Method for Inductive Heating of a Workpiece

Abstract

A method for inductive heating of an electrically conducting workpiece, by rotating the workpiece in a magnetic field of a direct-current carrying coil arrangement comprising superconductive windings about a rotation axis that forms an angle with the principal axis of the magnetic field, allows temperatures that differ from each other along the workpiece to be obtained when the flux density of the magnetic field permeating the workpiece is set differently along the rotation axis.

Description

BACKGROUND

A method for inductively heating of an electrically conductive workpiece by rotating the workpiece in a magnetic field is known from “Temperature distribution in aluminum billets heated by rotation in a static magnetic field produced by superconducting magnets” (Preprint COMPEL; Vol. 24, No. 1, pages 281 to 290, (2004)). However, the document does not reveal how the method may be put into practice technically.

From WO 2004/066681 A1 it is known to rotate a workpiece in a magnetic field of a direct-current carrying coil arrangement. This makes possible a uniform inductive heating of the workpiece in a static magnetic field. The latter is generated without losses by means of a high-temperature superconducting coil arrangement. The workpiece may be, in particular, a block or billet, for example, of aluminum, copper, or corresponding alloys. Usual diameters are between 50 mm and 400 mm, and usual lengths between 20 mm and 1,000 mm The rotation axis of the workpiece forms an angle of 90° with the principal axis of the magnetic field. According to the known Law of Induction, the increase of temperature per unit of time becomes greater as the flux density of the magnetic field becomes higher, and as the rotation number of the workpiece becomes higher.

From “Strangpressen”, Aluminium-Verlag Düsseldorf, 2001, 553 to 555, it is known to heat a block inductively so that it has, along an axial direction, a temperature profile which in a subsequent transformation zone leads to an optimum temperature that is the same along the length of the block. With light metals, a block starting-end or block head should therefore have a temperature that is, for example, up to 100° C. higher than that of the block end. With copper alloys, an inverse temperature distribution is frequently desired. For this, the block that is moved linearly through an elongate coil arrangement generating an alternating field is additionally heated following uniform heating to a base temperature by switching on partial coils in desired regions. This method is costly, for reasons of the ohmic losses in the coil arrangement, and the outlay of control technology, amongst others.

From DE 1215276A, a method is known for inductive heating of an electrical workpiece inside an alternating-current fed induction coil which in turn is surrounded by at least one electrical short-circuit ring. By varying the diameter of the short-circuit ring, its reactive or effective power consumption can be regulated in order to achieve a steady, spatially limited variation of the specific heating power of the induction coil.

SUMMARY

A method is described herein for inductive heating of an electrically conducting workpiece by rotating the workpiece in a magnetic field of a direct-current carrying coil arrangement comprising superconductive windings, about a rotation axis that forms an angle with the principal axis of the magnetic field. The flux density of the magnetic field permeating the workpiece is set differently along the rotation axis. Furthermore, the described method allows temperatures that differ from each other along the workpiece to be obtained when the flux density of the magnetic field permeating the workpiece is set differently along the rotation axis.

The above and still further features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof, wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the method and schematically simplified arrangements for its performance are illustrated in the following for example with the aid of the drawings, where:

FIG. 1 is a plan view and a side view of a superconducting race-track coil with a magnetic short-circuit;

FIG. 2 is the same coil, but with an additional coil displaced parallel to the axis;

FIG. 3 is the same coil, but with an additional coil fed with alternating current;

FIG. 4 is the same coil, but with an added yoke enclosing a coil limb;

FIG. 5 is a cross-section through the superconducting coil with a surrounding yoke;

FIG. 6 a is another embodiment of a superconducting coil arrangement with a yoke in an end-face view and a partial sectional side view;

FIG. 6 b is the same coil arrangement as in FIG. 6a, but with a tilted rotation axis of the workpiece;

FIG. 7 a is a superconducting coil on a limb of a C-shaped yoke in an end-face view and a partial sectional view rotated through 90°;

FIG. 7 b is an end-face view of a C-shaped yoke with an arrangement of two superconducting coils;

FIG. 8 a is a race-track coil similar to FIG. 1, but with a tilted rotation axis of the workpiece;

FIG. 8 b is a sectional view of an arrangement of two superconducting coils having a common axis;

FIG. 9 is a race-track coil as in FIG. 1, but with a workpiece that is displaced linearly along its rotation axis within the inner space of the coil;

FIG. 10 a is a workpiece with points of temperature measurement;

FIG. 10 b is the same workpiece with a rotation axis tilted by 6° with respect to an axis orthogonal to the axis of a magnetic field; and

FIG. 11 is a simplified, but perspective illustration of a cylindrical workpiece, the longitudinal and rotational axis of which is tilted with respect to the plane of a surrounding race-track coil.

DETAILED DESCRIPTION

Described herein is a method to inductively heat a workpiece such that the temperature of a typical cylindrical workpiece along its central axis coinciding with the rotation axis of the workpiece follows a desired course, i.e., has a temperature gradient that differs from zero, but is not necessarily constant.

The flux density of the magnetic field permeating the workpiece is set differently along the rotation axis. This may be performed either by specifically affecting the local flux density, and/or by suitably positioning the rotating workpiece relative to the inhomogeneous magnetic field.

In the following, for the sake of simplicity the regions of lower flux density are designated as being a (relatively) weaker magnetic field, and conversely, regions of higher flux density as being a (relatively) stronger magnetic field.

The coil arrangement generating the magnetic field is preferably high-temperature superconducting. In particular, it may consist of one or a plurality of dipole magnetic-field generating coils which in the latter case are disposed adjacently to be mechanically parallel, and which enclose an approximately oval space, and which are so-called race-track coils. The workpiece rotates in this space about a rotation axis coinciding approximately with the long axis of the oval.

A flux density that is specifically different along the rotation axis can be generated, for example, via a magnetic short circuit introduced into a partial region of the magnetic field. The magnetic short circuit may consist of a ferromagnetic body. The magnetic field is weaker in the vicinity of this body. The region of the workpiece lying within this magnetic field is accordingly heated less intensely.

The flux density that is different along the rotation axis may also be generated via an additional coil.

This additional coil may be positioned, for example, to be displaced parallel to the axis of the superconducting coil arrangement. The additional coil may be positioned, for example, to be laterally adjacent to the coil arrangement on a level with one or the other end of the oval space, in order to amplify the magnetic field which is already stronger in this region. The part of the rotating workpiece located within this region is then heated more intensely.

Optionally, the additional coil can be positioned on the same axis as the rotation axis to surround the workpiece concentrically in a partial region of the magnetic field. The workpiece is then permeated by both the magnetic field of the coil arrangement, and also the magnetic field, orthogonal to this, of the additional coil that in this case is fed with alternating current.

A flux density that differs in dependence upon locality may be generated also via a ferromagnetic yoke surrounding the coil arrangement on the outside. It is possible to affect the strength of the magnetic field along the rotation axis by appropriately configuring the geometry of the yoke along the straight long coil sides. At the same time, the yoke has the advantage of screening-off the magnetic field of the coil arrangement to the outside, and of increasing the flux density within the space enclosed by the coil arrangement and therewith through the coil arrangement at the same number of ampere turns.

To further increase the flux density, the yoke can be optionally configured in a shape similar to a torus that is open on the inside.

Alternatively, the yoke also may have a closed or an open, circular or C-shaped cross-section with at least one pole-piece on each of both sides of the rotation axis. In the case of an open cross-section (at right angles to the rotation axis), or more exactly, of a hollow cylinder that is open along a surface line, the rotation axis of the workpiece is located between the faces of the hollow cylinder that define the slot-shaped opening and form the pole-pieces, or are configured to be pole-pieces.

Basically, the coil arrangement may be seated at any desired place on the yoke. The magnetic field, however, may be generated also via one superconducting coil on each one of the pole-pieces.

The flux density that differs along the rotation axis may be optionally generated via changing a spacing of the pole-faces of the pole-pieces of the yoke along the rotation axis.

A flux density of the magnetic field permeating the workpiece, which differs along the rotation axis, can be set in particular also by changing the angle between the rotation axis of the workpiece and the principal axis of the magnetic field. This angle then deviates from 90°. The point about which the rotation axis is tilted from the principal axis of the magnetic field can be chosen in dependence upon the temperature distribution required along the length of the workpiece. If the rotation axis is tilted, for example, around a point located in the region of an end-face of a cylindrical workpiece, then, this region of the workpiece remains in the region of the strong magnetic field, while the opposite end-face region is located in a weaker magnetic field and is therefore heated less intensely. The angle of tilt may be between about 2° and about 20°, in accordance with an angle between about 88° and 70° formed by the rotation axis and the principal axis of the magnetic field.

In the following paragraphs, exemplary embodiments of the method are described in connection with the figures.

FIG. 1 shows a schematically simplified superconducting race-track coil S. It comprises a number of windings (not shown) and carries a direct current, so that it generates a dipole magnetic field. This permeates a cylindrical workpiece W of an electrically conducting material. The workpiece may be, for example, an aluminum bar or billet. The workpiece W is driven to be rotated about its longitudinal axis D. The drive is not illustrated. As is known, the workpiece W becomes inductively heated in this manner. In order to produce a temperature gradient along the workpiece, a magnetic short-circuit K is located in the upper part of the oval space, here in the form of a short cylinder of a ferromagnetic material. The magnetic field B permeating the workpiece W is weakened in the vicinity of the short-circuit K. The upper-end region of the workpiece W is therefore subjected to less heating than those regions of the workpiece which are permeated by the unweakened magnetic field of the coil S.

FIG. 2 shows an arrangement which in principle is the same as that of FIG. 1, however, an additional coil Z is disposed to be displaced axially parallel to the coil S, the windings of which also carry a direct current. With same direction of windings of the additional coil Z and the coil S, the magnetic fields are superimposed to increase the total magnetic field permeating the upper part of the workpiece W. This part of the workpiece W is therefore heated more intensely than the remaining part. If another region of the workpiece W is to be heated more intensely than the remaining regions, then the additional coil Z is shifted in the direction of the double arrow to the desired place. The desired temperature difference or excess increase of temperature may be set by changing the exciting current of the additional coil Z.

According to FIG. 3, the same effect is achieved with an alternating-current fed additional coil Z1 which is disposed in the space enclosed by the coil S to surround the workpiece W concentrically, and also to be displaceable along the double arrow.

Instead of providing, as in FIG. 1, merely a magnetic short-circuit in the space enclosed by the coil S, according to FIG. 4 a closed yoke J can be disposed around the upper short limb of the coil S. The yoke J improves the magnetic short-circuit and simultaneously screens off the magnetic field of the coil S at this place towards the outside. Accordingly, in this embodiment too the upper region of the workpiece W is heated less than the remaining region.

A modification of this embodiment is illustrated by FIG. 5. A yoke J1 encloses the entire coil arrangement and thereby substantially screens-off the magnetic field totally towards the outside. At the same time, the excitation power needed to generate the magnetic filed with the flux direction B, or in other words, the excitation current through the coil S, is reduced. Differences of heating of the workpiece W, i.e., a temperature gradient along its axis, may be achieved with this arrangement also via the measures illustrated with the aid of the FIGS. 1 to 3.

The arrangement illustrated in FIG. 6a starts out from a closed yoke J2 with pole-pieces P1 and P2 which each bear a superconducting coil S1 and S2, respectively, and which are electrically connected in series and carry a direct current. The different strengths of the magnetic field are denoted by the line widths of the arrows symbolizing the field lines. As is evident from the side view, displacing the workpiece W to a greater or lesser extent along its rotation axis D makes is possible to achieve that one end of the workpiece W rotates in a stray field which becomes progressively weaker outside the yoke J2, and accordingly becomes heated less than the remaining region of the workpiece W.

FIG. 6 b shows an arrangement similar to that of FIG. 6a, however, in this case the workpiece W is variably heated not by displacing it along the rotation axis D, but by tilting this rotation axis with respect to the long axis of the coil arrangement S1, S2, J. This is indicated by the semi-perspective illustration of the cylindrical workpiece W in the end-face view of FIG. 6b.

FIG. 7 a shows an arrangement in which a superconducting coil S3 encloses the long limb of a C-shaped yoke J3, between the pole-pieces P3 and P4 of which the workpiece rotates. The sectional view and the rotated plan view clearly show that the pole pieces P3 and P4 define a space around the workpiece W, which narrows from the right-hand side to the left-hand side, so that the workpiece W becomes heated progressively more intensely along its extent from its right-hand side to its left-hand side, in accordance with the decrease of the air-gap. This arrangement has the advantage of an approximately constant temperature gradient along the length of the workpiece.

The arrangement of FIG. 7b operates according to the same principle with the only difference that here, instead of one coil, two superconducting coils S4 and S5 are employed, each of which surrounds a pole-piece P5 and P6, respectively.

As noted above, in an embodiment of the invention, the rotating workpiece W is positioned relative to the magnetic field such that the flux density of the magnetic field permeating the workpiece varies along the rotational axis of the workpiece. This is achieved, for example, by changing the spacing between the pole-pieces (e.g., the pole faces) of the coil arrangement (e.g., the yoke) and the workpiece, and in particular, orienting the workpiece within the coil arrangement such that the angle formed between the workpiece rotational axis and the principal axis of the magnetic field B is offset from 90°.

In an embodiment, the coil arrangement includes a yoke have a first pole piece spaced in opposed relation from a second pole piece. By way of example, the yoke is a C-shaped ferromagnetic yoke defined by a first pole piece P5, and intermediate or long limb, and a second pole piece P6, as illustrated in FIG. 7b. As shown, the first pole face F1 of the first pole piece P5 is generally aligned with the face F2 of the second pole piece P6, and the workpiece W is disposed between the pole pieces P5, P6, with the first pole face F1 being disposed along one lateral side of the workpiece rotational axis D and the second pole face F2 being disposed along a second, opposite lateral side of the workpiece rotational axis. One or both pole pieces P5, P6 may include a superconducting coil S4, S5 disposed thereon. Alternatively, the yoke intermediate arm may include a superconducting coil S3 centrally disposed thereon.

The rotational axis D of the workpiece W may be offset from 90° with respect to the principal axis of the magnetic field B by, e.g., changing the spacing between a pole face F1, F2 and the workpiece rotational axis D. In an embodiment, this is achieved by sloping at least one pole face, as shown in FIG. 7a, such that the spacing between the pole face and the workpiece narrows. In another embodiment, the pole faces F1, F2 are substantially planar; however, a portion of the workpiece W is tilted toward one of the pole faces. In either embodiment, a first region of the workpiece W (e.g., one longitudinal end portion) may be oriented closer to the first pole piece P5 (and, in particular, the first pole face F1) than a second region of the workpiece (e.g., the other longitudinal end portion). In addition or alternative to, the second region of the workpiece W may be oriented closer to the second pole piece P6 (and, in particular, the second pole face F2) than the first region of the workpiece to generate the temperature gradient. As such, the first region of the workpiece W may be heated more intensely than the second region of the workpiece, or vice versa, generating a temperature gradient along the workpiece.

With the above described embodiments, the spacing between the pole faces F1, F2 and the workpiece W changes along the rotational axis of the workpiece (e.g., along the length of the workpiece) such that the rotational axis of the workpiece is oriented nonparallel to one or both of the pole piece faces. As a result, the flux density of the magnetic field permeating the workpiece varies along the rotational axis such that the flux density of the magnetic field permeating the first portion of the workpiece differs from the flux density of the magnetic field permeating the second portion of the workpiece. It is this variation of the flux density permeating the workpiece along the rotational axis that generates a temperature gradient along the workpiece rotational axis.

The arrangement illustrated in FIG. 8a operates with a race-track coil S in analogy with FIG. 1, however, differences of heating of the workpiece W along its rotation axis D are achieved by this rotation axis being tilted with respect to the center plane of the coil S through an angle α about a point lying on the center axis M. Consequently, the flux density B decreases from the lower to the upper end of the workpiece W, so that the upper end of the workpiece becomes heated less intensely than its remaining region.

The arrangement of FIG. 8b operates according to the same principle, however, with two superconducting coils S6 and S7 disposed on a common axis adjacently or in series, whereby a higher flux density B is achieved.

FIG. 9 also shows a race-track coil S enclosing the workpiece W. However, the workpiece is displaced upwards along the rotation axis D from its symmetrical position within the space enclosed by the coil S. As a consequence of this, the upper part of the workpiece W is located in a region of higher flux density B than the remaining region of the workpiece, and is therefore more intensely heated. In addition, and in analogy with the arrangement in FIG. 8a, the workpiece can be tilted, if desired, out of the center plane of the coil S about a point that is then expediently located in the region of the upper end-face (not illustrated).

The following table illustrates on a numerical example the attainable temperatures and temperature differences. The workpiece consists of a billet having a length of 800 mm and a diameter of 250 mm In the table, the term “Equilibrium” denotes a waiting time following the end of the inductive heating and prior to a determination of the temperatures at the points as drawn in FIG. 10a. The angle of tilt α in the first column is defined as in FIGS. 8a and 10b. The linear displacement in the second column refers to the displacement of the workpiece along the rotation axis D as explained with the aid of FIG. 9. Particularly the entries in the last five lines show that it can be of advantage to apply both of the basically separately applicable measures of a displacement of the workpiece and a tilting of its rotation axis also in combination with each other.

	Coil
	Linear
Billet	displacement	Inside	Rotation		Temperature
α	from center	length	number	Equilibrium	a	b	c	d
[°]	[mm]	[mm]	[Hz]	[s]	[° C.]	[° C.]	[° C.]	[° C.]
0	0	1500	4	50	350	350	380	405
2	0	1500	4	50	355	360	385	420
3	0	1500	4	50	360	350	385	415
5	0	1500	4	50	350	305	360	393
6	0	1500	4	50	350	280	340	366
10	0	1500	4	50	312	200	255	284
6	0	1500	4	50	350	280	340	366
6	0	1500	5	50	445	360	420	460
6	0	1500	6	50	550	435	500	550
6	0	1500	5	150	460	375	430	440
6	0	1500	6	150	545	445	495	505
0	0	1500	5	150	470	470	475	490
0	0	1500	5	150	470	470	475	490
6	0	1500	5	150	470	375	430	440
6	−50	1500	5	150	480	370	430	445
6	−100	1500	5	150	490	370	440	440
6	−200	1500	5	150	535	370	450	450

FIG. 11 illustrates in perspective, but schematically simplified, a billet with a tilted rotation axis in a race-track coil.

While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one of ordinary skill in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Accordingly, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. A method of inductively heating an electrically conducting workpiece such that it possesses a temperature gradient along its axis, the method comprising: positioning an electrically conducting workpiece in a magnetic field having a flux density, the magnetic field being generated by a direct-current carrying coil arrangement comprising at least one superconducting coil disposed on a ferromagnetic yoke including a pole piece;directing the magnetic field toward the workpiece along a principal axis such that the magnetic field permeates the workpiece;rotating the workpiece about a rotational axis, wherein the rotational axis of the workpiece forms an angle with the principal axis of the magnetic field; andsetting the angle formed between the rotational axis of the workpiece and the principal axis of the magnetic field at a value that is offset from 90° to position a first portion of the workpiece closer to the pole piece than a second portion of the workpiece,wherein the flux density of the magnetic field permeating the workpiece varies along the rotational axis such that the flux density of the magnetic field permeating the first portion of the workpiece differs from the flux density of the magnetic field permeating the second portion of the workpiece,and wherein the variation of the flux density permeating the workpiece along the rotational axis generates a temperature gradient along the workpiece rotational axis.

2. The method according to claim 1, wherein: the ferromagnetic yoke includes a plurality of pole pieces; anda pole piece is disposed on each side of the workpiece rotational axis.

3. The method according to claim 2, wherein the coil arrangement that generates the magnetic field comprises a superconductive coil disposed on each pole piece.

4. The method according to claim 1, wherein: the yoke comprises a C-shaped yoke having a long limb, a first pole piece, and a second pole piece;the pole pieces define a space;the workpiece is positioned within the space such that the rotational axis of the workpiece is oriented between the pole pieces;the coil arrangement comprises a superconducting coil disposed on each pole piece such that the superconducting coils generate the magnetic field along the principal axis;the method further comprises positioning the rotational axis of the workpiece between the first and second pole pieces such that the angle formed between the rotational axis of the workpiece and the principal axis of the magnetic field is from about 70° to about 88°.

5. The method according to claim 1, wherein: the yoke comprises a C-shaped yoke having a long limb, a first pole piece, and a second pole piece;the pole pieces define a space;the workpiece is positioned within the space such that the rotational axis of the workpiece is oriented between the pole pieces;the superconducting coil is disposed on the long limb of the yoke, the superconducting coil generating the magnetic field along the principal axis.

6. The method according to claim 1, wherein: the yoke comprises a C-shaped yoke having a long limb, a first pole piece, and a second pole piece disposed opposite the first pole piece;the opposed pole pieces cooperate to define a workpiece space having a space first end and a space second end;the workpiece is positioned within the workpiece space such that the rotational axis of the workpiece is oriented between the pole pieces; andthe workpiece space narrows from the space first end to the space second end.

7. The method according to claim 6, wherein the narrowing workpiece space creates a substantially constant temperature gradient along the length of the workpiece.

8. The method according to claim 1, wherein: the yoke comprises a C-shaped yoke having a long limb, a first pole piece, and a second pole piece;the first and second pole pieces cooperate to define a space having a space first end and a space second end;the workpiece is positioned within the space such that the rotational axis of the workpiece is oriented between the pole pieces;the coil arrangement comprises a superconducting coil disposed on at least one of the long limb, the first pole piece, and the second pole piece, wherein the superconducting coil generates the magnetic field along the principal axis;the space defined by the pole pieces narrows from the space first end to the space second end; andthe narrowing space generates the different flux densities.

9. The method according to claim 8, wherein the narrowing space creates a substantially constant temperature gradient along the length of the workpiece.

10. The method according to claim 1, wherein: the yoke comprises an open yoke including a first arm, a second arm, and an intermediate limb extending from the first arm to the second arm;the limb and the arms cooperate to define a yoke space;the workpiece is positioned within the yoke space such that the rotational axis of the workpiece is oriented between the arms;the superconducting coil generates the magnetic field along the principal axis; andthe method further comprises: directing the magnetic field toward the workpiece along the principal axis, andselectively positioning the workpiece such that the angle formed between the rotational axis of the workpiece and the principal axis of the magnetic field is from about 70° to about 88°.

11. The method of claim 10, wherein the superconducting coil is disposed on the intermediate limb.

12. The method according to claim 1, wherein the pole piece is a first pole piece;the ferromagnetic yoke further comprises a second pole piece disposed in opposed relation from the first pole piece; andthe workpiece is oriented between the pole pieces such that workpiece first portion is positioned closer to the first pole piece than the second workpiece portion.

13. The method according to claim 1, wherein: the pole piece is a first pole piece;the ferromagnetic yoke further comprises a second pole piece disposed in opposed relation from the first pole piece;each pole piece defines a pole face; andthe rotational axis of the workpiece is oriented nonparallel to each of the pole faces.

14. The method according to claim 13, wherein: the first pole face is oriented in opposed spaced relation to the first pole face; andthe magnetic field is directed between the first and second pole faces.

15. The method according to claim 1, wherein the angle formed between the rotational axis of the workpiece and the principal axis of the magnetic field is from about 70° to about 88°.

16. A method of inductively heating an electrically conducting workpiece, the method comprising: generating a magnetic field via a direct-current carrying coil arrangement comprising a superconductive coil, the magnetic field possessing a flux density;directing the magnetic field toward the workpiece along a principal axis;positioning a workpiece within the magnetic field;rotating the workpiece in the magnetic field about a workpiece rotational axis, wherein the workpiece rotational axis forms an angle with the principal axis of the magnetic field; andorienting the workpiece within the coil arrangement such that the angle formed between the workpiece rotational axis and the principal axis of the magnetic field is offset from 90° and the flux density permeating the workpiece varies along the workpiece rotational axis;wherein the variation in the flux density generates a temperature gradient in the workpiece along the workpiece rotational axis.

17. The method according to claim 16, wherein the flux density of the magnetic field permeating a first portion of the workpiece differs from the flux density of the magnetic field permeating a second portion of the workpiece.

18. The method according to claim 16, wherein: the superconducting coil is disposed on a ferromagnetic yoke having a first pole piece oriented in spaced relation from a second pole piece;the first pole piece comprises a first pole face;the second pole piece comprises a second pole face;the workpiece comprises a first workpiece portion and a second workpiece portion;the workpiece is positioned between the pole faces such that the first workpiece portion is oriented closer to the first pole face than the second workpiece portion; andthe magnetic field is directed between the first and second pole faces.

19. The method according to claim 16, wherein: the superconducting coil is disposed on a ferromagnetic yoke comprising: a first pole piece having a first pole face, anda second pole piece having a second pole face,wherein the first pole face is disposed in opposed spaced relation from the second pole face;the workpiece comprises an elongated workpiece having a first workpiece portion and a second workpiece portion; andthe workpiece is positioned between the pole faces such that the first workpiece portion is oriented closer to the first pole face than the second workpiece portion,wherein the magnetic field is directed between the pole faces.

20. The method according to claim 16, wherein: the superconducting coil is disposed on a ferromagnetic yoke comprising: a first pole piece having a first pole face, anda second pole piece having a second pole face,wherein the first pole face is disposed in opposed spaced relation from the second pole face;the workpiece comprises an elongated workpiece having a first workpiece portion and a second workpiece portion; andthe workpiece is positioned between the pole faces such that the rotational axis of the workpiece is oriented nonparallel to each of the pole faces.