Passive reduction of temperature-induced shim drift in NMR magnet systems

Abstract

To provide a nuclear magnetic resonance (NMR) device that maintains a homogeneous magnetic field almost stably and constantly even when a temperature state inside and around the device is changed.SOLUTION: A nuclear magnetic resonance (NMR) device has a superconducting magnet 1 which is arranged in a cold region of a cryostat within a vacuum container 8), and has a shim system 7 which contains a plurality of shim elements 6 arranged outside the vacuum container. The magnet has a first mechanical connection point 11 with the vacuum container via a magnetic suspension device 3, and the shim system uses only a material having a coefficient of thermal expansion at an operation temperature smaller than 5 ppm / K. The NMR device can thus maintain a homogeneous magnetic field almost stably and constantly even when a temperature state inside and around the device is changed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an NMR apparatus having a magnet coil system for generating a uniform magnetic field, the magnet coil system having a superconducting magnet arranged inside a vacuum vessel in a low temperature region of a cryostat, and a shim system including a plurality of shim elements arranged outside the vacuum vessel, wherein the superconducting magnet has a first mechanical connection point with the vacuum vessel via a magnet suspension device, and the shim system has a second mechanical connection point with the vacuum vessel via a positioning element.

[0002] Such an NMR device is known from DE 10104365 (=document [1]). [Background technology]

[0003] The present invention relates generally to the field of nuclear magnetic resonance ("NMR"), and more particularly to cooled NMR magnet systems that are always superconducting during operation, and in which the homogeneity of the NMR field is further improved by a shim system.

[0004] NMR spectroscopy is a highly efficient method widely used in instrumental analysis, which can be used to study the electronic environment of individual atoms and their interactions with neighboring atoms in test substances, such as hydrocarbons or complex bioinorganic compounds, thus elucidating, for example, the composition, structure, and dynamics of test substances, as well as determining their concentrations.

[0005] In an NMR measurement, a material is exposed to a strong, uniform static magnetic field B0, which aligns the nuclear spins within the material. A high-frequency electromagnetic pulse is then emitted into the material under test. This generates a similarly high-frequency electromagnetic field that is detected by an NMR spectrometer, from which information about the properties of the material under test can be obtained.

[0006] The requirements for magnetic field homogeneity are very high in both high-resolution magnetic resonance spectroscopy and magnetic resonance imaging. To achieve the homogeneity specifications, electric cryoshims are often used. Their coils generate the basic magnetic field profile. By supplying the appropriate current to the coils, the homogeneity of the NMR magnet at the sample position can be improved.

[0007] Low-temperature ferromagnetic materials (e.g., iron or steel alloys) can also be used as shim elements to improve uniformity, as described, for example, in German Patent No. 102015225731 (= Reference [7]). Regardless of how a magnet is cryo-shimmed, inhomogeneities ultimately remain that need to be corrected in the magnet bore outside the vacuum vessel. For this purpose, so-called room-temperature shims (hereinafter referred to as "RT shims") are used. These room-temperature shims, like cryo-shims, consist of coils, ferromagnetic materials, or a combination of both. While the name "room-temperature shims" suggests that these shims are at laboratory temperature, this is not necessarily the case; the power dissipated in the shim coils often causes the shim temperature to be slightly higher than laboratory temperature. Additionally, NMR experiments can be performed with sample materials at various temperatures. Nevertheless, for simplicity, we will always refer to "RT shims" and "room temperature" below. That is, RT shims do not necessarily have to operate at room temperature.

[0008] The superconducting NMR magnet is mechanically connected to the RT shim via a long path. Part of this path is in the cryostat's low temperature region, and another part is at room temperature. The latter path changes length due to thermal expansion as the ambient temperature fluctuates. As a result, relative movement occurs between the superconducting magnet and the RT shim. In this case, the RT shim is no longer ideally aligned to cancel out magnetic field inhomogeneities, resulting in poor quality magnetic resonance images or spectra.

[0009] If these RT shims have coils or coil systems, adjusting their currents can restore them to their original quality. Similar deviations in RT shim currents can be observed if these currents themselves drift, for example, due to temperature-dependent current sources. Therefore, hereafter, we sometimes (rather informally) say that RT shim currents "drift" with ambient temperature.

[0010] Sensitive instruments are often used in air-conditioned rooms, where it is not uncommon for the temperature to fluctuate by 1°C during the day. In many applications, the RT shim currents can be adjusted automatically. If this is not possible, variations in uniformity during the measurement can be problematic.

[0011] Similar uniformity fluctuations can also occur if the temperature in the magnet bore changes over time for other reasons. This situation occurs, for example, when the NMR sample is replaced and new shim adjustments are required. The power dissipated by the RT shim may be different from the power of the previous RT shim, potentially creating new temperature conditions in the magnet bore. In this case, a certain amount of time is required for the temperature to settle to the new level. During this time, the shim currents must be repeatedly adjusted. A similar problem occurs when the temperature of the NMR sample changes.

[0012] The obvious solution to this problem would be to, in the first approach, keep the ambient temperature fluctuations as small as possible: the larger the laboratory, the more expensive and difficult it becomes to operate an air-conditioning system with sufficient precision.

[0013] A second, more sophisticated approach involves automatically adjusting the shim currents to accommodate varying uniformity, although this is only possible if a locking material is available, which is not the case, for example, in solid-state experiments.

[0014] A high-resolution NMR spectrometer with a superconducting NMR magnet coil system cooled to extremely low temperatures in a pulse tube cooler and placed in the low temperature region of a cryostat inside a vacuum vessel is described in EP 0 780 698 (= Reference [2]).

[0015] European Patent Application Publication No. 2015092 (=Reference [3]) addresses the problem of shim drift due to atmospheric pressure fluctuations. As with temperature fluctuations, a relative movement occurs between the RT shim and the superconducting magnet. To minimize this relative movement, a bridge is used, which is fixed to the outer edge of the cryostat and does not follow the movements of the central region of the cryostat that occur during pressure fluctuations. The RT shim system is fixed to this bridge.

[0016] A quantitative clinical NMR analysis instrument that automatically compensates for temperature sensitivity over a wide temperature range is described in EP 3686620 (=reference [4]).

[0017] The publication "Acyclic Acids - Advances in Research and Application: 2013 Edition: Scholarly Brief," by Q. Ashton Acton, PhD, Scholarly Editions, 2013, p. 139 (Reference [5]) also mentions the temperature sensitivity of the electronics in NMR analyzers, suggesting that temperature regulation of the electronics could be a remedy.

[0018] The publication "Modern Instrumental Analysis", Satinder Ahuja, Neil Jespersen, Elsevier, 2006, p. 270 (= Reference [6]) mentions the temperature sensitivity of sample materials. This effect can be counteracted by adjusting the temperature of the material placement location.

[0019] From the above-mentioned German patent application DE 102015225731 (= document [7]) a helium-cooled superconducting magnet coil system for an NMR apparatus with an easily accessible and likewise cryogenically cooled NMR shim assembly is known, which has a magnetic field former between the helium inner tube and the radiation shielding inner tube, which magnetic field former is in mechanically firm contact with the helium vessel without contacting the radiation shielding inner tube.

[0020] The same document [1] mentioned above also discloses a universal NMR apparatus according to the present invention, which includes a universal shim system and has all the characteristics defined at the beginning, but even in this apparatus the above-mentioned problems related to the fluctuation of the magnetic field homogeneity due to temperature changes in the range that is typical in laboratories still exist. [Prior art documents] [Patent documents]

[0021] [Patent Document 1] German Patent Invention No. 10104365 [Patent Document 2] German Patent Invention No. 102015225731 [Patent Document 3] European Patent No. 0780698 [Patent Document 4] European Patent Application Publication No. 2015092 [Patent Document 5] European Patent Application Publication No. 3686620 [Non-patent literature]

[0022] [Non-Patent Document 1] “Acyclic Acids-Advances in Research and Application:2013 Edition:ScholarlyBrief”, Q. Ashton Acton, PhD, ScholarlyEditions, 2013, p139 [Non-patent document 2] "Modern Instrumental Analysis", Satinder Ahuja, Neil Jespersen, Elsevier, 2006, p270 Summary of the Invention [Problem to be solved by the invention]

[0023] In contrast to this, the present invention is based on the object of improving an NMR apparatus having the characteristics defined at the outset in such a way that, using particularly simple and readily available technical means and at the lowest possible cost, the magnetic field homogeneity remains approximately stable and constant even when the temperature conditions inside and around the apparatus change. [Means for solving the problem]

[0024] The above problem is solved in a surprisingly simple and effective manner according to the invention by using only materials having a thermal expansion coefficient of less than 5 ppm / K at operating temperatures for at least a portion of the path along the vacuum vessel from a first mechanical connection point between the magnet suspension and the vacuum vessel to a second mechanical connection point between a positioning element for the shim system and the vacuum vessel, and / or for at least a portion of the path along the positioning element from the second mechanical connection point to the shim system.

[0025] That is, the present invention proposes to achieve stable magnetic field homogeneity by selecting a predetermined material along the above-mentioned path.

[0026] The above-mentioned configuration of the path according to the present invention, in particular by selecting materials with low thermal expansion coefficients, minimizes the expansion changes of the corresponding path sections during temperature fluctuations, thereby keeping the relative movement between the superconducting magnet and the shim system small, so that the magnetic field homogeneity remains stable.

[0027] The thermal expansion coefficient of conventional metallic materials such as steel and aluminum is approximately 20 ppm / K at room temperature. To reduce the relative movement between the shim system and the superconducting magnet when the room temperature fluctuates, a material with much lower thermal expansion can be used for the connection path between these two parts.

[0028] Carbon reinforced plastic (CFK) has a coefficient of thermal expansion along its fibers of less than 1 ppm / K. This material can be used, for example, in a magnet bore as part of a positioning element for a shim system.

[0029] A type of steel called Invar has a thermal expansion coefficient of less than 2 ppm / K. This steel can be used in parts of vacuum vessels. In contrast, CFK cannot be welded, making it unsuitable as a material for vacuum vessels.

[0030] Relative movement can also be reduced by maintaining a constant temperature in a portion of the connection path between the superconducting magnet and the shim system using a control circuit. The temperature or strain is measured at several locations, and heating is applied if necessary. An area of ​​the heater is artificially kept warmer than the ambient temperature. Alternatively, cooling can be achieved using a TEC element (thermoelectric cooler). In that case, an area of ​​the TEC element is artificially kept cooler than the ambient temperature.

[0031] A constant temperature can also be maintained in the magnet bore using a temperature-controlled gas.

[0032] In a very particularly preferred embodiment of the NMR apparatus according to the invention, the length of the path portion along the vacuum vessel from the first mechanical connection point to the second mechanical connection point and / or the length of the path portion along the positioning element from the second mechanical connection point to the shim system is configured to be more than 50% of the total length of the corresponding path, respectively.

[0033] Shim coils are typically wound on an aluminum support. Aluminum has the advantage of good thermal conductivity, allowing heat dissipated within the coil to be efficiently removed. Unfortunately, however, aluminum has a high thermal expansion coefficient of approximately 23 ppm / K. In an embodiment of the present invention, only the connection portion of the shim coil from the aluminum support to the second mechanical connection point is made of a material with a low thermal expansion coefficient. If the shim element is not a coil but a plate made of a ferromagnetic material, it is even possible to make the entire positioning element from a material with a low thermal expansion coefficient. Outside the magnet bore, the hemispherical cap of the vacuum vessel and its connecting flange to the inner tube are not necessarily made of a material with a low thermal expansion coefficient. Given this background, however, it is easy to use a material with a low thermal expansion coefficient for more than 50% of the total length of the path.

[0034] Also particularly advantageous is an embodiment of the NMR device according to the invention in which, in each of the paths along the vacuum vessel from the first mechanical connection point to the second mechanical connection point and / or along the positioning element from the second mechanical connection point to the shim system, multiple materials with different thermal expansion coefficients are used in part, so that the thermal expansion of these materials cancels each other out.

[0035] In that case, all that remains is a significantly lower absolute thermal expansion, which can ultimately be further minimized within the scope of fine tuning according to the invention.

[0036] Also preferred are embodiments in which Invar is used as the material for at least a portion of the path along the vacuum vessel from the first mechanical connection point to the second mechanical connection point.

[0037] In this case, the relative movement between the superconducting magnet and the shim system is advantageously reduced. The thermal expansion coefficient of Invar is typically less than 2 ppm / K. In addition, Invar has the advantage that it can be welded. The vacuum vessel must be vacuum-tight, which places high demands on all mechanical connections. In the case of a typical cryostat with a vertically extending magnet bore, the sections described here will extend over one or more suspension towers.

[0038] However, Invar is magnetized by the background magnetic field of the superconducting magnet. Since Invar exists only in the small stray magnetic field region of the superconducting magnet, no problems are expected, at least in actively shielded magnets. In any case, the magnetizing force and the interfering magnetic field are small.

[0039] Alternatively or additionally, in another embodiment of the invention, CFK (= carbon fiber reinforced plastic) is used as material for at least part of the path along the positioning element from the second mechanical connection point to the shim system.

[0040] Typically, the thermal expansion coefficient of CFK along the fibers is less than 1 ppm / K. The low thermal expansion coefficient minimizes expansion changes when the temperature fluctuates, thereby keeping the relative movement between the superconducting magnet and the shim system small, thereby maintaining a stable magnetic field homogeneity.

[0041] In preferred developments of these embodiments, the distance between the first mechanical connection point and the second mechanical connection point is less than 10 cm.

[0042] Advantageously, the thermal expansion of CFK or similar materials is less than that of all metals, including Invar. Therefore, it is expedient to use such materials for as large a portion of the connection length as possible between the shim system and the first mechanical connection point. In practice, the positioning element for the shim system can be attached to the suspension tower using a frame made of CFK.

[0043] Also advantageous is an embodiment of the invention in which a material, in particular copper, having a thermal conductivity at operating temperature of greater than 50 W / (mK) is used for at least a part of the further path along the positioning element from the second mechanical connection point to the shim system.

[0044] This means that there is a risk of overheating if a high current is required in the shim element. This problem can be addressed by inserting sliding rails made of a material with good thermal conductivity, for example copper, into the positioning element, which does not or only slightly contributes mechanically to the thermal expansion of the positioning element. Sliding tubes made of aluminum or copper can also be installed in parallel to the positioning element. The use of flushing gas also contributes to heat dissipation.

[0045] Another advantageous embodiment of the NMR device according to the invention is characterized in that the shim elements are formed as electric coils and / or ferromagnetic elements.

[0046] Ferromagnetic elements have the advantage of contributing to the homogenization of the magnetic field without dissipating heat. These ferromagnetic elements are typically used in magnetic resonance imaging equipment. Electrical shim coils are characterized by their variable current and therefore their ability to dynamically respond to homogeneity variations. Shim coils are essential for high-resolution nuclear magnetic resonance imaging. However, they can also be combined with ferromagnetic elements.

[0047] Also preferred is an embodiment of the invention in which the vacuum vessel has a vertically extending room temperature bore, the shim system is arranged within the room temperature bore, the positioning element includes a clamp ring, and the contact surface of the clamp ring located on the upper end of the room temperature bore of the vacuum vessel forms the second mechanical connection point.

[0048] In a vertical cryostat, the length of the mechanical connection between the first and second mechanical connections located in the upper region of the vacuum vessel is minimized by also locating the second mechanical connection in the upper region of the vacuum vessel, with a shorter length varying less during temperature fluctuations than a longer length.

[0049] A preferred type of embodiment of the NMR apparatus according to the invention is characterized in that an adjusting element is arranged in a part of the path along the vacuum vessel from the first mechanical connection point to the second mechanical connection point and / or in a part of the path along the positioning element from the second mechanical connection point to the shim system for adjusting thermally induced length changes in said path, preferably the adjusting element comprising a heating element and / or a cooling element and / or a heat exchanger.

[0050] The adjustment element allows the temperature fluctuations in that area to be kept small, which minimizes the relative movement between the superconducting magnet and the shim system and keeps the magnetic field homogeneity stable.

[0051] In a preferred development of this type of embodiment, in addition to the adjusting element a sensor element is arranged on the same path, the sensor element preferably comprising a thermometer, in particular a PT-100 sensor, and / or a strain gauge.

[0052] A typical sensor element is a platinum resistance thermometer (typically PT-100), which is particularly suitable as it allows accurate temperature measurement in the room temperature range.

[0053] The advantage here is that controlling the expansion may reduce the relative movement between the superconducting magnet and the shim system. The portion of the path along the vacuum vessel here may include one or all of the suspension towers in a typical cryostat. The sensor element outputs a variable that needs to be kept constant, which is achieved by controlling the current in the heating or cooling element.

[0054] Further developments of these embodiments are also preferred, characterized in that the sensor element comprises a laser, with which a change in position of the path section of the object can be detected by electro-optical distance measurement, in particular by interferometry.

[0055] Similar to thermometers and / or strain gauges, lasers can be used to measure changes in length, which can be adjusted using current in heating or cooling elements. Lasers can monitor changes in length of positioning elements or suspension towers, for example.

[0056] Also preferably, in these further developments, at least one piezoelectric element is provided, which can be used to correct expansion changes in the path section to be observed.

[0057] A piezoelectric element can be incorporated into the positioning element, for example, and its length can be adjusted by applying a voltage so that the overall length of the positioning element remains constant over time.

[0058] In another, particularly preferred type of embodiment of the NMR apparatus according to the invention, the superconducting magnet is arranged inside the vacuum vessel in a helium vessel of a cryostat which is filled with liquid helium during operation, the helium vessel being radially surrounded by a nitrogen vessel of the cryostat which is preferably filled with liquid nitrogen.

[0059] In practice, most cryostats operate in this manner. In the case of a vertical magnet, the first mechanical connection involves one or more weld seams connecting the magnet suspension, in this case the helium suspension tube, to the vacuum vessel.

[0060] Also advantageous may be embodiments in which the cryostat includes a cryocooler capable of cooling the superconducting magnet to its operating temperature.

[0061] Advantageously, in this case, there is no need to refill with helium, so there is no problem if there is a shortage of helium.

[0062] In principle, an embodiment of the NMR apparatus according to the invention may also be advantageous in which the vacuum vessel is completely surrounded by a temperature-controlled box, preferably located in an air-conditioned room. An air-conditioned room is in principle already such a box. However, as the examples show, the temperature control can be significantly improved by adding an insulating box.

[0063] Other advantages of the present invention will become apparent from the following description and drawings. Likewise, the present invention allows the use of the above and below features, either alone or in any combination. The illustrated and described embodiments should not be considered as an exhaustive list, but rather have an exemplary character for explaining the present invention. [Brief explanation of the drawings]

[0064] [Figure 1] 1 is a schematic vertical cross-sectional view of an embodiment of an NMR apparatus according to the invention, in which the positioning element comprises a clamp ring, the contact surface of which forms the second mechanical connection point on the upper end of the room temperature bore of the vacuum vessel. FIG. [Figure 2] 2 is a schematic vertical cross-sectional view of an embodiment of an NMR apparatus similar to that of FIG. 1, but in which a mechanical actuator is arranged in a path along the positioning element from the second mechanical connection point to the shim system, according to the present invention; FIG. [Figure 3] 3 is a diagram of a further embodiment of an NMR apparatus according to the invention, in which the positioning element comprises a frame, the contact surface of which is located on the upper end of the suspension tower of the magnet suspension forming the second mechanical connection point. [Figure 4] 1 is a partial view of an NMR device according to the present invention, including wiring diagrams of several adjustment and sensor elements. [Figure 5] 1 is a partial view of an NMR device according to the invention showing a wiring diagram of multiple regulation and sensor elements with only a single control circuit. [Figure 6] 1 is a schematic vertical cross-sectional view of an embodiment of an NMR apparatus according to the present invention that uses a temperature-controlled fluid to keep the suspension tower and positioning elements at a constant temperature. DETAILED DESCRIPTION OF THE INVENTION

[0065] The invention is illustrated in the drawings and will be explained in more detail with reference to example embodiments.

[0066] Generally, the present invention relates to an improved NMR apparatus with a magnet coil system for generating a uniform magnetic field, the NMR apparatus comprising a superconducting magnet 1 disposed in a low temperature region of a cryostat inside a vacuum vessel 8. The NMR apparatus further comprises a shim system 7 including a plurality of shim elements 6 disposed outside the vacuum vessel 8, which may be formed as electric coils and / or ferromagnetic elements. The superconducting magnet 1 also has a first mechanical connection 11 with the vacuum vessel 8 via a magnet suspension 3, which is itself disposed in a suspension tower 4. The shim system 7 has a second mechanical connection 10; 12 with the vacuum vessel 8 via a positioning element 5.

[0067] In addition to the shim elements 6, the shim system 7 typically also includes a mounting structure for the shim elements 6, so that the shim system 7 forms a unit not only functionally but also structurally. This mounting structure can mechanically connect the entire shim element 6 to the positioning element 5, or it can serve as a conductor for an electrical connection to the shim element 6 or as a mechanical interface to the NMR probehead. A conceptual distinction between the positioning element 5 and other structural parts, such as the mounting structure for the shim elements 6, should be made in that all structural parts in the path between the second mechanical connection points 10; 12 to the vacuum vessel 8 on the one side and the at least one shim element 6 on the other side are to be interpreted as the positioning element 5 if their expansion affects the position or orientation of this shim element 6 relative to the superconducting magnet 1.

[0068] The superconducting magnet 1 is typically disposed inside the vacuum vessel 8 within a cryostat helium vessel 2 that is filled with liquid helium during operation, and the helium vessel 2 is radially surrounded by the cryostat nitrogen vessel 9 that is preferably filled with liquid nitrogen. The cryostat may also include a cryocooler capable of cooling the superconducting magnet 1 to its operating temperature.

[0069] In contrast, the present invention is characterized in that only materials having a thermal expansion coefficient of less than 5 ppm / K at operating temperatures are used in at least a portion of the path along the vacuum vessel 8 from the first mechanical connection point 11 to the second mechanical connection point 10;12 and / or in at least a portion of the path along the positioning element 5 from the second mechanical connection point 10;12 to the shim system 7.

[0070] It may also be advantageous if, in each case, several materials with different thermal expansion coefficients are used partly in the path along the vacuum vessel 8 from the first mechanical connection point 11 to the second mechanical connection point 10;12 and / or in the path along the positioning element 5 from the second mechanical connection point 10;12 to the shim system 7, so that the thermal expansion of these materials cancels each other out.

[0071] Preferably, the length of the path portion along the vacuum vessel 8 from the first mechanical connection point 11 to the second mechanical connection point 10;12 and / or the length of the path portion along the positioning element 5 from the second mechanical connection point 10;12 to the shim system 7 each exceeds 50% of the total length of the corresponding path.

[0072] FIG. 1 shows a particularly advantageous two-part mechanical structure for positioning shim elements 6 in the magnet bore. The upper part of the mechanical structure establishes a connection path from the shim system 7 to the second mechanical connection point 10 between the clamping ring and the vacuum vessel 8 and, by definition, serves as the positioning element 5. The lower part of the mechanical structure belongs to the shim system 7 and serves to hold the entire shim elements 6 for insertion into the magnet bore from below, as well as to receive the NMR probe head and route electrical supply lines to the shim elements 6. The two-part mechanical structure for positioning the shim elements 6 simplifies installation in the magnet bore. In operation, the two structures are mechanically connected, for example, by screws. Alternatively, a one-piece structure is conceivable in which the shim system 7 as a whole of all shim elements 6 does not have any structural parts, and the positioning element 5 also serves as a holder for the shim elements 6, thereby fulfilling the aforementioned functions.

[0073] The positioning element 5 of FIG. 1 comprises a clamping ring, the contact surface of which rests on the upper end of the room temperature bore of the vacuum vessel 8 and forms a second mechanical connection point 10 .

[0074] FIG. 2 shows an embodiment of an NMR apparatus similar to that of FIG. 1, but in addition includes a mechanical actuator 15 located below the clamp ring 10.

[0075] As specifically shown in Figure 2, a piezoelectric element 15 can be integrated into the positioning element 5. If the position of the shim system 7 is to be changed, a corresponding voltage can be applied to the piezoelectric element 15. The length change of the piezoelectric crystal is then proportional to the applied voltage, with negative voltages also possible. Depending on the thickness of the sandwich-type piezoelectric stack, the possible displacement range can be adapted to the displacement requirements.

[0076] 3, on the other hand, the positioning elements for the shim system 7 are suspended from the suspension tower via a frame. In this way, the first and second mechanical connection points are approximately coincident, and the path along the vacuum vessel connecting the two mechanical connection points to each other is minimized. In particular, the connection elements 12 can be made from a material with a low thermal expansion coefficient, so that they do not expand significantly when the temperature fluctuates.

[0077] 4 shows an example of a control device on a suspension tower 4 (here three control variables and three adjustment elements per tower) in a schematic vertical cross-section of an NMR device according to the invention with several adjustment elements 13 and sensor elements 14. It shows how several control circuits can be used to keep the local strain variations small on the different suspension towers 4. By using such a solution, maximum control over the strain is obtained. The effort involved is of course very high, since several current sources and several control devices are required.

[0078] 4 shows an example of a specific embodiment with an adjustment element 13 formed as a heating element on a suspension tower 4 and a sensor element 14. The expansion or temperature of the suspension tower 4 is measured using the sensor element 14, and the heater can be used to counteract the measured expansion changes.

[0079] 4 also shows, in a schematic manner, an electronic control unit 17 (here in the form of a PI controller) which controls the current of the heating element as a function of the measured strain. The PI controller is, of course, only one example of a wide variety of possible controllers known per se from control technology. In control technology, there are many other controllers, which will not be mentioned in detail here. As sensor element 14, for example, a PT-100 temperature sensor can be used.

[0080] The PT-100 sensor is particularly sensitive in the room temperature range and is therefore suitable for temperature measurements. Alternatively, other temperature sensors or strain gauges can be used. The heating element often comprises a serpentine wire that covers as large an area as possible.

[0081] Furthermore, FIG. 4 shows diagrammatically the current supply of each sensor element 14 .

[0082] The suspension tower 4, together with the sensor element 14 and the adjusting element 13, is usually covered with a thermal insulator (not specifically shown in the figures) to reduce fluctuations in the ambient temperature. In order to be able to react to increases in the ambient temperature, the temperature of the suspension tower 4 is artificially kept slightly higher than the ambient temperature in this example by a heater. Instead of a heating element, a cooling element, in particular a so-called thermoelectric cooler (=TEC), can also be used. In this case, the temperature of the suspension tower 4 is kept reasonably lower than the ambient temperature.

[0083] While Figure 4 shows an example using multiple PI controllers, it is possible to reduce the number by connecting multiple heating or cooling elements in series. It is also possible to connect PT-100 sensors in series, parallel, or a combination of both and use only one average temperature for control. This reduces the complexity and cost of the required electronics, often without any significant drawbacks.

[0084] Such an embodiment is shown in Figure 5. However, the control over the expansion of the suspension tower 4 is slightly less precise than in the example embodiment shown in Figure 4. However, if a material with good thermal conductivity is used for the suspension tower 4, the temperature gradient across the suspension tower 4 can be reduced. When combined with good insulation of the suspension tower 4 from the ambient temperature, strain control can be further improved.

[0085] Depending on the application, there are many possibilities between the two solutions presented here.

[0086] A similar arrangement to that of the suspension tower 4 is also possible for the positioning elements 5.

[0087] FIG. 6 shows an embodiment with a liquid circuit arranged along the suspension tower 4 and the positioning element 5. The advantage of this embodiment is that only one controlled heating or cooling element is needed to stabilize the temperature of several components. The temperature control fluid can transport a large amount of heat, thereby keeping the temperature gradient within the temperature-controlled object small. This improves the accuracy of controlling thermal expansion. Thermal insulation from the external space can further reduce the temperature gradient within the temperature-controlled object. A gas can also be used as the temperature control fluid. The advantage of a gas is that it can move freely within the magnet bore and along the positioning element 5.

[0088] Bibliography Publications to be considered in determining patentability: [1] German Patent No. 10104365 ≒ British Patent No. 2411238 ≒ US Patent Application Publication No. 2005 / 0174118 [2] European Patent No. 0780698 ≒ U.S. Patent No. 5,744,959 [3] European Patent Application Publication No. 2015092 [4] European Patent Application Publication No. 3686620 [5]Acyclic Acids-Advances in Research and Application:2013 Edition:ScholarlyBrief, Q.Ashton Acton,PhD,ScholarlyEditions,2013,p139 [6]Modern Instrumental Analysis,Satinder Ahuja,Neil Jespersen,Elsevier,2006,p270 [7] German Patent Invention No. 102015225731 ≒ European Patent No. 3182147 ≒ U.S. Patent No. 9,766,312 ≒ ≒ Chinese Patent No. 106898452 ≒ Patent No. 6340403 [Explanation of symbols]

[0089] 1. Superconducting magnet 2. Helium container 3 Magnetic suspension system 4 Suspension Tower 5 Locating elements for shim systems 6 Shim Elements 7 Shim System 8 Vacuum container 9 Nitrogen container 10 Second mechanical connection point 11 First mechanical connection point 12 Alternate second mechanical connection point 13 Adjustment element 14 Sensor element 15 Mechanical Actuators 17 Electronic Control Unit

Claims

1. 1. An NMR apparatus comprising a magnet coil system for generating a uniform magnetic field, the magnet coil system having a superconducting magnet (1) arranged inside a vacuum vessel (8) in a low temperature region of a cryostat, and a shim system (7) including a plurality of shim elements (6) arranged outside the vacuum vessel (8), 1. An NMR apparatus, wherein the superconducting magnet (1) has a first mechanical connection (11) with the vacuum vessel (8) via a magnet suspension device (3), and the shim system (7) has a second mechanical connection (10; 12) with the vacuum vessel (8) via a positioning element (5), 1. An NMR apparatus, characterized in that only materials having a thermal expansion coefficient of less than 5 ppm / K at operating temperatures are used in at least a portion of a path along the vacuum vessel (8) from the first mechanical connection point (11) to the second mechanical connection point (10; 12) and / or in at least a portion of a path along the positioning element (5) from the second mechanical connection point (10; 12) to the shim system (7).

2. 2. The NMR apparatus according to claim 1, wherein the length of the at least one portion of the path along the vacuum vessel from the first mechanical connection point to the second mechanical connection point and / or the length of the at least one portion of the path along the positioning element from the second mechanical connection point to the shim system are each greater than 50% of the total length of the corresponding path.

3. 3. The NMR apparatus according to claim 1, wherein materials having different thermal expansion coefficients are used in part along the path along the vacuum vessel from the first mechanical connection point to the second mechanical connection point and / or along the positioning element from the second mechanical connection point to the shim system, so that the thermal expansions of the materials cancel each other out.

4. 2. The NMR apparatus according to claim 1, characterized in that invar is used as material for at least a portion of the path along the vacuum vessel (8) from the first mechanical connection point (11) to the second mechanical connection point (10; 12).

5. 2. The NMR device according to claim 1, characterized in that CFK (= carbon fiber reinforced plastic) is used as material for at least a portion of the path along the positioning element (5) from the second mechanical connection point (10; 12) to the shim system (7).

6. NMR device according to claim 5, characterized in that the distance between the first mechanical connection point (11) and the second mechanical connection point (10; 12) is less than 10 cm.

7. 2. The NMR device according to claim 1, wherein at least a portion of another path along the positioning element (5) from the second mechanical connection (10; 12) to the shim system (7) is made of a material having a thermal conductivity of more than 50 W / (mK) at the operating temperature.

8. NMR device according to claim 1, characterized in that the shim element (6) is formed as an electric coil and / or as a ferromagnetic element.

9. 2. The NMR apparatus of claim 1, wherein the vacuum vessel (8) has a vertically extending room-temperature bore in which the shim system (7) is arranged, and the positioning element (5) comprises a clamp ring, the contact surface of which is located on an upper end of the room-temperature bore of the vacuum vessel (8) forming the second mechanical connection point (10).

10. 2. The NMR apparatus according to claim 1, characterized in that an adjusting element (13) is arranged on a part of the path along the vacuum vessel (8) from the first mechanical connection point (11) to the second mechanical connection point (10; 12) and / or on a part of the path along the positioning element (5) from the second mechanical connection point (10; 12) to the shim system (7) for adjusting a thermally induced length change in the path.

11. NMR device according to claim 10, characterized in that in addition to the adjustment element (13) a sensor element (14) is arranged on the same path.

12. NMR device according to claim 11, characterized in that the sensor element (14) comprises a laser, which can be used to detect changes in the position of the path section being observed by electro-optical distance measurement.

13. 12. An NMR apparatus according to claim 11, characterized in that at least one piezoelectric element (15) is provided, by means of which it is possible to correct positional changes of the path portion to be observed.

14. 2. The NMR apparatus according to claim 1, characterized in that the superconducting magnet (1) is arranged inside the vacuum vessel (8) in a helium vessel (2) of the cryostat which is filled with liquid helium during operation.

15. NMR apparatus according to claim 1, characterized in that the cryostat comprises a cryocooler capable of cooling the superconducting magnet (1) to an operating temperature.

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