Superconducting magnet apparatus and method for magnetizing a superconductor bulk magnet by field cooling through a ferromagnetic shield

Abstract

A superconductor magnet apparatus (2) includes a superconductor bulk magnet (9), a cryostat (7) and a ferromagnetic shielding body (11). The bulk magnet has a superconductor bore (10), an axis (z) of rotational symmetry, and a maximum outer diameter OD.sub.bm in a plane perpendicular to the z axis. The superconductor bore has a minimum cross-sectional area S.sub.bo in a plane perpendicular to the z axis. The cryostat has a room temperature bore (8), the bulk magnet is arranged within the cryostat and the room temperature bore is arranged within the superconductor bore. The shielding body has a shielding bore (12), the bulk magnet is arranged within the shielding bore and the shielding body extends beyond the bulk magnet at each axial end by at least OD.sub.bm/3. For an average cross-sectional area S.sub.fb of the shielding body, S.sub.fb≥2.5*S.sub.bo, and the shielding body is arranged within the cryostat.

Claims

1. A superconductor magnet apparatus, comprising: a superconductor bulk magnet with a superconductor bore, wherein the superconductor bulk magnet has an axis (z) of rotational symmetry, and a maximum outer diameter OD.sub.bm in a plane perpendicular to the axis (z) of rotational symmetry, and the superconductor bore has a minimum cross-sectional area S.sub.bo in the plane perpendicular to the axis (z) of rotational symmetry, a cryostat with a room temperature bore, wherein the superconductor bulk magnet is arranged within the cryostat, and the room temperature bore is arranged within the superconductor bore, and a ferromagnetic shielding body with a shielding bore, wherein the superconductor bulk magnet is arranged within the shielding bore of the ferromagnetic shielding body and the ferromagnetic shielding body extends beyond the superconductor bulk magnet at each axial end with respect to the axis (z) of rotational symmetry by at least OD.sub.bm/3, wherein for an average cross-sectional area S.sub.fb of the ferromagnetic shielding body, defined as the average of the cross-sectional areas of the ferromagnetic shielding body in all planes perpendicular to the axis (z) of rotational symmetry and intersecting the ferromagnetic shielding body, S.sub.fb≥2.5*S.sub.bo applies, and wherein the ferromagnetic shielding body is arranged within the cryostat.

2. A superconductor magnet apparatus according to claim 1, wherein for a minimum inner diameter ID.sub.bm of the superconductor bulk magnet: ID.sub.bm≥20 mm.

3. A superconductor magnet apparatus according to claim 1, wherein the superconductor bulk magnet has an axial length L.sub.bm along the axis (z) of rotational symmetry, with L.sub.bm≥2.5*ID.sub.bm, wherein ID.sub.bm is a minimum inner diameter of the superconductor bulk magnet.

4. A superconductor magnet apparatus according to claim 1, wherein the ferromagnetic shielding body comprises ferromagnetic end caps which reach radially inwards over at least a part of a radial thickness of the superconductor bulk magnet at each axial end of the ferromagnetic shielding body.

5. A superconductor magnet apparatus according to claim 1, wherein the cryostat comprises a controller device for controlling a temperature of the ferromagnetic shielding body.

6. A superconductor magnet apparatus according to claim 1, wherein the ferromagnetic shielding body has a substantially cylindrical wall shape having a varying outer diameter and/or a varying inner diameter along the axis (z), and/or a varying radial thickness as a function of an azimuthal angle position, and/or boreholes.

7. A superconductor magnet apparatus according to claim 1, wherein the ferromagnetic shielding body has a cylindrical wall shape.

8. A superconductor magnet apparatus according to claim 6, wherein the varying radial thickness as a function of the azimuthal angle position comprises grooves along the axis (z).

9. A superconductor magnet apparatus according to claim 1, wherein the superconductor bulk magnet is in a charged state, and wherein a residual magnetic flux strength B.sub.bo is conserved by the superconductor bulk magnet at a magnetic center of the superconductor bulk magnet.

10. A superconductor magnet apparatus according to claim 9, wherein B.sub.bo≥3.5 Tesla.

11. A superconductor magnet apparatus according to claim 9, configured such that a magnetic stray field outside the room temperature bore and outside the cryostat has a magnitude of 15 Gauss or less.

12. A superconductor magnet apparatus according to claim 11, wherein the ferromagnetic shielding body is configured such that a magnetic stray field outside the room temperature bore and outside the cryostat has a magnitude of 15 Gauss or less.

13. A superconductor magnet apparatus according to claim 9, wherein the ferromagnetic shielding body is at a magnetization of at least 70% of a maximum magnetization of the ferromagnetic shielding body.

14. A charging arrangement, comprising an electrical charger magnet having a charger bore, and a superconductor magnet apparatus according to claim 1, wherein the superconductor magnet apparatus is arranged at least partially within the charger bore.

15. A method for charging a superconductor bulk magnet within a superconductor magnet apparatus, said superconducting magnet apparatus comprising: a superconductor bulk magnet with a superconductor bore, wherein the superconductor bulk magnet has an axis (z) of rotational symmetry, and a maximum outer diameter OD.sub.bm in a plane perpendicular to the axis (z) of rotational symmetry, and the superconductor bore has a minimum cross-sectional area S.sub.bo in the plane perpendicular to the axis (z) of rotational symmetry, a cryostat with a room temperature bore, wherein the superconductor bulk magnet is arranged within the cryostat, and the room temperature bore is arranged within the superconductor bore, and a ferromagnetic shielding body with a shielding bore, wherein the superconductor bulk magnet is arranged within the shielding bore of the ferromagnetic shielding body and the ferromagnetic shielding body extends beyond the superconductor bulk magnet at each axial end with respect to the axis (z) of rotational symmetry by at least OD.sub.bm/3, wherein for an average cross-sectional area S.sub.fb of the ferromagnetic shielding body, defined as the average of the cross-sectional areas of the ferromagnetic shielding body in all planes perpendicular to the axis (z) of rotational symmetry and intersecting the ferromagnetic shielding body, S.sub.fb≥2.5*S.sub.bo applies, and wherein the ferromagnetic shielding body is arranged within the cryostat, said method comprising: a) placing the superconductor magnet apparatus at least partially within a charger bore of an electrical charger magnet, b) applying at least one electrical current I.sub.0 to the electrical charger magnet, to generate a magnetic flux density within the charger bore such that an applied magnetic flux density B.sub.app is present at a magnetic center of the superconductor bulk magnet, wherein a temperature T.sub.bm of the superconductor bulk magnet is above a critical temperature T.sub.crit of the superconductor bulk magnet; c) lowering the temperature of the superconductor bulk magnet T.sub.bm below T.sub.crit; d) turning off the at least one electrical current I.sub.0 at the electrical charger magnet, wherein T.sub.bm<T.sub.crit, such that a residual magnetic flux density B.sub.bo is conserved at the magnetic center; and e) removing the superconductor magnet apparatus from the charger bore and keeping T.sub.bm<T.sub.crit.

16. A method according to claim 15, wherein in said step b), the at least one electric current I.sub.0 is selected such that B.sub.app≥3.5 Tesla.

17. A method according to claim 15, further comprising: after said step e), maintaining the magnetic flux density on the axis (z) of rotational symmetry at a position of up to +5 mm and −5 mm with respect to the magnetic center within 100 ppm of B.sub.bo by selecting a shape of the superconductor bulk magnet and/or a shape of the ferromagnetic shielding body, and by controlling a temperature of the ferromagnetic shielding body in the cryostat after said step e).

18. A method for performing a nuclear magnetic resonance (NMR) measurement of a sample, comprising: charging a superconductor bulk magnet according to claim 15, and after said step e), arranging the sample within the room temperature bore, exposing the sample to the residual magnetic flux density B.sub.bo conserved by the superconductor bulk magnet at the magnetic center, and performing the NMR measurement on the sample in the room temperature bore.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are shown in the drawing.

(2) FIG. 1 shows a longitudinal section along the z axis of an exemplary embodiment of an inventive charging arrangement, comprising an inventive superconducting magnet apparatus and an electrical charger magnet;

(3) FIG. 2 shows a schematic cross-section of the charger apparatus of FIG. 1;

(4) FIG. 3 shows schematically an inventive charger arrangement in a first stage of an exemplary variant of an inventive charging method, before loading the charger magnet;

(5) FIG. 4 shows the charger arrangement of FIG. 3 in a second stage, with a charger current so low that the ferromagnetic shielding body is not yet saturated;

(6) FIG. 5 shows the charger arrangement of FIG. 3 in a third stage, with the charger current high enough such that the ferromagnetic shielding body has become saturated and some magnetic flux penetrates the superconductor bulk magnet;

(7) FIG. 6 shows the charger arrangement of FIG. 3 in a fourth stage, with the charger current having achieved its highest value, when the superconductor bulk magnet is to be cooled below its critical temperature;

(8) FIG. 7 shows the charger arrangement of FIG. 3 in a fifth stage, with the charger current somewhat reduced again;

(9) FIG. 8 shows the charger arrangement of FIG. 3 in a sixth stage, with the charger current further reduced;

(10) FIG. 9 shows the charger arrangement of FIG. 3 in a seventh stage, with the charger current reduced to zero, and a the residual magnetic field trapped by the superconductor bulk magnet;

(11) FIG. 10 shows the superconductor magnet apparatus of FIG. 3, after having been removed from the electric charger magnet;

(12) FIG. 11 shows a schematic diagram of the charger magnet current, the temperature of the superconductor bulk magnet and the magnetic flux densities at locations MC and LB (compare FIG. 3) as a function of time for the inventive method illustrated in FIGS. 3-10;

(13) FIG. 12 shows an exemplary arrangement of a superconductor bulk magnet and a ferromagnetic shielding body for an inventive superconductor magnet apparatus, with the ferromagnetic shielding body showing circumferential grooves;

(14) FIG. 13 shows a further exemplary arrangement of a superconductor bulk magnet and a ferromagnetic shielding body with end caps for an inventive superconductor magnet apparatus, with the superconductor bulk magnet showing a circumferential groove.

DETAILED DESCRIPTION

(15) It should be noted that the figures are schematic in nature, and some features may be shown in an exaggerated or understated way, in order to accentuate particular features of the invention.

(16) FIG. 1 shows schematically by way of example an inventive charging arrangement 1, comprising an inventive superconductor magnet apparatus 2 and an electric charger magnet 3. Note that a cross-section of the charging arrangement 1 at plane II is shown in FIG. 2 (without the charger cryostat, for simplicity); plane II is perpendicular to the axis z, which is the axis of rotational symmetry of the superconductor bulk magnet 9 of the superconductor magnet apparatus 2, and goes through the magnetic center of the superconductor bulk magnet 9.

(17) In the example shown, the electrical charger magnet 3 comprises a basically cylinder-wall shaped charger coil 4 of here superconducting type, arranged in a charger cryostat 5. Inside the charger cryostat 5, a cryogenic temperature is present; however note that in other embodiments, also non-superconducting charger magnets may be used. The electrical charger magnet 3 generates, depending on the electrical current flowing through it, in its charger bore 6 a (charger) magnetic flux density. The electrical current at the charger magnet 3 may be controlled by an electronic control device (not shown).

(18) Within the charger bore 6 is arranged the superconductor magnet apparatus 2. The superconductor magnet apparatus 2 comprises a cryostat 7 with a room temperature bore 8, which is here open to one side only (here top side). Note that for simplicity, a lower part of the cryostat 7 is not shown in FIG. 1. Inside the cryostat 7, there is arranged a superconductor bulk magnet 9, which is comprised here of four high temperature superconductor (HTS) rings arranged coaxially with the z-axis, so that in its entirety of all four rings, the superconductor bulk magnet 9 has a basically cylinder-wall shape with a rotational symmetry about axis z. The room temperature bore 8 of the cryostat 7 reaches into a superconductor bore 10 of the superconductor bulk magnet 9. Further, within the cryostat 7, there is arranged a ferromagnetic shielding body 11 of basically cylinder-wall shape. The superconductor bulk magnet 9 is arranged within a shielding bore 12 of the ferromagnetic shielding body 11.

(19) The charger magnet 3 with its charger bore 6, the cryostat 7 with its room temperature bore 8, the ferromagnetic shielding body 11 with its shieling bore 12 and the superconductor bulk magnet 9 with its superconductor bore 10 are all arranged coaxially with the z axis.

(20) Inside the cryostat 7, at least where the superconductor bulk magnet is located, the temperature may be varied from above a critical temperature T.sub.crit at which the superconductor bulk magnet 9 becomes superconducting, to below T.sub.crit, for example by adding and removing a cryogenic fluid such as LN.sub.2 or LHe in the cryostat 7 or by controlling a cooling head (not shown for simplicity), and if necessary activating and deactivating some heating in the cryostat 7, typically an electrical heating (not shown for simplification). The cryostat 7 typically comprises a vacuum insulation (not shown for simplification) or is evacuated.

(21) In the example shown, the ferromagnetic shielding body 11 encompasses spacers 13 here made of non-magnetic material e.g. copper, which are roughly of ring shape and are placed below and on top of the superconductor bulk magnet 9, and reach radially inwards here over the entire radial thickness of the superconductor bulk magnet 9. In other embodiments, the spacers 13 may be replaced with ferromagnetic elements of the same size, then acting as ferromagnetic end caps and constituting part of the ferromagnetic shielding body 11. The spacers 13 and the ferromagnetic shielding body 11 are here axially encompassed by base structures 16, and the bottom base structure 16 is connected to a rod 17, for mechanical support and/or cooling. The base structures 16 and the rod 17 are made of non-magnetic material such as copper.

(22) The ferromagnetic shielding body 11 extends in axial direction by an extension EX.sub.sb beyond the superconductor bulk magnet 9, both at its upper end and at its lower end. The superconductor bulk magnet 9 has a (maximum, with respect to all z positions) outer diameter of OD.sub.bm, and a (minimum, with respect to all z positions) inner diameter ID.sub.bm, which are both constant along z here. In the example shown, EX.sub.sb=OD.sub.bm/2 applies, and further, EX.sub.sb=(OD.sub.bm-ID.sub.bm) applies. Note that EX.sub.sb≥(OD.sub.bm-ID.sub.bm), and in particular EX.sub.sb≥2*(OD.sub.bm-ID.sub.bm) are preferred geometries, in accordance with the invention. Moreover, for the axial length L.sub.bm of the superconductor bulk magnet 9 here further applies L.sub.bm=3.5*ID.sub.bm.

(23) The ferromagnetic shielding body 9 has a ring-shaped average cross-sectional area of S.sub.fb, which can be directly seen in FIG. 2 since the cross-sectional area of the ferromagnetic shielding body 9 is constant here over z (note that in case of the cross-sectional area varying over z, an averaging should be performed to establish S.sub.fb). The minimum cross-sectional area of the superconductor bore 10, i.e. the entire area within the inner edge of the superconductor bulk magnet 9, is S.sub.bo, which again can be seen directly in FIG. 2 since the cross-sectional area of the superconductor bore 10 of the superconductor bulk magnet 9 is also constant here over z (note that in case of the cross-sectional area varying over z, the minimum cross sectional area should be chosen to establish S.sub.bo). In the example shown, about S.sub.fb=10*S.sub.bo applies, i.e. the average cross-section area S.sub.fb of the ferromagnetic shielding body 11 is much larger than the minimum cross-sectional area S.sub.bo of the superconductor bore 10.

(24) Inside the room temperature bore 8, at the magnetic center MC of the superconductor bore 10, a sample volume 14 is located, at which a sample 15 to be investigated may be placed (typically after charging).

(25) The charging arrangement 1 is used to charge (load) the superconductor bulk magnet 9 inductively with a superconducting current by a “field cooling” type procedure, in order to establish a residual magnetic flux density B.sub.bo at the magnetic center MC of the superconductor bulk magnet 9 or of the superconductor magnet apparatus 2, respectively. For this purpose, a charger magnetic field is applied to the superconductor bulk magnet 9 through the ferromagnetic shielding body 11, which are both located inside the cryostat 7, resulting in an applied magnetic flux density B.sub.app inside the superconductor bore 12 (see FIGS. 3-10 and FIG. 11 below), at the magnetic center MC. The superconductor magnet apparatus 2 may then be removed from the electrical charger magnet 3 and be transported to a site of application, where samples 15 in the sample volume 14 are investigated.

(26) FIGS. 3 through 10 illustrate by way of example an inventive charging method, for charging a superconducting magnet apparatus 2 with an electrical charger magnet 9, as shown for example in FIG. 1 and FIG. 2. In FIGS. 3 through 10, a charger cryostat and a cryostat containing the ferromagnetic shielding body 11 and the superconductor bulk magnet 9 are not shown for simplification. FIG. 11 illustrates the electric current I applied at the electrical charger magnet 3 (in arbitrary units, top part), as well as the magnetic flux density B (in arbitrary units) at the magnetic center MC (bold curve bottom part) and LB (outside the ferromagnetic shielding body 11, but within the charger bore, dashed curve bottom part), as well as the superconductor bulk temperature T.sub.bm (in arbitrary units, dotted line middle part) as a function of time during the course of method.

(27) In step a), the superconductor magnet apparatus 2 is placed within the charger bore of the charger magnet 3, see FIG. 3. Before the charging of the charger magnet 3 begins, no current I is applied to the charger magnet 3, and a zero magnetic flux density B is present at both the magnetic center MC and the location LB, see FIG. 11. The superconductor bulk magnet 9 is at a temperature T.sub.bm above T.sub.crit, and accordingly is not superconducting.

(28) In step b), the electrical current I is increased (here linearly); this increases the magnetic flux density inside the charger bore. Initially, this does not result in a magnetic flux density at the magnetic center, since the ferromagnetic shielding body 11 shields its inside including the superconductor bulk magnet 9 and the magnetic center MC, and the magnetic flux density is also largely reduced at location LB, compare the field lines 20 in FIG. 4. When the electric current I increases further, the ferromagnetic shielding body 11 becomes saturated, and some magnetic flux density enters the superconductor bulk magnet 9, compare FIG. 5; Note that the superconductor bulk magnet 9 has no significant shielding effect in this situation, since it is not (yet) superconducting. Eventually, the electric current I at the charger magnet reaches its maximum I.sub.0, and accordingly B.sub.app is reached at the magnetic center MC, compare FIG. 6. The relative shielding effect of the ferromagnetic shielding body 11 is rather weak in this situation, such that the penetrating magnetic flux density B.sub.app is close to the magnetic flux density inside the charger magnet 3 that would be present without the ferromagnetic shielding body 11. If a specific value of B.sub.app is desired within the superconductor bore, a slightly stronger electric current at the electrical charger magnet is required in the inventive field cooling method, as compared to a conventional field cooling without a ferromagnetic shielding body within the electrical charger magnet. The magnetic flux density at location LB basically corresponds to the magnetic flux density at the magnetic center MC in this stage.

(29) In step c), the temperature T.sub.bo of the superconductor bulk magnet 9 is lowered below T.sub.crit, so the superconductor bulk magnet 9 becomes superconducting. The electric current I at the charger magnet 3 remains unchanged at I.sub.0, see FIG. 11. The magnetic flux density distribution basically remains as shown in FIG. 6.

(30) In step d), the electric current I at the charger magnet 3 is lowered; accordingly the magnetic flux density generated by the charger magnet 3 decreases, which can be spotted by the magnetic flux density B at location LB, compare FIG. 11. The superconductor bulk magnet 9, now superconducting, keeps the magnetic flux it encloses in its superconductor bore constant by corresponding superconducting currents induced in the superconductor bulk magnet 9. Outside the superconductor bulk magnet 9, the magnetic field decreases, compare FIG. 7 and at a further decrease FIG. 8. In the final state, when the electric current I reaches zero, the magnetic field lines 20 generated by the superconductor bulk magnet 9 loop through the ferromagnetic shielding body 11, without significant spreading into the environment, compare FIG. 9. The magnetic flux density at the magnetic center MC is at B.sub.bo, which basically corresponds to B.sub.app present before. The magnetic flux density outside the ferromagnetic shielding body 11, for example at location LB, has a low absolute value, and has here an opposite sign at location LB, as compared to the magnetic flux density before, at step c) for example, compare FIG. 11.

(31) As long as the temperature T.sub.bm of the superconductor bulk magnet 9 is kept far enough below T.sub.crit, the magnetic flux density B.sub.bo trapped inside the superconducting bulk magnet 9 or its superconductor bore, respectively, will remain constant.

(32) In step e), the superconductor magnet apparatus 2, including the superconductor bulk magnet 9 and the ferromagnetic shielding body 11 as well as the cryostat (not shown) containing them is removed from the electrical charger magnet 3; FIG. 10 shows the superconductor magnet apparatus 2 already removed. The superconductor magnet apparatus 2 is then brought to a site of application, such as a laboratory, where it may be used for example in NMR experiments. The magnetic flux density at the magnetic center MC within the superconductor bore, and outside the ferromagnetic shielding body 11 at location LB (assuming that LB is chosen relative to the superconductor magnet apparatus 2), do not change upon this removal or the transport, compare FIG. 11. Accordingly, the superconductor magnet apparatus 2 is well shielded to protect the environment from stray fields, without the need to place a ferromagnetic shielding body after the charging requiring control of strong magnetic forces, and the superconductor magnet apparatus provides a strong magnetic field inside the superconductor bore. Further, since the ferromagnetic shielding body in accordance with the invention may be fixed inside the cryostat before the charging with high accuracy, the apparatus 2 allows for a good homogeneity of the magnetic field in the sample volume or inside the superconductor bore, respectively. Also, since the ferromagnetic shielding body is inside the cryostat and cooled together with the superconductor bulk magnet, the apparatus allows for a good field stability in the sample volume or inside the superconductor bore, respectively. Moreover, with the ferromagnetic shielding body inside the cryostat, a particularly compact design of the superconductor magnet apparatus 2 is possible.

(33) FIG. 12 shows by way of example an arrangement of a ferromagnetic shielding body 11 and a superconductor bulk magnet 9 placed in the shielding bore 12, for an inventive superconductor magnet apparatus 2. In the example shown, the ferromagnetic shielding body 11 is basically cylinder-wall shaped, but exhibits a profile on its outside with here two circumferential grooves 30, 31. In other words, the radial thickness varies here as a function of the axial position (z-position) at the ferromagnetic shielding body 11. By such a structuring, the homogeneity of the magnetic flux density in the superconductor bore 10 or, more specifically, at the sample volume 14 where the sample 15 is placed, may be influenced and in particular improved. Further structuring of the ferromagnetic shielding body 11 may include grooves in axial direction or boreholes (not shown).

(34) It should be noted that, alternatively or in addition, the superconductor bulk magnet 9 may comprise a structuring, too, in particular a varying radial thickness along the axial direction (z direction), compare FIG. 13. Here, the superconductor bulk magnet 9 comprises a circumferential groove 32 at its inside. The ferromagnetic shielding body 11 here comprises a main body 33 which is cylinder-wall shaped, and two ferromagnetic end caps 34, 35 (here shown lifted up, for better understanding), which are in use preferably axially adjacent or at least placed in close axial vicinity to the superconductor bulk magnet 9 and reach radially inward over the superconductor bulk magnet 9.

LIST OF REFERENCE SIGNS

(35) 1 charging arrangement 2 superconductor magnet apparatus 3 electrical charger magnet 4 charger coil 5 charger cryostat 6 charger bore 7 cryostat (for ferromagnetic shielding body and superconductor bulk magnet) 8 room temperature bore 9 superconductor bulk magnet 10 superconductor bore 11 ferromagnetic shielding body 12 shielding bore 13 spacer 14 sample volume 15 sample 16 base structure 17 rod 20 magnetic field line 30 groove 31 groove 32 groove 33 main body 34 end cap 15 end cap B magnetic flux density B.sub.app applied magnetic flux density B.sub.bo residual magnetic flux density EX.sub.sb axial extension of the ferromagnetic shielding body over the superconductor bulk magnet I electric current (applied at the charger magnet) I.sub.0 maximum electrical current (applied at the the charger magnet) ID.sub.bm (minimum) inner diameter of the superconductor bulk magnet LB location for magnetic flux density measurement (inside charger magnet, but outside of ferromagnetic shielding body) L.sub.bm axial length of the superconductor bulk magnet MC magnetic center (of the superconductor bulk magnet/superconductor magnet apparatus) OD.sub.bm (maximum) outer diameter of the superconductor bulk magnet S.sub.fb average cross-sectional area of the ferromagnetic shielding body S.sub.bo minimum cross-sectional area of the superconductor bore T.sub.bm temperature of the superconductor bulk magnet T.sub.crit critical temperature of the superconductor bulk magnet z axis of rotational symmetry of the superconductor bulk magnet/axial direction