Magnetic arrangement for a magnetic resonance apparatus with correction device with superconducting coated foil, magnetic resonance arrangement and method for operating a magnetic arrangement

Abstract

The invention relates to a magnet arrangement for a magnetic resonance apparatus, wherein the magnet arrangement comprises: a main magnet for generating a magnetic field in the direction of a main magnet longitudinal axis (z) in a measuring volume arranged around the main magnet longitudinal axis (z) with at least one main field coil, wherein the main field coil is a coil wound from a conductor made of superconducting material, and a correction device (4) for homogenizing the magnetic field generated by the main magnet in the measuring volume, wherein the correction device (4) is arranged radially inside the main magnet (2) around the main magnet longitudinal axis (z).The magnetic arrangement is characterized in that the correction device (4) comprises a foil arrangement (7) with at least one foil (8) coated with superconducting material, wherein the foil arrangement (7) covers at least 90% of a cylindrical surface around the measuring volume (5), the cylindrical surface having a cylinder axis parallel to the main magnetic field axis (z), wherein at least one foil (8) of the foil arrangement (7) is at least 40 mm long in each of two mutually perpendicular directions, and wherein the number of turns around the main magnet longitudinal axis (z) of each electrically closed current path that runs exclusively through superconducting material of the foil arrangement (7) is zero. The magnetic arrangement according to the invention features improved homogeneity, a simple design, and easy handling.

Description

Background of the invention

[0001] The invention relates to a magnet arrangement for a magnetic resonance apparatus, wherein the magnet arrangement comprises: a main magnet for generating a magnetic field in the direction of a main magnet longitudinal axis in a measuring volume arranged around the main magnet longitudinal axis with at least one main field coil, wherein the at least one main field coil is a coil wound from a conductor made of superconducting material, and a correction device for homogenizing the magnetic field generated by the main magnet in the measuring volume, wherein the correction device is arranged radially inside the main magnet around the main magnet longitudinal axis.

[0002] Magnetic arrangements for a magnetic resonance apparatus with wound main magnetic field coils and correction devices are known from [4], [5] and [6].

[0003] In certain applications of superconducting magnets, such as high-resolution magnetic resonance spectroscopy and magnetic resonance imaging, the requirements for the magnetic field homogeneity of the main magnetic field generated by the main magnet are very high. Even if the homogeneity of known main magnets meets the required specifications according to the design, in practice small manufacturing inaccuracies inevitably lead to field profiles that must be improved. Corrective devices in the form of active and / or passive shims are used for this purpose.

[0004] In passive shimming, ferromagnetic materials are attached at specific points along the inner bore of the magnet. These ferromagnetic materials are magnetized by the main magnetic field and generate an additional field that improves the homogeneity of the main magnetic field. In contrast, active shimming involves passing currents through special shim coils to generate an additional, "correcting" magnetic field.

[0005] NMR main magnets can be in the form of wound coils (made of superconducting wires or tape conductors) or bulk magnets (made of high-temperature superconductors (HTS), e.g., REBCO). Preferably, HTS materials are used in the radially inner region of the main magnet to generate high field strengths, as these materials exhibit a very high critical field.

[0006] [1] discloses a correction device for an NMR main magnet made of HTS bulk material, wherein the high-temperature superconductor has the form of thick HTS rings stacked on top of each other. Due to "defects," the current flowing through the superconducting HTS material does not flow in a perfectly circular path within the rings, resulting in an inhomogeneous magnetic field in a volume around the axis of the rings. To homogenize the magnetic field, [1] proposes winding several layers of an HTS tape conductor radially inside the bulk magnet.

[0007] Reference [7] also describes a shim device for an NMR main magnet made of HTS bulk material. Here, the shim device is made from a wound HTS tape conductor arranged radially inside the main coil.

[0008] In [2] the effect of a thin superconducting hollow cylinder inserted into a superconducting NMR bulk magnet is investigated. [2] reveals that an axial slit in this hollow cylinder can adversely affect the field homogeneity.

[0009] Bulk magnets require an outer magnet, which is conventionally wound and charged with electricity. Only after the outer magnet is charged is the bulk magnet cooled below its critical temperature (a process known as "field cooling"). The outer magnet is then discharged, inducing a current in the bulk magnet. If a shim is integrated inside the bulk magnet, it is cooled along with the magnet. During the charging process, the shim experiences only a small change in the magnetic field, as the bulk magnet gradually takes over the field of the main magnet.

[0010] HTS conductors in wound main magnets are preferably composed of a substrate and a superconducting layer deposited on it, in particular REBCO (Rare Earth Barium Copper Oxide). They are usually available in ribbon form (strip conductors). In main magnets wound with HTS conductors, the HTS conductor generates a rather inhomogeneous field due to its high current density and its small distance from the magnet axis.

[0011] Furthermore, the magnetic field often remains unstable after charging when using ribbon conductors. The current flows in a thin (a few µm) but wide (several mm) layer. It is not homogeneously distributed, but rather flows predominantly along the edges of the wide superconducting layer after the magnet is charged, thereby shielding part of the magnetic field in the center of the conductor. If one waits months to years, the current distribution across the conductor's cross-section becomes increasingly homogeneous, which can be accompanied by a change in the magnetic field homogeneity over time. The wider the conductors, the more pronounced this effect. However, manufacturing narrow ribbon conductors is difficult. Moreover, narrower ribbon conductors carry a lower current, which would negatively impact the magnet design.

[0012] In [8], the drift, i.e., the change in the field over time after charging, was reduced by appropriate overshoots and waiting periods. An overshoot is a procedure in which the magnet is first charged with a higher current before being discharged to the nominal current. In [8], the drift of the magnet after an overshoot of 10% and 20% is investigated. These higher currents intentionally introduce a quench risk. This approach is not practical for most applications because the current reserve is smaller. Furthermore, while the method in [8] can reduce the drift after charging, it cannot optimize the homogeneity immediately after charging.

[0013] To improve the field homogeneity of NMR main magnets wound from HTS conductors, it is known to first insert ferromagnetic material radially inside the main magnet and / or electric cryoshims to improve the field homogeneity until it can finally be shimbed with electric room-temperature shims in a second step. The magnetic field of the cryoshims, which are located at a greater distance from the magnet axis than the innermost coils of the main magnet, is largely shielded by the HTS conductor of the main magnet because shielding currents flow in it that oppose the field of the cryoshims. Therefore, before electric room-temperature shims can be used, the homogeneity must be improved with ferromagnetic material that is located closer to the magnet axis than the HTS conductor of the main magnet.Due to the required field stability, at least a large portion of this ferromagnetic material is used in the low-temperature range. However, the close proximity of the cold ferromagnetic material and the high-temperature shielding conductor of the main magnet causes shielding currents to flow in the HTS conductor, distorting the field profile of the ferromagnetic material in unpredictable ways. Therefore, several shim iterations with cold ferromagnetic material are usually necessary to achieve the desired field homogeneity. These iterations are complex and costly.

[0014] In high-resolution magnetic resonance spectroscopy, room-temperature shims (see, e.g., [9]) are also used because changes in temperature or pressure in the laboratory negatively affect homogeneity. Shimming is required every time the sample is changed. These room-temperature shims can be used to a certain extent to compensate for the time-varying homogeneity of band conductors. However, if this homogeneity is too great, the current in certain room-temperature shims will eventually reach its limits.

[0015] In the context of accelerator magnets, [3] proposes making the field of a dipole coil more homogeneous using HTS stacks made of HTS tape conductors. In the experimental setup described in [3], a modest field swing of 0.1 T is applied at the location of the HTS stacks, and an improvement in magnetic field homogeneity of up to a factor of 4 is achieved. However, for an NMR application, additional measures would be necessary, such as iron shimming or room-temperature shimming.

[0016] Active shim devices are known from [4], [5], [6], which are arranged radially inside a main magnet whose field is to be influenced by the respective shim device. The shim devices comprise closed HTS conductor tracks. However, the shim currents would have to be regularly adjusted to correct the time-varying homogeneity, which is very complex. Object of the invention

[0017] The purpose of the invention is to improve the homogeneity of a magnet arrangement for a magnetic resonance apparatus while simultaneously achieving a simple design and easy handling. Description of the invention

[0018] This problem is solved according to the invention by a magnet arrangement according to claim 1, a magnetic resonance arrangement according to claim 11 and a method according to claim 13.

[0019] According to the invention, the correction device comprises a foil arrangement with at least one foil coated with superconducting material, wherein the superconducting material of the foil arrangement covers at least 90% of a cylindrical surface around the measuring volume. The cylindrical surface has a cylinder axis that is parallel to the main magnetic field axis. At least one of the foils of the foil arrangement is at least 40 mm long in each of two mutually perpendicular directions. The correction device thus has a planar HTS layer within which current can be induced by time-varying inhomogeneities of the main magnet, wherein the length and width of the foil have similar dimensions (in contrast to HTS tape conductors, where the width is much smaller than the length).

[0020] For the efficiency of the film, it is advantageous if the current density of the HTS material remains well below the critical current density.

[0021] According to the invention, the number of turns around the longitudinal axis of the main magnet in each electrically closed current path that runs exclusively through superconducting material of the foil assembly is zero. This means that there is no superconducting short-circuited current path in any of the foils that circles the longitudinal axis of the main magnet one or more times. In other words, the closed current path circles the longitudinal axis of the main magnet clockwise as many times as it does counterclockwise. This prevents the induction of a circumferential current (a circular current flowing around the entire circumference of the cylinder formed by the cylindrical surface of the foil coated with superconducting material). Such a circumferential current would, for example, attempt to compensate for changes in the main magnet's field during charging, until the foil itself would quench. The above condition thus reduces the risk of quenching.

[0022] A further advantage of the magnet arrangement according to the invention is that the correction device maintains the field homogeneity stably over time after the main magnet has been charged. If the homogeneity of the main magnet changes over time, currents are induced in the HTS foil of the correction device that counteract this change.

[0023] In the device according to the invention, fields that run perpendicular to the cylindrical surface of the correction device are compensated by circular currents within the foil coated with superconducting material. The correction device according to the invention thus prevents / minimizes radial fields. When no radial fields are possible, the magnetic field automatically becomes homogeneous.

[0024] According to the invention, the foil assembly of the correction device is coated with superconducting material. This results in the generated currents not decaying over time and the field remaining stable over time. Partial coverage of the cylindrical surface leads to a reduced compensation capacity. The foil(s) of the foil assembly coated with superconducting material comprise a flexible substrate and a superconducting layer deposited thereon. Preferably, this is a REBCO layer.

[0025] The main magnet comprises at least one main field coil, which carries current during operation and is preferably designed as a solenoid coil. This at least one main field coil is configured to generate at least 10 Tesla in a measuring volume arranged around a point z=0 (center of the measuring volume) located on the longitudinal axis. Preferably, the main magnet comprises several main field coils wound with HTS conductors, in particular HTS tape conductors.

[0026] The correction device is arranged radially inside the main field coil. This ensures that even the inhomogeneities of the radially innermost sub-coils of the main field coil are compensated.

[0027] The main field coil and the correction device are arranged coaxially to each other.

[0028] Preferably, the superconducting material of the foil arrangement covers at least 98%, and in particular 100%, of the cylindrical surface surrounding the measuring volume in the axial region. Complete coverage of the cylindrical surface by the foil arrangement ensures maximum compensation of the radial fields by generating closed circular currents within the superconducting foil, around an axis other than the main magnet's longitudinal axis. If the coverage is less than 100%, only a lesser degree of compensation can be achieved.

[0029] In a preferred embodiment of the magnet arrangement according to the invention, one of the foils of the foil arrangement coated with superconducting material is wound around the main magnet longitudinal axis by more than 360°, preferably by n*360°+ 180°, where n is an integer.

[0030] Since the current must flow back at the ends of the film, causing local field distortions, it is advantageous for the ends to be as far apart as possible, ideally offset by 180°. A film of the film assembly coated with superconducting material is therefore preferably wound more than one full turn around the longitudinal axis, forming a cylindrical surface that extends around the measurement volume, the axial length of which is predetermined. The superconducting material-coated film thus partially covers the cylindrical surface multiple times in the axial region around the measurement volume. This multiple coverage allows the current density required for field homogenization to be distributed across several layers within the film.

[0031] In a further preferred embodiment, the film arrangement comprises several films, the films preferably overlapping each other. The individual films are preferably arranged offset from one another around the cylindrical surface such that the entire cylindrical surface is covered by film material. The individual films are preferably arranged one above the other on a common cylindrical support in a space-saving manner. The ends of the individual films are then spaced apart from each other in a circumferential direction. Preferably, each film is wound by less than 360° around the longitudinal axis of the main magnet.

[0032] Preferably, at least one film of the film assembly has two outer edges aligned parallel to the main magnetic field axis. The simplest available films have a rectangular shape.

[0033] Preferably, the axial extent of the cylindrical surface in the direction of the main magnet's longitudinal axis is at least as large as its diameter. A measuring volume is located within the main magnet; that is, the area in which the samples to be measured are arranged during the measurement. Good homogeneity must be ensured within the measuring volume. Therefore, a sufficiently long area in the axial direction along the main magnet's longitudinal axis must be covered by the foil arrangement.

[0034] Preferably, the extent of the foil coated with superconducting material in the direction of the main magnet's longitudinal axis corresponds at least to the axial extent of the measuring volume. Thus, the foil arrangement provides a cylindrical surface extending beyond the axial extent of the measuring volume, which is largely, and in particular completely, covered with high-temperature superconducting material to enable the induction of superconducting correction currents in this conductor in the event of any inhomogeneity.

[0035] Preferably, the superconducting material with which the at least one film of the film arrangement is coated is a high-temperature superconducting material, in particular REBCO.

[0036] The film(s) of the film assembly can be sheathed with an insulating material. The film(s) can then be wound without inserting a separate intermediate layer, without a conductive connection between the individual films or (in the case of a single film) without the ends of the films being conductively connected to each other (in addition to the conductive connection provided by the film itself).

[0037] Preferably, the at least one film of the film arrangement coated with superconducting material is wound onto a cylindrical support, preferably a circular cylindrical support. This increases the mechanical stability of the correction device.

[0038] A particularly preferred embodiment provides that the at least one superconducting material-coated foil of the correction device and the conductor of the at least one main field coil are wound on a common support, wherein the at least one superconducting material-coated foil of the correction device is arranged radially between the support and the conductor of the at least one main field coil. In this way, a particularly space-saving magnet arrangement can be realized.

[0039] Preferably, the conductor of the at least one main field coil is an HTS conductor. With main magnets made of HTS conductors, both the expected inhomogeneity after charging and its change over time are particularly large, the latter because the current redistributes itself within the current-carrying HTS conductor over time. The inventive design of the correction device allows even large inhomogeneities, such as those occurring in HTS coils, to be compensated.

[0040] The magnet arrangement according to the invention is based on the fact that a large area of ​​the cylindrical surface is covered by the HTS material of the HTS foil(s), which is achieved on the one hand by using a foil continuously coated with HTS material and on the other hand by a correspondingly large wrapping or overlapping wrappings. In addition, circumferential currents around the main field longitudinal axis are avoided by ensuring that the number of turns in each electrically closed current path around the main magnet longitudinal axis is zero, so that no circular current can occur beyond the ends of the foil(s).

[0041] A similar result can be achieved if, instead of an HTS foil, a tightly wound HTS tape conductor is used, wound around the main field magnet axis on a cylindrical surface. The correction device comprises at least two winding layers, preferably helical windings. The winding layers have pitches with opposite polarities. Providing multiple winding layers ensures a high degree of coverage, as any gaps between the windings are covered by the subsequent layer(s). It is important that the ends of the HTS conductor are not connected to each other (except via the HTS conductor itself) to prevent the formation of transport currents that would cause a quench when the main field magnet is charged. The HTS tape conductor is preferably wound with maximum density, meaning the pitch and inclination of the windings are constant, so that the windings touch each other. The HTS tape conductor can be electrically insulated.This prevents currents from being induced via resistive paths, i.e., between the turns of the HTS tape conductor, during external field changes, thus ensuring field stability over longer periods. The tape conductor can also be sheathed with an insulating material.

[0042] The invention also relates to a magnetic resonance apparatus with a cryostat having a room temperature bore, and a magnet arrangement arranged in the cryostat as described above.

[0043] Preferably, it is a magnetic resonance spectroscopy setup.

[0044] In a method for operating a previously described magnetic arrangement, the correction device, together with the main field coil, can be cooled below the critical temperature of the superconducting material of the main field coil conductor before the main field coil is charged. Thus, the main field coil and the correction device are cooled together. This simultaneous cooling enables a compact design of the magnetic coil system, as the correction device can then be located close to the main field coil in the same cryostat.

[0045] However, a method according to the invention provides that the correction device is only cooled below its critical temperature after the at least one main field coil has been charged. In this case, the foil(s) of the correction device coated with superconducting material are completely currentless after the main field magnet has been charged and immediately after cooling. This condition is ideal for maintaining the field homogeneity over time after the main field magnet has been charged. If the homogeneity of the main field magnet changes over time, currents are induced in the foils coated with superconducting material, which counteract this change. Thus, the field homogeneity of the main field magnet can be maintained stably over time after charging.

[0046] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those described in more detail below can each be used individually or in any combination according to the invention. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples for illustrating the invention. Fig. Figure 1 shows a magnetic resonance apparatus according to the invention. Fig. Figure 2 shows an exemplary embodiment of a foil arrangement for a correction device of a magnetic arrangement according to the invention, using a single HTS foil. Fig. Figure 3 shows an exemplary embodiment of a foil arrangement for a correction device of a magnetic arrangement according to the invention, using two overlapping HTS foils. Fig. Figure 4 shows an exemplary embodiment of a magnet arrangement in which the HTS foil of the correction device and the conductor of the main field coil are wound on a common support. Fig. Figure 5a shows an example of a first simulated current profile on the cylindrical surface of a simulation body, which counteracts a linear field inhomogeneity in the z-direction. Fig. Figure 5b shows an example of another simulated current profile on the cylindrical surface of a simulation body, which counteracts a square field inhomogeneity in the z-direction. Fig. Figure 6 shows a typical current distribution in an HTS tape conductor and how it changes over time after the main magnet has been charged.

[0047] Fig. Figure 1 shows the structure of a magnetic resonance apparatus 1 according to the invention with a magnet arrangement 10, which is arranged in a cryostat 3.

[0048] The cryostat 3 has a room temperature bore 6 in the direction of a main magnet longitudinal axis z, within which a measuring volume 5 is arranged.

[0049] The magnetic arrangement 10 comprises a main magnet 2 (here comprising two main field coils 2a and 2b) for generating a main magnetic field within the measuring volume 5 in the direction of the main magnet's longitudinal axis z, and a correction device 4, 4' (shim device) arranged radially within the main magnet 2 and coaxially to it. The correction device 4, 4' serves to homogenize the main magnetic field generated by the main magnet 2 in the measuring volume 5.

[0050] Fig. 2 and Fig. Figure 3 shows exemplary embodiments of the correction device 4, 4'. The correction device 4, 4' comprises a foil arrangement 7, 7' with at least one foil 8, 8a', 8b' (HTS foil) coated with superconducting material, which forms a cylindrical surface around the longitudinal axis z of the main magnet. According to the invention, the HTS foil(s) 8, 8a', 8b' of the foil arrangement 7, 7' surround the measuring volume 5 in such a way that no electrically superconducting closed current path (circular current) can form around the longitudinal axis z of the main magnet.

[0051] The HTS foils 8, 8a', 8b' have outer edges 9, 9a', 9b' which, after installation in the magnet arrangement 10, are oriented essentially parallel to the main magnet longitudinal axis z, i.e. the orientation of the outer edges 9, 9a', 9b' has a principal component in the direction of the main magnet longitudinal axis z.

[0052] To prevent circular currents from forming around the main magnet longitudinal axis z, the outer edges 9, 9a', 9b' of the HTS foil(s) 8, 8a', 8b' form free ends, i.e. they are only connected by the foil 8, 8a', 8b', but not themselves.

[0053] The outer edges 9, 9a', 9b' are spaced apart from each other in the circumferential direction. In this way, the current flow around the main magnet's longitudinal axis z is interrupted and redirected in the opposite direction (no ring current beyond the ends of the foil).

[0054] In Fig. Figure 2 shows a first embodiment of the foil arrangement 7, in which a single HTS foil 8 is wound more than one turn (here: 1.5 turns) around the main magnet longitudinal axis z, such that the ends of the foil 8 overlap. The foil arrangement 7 does not form a closed cylinder, but is wound in a spiral. In the Fig. In the embodiment shown in Figure 2, the HTS foil 8 is depicted as an exemplary rectangular shape. The HTS foil 8 has a pair of circumferential edges 11, the length of which corresponds to 1.5 times the circumference of the cylindrical surface, so that the foil is wound by 540° around the main magnet's longitudinal axis z. The outer edges 9 of the HTS foil 8 (or the edges of the ends) are thus arranged offset by 180° on the cylindrical surface.

[0055] However, other embodiments are also conceivable. The length of the edge pair should at least exceed the circumference of the cylindrical surface.

[0056] In Fig. Figure 3 shows a further embodiment of the film arrangement 7'. The film arrangement 7' has two overlapping HTS films 8a' and 8b'. The film arrangement 7' does not form a closed cylinder, but comprises partially cylindrical films 8a', 8b' that overlap without contact. The individual HTS films 8a', 8b' have circumferential edges 11a, 11b, the length of which is less than the circumference of the cylindrical surface. However, the sum of the lengths of each circumferential edge 11a, 11b is at least equal to the circumference of the cylindrical surface. Each HTS film 8a', 8b' is in the Fig. In the example shown, the HTS foils 8a' and 8b' of the foil arrangement 7' are wound by less than 360° around the main magnet longitudinal axis z, with the HTS foils 8a' and 8b' of the foil arrangement 7' overlapping in such a way that they completely encompass the cylindrical cross-section of the cylindrical surface, i.e. the foils 8a', 8b' completely cover the cylindrical surface.

[0057] In this embodiment, the HTS films 8a' and 8b' each have two outer edges 9a' and 9b'. These four outer edges 9a', 9b' are formed as free ends, meaning they are connected only by the respective film 8a', 8b' itself and not to each other. Furthermore, the HTS films 8a' and 8b' are not electrically connected to each other.

[0058] To simplify the presentation, the following are included in the Fig. 2 and Fig. Figure 3 shows only rectangular HTS sheets as examples. However, for sheet arrangement 7, 7', other geometries of the HTS sheets 8, 8a', 8b' are also conceivable, in particular polygons with n vertices (where n>4 can be assumed).

[0059] According to the invention, the correction device 4, 4' is arranged radially inside the main magnet 2. Fig. Figure 4 shows a particularly compact embodiment of the magnet arrangement 10 according to the invention, in which the foil arrangement 7, 7' and the conductor of the innermost main field coil 2a (symbolically represented as a cross) are wound on a common support 14.

[0060] The Fig. 5a and Fig. Figure 5b shows the result of a simulation of current profiles 12, 12' (represented by arrows), which was carried out in the Fig. The foil arrangement 7 shown is induced due to two exemplary changes in magnetic field homogeneity after charging the main magnet 2.

[0061] The foil arrangement 7 is replicated in the simulations using a cylindrical simulation body 13. The outer edges 9 are somewhat hidden in these figures, but their influence on the flow profile of the foil layer above and below them can be seen.

[0062] In practical application, the induced currents of the current profiles 12, 12' would counteract a change in the homogeneity of the main magnetic field generated by the main magnet 2, thus keeping the field homogeneity stable over time.

[0063] For the simulations, it is assumed that at time t = 0 (after the main magnet has been loaded) the magnetic field, i.e. the magnetic flux density B, in the measurement volume 5 (see Fig. 1) is homogeneous and points in the main magnet longitudinal axis z, and that the magnetic field in the measurement volume loses homogeneity for times t > 0.

[0064] The simplest inhomogeneity that occurs is linear in the direction of the main magnet's longitudinal axis z, i.e. dBz=H1(t)*z; dBx=−H1(t)*x / 2; dBy=−H1(t)*y / 2 H1=H1(t) H1(t) can be any monotonically increasing function such that H1(t=0) = 0. The x and y components of the main magnetic field are chosen such that Maxwell's equations are satisfied. The symbol dB indicates that this is a field variation superimposed on the homogeneous initial field (B(t=0)). This field variation is represented by the Fig. The current profile shown in Figure 5a is compensated.

[0065] Fig. 5b illustrates, analogous to Fig. 5a, the simulated current profile on the simulation body 13, however under the assumption that the field variation is quadratic in z, i.e. dBx=−H2(t)*x*z; dBy=−H2(t)*y*z: dBz=H2(t)*(z2−x2 / 2−y2 / 2)

[0066] The corresponding magnetic field variation is caused by the in Fig. Current profile 12' shown in 5b is compensated.

[0067] Fig.Figure 6 shows a typical current distribution in an HTS tape conductor and how this changes over time after the main magnet is charged. This change is accompanied by a time-varying field homogeneity, which can be compensated for by the correction device. The invention is therefore particularly advantageous for main magnets with HTS tape conductors. Reference symbol list 1 Magnetic resonance imaging apparatus 2 Main magnet 2a, 2b Main field coils of the main magnet 3 Cryostat 4; 4' Correction device 5 measuring volumes 6 Room temperature bore 7; 7' Slide arrangement 8; 8a', 8b' HTS foils of the foil arrangement 9; 9a', 9b' Outer edges of the HTS foils 10 Magnet arrangement 11, 11a, 11b Circumferential edges 12; 12' induced current profiles 13 Slide arrangement of representing simulation bodies 14 carriers z Hauptmagnet-Längsachse Referenzliste [1] ITOH, Y. et al., Improved Magnetic-Field Homogeneity of NMR HTS Bulk Magnet Using a New Stacking Structure and Insertion of an HTS Film Cylinder into a Bulk Bore, Super. Soc. Jpn., Vol. 52 No. 1, p 25-32 (2017); DOI: 10.2221 / jcsj.52.25 [2] FUJISHIRO, H. et al, Numerical Simulation of Trapped Field Homogeneity in an NMR Superconducting Bulk Magnet after Inserting a High-Jc HTS Thin Cylinder. J. Cryo. Super. Soc. Jpn., Vol. 52 No. 1, p. 33-37 (2017); DOI: 10.2221 / jcsj.52.33 [3] BORTOT, L. et al., High-Temperature Superconducting Screens for Magnetic Field-Error Cancellation in Accelerator Magnets, Supercond. Sci. Technol. Vol. 34, No. 10 p. 105001 (2021); DOI: 10.1088 / 1361-6668 / ac1c13 [4] US 11 199 599 B2 ≙ DE 10 2016 225 017 A1 [5] US 11 393 615 B2 [6] US 11 391 800 B2 ≙ DE 10 2020 204 196 B3 [7] OTSUKA, A. et al, HTS Magnetic Field Damper for Short-Term Fluctuations in the Driven-Mode, IEEE Trans. On Appl. Supercond., Vol. 18, No. 2, p. 848-851 (2008); DOI: 10.1109 / TASC.2008.920562 [8] UEDA, S. et al., Numerical Evaluation of Screening Current-Induced Magnetic Field in No-Insulation REBCO Coil Systems, IEEE Trans. On Appl. Supercond., Vol. 31, No. 5, p. 1-5, (2021); DOI: 10.1109 / TASC.2021.3064540 [9] EP 2 015 092 B1

Claims

A magnetic arrangement (10) for a magnetic resonance apparatus (1), wherein the magnetic arrangement (10) comprises: a main magnet (2) for generating a magnetic field in the direction of a main magnet longitudinal axis (z) in a measuring volume (5) arranged around the main magnet longitudinal axis (z) with at least one main field coil (2a, 2b), wherein the at least one main field coil (2a, 2b) is a coil wound from a conductor made of superconducting material; a correction device (4; 4') for homogenizing the magnetic field generated by the main magnet (2) in the measuring volume (5), wherein the correction device (4; 4') is arranged radially inside the main magnet (2) around the main magnet longitudinal axis (z), characterized in that the correction device (4; 4') comprises a foil arrangement (7, 7') with at least one foil (8) coated with superconducting material.8a', 8b') comprising, wherein the superconducting material of the foil arrangement (7, 7') covers a cylindrical surface around the measuring volume (5) to at least 90%, wherein the cylindrical surface has a cylinder axis parallel to the main magnetic field axis (z), wherein at least one foil (8; 8a', 8b') of the foil arrangement (7, 7') is at least 40 mm long in each of two mutually perpendicular directions, and wherein the number of turns around the main magnet longitudinal axis (z) of each electrically closed current path passing exclusively through superconducting material of the foil arrangement (7, 7') is zero. Magnet arrangement (10) according to claim 1, characterized in that the superconducting material of the foil arrangement (7, 7') covers at least 98%, in particular 100%, of the cylindrical surface area around the measuring volume (5) in the axial region around the measuring volume (5). Magnet arrangement (10) according to one of the preceding claims, characterized in that one of the foil (8) of the foil arrangement (7) coated with superconducting material is wound around the main magnet longitudinal axis (z) by more than 360°, preferably by n*360° + 180°, where n is an integer. Magnetic arrangement (10) according to one of the preceding claims, characterized in that the foil arrangement (7') comprises several foils (8a', 8b') wherein the foils (8a', 8b') preferably overlap each other. Magnet arrangement (10) according to one of the preceding claims, characterized in that a foil (8; 8a', 8b') of the foil arrangement (7, 7') has two outer edges (9; 9a', 9b') which are aligned parallel to the main field magnetic axis (z). Magnet arrangement (10) according to one of the preceding claims, characterized in that the axial extent of the cylindrical surface in the direction of the main magnet longitudinal axis (z) is at least as large as its diameter. Magnet arrangement (10) according to one of the preceding claims, characterized in that the superconducting material with which the at least one foil (8; 8a', 8b') of the foil arrangement (7, 7') is coated is a high-temperature superconducting material, in particular REBCO. Magnet arrangement (10) according to one of the preceding claims, characterized in that the at least one foil (8; 8a', 8b') coated with superconducting material of the foil arrangement (7, 7') is wound onto a cylindrical support (14). Magnet arrangement (10) according to one of the preceding claims, characterized in that the at least one superconducting material coated foil (8; 8a', 8b') of the correction device (4; 4') and the conductor of the at least one main field coil (2a, 2b) are wound on a common support (14), wherein the at least one superconducting material coated foil (8; 8a', 8b') of the correction device (4; 4') is arranged radially between the support (14) and the conductor of the at least one main field coil (2a, 2b). Magnet arrangement (10) according to one of the preceding claims, characterized in that the conductor of the at least one main field coil (2a, 2b) is an HTS conductor. Magnetic resonance apparatus (1) with a cryostat (3) having a room temperature bore (6), a magnet arrangement (10) arranged in the cryostat (3) according to one of the preceding claims. Magnetic resonance apparatus (1) according to claim 11, characterized in that it is a magnetic resonance spectroscopy arrangement. Method for operating a magnetic arrangement (10) according to one of claims 1 to 10, characterized in that the correction device (4; 4') is cooled below its critical temperature only after the at least one main field coil (2a, 2b) has been charged.

Images