of terminals 611A-B provided at either end of the superconductor magnet 602. In use, the primary electric current source 610 supplies a DC electric current to the superconductor magnet 602 such that the current passes through a first of the terminals 611A, circulates around successive turns of each of the field coils 604A-C in the stack, and then leaves the superconductor magnet 602 through the second of the terminals 611B.

[0066] A secondary electric current source 612 is connected across the second (middle) field coil 604B of the superconductor magnet 602 using a first terminal 613A located between the first and second field coils 604A, B and a second terminal 613B located between the second and third field coils 604B,C. In the present example, the secondary electric current source 612 is configured to supply an additional DC electric current to the second field coil 604B. In use, the additional DC electric current flows around the turns of the second field coil 604B to increase the transport current flowing within the HTS material, over and above the DC electric current supplied by the primary electric current source 610. The magnetic field produced by the superconductor magnet 602 has greater curvature towards the ends of the stack of pancake coils 604A-C as compared to the middle of the stack. This greater curvature means that the magnetic field is generally less well aligned with a crystal axis (e.g. ab-axis) of the HTS material in the first and third field coils 604A-C located at either end of stack as compared to the second field coil 604B located in the middle of the stack. The HTS material in the second field coil 604B therefore generally has a higher critical current than the HTS material in the first and third field coils 604A,C and can therefore accommodate greater transport currents without loss of superconductivity. The secondary electric current source 612 may have the same polarity of as the primary electric current source 610 so that a greater electric current flows within the HTS material in the turns of the second field coil 604B than the electric current that flows in the HTS material in the turns of the first and third field coils 604A, C. One or both of the electric current sources 610, 612 may be tuneable such the absolute and/or relative amounts of current supplied by the primary and secondary electric current sources 610, 612 can be varied. For example, the currents may be adjusted so that the ratio of transport current to critical current in the HTS material in each of the field coils 604A-C is approximately constant, thereby allowing efficient use to be made of the superconducting capacity of the field coils and/or to reduce the magnitude of screening currents in the HTS material. Alternatively, the absolute and/or relative amounts of transport current flowing in the field coils 604A-C may be tuned to control the contributions to the magnetic field provided by each of the field coils 604A-C, thereby altering the magnitude and/or shape of the magnetic field generated by the superconductor magnet 602 as a whole. Such tuning may eliminate the need for separate shim coils to achieve a desired (e.g. more uniform) magnetic field.

[0067] FIG. 7 shows a circuit diagram of another superconductor magnet system 700 that is identical to the superconductor magnet system 600 of FIG. 6, except that the secondary electric current source 712 is configured to supply an AC electric current to the second field coil 604B. In a partially insulated magnet of the present example, the AC electric current flows preferentially between the turns of HTS material via the electrically conductive layer of the partially insulated coil 604B, which is represented in FIGS. 6 and 7 by the resistor 606B. By contrast, very little AC electric current flows around the superconducting spiral path in the field coil 604B, as represented by the inductor 608B, because the field coil 604B has a long time constant relative to the frequency of the AC electric current. It will be appreciated that, in other implementations in which insulated coils are used (in which the turn-to-turn resistance is very high), AC electric current would flow within the spiral path of the coil. Resistive heating of the electrically conductive layer (i.e. the resistor 606B) by the AC current raises the temperature of the layer and the HTS material in the turns on either side of the layer. As the critical current of the HTS material decreases as its temperature increases, the ratio of transport current to critical current in the HTS material increases after the AC current is applied, meaning that screening currents in the HTS material are suppressed. The magnetic field produced by the superconductor magnet 602 may therefore more closely match the magnetic field intended by the designer of the magnet system, be more stable and/or reduce drift in the magnetic field. One or more parameters of the AC current, such as its amplitude, frequency and/or waveform, may be varied in order to control the temperature of the HTS material and hence the ratio of transport current to critical current in the HTS material.

[0068] In some implementations, the secondary electric current sources 612, 712 may be configured to provide both a DC and AC electric current to the second field coil 604B, either simultaneously or separately. For example, the secondary electric current sources 612, 712 may provide a DC current to the second field coil 604B to adjust (e.g. maximise) the transport current in the HTS material in the turns of the second field coil 604B, whilst simultaneously providing an AC current that decreases the critical current of the HTS material. Therefore, the local ratio of the transport current to critical current ratio can be increased (i.e., brought closer to one without quenching the magnet or any of the field coils) in different coils of the magnet. In certain applications, the primary electric current source 610 may also be configured to supply an AC electric current in addition to the DC electric current.

[0069] In general, the majority of the electric current supplied to the coils is supplied by the primary electric current source 610, with the secondary electric current source(s) 612, 712 providing a smaller amount of current in order to allow the magnetic field generated by the superconductor magnet 602 to be corrected or modified by a relatively small amount.

[0070] The superconductor magnet system 600, 700 may be housed within a cryostat (not shown) which cools the superconductor magnet 602 so that the superconductor material becomes and remains superconducting. The primary and secondary electric current sources 610, 612, 712 may be supplied with electrical power through feedthroughs passing from the relatively higher temperature exterior of the cryostat to the lower temperature interior of the cryostat. Separate pairs of feedthroughs may be provided for both of the electric current sources 610, 612, 712, or alternatively a single pair of feedthroughs may be used to supply electrical power to both the primary and secondary electric current sources 610, 612, 712.

[0071] It will be appreciated that, in general, the superconductor magnet 602 may have any number of field coils 604A-C greater than one, such that that the secondary electric current source 612, 712 can be connected across a subset of the field coils (the subset containing at least one, but not all, of the field coils, i.e. a strict subset). For example, the superconductor magnet 602 may have only two field coils 604A-C or may have 3, 4, 5 or 10 or more field coils 604A-C. The field coils 604A-C are also not required to be identical to one another, although in some embodiments and use cases this may be preferred. More than one secondary electric current source 612, 712 may also be provided, with each secondary electric current source 612, 712 being connected across a different respective subset of the field coils. Such an arrangement may allow greater control over the magnetic field generated by the superconductor magnet 600, 700, and/or more effective elimination of screening currents, for example. The subsets may be overlapping such that one or more of the field coils belongs to more than one subset and therefore receives DC and/or AC electric current from more than one secondary electric current source. In some cases, a secondary electric current source 612, 712 may be connected across a subset of the field coils, with another secondary electric current source 612, 712 connected across some but not all of the field coils in the subset (i.e. a strict subset of the subset). This type of nested arrangement of secondary electric current sources 612, 712 may allow successively greater currents to be provided to the field coils near the centre of the stack, without exceeding the critical current of the HTS material in any of the field coils towards the ends of the stack (where the critical current is lower).

[0072] FIG. 8 shows a superconductor magnet system 800 comprising a superconductor magnet 802 having a central axis A-A and comprising a stack of 10 pancake coils 804A-J connected in series with one another via joints 814 arranged such that the electric current flows around the turns of each coil in succession, with the direction of the current with respect to the central axis A-A reversing on passing from one coil to the next. A primary electric current source 810 is connected across the stack of pancake coils 804A-J using a pair of conductive plates 811A-B which act as terminals to allow an electrical connection to be made to the radially outermost end of each of the coils 804A, 804J at either end of the stack. Four secondary electric current sources 812A-D are connected in parallel across different subsets of the coils, with a first of the secondary electric current sources 812A being connected across all but the two field coils 804A, 804J at the end of the stack, a second of the secondary electric current sources 812B being connected across all but the four field coils 804A-B, 804I-J at the ends of the stack and so on. In this example, the electrical connections to these coils 804B-I are made using a series of conductive plates interposed between the coils. The superconductor magnet 802 is cooled by a cryostat 816 that is thermally coupled to the pancake coils 804A-J by the series of plates interposed between the coils and by the plates 811A-B.

[0073] The cross sectional area of the leads used to connect the electric current sources 810, 812A-D across the coils 804A-J may differ from one another according to how much electric current each source is required to supply to the field coils. For example, as the primary electric current source 810 supplies the majority of the current (e.g. 400 A in the present example), it may use leads with a greater cross-sectional area compared to the leads used for the secondary electric current sources 812A-D which supply less current (e.g. 100 A). In the example shown in FIG. 8, the primary and secondary electric current sources are located outside of the cryostat and each pair of leads therefore places a thermal load on the cryostat 816. However, as each pair of leads carries only a fraction of the total current needed to power the magnet 802, the amount of resistive heating caused by using two pairs of leads may remain similar to when just a single pair of leads is used to supply the current to the magnet 802. Thus, using multiple electric current sources may allow increased flexibility in the design and operation of the magnet without adversely affecting its cooling and temperature stability.

[0074] In the present example, temperature sensors T1-T5 (such as thermocouples) are provided at various positions within the magnet 802 to measure the temperature of the field coils 804A-J. Measurements from one or more of the temperature sensors T1-T5 are provided to a feedback controller 818 (such as a proportional integral derivative (PID) controller), which controls the one or more of the electric current sources 810, 812A-D so as to maintain the temperature of the field coils. For example, as shown in FIG. 8, a temperature sensor T1 provided in the conductive plate 811A may be used to measure the temperature of the first field coil 804A, provide measurements of the temperature to the controller 818, which adjusts the primary electric current source 810 based on the measurements to maintain the temperature of the field coil 804A at a particular setpoint. The field coils 804A-J may also each comprise one or more heaters (not shown), each heater having an associated feedback controller to assist in maintaining the temperatures of the field coils 804A-J. In such implementations, the associated feedback controllers and the controller 818 may be configured such that an increase in radial electric current supplied to a field coil 804A-J is compensated for by decreasing the electric current supplied to the heater of that coil (and vice versa) to maintain the setpoint (i.e. target) temperature.

[0075] It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.