Magnetic field sensor

Abstract

A magnetic field sensor for use in a tokamak power plant is disclosed. The sensor comprises a Hall sensor 4 made of a radiation-hard semiconductor such as diamond or silicon carbide with a doped regio

Description

Field The present invention relates to a magnetic field sensor for measuring a magnetic field 5 in a high-radiation environment, such as 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 magnetic field sensors, such as induction coils, generally are not sensitive enough, particularly for the slowly varying magnetic fields from a long-pulse or continuously operating tokamak. Other types of magnetometers, such as 15 semiconductor-based Hall sensors, fluxgates, and vapour cells, are not sufficiently radiation hard. For example, the plasma can emit a high flux of high-energy (14 MeV) neutrons which is likely to destroy many types of equipment within a 50 m radius of the plasma. Summary According to a first aspect of the present invention there is provided a magnetic field sensor comprising a Hall-effect device installable or installed to measure a magnetic field generated by a tokamak power plant, the Hall-effect device comprising a radiation-hard semiconductor material having a doped region and at least four terminals. The radiation-hard semiconductor material may be diamond or silicon carbide. The doped region may be doped with boron, or phosphorous. The Hall-effect device may be a Hall bar having four terminals. The Hall-effect device may be a Hall bar having six terminals. The doped region may have a width w of between 0.25 to 5 mm. The width of the doped region may be between 0.25 mm and 1 mm. An active region of the Hall-effect device may have an area of between 0.0625 mm2 and 25 or 75 mm2. An active area of the Hall-effect device may have an area of between 0.0625 mm2 and 1 mm2. The shape of the doped region of the Hall-effect device may be defined by doping. The shape of the doped region of the Hall-effect device may be defined by a physical boundary, for example, a side wall which is etched or machined. The Hall-effect device may be defined by a mesa. According to a second aspect of the present invention there is provided apparatus comprising the magnetic field sensor of the first aspect and a measurement system configured to perform a Hall effect measurement of the magnetic field sensor. The measurement system may be separated from the magnetic field sensor by a distance D of at least 10 m. According to a third aspect of the present invention there is provided use of the magnetic field sensor of the first aspect 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 magnetic field sensor of the first aspect to measure a magnetic field generated by a tokamak power plant. 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 block diagram of a magnetic field source, a magnetic field sensor and measurement electronics; Figure 2 is a schematic perspective view of a first magnetic field sensor; Figure 3 is a schematic perspective view of a second magnetic field sensor; Figure 4 is a schematic perspective view of a third magnetic field sensor; Figure 5 is a schematic circuit diagram which includes a magnetic field sensor, a current source, and a voltmeter for measuring Hall voltage; and Figure 6 is a schematic plan view of a fourth magnetic field sensor. Detailed Description Referring to Figure 1, an arrangement 1 is shown in which a magnetic field B generated by a magnetic field source 2 is measurable by a magnetic field sensing system 3 comprising a radiation-hard magnetic field sensor 4 (or "magnetometer") and a measurement system 5 (or "measurement electronics"). To protect the measurement system 5, the measurement system 5 may be located away from the magnetic field sensor 4, for example, at least 10 m from the magnetic field sensor 4. Referring to Figure 2, the radiation-hard magnetic field sensor 4, 4i takes the form of a Hall probe which comprises a Hall bar 6 consisting of a cruciform doped region 7 of a radiation-hard semiconductor material formed in a layer 8 of radiation-hard semiconductor material, at its surface 9. The Hall bar 6 may be defined by implantation. The radiation-hard semiconductor material is diamond and the Hall bar 6, 62 consists of a boron-doped diamond formed by ion implantation in a cruciform-shaped area of a diamond layer and annealing. This leaves co-planar undoped regions of diamond around the Hall bar. Referring to Figure 3, the Hall probe 4, 42 may be defined by a physical boundary by patterning a substrate. The radiation-hard semiconductor material is diamond, and a surface layer (not shown) of the diamond is doped, for instance, across the whole surface, with boron. The doped surface layer (not shown) is then patterned using an etch mask (not shown) and dry etching, for example inductively coupled plasma (ICP) etching, into the underlying undoped region of the diamond to define a cruciformshaped mesa 10. The Hall bar could be formed in other ways, for example, by laser cutting. The doped region could be doped by ion implantation or grown, in other words, by growing doped material in situ, such as boron-doped diamond. Referring to Figure 4, the Hall probe 4, 43 may be defined by a physical boundary by cutting or machining, for example, by laser cutting, without a substrate or using a sacrificial substrate (or "handle substrate"). For instance, a layer of boron-doped diamond can be grown as a cuboidal plate (not shown) on a sacrificial substrate (not shown). The plate (not shown) can then cut into the shape of a Hall bar, for instance by laser cutting, and the sacrificial substrate removed. The Hall probe 4, 4s can be mounted on another substrate (not shown) for subsequent handling and / or installation. Other radiation-hard materials can be used instead of diamond, such as silicon carbide. The radiation-hard material may be single crystal or polycrystalline. The Hall bar 6 may take the form of a four-lead device comprising a channel 11 running between first and second electrodes 12, 13 (through which a current, I, can flow and lateral electrodes 14, 15 (or "probe leads"). Referring also to Figure 5, the measurement electronics 5 includes a bias source 18 for driving a current through the channel 11 of the device and a voltage sensor 19 for measuring the Hall voltage, Vh, across the channel 11. Referring also to Figure 6, the radiation-hard magnetic field sensor 4 may take the form of a six-lead device comprising two pairs of lateral electrodes 14i, 15i, 14a, 15a. A six-lead device allows measurement of diagonal Hall effects and can also provide redundancy. The probe leads 14i, 15i, 14a, 15a may have a length I of between 0.25 and 5 mm, for example, between 0.25 and 1 mm, and adjacent probe leads 14i, 14a, 14i, 14a may be separated by a separation s of between 0.25 and 5 mm, for example, between 0.25 and 1 mm. The doped region 7 has a dopant concentration which is sufficiently high that it behaves as a semiconductor, but not so high that the region 7 becomes metallic. The doping concentration is less than 1% and is preferably between 0.1 to 0.5%. To help improve radiation hardness, the lead width, w, and probe length, I, can be between 0.25 to 1 mm or more (e.g., 5 mm), and / or the device 4 can have an active region (for example, defined by the lead width w, the probe lead length I and, if more than two pairs of probe leads, probe lead separation s) having an area A of at least 0.0625 mm2 and which may be up to 1 mm2 or more (e.g., 25 or 75 mm2). In this way, if radiation damages one part of the device 4, then the rest of the device can still work. Metal leads (not shown) are used to contact to the doped region 7. Ohmic contacts (not shown) in the form of annealed titanium-gold contacts or annealed titanium-platinum-gold contacts can used. Other dopants could be used instead of boron in the diamond such as phosphorous. Other radiation-hard materials could be used instead of diamond such as silicon carbide. Silicon carbide can be made more cheaply with larger wafers. It will be appreciated that many modifications may be made to the embodiments hereinbefore described. For example, a van der Pauw geometry can be used instead of a Hall geometry. Other dopants can be used instead of boron in the diamond, such as phosphorous. Other radiation-hard materials can be used instead of diamond, such as silicon carbide. Silicon carbide can be made more cheaply with larger wafers.

Claims

1. A magnetic field sensor comprising a Hall-effect device installable or installed to measure a magnetic field generated by a tokamak power plant, the Hall-effect device comprising a radiation-hard semiconductor material having a doped region and at least four terminals.

2. The magnetic field sensor of claim 1, wherein the radiation-hard semiconductor material is diamond.

3. The magnetic field sensor of claim 2, wherein the doped region is doped with boron.

4. The magnetic field sensor of claim 2, wherein the doped region is doped with phosphorous.

5. The magnetic field sensor of claim 1, wherein the radiation-hard semiconductor material is silicon carbide.

6. The magnetic field sensor of claim 1 or any one of claims 2 to 5, wherein the Hall-effect device is a Hall bar having four terminals.

7. The magnetic field sensor of claim 1 or any one of claims 2 to 5, wherein the Hall-effect device is a Hall bar having six terminals.

8. The magnetic field sensor of claim 1 or any one of claims 2 to 7, wherein the doped region has a width w of between 0.25 to 5 mm.

9. The magnetic field sensor of claim 8, wherein the width of the doped region is between 0.25 mm and 1 mm.

10. The magnetic field sensor of claim 1 or any one of claims 2 to 9, wherein an active region of the Hall-effect device has an area of between 0.0625 mm2 and 25 or 75 mm2.

11. The magnetic field sensor of claim 10, wherein an active area of the Hall-effect device has an area of between 0.0625 mm2 and 1 mm2.

12. The magnetic field sensor of claim 1 or any one of claims 2 to 11, wherein the shape of the doped region is defined by doping.

13. The magnetic field sensor of claim 1 or any one of claims 2 to 11, wherein the shape of the doped region is defined by physical boundaries.

14. Apparatus comprising:• the magnetic field sensor of claim 1 or any one of claims 2 to 13; and• a measurement system configured to perform a Hall effect measurement of the magnetic field sensor.

15. The apparatus of claim 14, wherein the measurement system is separated from the magnetic field sensor by a distance D of at least 10 m.

16. Use of the magnetic field sensor of claim 1 or any one of claims 2 to 13 for measuring a magnetic field generated by a tokamak power plant.

16. A method of using the magnetic field sensor of claim 1 or any one of claims 2 to 13 to measure a magnetic field generated by a tokamak power plant.

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