Tokamak power plant magnetic field measurement

Abstract

The apparatus 10 comprises a sensor 8 comprising diamond, and a system 14 which measures optically detected magnetic resonance (ODMR) of defect centres 13 in the diamond, for example comprising nitrog

Description

Field The present invention relates to apparatus for measuring a magnetic field generated 5 by a tokamak power plant. Background In a tokamak power plant, accurate and reliable monitoring of magnetic fields is required for feedback control of magnetic field coils to keep the plasma in the tokamak 10 stable. Some types of magnetometers, such as induction coils, are generally not sensitive enough, particularly for detecting the slowly varying magnetic fields coming from long-pulse or continuously operating tokamaks. Other types of magnetometers, such as 15 semiconductor-based Hall sensors, fluxgates, and vapour cells, are not sufficiently radiation hard. Moreover, many magnetometers lack the required dynamic range, particularly at high (> IT) magnetic fields. Summary According to a first aspect of the present invention there is provided apparatus for measuring a magnetic field generated by a tokamak power plant. The apparatus comprises a diamond, having defect centres, installable or installed to measure a magnetic field generated by a tokamak power plant, a measurement system for measuring optically detected magnetic resonance of defect centres in the diamond, a light transmission arrangement configured to couple the diamond and the measurement system, and a waveguide transmission arrangement configured to couple the diamond and the measurement system. The apparatus can provide a sensitive, radiation-hard arrangement for measuring magnetic fields generated by a tokamak power plant. Furthermore, a diamond-based magnetometer can also achieve a large dynamic range at high (> IT) magnetic fields. The light transmission arrangement may be arranged to transmit light between the diamond and the measurement system through free space. The light transmission arrangement may include at least two curved mirrors. The curved mirrors may be parabolic mirrors. The light transmission arrangement may include an optical waveguide. The optical waveguide may comprise a liquid- or gel-core optical fibre. The optical waveguide may comprise a radiation-hard optical fibre. The radiation-hard optical fibre may have a pure silica core. The optical waveguide may comprise a hollow-core fibre. The light transmission arrangement may include a first portion through which light is transmitted through free space and a second portion through which light is transmitted through an optical waveguide. The waveguide transmission arrangement may comprise a coaxial cable comprising an inner conductor and a concentric conducting shield separated by an insulator. The insulator may be air or a dielectric material. The waveguide transmission arrangement may comprise a rectangular or cylindrical waveguide. The waveguide transmission arrangement may include an antenna structure for concentrating microwave fields. The optical and microwave transmission arrangements may be integrated. For example, the optical transmission arrangement may be disposed inside the microwave transmission arrangement. Alternatively, the optical transmission arrangement may comprise inner walls or surfaces of the microwave transmission arrangement. The defect centres may comprise, consist predominantly of, or consist of nitrogen-15. The defect centres may comprise, consist predominantly of, or consist of nitrogen-14. The apparatus may further comprise a magnetic flux guide for guiding magnetic flux from the tokamak power plant to the diamond. The magnetic flux guide may comprise soft magnetic material. The soft magnetic material may be ferrite. The apparatus may further comprise a magnetic shield arranged such that the magnetic flux guide is disposed inside the magnetic shield. The magnetic shield may comprise a superconducting material. The magnetic shield may comprise soft magnetic material. The apparatus may further comprise a further magnetic shield arranged such that the magnetic shield is disposed inside the further magnetic shield. The magnetic shield and the further magnetic shield may be separated by a non-magnetic material, such as air or aluminium. The measurement system may be configured to perform optically detected magnetic resonance measurements using one or more known constant microwave frequencies. The diamond may be aligned so that some or all of the defect centres, for instance NV centres, are aligned to be along the direction of the magnetic field to be detected. This can help improve magnetic sensitivity. According to a second aspect of the present invention there is provided a system comprising a tokamak power plant and at least one diamond having defect centres installed Inside and / or around the tokamak power plant for measuring a magnetic field generated by a tokamak power plant. According to a third aspect of the present invention there is provided the use of the apparatus of first aspect of the present invention for measuring a magnetic field generated by a tokamak power plant. According to a fourth aspect of the present invention there is provided a method of using the apparatus of first aspect of the present invention to measure a magnetic field generated by a tokamak power plant. According to a fifth aspect of the present invention there is provided a method comprising installing at least one diamond having defect centre inside and / or around the tokamak power plant for measuring a magnetic field generated by a tokamak power plant. According to a sixth aspect of the present invention there is provided apparatus for 5 measuring a magnetic field, comprising a magnetic field sensor, and a magnetic flux guide for guiding magnetic flux from a magnetic source to the magnetic field sensor. According to a seventh aspect of the present invention there is provided apparatus comprising a magnetic flux guide for guiding magnetic flux from a magnetic source to 10 a magnetic field sensor, and a magnetic shield disposed around the magnetic flux guide. Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view of a part of a tokamak power plant; Figure 2 is a schematic block diagram of a magnetic field sensing apparatus using a diamond having an ensemble of defects, an optical transmission arrangement, a microwave transmission arrangement and an optically detected magnetic resonance measurement system; Figure 3 is a schematic view of an optical transmission arrangement which includes curved mirrors; Figure 4 is a schematic view of an optical transmission arrangement in the form of an optical waveguide; Figure 5 is a schematic cross-sectional view of a first optical waveguide in the form of a liquid- or gel-filled waveguide; Figure 6 is a schematic cross-sectional view of a second optical waveguide in the form of radiation-hard optical fibre; Figure 7 is a schematic cross-sectional view of a third optical waveguide in the form of hollow-core optical fibre; Figure 8 is a schematic view of an optical transmission arrangement in which light is transmitted though free space close to a tokamak and through an optical waveguide further away from the tokamak; Figure 9 Is a schematic view of a microwave transmission arrangement; Figure 10 is a schematic view of a microwave transmission arrangement in the form of an air-filled coaxial cable; Figure 11 is a schematic view of a microwave transmission arrangement in the form of a dielectric-filled coaxial cable; Figure 12 is a schematic view of a microwave transmission arrangement in the form of a waveguide; Figure 13A is a schematic view of a first integrated optical and microwave transmission arrangement; Figure 13B is a schematic view of a second integrated optical and microwave transmission arrangement; Figure 14 schematically illustrates a nitrogen-vacancy-centre diamond arranged locally at the tokamak; Figure 15 schematically illustrates a nitrogen-vacancy-centre diamond arranged remotely from the tokamak and using a magnetic flux guide a guide flux from the tokamak to the diamond; Figure 16 is a schematic cross-sectional view of a magnetic flux guide; Figure 17 is a schematic cross-sectional view of a magnetic flux guide and a first shielding arrangement in the form of a superconducting material shield; and Figure 18 is a schematic cross-sectional view of a magnetic flux guide and a second shielding arrangement in the form of a soft magnetic material shield; and Figure 19 is a schematic cross-sectional view of a magnetic flux guide and a third shielding arrangement in the form of a first, inner shield and a second, outer shield. Detailed Description Referring to Figure 1, a tokamak power plant 1 is shown. The tokamak power plant 1 comprises a vacuum vessel 2 in which a plasma (not shown) can be confined by magnetic fields (not shown) generated using a set of magnetic field coils 4, 5, 6, 7. The magnetic field coils 4, 5, 6, 7 include toroidal field coils 4, a central solenoid coil 5, poloidal field coils 7 and correction field coils 7. For clarity, other parts of the tokamak power plant 1 such as a support structure, cold shields, divertor plates, vacuum pumps, ports, and cryostat are not shown. Also, for clarity, a single vacuum vessel wall is shown, although the tokamak comprises inner and outer vacuum vessels. Magnetic fields around the tokamak power plant 1, particularly in the vicinity of the vacuum vessel 2, are monitored using a set of magnetic field sensors 8 disposed on outer walls of the vacuum vessel 2. There may be, for example, at least 100 sensors 8, sufficient to be able to form a three-dimensional image of the magnetic field around the tokamak. Accurate and reliable monitoring of magnetic fields is required for feedback control of magnetic field coils 3, 4, 5, 6 to keep the plasma (not shown) stable. Referring to Figure 2, apparatus 10 for measuring a magnetic field generated by a tokamak power plant 1 is shown. The apparatus 10 comprises a sensor 8 which takes the form of a diamond containing an ensemble of defect centres 13, in particular, nitrogen vacancy centres (or "NV centres"). The sensor 8 is provided in or near to the tokamak power plant 1. For example, in some arrangements, the diamond 8 may be attached to the outer wall of the vacuum vessel 2. In some arrangements, however, the diamond 8 can be located remotely from the vacuum vessel 2, for example, greater than 10 meters away from the vacuum vessel 2 and / or outside the cryostat (not shown). The diamond 8 may be aligned so that some or all of the NV centres are aligned to be along the direction of the magnetic field to be detected, or close to this within about one degree. There are four equivalent

[111] crystallographic directions in a diamond lattice. Typically, one quarter of the NV centres are aligned along each of these. Higher sensitivity can be achieved when the magnetic field Is along the direction of an NV centre, and this can be particularly important for tokamaks when higher magnetic fields are encountered. The diamond 8 may be configured to have more than a quarter of its NV centres or all of its NV centres aligned along just one of the

[111] directions to improve the magnetic sensitivity. The apparatus 10 includes a measurement system 14 for measuring optically detected magnetic resonance (ODMR) of a defect centre 13 in the diamond 8. The measurement system 14 includes an optical system 15 which includes a laser or alternative light source (not shown) for generating an excitation signal 17, a signal detector (not shown) comprising photodiodes (not shown) for comparing a sample of the excitation signal 17 and a response signal 21 from the diamond 8, and signalhandling optics (not shown) such as beam splitters and a dichroic mirror (not shown). The measurement system 14 also includes a microwave system 25 which includes a microwave source (not shown) which generates a microwave signal 27. The optical and microwave systems 15, 25 are controlled by a control system 30, for example, in the form of a computer system. The apparatus 10 includes a light transmission arrangement 40 configured to couple the diamond 8 and the measurement system 14, in particular, to transmit the excitation signal 17 from the optical system 15 to the diamond 8 and to transmit the response signal 21 from the diamond 8 to the optical system 15. The apparatus 10 includes a waveguide transmission arrangement 50 configured to couple the diamond 8 and the measurement system 14, In particular, to transmit the microwave signal 27 from microwave system 25 to the diamond 8. Magnetic fields can be sensed using the sensor 8 by optically detected magnetic resonance (ODMR.) of the defect centres 13 in the diamond 8. An excitation signal 17 in the form of green light is sent to the diamond 8 and a response signal 21 in the form of red fluorescence is collected from the diamond 8 and measured. Light transmission Referring to Figure 3, a first light transmission arrangement 40i, 40 is shown. The excitation signal 17 and the response signal 21 can be sent through free space 41. This has the benefit that free space is not damaged by the high neutron flux generated by the tokamak power plant 1. To transmit signals 17, 21, a mirror arrangement 42 comprising curved mirrors 43 can be used to prevent light from diverging. For example, the curved mirrors 43 can take the form of pairs of parabolic mirrors. Using mirrors, as opposed to lenses, helps to minimise or even avoid chromatic aberration. Thus, the same set of mirrors 43 can be used to transmit both green and red light. The diamond 8 can be placed at one of the foci of a parabolic mirror to collimate the red fluorescence that is emitted. Referring to Figure 4, a second light transmission arrangement 402, 40 is shown. Instead of transmitting light 17, 21 (Figure 2) through free space, the light 17, 21 (Figure 2) can be transmitted through a light guide 44. Referring to Figures 5 to 7, different types of light guides 44i, 442, 44s can be used. A light guide 44i, 44 can take the form of an optical fibre comprising a liquid or gel core 45i, 45 and a silica or plastic cladding 46i, 46. A light guide 442, 44 can take the form of a radiation-hard optical fibre comprising a high-purity silica core 452, 45 and a silica cladding 462, 46. A light guide 443, 44 can take the form of hollow-core optical fibre comprising a hollow core 45a, 45 and a silica cladding 463, 46. Referring to Figure 8, light 17, 21 (Figure 2) can be transmitted through free space 41 over a first section 47 close to the tokamak 2 where the radiation is highest and through a waveguide 44 in a second section 48 further away. In Figure 8, mirrors are not shown for clarity. The length of the first section 47 may be 10 or more meters. Microwave transmission Referring to Figure 9, a microwave transmission arrangement 50 is shown. Microwaves 27 (Figure 2) are transmitted through a microwave guide 51. Referring to Figures 10 to 12, different types of microwave guides 51i, 512, 51s can be used. A microwave guide 51i, 51 can take the form of coaxial cable comprising a central conductor 52i, 52, an outer, co-axial conductor 53i, 53 and electrical insulation 54i, 54 between the conductors in the form of an air gap. Alternatively, a microwave guide 512, 51 can take the form of coaxial cable comprising a central conductor 522, 52, an outer, co-axial conductor 532, 53 and electrical insulation 542, 54 between the conductors in the form of silica glass. Air and silica glass tend to be more radiation hard than plastic. A microwave guide 513, 51 can take the form of hollow pipe, for instance a rectangular waveguide in the form of a hollow metal structure 55 having a rectangular cross section. A circular cross-section could be used instead of a rectangular cross-section. Integrated optical and microwave transmission Referring to Figure 13A, the optical and microwave transmission arrangements may be integrated into an integrated transmission arrangement 60. Thus, the same structure 60 can be used to get light 17, 21 (Figure 2) to and from the diamond 8 and to get microwaves 27 (Figure 2) to the diamond 8. For example, in a first arrangement 60, 60i, curved mirrors 43 can be placed inside a metal microwave waveguide 55 so allowing the structure 60 not only to deliver microwaves 27 to the diamond 8, but also to bring the excitation light 17 to the diamond 8, and to collect the red fluorescence light from the diamond 8. Referring to Figure 13B, in a second integrated transmission arrangement 60, 6O2, edges of the microwave waveguide 55', for instance, inner walls of the microwave waveguide 55', provide the curved mirrors 43'. Magnetic flux guide Referring to Figure 14, in some embodiments, the diamond 8 is mounted on or near to vacuum vessel 2, and so the light guide 44 (if used) and the microwave guide 51 run up to the vacuum vessel 2. Referring to Figure 15, a magnetic flux guide 70 can be used to pipe magnetic flux from a region of interest, in a central region of the tokamak 1, past devices, structures and other objects which may generate magnetic noise. Referring also to Figure 16, the magnetic flux guide 70 can be cylindrical or rectangular cuboid. The magnetic flux guide 70 can have a length L between, for example, 0.5 to 50 meters and a diameter or width D of between, for instance, 1 to 20 cm. The magnetic flux guide 70 Is formed from a soft magnetic material, such as ferrite, or other suitable material for transmitting the magnetic signal. Referring also to Figure 17, a shield 71, for example, in the form of a cylinder, formed of superconducting material can be provided around the magnetic flux guide 70 to help keep the magnetic signal inside the magnetic flux guide 70 and to prevent magnetic noise from entering into the magnetic flux guide 70 from outside. Referring to Figures 18 and 19, instead of or as well as, the shield 71, a non-magnetic shield 72, such as a hollow aluminium cylinder, can be provided around the magnetic flux guide, and then another shield 73 which is magnetic, such as a hollow ferrite cylinder, provided around the non-magnetic shield 72. Magnetic flux noise would be absorbed by the magnetic shield 73 without reaching the inner magnetic flux guide 70. An advantage of the magnetic flux guide 70 is that the magnetometer can be located further from the tokamak 1, where neutron flux is lower and so help to reduce potential radiation damage. Nitrogen-15 vacancy centres As explained earlier, the sensor 8 may take the form of a diamond containing an ensemble of nitrogen vacancy centres 13. The nitrogen impurity used to provide the nitrogen vacancy centres can take the form of nitrogen-14 (14N) isotope. Alternatively, nitrogen-15 (15N) isotope can be used. Nitrogen-15 can be used instead of nitrogen-14 such there is no nitrogen-14 or substantially no nitrogen-14. In some cases, there may be a mixture of nitrogen-15 and nitrogen-14, such that there is more nitrogen-15 than nitrogen-14 (that is, more than 50% nitrogen-15). In some cases, there may be a mixture of nitrogen-15 and nitrogen-14 such that there are about equal amounts of nitrogen-14 than nitrogen-15 (that is, about 50% nitrogen-15) or there is more nitrogen-14 than nitrogen-15, but the level of nitrogen-15 is higher than is normally found (that is, between 1% and 49% nitrogen-15). Using nitrogen-15 instead of nitrogen-14 can remove or reduce unwanted forbidden transitions in the optically detected magnetic resonance spectrum. These forbidden transitions are present because of the nuclear quadrupole term in the spin Hamiltonian for the 14N isotope. The 15N isotope has no nuclear quadrupole term as it only has a nuclear spin of I = 1 / z. At the high magnetic fields used for tokamak diagnostics, the 14N forbidden transitions for NV centres become much larger than they are at 1 mT to 10 mT which is the more commonly used magnetic field range for NV centres. ODMR - Field swept mode A tokamak plant may generate fast magnetic-field pulses and / or quickly changing magnetic fields. To measure such fast pulses and quickly changing fields, ODMR can be performed using multiple constant microwave frequencies as a field-swept measurement. During a normal ODMR measurement, the microwave frequency tracks the ODMR resonance. The microwave frequency that is required to be on resonance corresponds to the magnetic field experienced by the diamond. Alternatively, during a normal ODMR measurement, the microwave frequency can be kept constant. No microwave frequency tracking is required as long as any changes in the magnetic field are small compared to the ODMR linewidth. If, however, the magnetic field from the tokamak changes too quickly and by more than the ODMR linewidth, then it may not be possible to stay locked onto the resonance frequency. To address this, one or more known constant microwave frequencies can be used. Thus, as the magnetic field from the tokamak changes, ODMR shows resonances as the resonance condition is met. The measurement system 14 can, therefore, be operated in two modes, namely "normal" and "field swept" modes, and the control system 30 can be used to switch the measurement system 14 between these two modes. The field-swept mode is easier to implement in that using fixed microwave frequencies makes microwave transmission simpler. In the field-swept mode, the measurement system 14 extracts the value of magnetic field from the measured data. The fixed microwave frequency or frequencies can be chosen to match the expected magnetic field range which may change depending on the mode of operation of the tokamak magnetic field coils. It will be appreciated that many modifications may be made to the embodiments hereinbefore described. For example, any one of the light transmission arrangements can be used in conjunction with any of the microwave transmission arrangements and vice versa.

Claims

1. Apparatus for measuring a magnetic field generated by a tokamak power plant, the apparatus comprising:• a diamond, having defect centres, installable or installed to measure a magnetic field generated by a tokamak power plant;• a measurement system for measuring optically detected magnetic resonance of defect centres in the diamond;• a light transmission arrangement configured to couple the diamond and the measurement system; and• a waveguide transmission arrangement configured to couple the diamond and the measurement system.

2. The apparatus of claim 1, wherein the light transmission arrangement is arranged to transmit light between the diamond and the measurement system through free space.

3. The apparatus of claim 1 or 2, wherein the light transmission arrangement includes at least two curved mirrors.

4. The apparatus of claim 1, 2 or 3, wherein the light transmission arrangement includes an optical waveguide.

5. The apparatus of claim 4, wherein the optical waveguide comprises a liquid- or gel-core optical fibre.

6. The apparatus of claim 1 or any one of claims 2 to 4, wherein the optical waveguide comprises a radiation-hard optical fibre.

7. The apparatus of claim 1 or 6, wherein the optical waveguide comprises a hollow-core fibre.

8. The apparatus of claim 1 or any one of claims 2 to 7, wherein the light transmission arrangement includes a first portion through which light is transmitted through free space and a second portion through which light is transmitted through an optical waveguide.

9. The apparatus of claim 1 or any one of claims 2 to 8, wherein the waveguide transmission arrangement comprises a coaxial cable comprising an inner conductor and a concentric conducting shield separated by an insulator.

10. The apparatus of claim 9, wherein the insulator is air.

11. The apparatus of claim 9, wherein the insulator is a dielectric material.

12. The apparatus of claim 1 or any one of claims 2 to 8, wherein the waveguidetransmission arrangement comprises a rectangular waveguide.

13. The apparatus of claim 1 or any one of claims 2 to 12, wherein the optical and microwave transmission arrangements are integrated.

14. The apparatus of claim 1 or any one of claims 2 to 13, wherein optical transmission arrangement is disposed inside the microwave transmission arrangement.

15. The apparatus of claim 1 or any one of claims 2 to 14, further comprising: ■ a magnetic flux guide for guiding magnetic flux from the tokamak power plant to the diamond.

16. The apparatus of claim 15, wherein the magnetic flux guide comprises soft magnetic material.

17. The apparatus of claim 15 or 16, further comprising:• a magnetic shield arranged such that the magnetic flux guide is disposed inside the magnetic shield.

18. The apparatus of claim 17, wherein the magnetic shield comprises a superconducting material.

19. The apparatus of claim 17, wherein the magnetic shield comprises soft magnetic material.

20. The apparatus of claim 18, further comprising:• a further magnetic shield arranged such that the magnetic shield is disposed inside the further magnetic shield.

21. The apparatus of claim 1 or any one of claims 2 to 20, wherein the defect centres comprise, consist predominantly of, or consist of nitrogen-15.

22. A system comprising:■ a tokamak power plant; and• at least one diamond having defect centres installed inside and / or around the tokamak power plant for measuring a magnetic field generated by a tokamak power plant.

23. Use of the apparatus of claim 1 or any one of claims 2 to 21 for measuring a magnetic field generated by a tokamak power plant.

24. A method of using the apparatus of claim 1 or any one of claims 2 to 21 to measure a magnetic field generated by a tokamak power plant.

25. A method, comprising:installing at least one diamond having defect centre inside and / or around the tokamak power plant for measuring a magnetic field generated by a tokamak power plant.

26. Apparatus for measuring a magnetic field, comprising:• a magnetic field sensor; and• a magnetic flux guide for guiding magnetic flux from a magnetic source to the magnetic field sensor.

27. Apparatus, comprising:■ a magnetic flux guide for guiding magnetic flux from a magnetic source to a magnetic field sensor; and• a magnetic shield disposed around the magnetic flux guide.

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