Nuclear magnetic resonance system

Abstract

A compact nuclear magnetic resonance (NMR) system 1 and a method of operating an NMR system 1, is disclosed. The NMR system 1 comprises a magnet subsystem 5. The magnet subsystem 5 comprises a superco

Description

BACKGROUND OF THE INVENTION This invention relates to nuclear magnetic resonance spectroscopy and to apparatus 5 used therein. Nuclear Magnetic Resonance (NMR) spectroscopy can be used to determine the molecular structure of a liquid or solid sample. For known compounds, NMR can be used to quantitatively analyse mixtures, whilst for unknown compounds, a spectrum 10 produced using NMR can either be matched to a result stored in a spectral library or can be used to infer the basic molecular structure directly. Once the basic structure is known, NMR can be used to determine molecular conformation in solution as well as studying physical properties at the molecular level such as conformational exchange, phase changes, solubility, and diffusion. 15 Nuclear Magnetic Resonance techniques rely on the fact that nuclei have nuclear spin and many nuclei are electrically charged. If an external magnetic field is applied (using a superconducting magnet for example) to a sample then it is possible to cause an energy transfer from a base energy level to a higher energy level (generally a single energy gap). This energy transfer takes place at a wavelength which corresponds to a 20 certain radio frequency and when the spin returns to its base level, energy is emitted at this same radio frequency. The signal that matches this transfer is measured and processed in order to yield an NMR spectrum relating to the nuclei concerned. NMR systems require a magnet in order to create a uniform magnetic field. There are known NMR systems which use permanent magnets, but these systems typically 25 operate below 100MHz (~ 2.3T). It is known for such permanent magnet NMR systems to be provided in a range of sizes, including “benchtop” systems, referred to because they are compact enough to be located on a standard lab bench so as to not occupy a large amount of space within a lab. These systems typically operate at 20, 40, 60 and 90MHz. Below 60MHz the systems tend to be used for relative relaxation 30 measurements (T1 / T2) to measure basic physical properties. At 60-100MHz the systems are used for structural conformation measurements and deriving information on molecular coupling but these systems only provide lower resolution spectra. An alternative kind of NMR system (high resolution FT-NMR) uses superconducting magnets which provide a medium-to-high strength field, for example 300MHz (~7T). Such superconducting magnets are cooled using liquid helium which is circulated through pipes from a remote tank by means of a pump. This requires a large helium storage tank, together with an array of pipes for transporting the liquid helium. Such a liquid helium cooled magnet will therefore take up a large space, due to the size of the tank and the required pipes. Furthermore, liquid helium presents Health and Safety considerations in terms of an asphyxiation risk. If the magnet quenches (changes state from superconducting to resistive), the liquid helium will evaporate to gas very quickly - typically around 1 minute. The coefficient of expansion for helium from the liquid to gas phase is around 750, thus a low-end 400 MHz magnet containing 80 litres of liquid will evolve around 60 cubic metres of gas. A superconducting magnet cooled using liquid helium therefore needs to be kept in a sufficiently large and well-ventilated room in order to minimize the asphyxiation risk. The present invention seeks to address the shortcomings of the currently available NMR systems. SUMMARY OF THE INVENTION From a first aspect, the invention provides a nuclear magnetic resonance system as claimed in claim 1. Thus it will be seen that, in accordance with the invention, a compact nuclear magnetic resonance (NMR) system is provided, which provides the advantages of a high power superconducting magnet NMR system, without occupying a large space. It is an advantage of a nuclear magnetic resonance system having a magnet subsystem of such a compact size that it is suitable for use on or under a standard laboratory bench. The Applicant has appreciated that, contrary to the long-held understanding in this field, it is in fact possible to produce an NMR system which is both compact and provides a medium-to-high magnetic field strength. The use of a cryocooler to provide cooling to the superconducting magnetic coil advantageously provides a system which does not require the circulation of liquid helium. No liquid helium is used to cool the magnet, and only a small amount of helium gas (or other suitable gas) is circulated internally within the cryocooler to provide cooling to the superconducting magnetic coil. Such systems can therefore be significantly more compact than known NMR systems, due to the smaller size of a cryocooler compared to a liquid helium cooling system, and the fact that a cryocooler can be arranged in closer proximity to the NMR system than a conventional liquid helium cooling system. Such compact systems in accordance with the invention may still have the capabilities of known, larger NMR systems. The superconducting magnetic coil is configured to produce at least a medium strength magnetic field, optionally a high strength magnetic field. The use of a medium-to-high strength magnetic field helps to provide an NMR system of high accuracy, e.g. capable of obtaining information about molecular coupling. For example, the superconducting magnetic coil may produce a magnetic field with a strength of at least 4T, preferably at least 5T further preferably at least 6T. Optionally the superconducting magnetic coil may produce a magnetic field with a strength of around 7T. This field strength can also be expressed as a frequency, which is the frequency which is observed in a given field for a single hydrogen atom 1H. In this example, a field of 4T can be expressed as ~174MHz proton resonance, whilst a field of 7T can be expressed as 300MHz proton resonance. It will be understood by the skilled person that a magnetic field produced by a superconducting magnetic coil will be considered to be “around” 7T, or300MHz, if the field is equal to 7T (300MHz), accounting for standard accepted tolerance ranges for such devices. The magnet subsystem has a height which is defined along the axis of the elongate central bore of the superconducting magnetic coil. The skilled person will understand that the “elongate” bore therefore has a length along the elongate direction which is significantly longer than its width across the perpendicular direction. This elongate direction defines an axis along its length, with the height of the magnet subsystem being defined as the length of the magnet subsystem along this dimension, regardless of the orientation of the magnet subsystem. The magnet subsystem may have a height of less than 1m, preferably less than 80cm, further preferably less than 70cm. The other two perpendicular directions - i.e. the width and depth of the of the magnet subsystem are the length of the subsystem along two mutually perpendicular axes, where both axes are also perpendicular with the axis of the elongate bore, defining the height axis of the subsystem. The magnet subsystem is preferably less than 1m wide, preferably less than 75cm wide, further preferably less than 50cm wide. The magnet subsystem is likewise preferably less than 1m deep, preferably less than 75cm deep, further preferably less than 50cm deep. The magnet subsystem is therefore compact. It will be understood by the skilled person that the term “compact” is to be understood 5 in relation to standard nuclear magnetic resonance systems which are known in the art i.e. in reference to industry standards. The magnet subsystem of the nuclear magnetic resonance system is sized so as to be able to fit under a standard laboratory bench or desk, which are approximately dimensioned to be at a working height for an average person. In some embodiments, the width, depth and height of the magnet subsystem 10 are such that the total volume of the magnet subsystem is less than 1 cubic metre. CM In some embodiments, the magnet subsystem may comprise an outer housing, partially or fully surrounding the other components of the magnet subsystem. The outer housing may have dimensions as described above. The outer housing may be approximately cylindrical. The outer housing may have a radius of less than 50cm, 15 preferably less than 30cm, preferably less than 25cm. The outer housing may be powder coated plastic. The plastic may, for example, be formed by injection moulding. 20 In some embodiments, the magnet subsystem further comprises a cryostat surrounding the superconducting magnetic coil. The cryostat is a device which substantially surrounds the superconducting coil and helps to maintain the superconducting magnet at a low, cryogenic temperature. The cryostat may provide the outer housing. It will be understood by the skilled person that the cryocooler provided in accordance 25 with the invention is a device generally known in the art. A cryocooler is a mechanical cooling device, which creates cooling typically by the pneumatic cycling of helium gas within a closed loop. The specific construction and design of various crycoolers would be known to the skilled person and are therefore not discussed in detail here. In some embodiments, the cryocooler cools the superconducting magnetic coil to a 30 temperature below 10K, preferably below 5K. In some embodiments, the cryocooler cools the superconducting magnetic coil to approximately 4.2K. The cryocooler is arranged to provide cooling to the superconducting magnetic coil. By this the skilled person will understand that it is the cryocooler itself, rather than another 35 cooling medium, which is responsible for maintaining the temperature of the superconducting magnetic coil. Thus, there is no requirement to circulate a cooling fluid e.g. liquid helium, around the system. Advantageously there is no fluid connection between the superconducting magnetic coil and the cryocooler. In order to reduce heat losses, the cryocooler is preferably located proximate to the superconducting magnetic 5 coil. In some embodiments, the cryocooler is in direct thermal contact with the superconducting magnetic coil. The cryocooler may be in direct mechanical connection with the superconducting magnetic coil. Alternatively, the cryocooler or the superconducting unit may additionally comprise a thermal bus such that the cryocooler may be connected to the superconducting magnetic coil via the thermal bus e.g. the 10 cryocooler may be in direct contact with the thermal bus and the superconducting magnetic coil may be in direct contact with the thermal bus. For example, the thermal bus may be a copper or aluminium bus e.g. a strip of highly pure aluminium. This helps to transfer heat from the magnet via direct contact of the thermal bus with the cryocooler, providing a thermal path from the cryocooler to the superconducting 15 magnetic coil, and helps to cool the magnet evenly. CM The Applicant has recognised that in some circumstances the cryocooler could generate vibrations and that it could be difficult to provide sufficient isolation between the cryocooler and the magnet to avoid having an adverse impact on the accuracy of 20 the magnet in application where this is important. Therefore, the magnet subsystem comprises an active-damping device, wherein the active-damping device acts to substantially cancel a vibration induced in the superconducting unit by the cryocooler. This advantageously allows the vibration of a cryocooler (or cold-head) to be damped using active vibration damping, helping to overcome the problem of vibrations which 25 may be present in a system which is cooled otherwise than with liquid helium. In some examples, the active-damping device is arranged to produce a vibration which is opposite in direction and substantially equal in magnitude to a vibration of the cryocooler. It will be understood that the term “substantially” indicates that the activedamping device may not be capable of perfectly cancelling the vibrations, and / or that 30 other much smaller vibrations may be present in addition to those of the cryocooler, and that the active-damping device acts to cancel these vibrations as well. By substantially equal it is meant that the vibration is equal to within standard tolerances e.g. within 5%, optionally within 1% of the magnitude of the vibration of the cryocooler. In some embodiments, in addition or alternatively to the mechanical connection (directly or via a thermal bus), the cryocooler is connected to the superconducting unit by an elastic connection. This elastic connection may advantageously help to damp the transmission of vibrations produced by the cryocooler to the superconducting unit 5 e.g. to the superconducting magnetic coil. In some embodiments, the elastic connection to the superconducting unit is provided by bellows and / or by springs. In some embodiments the elastic connection connects the cryocooler to the cryostat. In some embodiments, the magnet subsystem comprises a motion sensor, for 10 example a displacement sensor, a velocity sensor or an accelerometer, arranged to detect vibration of the cryocooler and / or the superconducting magnetic coil in order to provide feedback to the active damping device. In some embodiments, the activedamping device itself comprises such a motion sensor. In some embodiments, the active-damping device comprises at least two sensors, of the same or different types, 15 in order to help improve the accuracy of the vibration damping. In some embodiments, the sensor is directly connected to the cryocooler. Thus, in these embodiments, the motion of the cryocooler is sensed using the sensor, i.e. either directly by a sensor connected to the cryocooler or indirectly e.g. by measuring vibration of the superconducting magnetic coil, and the mechanical actuator is actuated to move in a 20 way that produces vibrations which are opposite in direction and substantially equal in magnitude to the vibrations of the cryocooler. This advantageously helps to cancel out the inherent vibrations of the cryocooler. The active-damping device preferably comprises a mechanical actuator. In some 25 embodiments, the active-damping device additionally comprises an inertial mass, having a mass such that the mass of the active-damping system is substantially equal to the mass of the cryocooler. By this it is meant that the mass of the inertial mass is such that the inertial mass together with the mass of any other components of the active damping system e.g. a housing or actuator, has a mass which is substantially 30 equal to the mass of the cryocooler. If the mass of the other components is negligible, the inertial mass may have a mass which is approximately equal to the mass of the cryocooler. This helps to allow the active-damping system to accurately produce a vibration which cancels the vibration induced in the superconducting unit by the crycooler. In some embodiments the mechanical actuator comprises at least one piezo-electric actuator. Optionally, the mechanical actuator comprises a stack of piezo-electric actuators. It will be understood by the skilled person that such piezo-electric actuators are well known in the art and the structure and operation of the piezo-electric actuator will hence not be discussed here in detail. In some embodiments, the piezo-electric actuator has a fast-acting movement capability of at least ±20 pm (40 pm in total). In some embodiments, the piezo-electric actuator is a lead zirconate titanate piezoelectric actuator. Where the active-damping device comprises a sensor, the piezoelectric actuator, or piezo-electric actuator stack, may comprise the sensor. In these examples, the sensor may be arranged to measure the strain within the piezo-electric actuator, or piezo-electric actuator stack, in order to determine the vibrations of the cryocooler. In some embodiments, the cryocooler is connected to the active-damping device, e.g. the cryocooler may be connected to the mechanical actuator of the active-damping device. This connection may be a rigid connection or an elastic connection. The connection of the cryocooler to the active-damping device may be a separate connection to the connection of the cryocooler to the superconducting unit. In some embodiments the cryocooler comprises a first side, on which the cryocooler is connected to the superconducting unit, and the cryocooler comprises a second e.g. opposite side, on which the cryocooler is connected to the active-damping device. Preferably the magnet subsystem further comprises the other components typically needed to make NMR measurements including any or all off: a radio frequency probe; high-power RF transmitter; RF power amplifier; RF gate; RF oscillator; RF receiver; RF receiver amplifier; a phase detector; or an analogue to digital converter. In some embodiments, the nuclear magnetic resonance system comprises a computer. The magnet subsystem may comprise the computer. Alternatively, the computer may be external to the magnet subsystem and in wired or wireless connection with the magnet subsystem. For example, if the magnet subsystem is placed, in use, under a laboratory bench, a computer which is present on the bench top may be connected to the magnet subsystem for use. The computer may comprise a processor. The computer may be arranged to control the magnet subsystem described herein and to carry out data processing on data acquired from the magnet subsystem. The computer may be arranged to carry out steps in the known process of collecting NMR data, such as: receiving RF signals, amplifying RF signals, carrying out phase detection, carrying out digital to analogue conversion, and / or carrying out pulse sequence generation to deliver exactly precise pulses of radio frequency energy with 5 the correct timings to appropriately excite the sample at the atomic nuclei level. Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that 10 these are not necessarily distinct but may overlap. BRIEF DESCRIPTION OF THE DRAWINGS Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 15 Figure 1 is a schematic drawing of an NMR system according to an embodiment of the present invention; Figure 2 is an image showing an outer view of an NMR system in accordance with the present invention in situ; Figure 3 is an image showing an NMR system which is cooled using liquid 20 helium, as is known in the art; Figure 4 is a schematic drawing showing the outer dimensions of an NMR system in accordance with the present invention; Figure 5 is a schematic drawing of a mechanical actuator according to an embodiment of the present invention. 25 Figure 6 is a schematic drawing of a mechanical actuator according to another embodiment of the present invention. Figure 7 is a schematic drawing representing an example feedback loop for a mechanical actuator in accordance with an embodiment of the present invention. 30 DETAILED DESCRIPTION There are typically two known kinds of NMR systems. The first are low magnetic field systems, for example using permanent magnets. These systems do not require cooling of the magnets; however, they produce magnetic fields of a maximum of usually around 100MHz (2.3T). Alternatively, there are known medium-to-high field NMR systems (for example as shown in Figure 3) which employ superconducting magnets which are cooled by the circulation of liquid helium. As seen in Figure 3, such systems occupy a very large amount of space within a laboratory. 5 Figure 1 is a schematic drawing showing an overview of an entire NMR system according to an embodiment of the present invention. The NMR system 1 includes a magnet subsystem 5. The magnet subsystem 5 includes a superconducting unit 3. The superconducting unit 3 includes superconducting magnet 2 surrounded by a cryostat 4. The superconducting magnet 2 has a 10 substantially cylindrical hollow shape and generates, within the hollow inner portion of the cylinder, a very strong and stable DC magnetic field. The hollow inner portion provides an elongate bore, as seen in the Figure. In this example, the magnet has a magnetic field strength of 7T. The cryostat 4 is arranged to house the superconducting magnet 2 and maintain the superconducting magnet 2 at a stable temperature of 15 around 4.2 Kelvin. The region 16 within the cryostat 4, including the superconducting magnet 2, will in use be at a temperature of approximately 4.2K, whilst the region 18 within the hollow centre of the superconducting magnet 2 and cryostat 4 will be at room temperate, approximately 300K. The superconducting unit 3 also includes a thermal bus 9, which is connected to the superconducting magnetic coil 2. In this example the 20 thermal bus 9 is a strip of highly pure aluminium In order to maintain the temperature of the superconducting magnet 2, the magnet subsystem 5 further includes a cryocooler, or cold-head 6 connected to the superconducting unit 3. Such cryocoolers are known in the art; the cryocooler could be, for example, Sumitomo model GM101D and HC-4E. The cryocooler 6 comprises a 25 mount part 7a, a first stage 7b and a second stage 7c. The second stage 7c of the cryocooler 6 is in direct contact with the thermal bus 9. Thus, the thermal bus transfers heat from the magnet to the second stage of the cryocooler 7c by thermal conduction. No circulation of liquid helium is required in order to cool the superconducting magnetic coil 2. In an alternative embodiment, the second stage 7c of the cryocooler 6 may be in 30 direct mechanical connection with the superconducting magnetic coil 2 - i.e. the thermal bus could be omitted. The crycooler 6 may be configured to pass through an opening in the cryostat 4 and / or may be in direct mechanical connection with the cryostat 4 e.g. fixed or bolted to the cryostat. The introduction of a cryocooler 6 to cool the superconducting magnet 2 allows a medium-to-high field superconducting magnet to be used in this NMR system, but in the absence of liquid helium, which allows the device to remain compact and also simple in structure. 5 In operation, the inner region 18 of the superconducting coil contains a sample 20, for which an NMR spectrum is to be produced. The inner region 18 is bounded by coils 22, which are at room temperature. The coils 22 are electrically resistive ‘shim’ coils which help to make the magnetic field within the coils 22, i.e. in the inner region 18, homogeneous. These coils 22 can be used to adjust and optimise the shape of the 10 magnetic field. The NMR system also includes a radio frequency probe 24, which is a small, low inductance coil used to excite and detect the NMR signal. The probe 24 is situated within the inner region 18. For optimum NMR measurement sensitivity, this coil must be as close as possible to the sample under investigation. CM 15 The NMR system also includes a high-power RF transmitter 26, including an RF power amplifier, an RF gate and an RF oscillator. The high-power RF transmitter 26 is capable of delivering precise, short pulses of radio frequency energy. The system also includes a very sensitive RF receiver 28, including an RF receiver, an RF amplifier, a 20 phase detector and an analogue to digital converter 30. The RF receiver 28 is capable of detecting NMR signals and amplifying them to useful levels for Fourier Transform and analysis. The digitiser 30 is used to convert analogue NMR signals into a data format that can be stored on a computer hard drive. A computer 32 is provided externally of the magnet subsystem and is used to control the NMR spectrometer sub- 25 systems described above and post-process the acquired data. The computer 32 is used for pulse sequence generation 34 to deliver precise pulses of radio frequency energy with the correct timings to appropriately excite the sample at the atomic nuclei level. 30 The superconducting unit 3, cryostat 4 and the associated electronics 26, 28 are built into a single, compact magnet subsystem 5. In the example shown in the Figures, the computer 32 is separate to the magnet subsystem and is connected by a wired or wireless data connection to it, to carry out the required functions. Since the system uses a relatively small 300Hz superconducting magnet, and, more importantly, is 35 cooled by a compact cryocooler 6, rather than an expansive liquid helium cooling system, the entire NMR system is a small, compact unit. This NMR system 1 described above may be small enough to be sited on a bench, or if this is not feasible, under a laboratory bench. This is illustrated in the image of Figure 2, in which the approximate size of the magnet subsystem 5 can be seen, located under the bench. 5 The contrast in size can be seen clearly with reference to Figure 3, which is an image showing the size of a liquid helium cooled NMR system, as is known in the art. The dimensions of this magnet subsystem are shown in more detail in Figure 4. In some embodiments, as shown, the magnet subsystem is approximately cylindrical. In this case the dimensions of the magnet subsystem 5 are its height 104, defined along 10 the length of the elongate bore, and the width 100 and depth 102 defined perpendicular to the height 104 and mutually perpendicular, as illustrated in Figure 4. In this particular example, the width and depth 100, 102 (i.e. the diameter of the NMR system) are less than around 80cm, and the height 104 is less than 100cm. As shown in Figure 1, there is a secondary connection of the cryocooler 6 to the 15 superconducting unit 3, e.g. in this case to the cryostat 4 which surrounds the coils, by bellows 8, which provide an elastic connection and act to damp some of the vibration of the cryocooler 6. Since the cryocooler 6 is directly connected to the superconducting unit 3, e.g. to the superconducting magnet 2 via direct connection to the thermal bus, the vibrations of the cryocooler 6 will be transferred to the superconducting magnet 2. 20 Some of this vibration will be damped by the bellows 8. In some cases, the vibrations produced by the cryocooler may be sufficiently small that they do not interfere with the measurements of the superconducting unit and / or the vibrations of the cryocooler may be sufficiently damped by the bellows 8. In other cases, it may be necessary or desirable to counter the vibrations produced by the cryocooler 6 . 25 In order to provide additional vibration damping, in the example shown in the Figures, the magnet subsystem 5 includes an active-damping device 10, arranged to actively damp the vibrations of the cryocooler 6 by producing vibrations which are substantially equal in magnitude and opposite in direction to the vibrations created by the cryocooler 6. The active-damping device 10 includes a piezo-electric actuator stack 12 and an 30 inertial damping mass 14. Figures 5 and 6 show two different embodiments of the active-damping device 10 in more detail. In both embodiments, the superconducting unit 3 is directly connected to the second stage of the cryocooler 7c and is also connected to the cryocooler 6 by bellows 8. The superconducting unit 3 is shown as a single unit for simplicity, however, as described with reference to Figure 3, the cryocooler 6 may be directly connected to the thermal 5 bus 9 or the superconducting magnetic coil 2 of the superconducting unit 3, whilst the bellows 8 may be connected to the cryostat 4 of the superconducting unit 3. The superconducting unit 3 may additionally or alternatively be connected to the cryocooler 6 by springs. The other side e.g. opposite side of the cryocooler 6 is connected to a piezo-electric actuator stack 12, and this piezo-electric actuator stack 12 is connected 10 to an inertial damping mass 14. The mass of the inertial damping mass 14 may be chosen to compensate accurately for the mass of the cryocooler 6. For example, if the fundamental frequency of oscillation for the cryocooler 6 is 1 Hz, with major harmonics at 50 Hz and 200 Hz, then a 7kg cryocooler 6 will vibrate with an amplitude of ±10 pm. If an inertial damping 15 mass 14 with a mass of 7kg is used (or a mass which is such that the mass of the inertial damping mass together with other components e.g. the piezo-electric stack and any housing is 7kg) then it too should be capable of moving in the region of ±10 pm, and thus compensating for the vibration of the cryocooler 6. In order to allow the cryocooler 6 and the inertial damping mass 14 to move in anti-phase relative to one 20 another, the piezo actuator has a fast-acting movement capability of at least ±20 pm (40 pm in total). In this example, the stiffness of the piezo-electric actuator stack 12 is approximately 40 N / pm with an intrinsic resonant frequency of 5.5 kHz. However, this effective resonance frequency is reduced to around 660 Hz as a result of connecting the piezo-25 electric actuator stack 12 to the cryocooler 6 and the inertial damping mass 14. The phase response of the active damping device 10 is thus about 34 degrees for a 200Hz driving frequency. The time for the piezo-electric actuator stack 12 to expand by 40 pm is ~0.5 ms. Dynamic force on the piezo actuator for 200 Hz actuation on 14 kg at 40 pm is 440 N (which is below both the pull maximum force - 3000 N and the push 30 maximum force - 700 N). The continual power requirement for this operation at 200 Hz is under 10 W. In the embodiment of Figure 5, a sensor 40 is attached to the cryocooler 6. This sensor 40 senses the motion of the cryocooler 6, either as displacement, velocity or as acceleration. Such motion sensors are well known in the art. Figures 5 and 6 show a simple one-dimensional representation of the active-damping device 10, damping vibration along a single axis. However, it will be understood by the skilled person that the active damping device 10 could be arranged to damp motion in two or three-dimensions. 5 An alternative embodiment of the active damping device 10 is shown in Figure 6, in which a stress sensor 50 is integrated within the body of the piezo-electric actuator stack 12. The stress sensor 50 measures the stress which is present within the piezoelectric actuator stack 12, which is proportional to the displacement of the cryocooler 6 relative to the inertial displacement of the inertial mass 14. The movement of the 10 inertial mass 14 provides a type of inertial vibration cancellation as the inertial mass 14 is not attached to a fixed structure. The vibration of the cryocooler 6 is cancelled by first taking the reading from a sensor 40, 50, which produces an output which is proportional to the vibration of the cryocooler 6 either directly or indirectly, and then by controlling the piezo-electric 15 actuator stack 12 to produce a motion which is (substantially) equal in magnitude and opposite in direction to the vibration detected by the sensor 40, 50. This feedback system is represented schematically in Figure 7. The vibration signal of the cryocooler 6 is first measured using external sensors 40, 50, the signal is then converted into a digital numeric value. The signal is then processed 20 by software that passes the signal through a feedback transfer function 60 and sends the resulting output voltage to the piezo-electric actuator stack 12 via a standard voltage amplifier. The feedback transfer function is tuned in such a way as to cancel the initial movement or acceleration of the cryocooler. By displacing the inertial mass, an equal and opposite force is applied to the cryocooler 6 which thereby substantially 25 cancels the vibration, at least to a level below the threshold sensitivity of the NMR system. This one-dimensional example (a single input, single output vibration control system) can be extended to two or three linear direction cancellations, or even to the full six degrees of freedom vibration cancellation (multi input, multi output), depending on the 30 level of vibration cancellation required for the NMR system. It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims. 29 11 24

Claims

1. A nuclear magnetic resonance system, comprising a magnet subsystem, the magnet subsystem comprising:a superconducting unit, comprising a superconducting magnetic coil5 configured to produce a magnetic field of at least 4T wherein thesuperconducting magnetic coil comprises an elongate central bore defining a central axis;a cryocooler, providing cooling to the superconducting magnetic coil; and10 an active-damping device;wherein the magnet subsystem has a height parallel to said central axis of less than 1,2m and both dimensions perpendicular thereto being less than OJ the height ahd wherein the active-damping device acts to substantially cancel avibration induced in the superconducting unit by the cryocooler.T"— 15 2. The nuclear magnetic resonance system of claim 1, wherein thesuperconducting magnetic coil produces a magnetic field with a strength of more than CM 4 Tesla.

3. The nuclear magnetic resonance system of claim 1 or 2, wherein the magnet 20 subsystem has a height of less than 70cm, with said both perpendicular dimensions being less than 50cm.

4. The nuclear magnetic resonance system of any preceding claim, wherein the height and said both perpendicular dimensions of the magnet subsystem are such that the total volume of the magnet subsystem is less than 1 cubic metre.25 5. The nuclear magnetic resonance system of any preceding claim, wherein themagnet subsystem comprises an outer housing, partially or fully surrounding the other components of the magnet subsystem.

6. The nuclear magnetic resonance system of any preceding claim, wherein the 30 magnet subsystem further comprises a cryostat surrounding the superconducting magnetic coil.

7. The nuclear magnetic resonance system of any preceding claim, wherein there is no fluid connection between the superconducting magnetic coil and the cryocooler.

8. The nuclear magnetic resonance system of any preceding claim, wherein the cryocooler is in direct thermal contact with the superconducting magnetic coil.

9. The nuclear magnetic resonance system of any preceding claim, wherein the superconducting unit comprises a thermal bus connecting the cryocooler to the superconducting magnetic coil.

10. The nuclear magnetic resonance system of any preceding claim, wherein the active-damping device is arranged to produce a vibration which is opposite in direction and substantially equal in magnitude to a vibration of the cryocooler.

11. The nuclear magnetic resonance system of any preceding claim, wherein the magnet subsystem comprises a motion sensor arranged to detect vibration of the cryocooler and / or the superconducting magnetic coil in order to provide feedback to the active damping device.

12. The nuclear magnetic resonance system of any preceding claim, wherein the active-damping device additionally comprise an inertial mass, having a mass such that the mass of the active-damping system is substantially equal to the mass of the cryocooler.

13. The nuclear magnetic resonance system of any preceding claim, wherein the active damping device comprises at least one piezo-electric actuator.

14. The nuclear magnetic resonance system of claim 13, wherein the piezo-electric actuator comprises a motion sensor, arranged to measure the strain within the at least one piezo-electric actuator in order to determine the vibrations of the cryocooler.

15. The nuclear magnetic resonance system of any preceding claim, further comprising a computer, wherein the computer is external to the magnet subsystem and is in wired or wireless connection with the magnet subsystem.

Images