Abstract:
A superconducting magnetic field coil ( 1; 21; 31; 41; 51; 61 ) comprising at least one coil section ( 42; 43 ) which is wound in layers, is characterized in that, in at least one layer ( 11, 12, 13, 14, 101, 102, 103, 104 ) of the coil section ( 42; 43 ) N (with N≧2), superconducting wire sections (A, B, C, D, E) are wound in parallel, such that the windings of the N wire sections (A, B, C, D, E) are adjacent to each other and the N wire sections (A, B, C, D, E) are connected in series. The inventive magnetic field coil can be produced at highly reduced costs, in particular, when the magnetic field coil has a comparatively large layer length.

Description:
[0001]    This application claims Paris Convention priority of DE 10 2006 035 751.5 filed Jul. 28, 2006 the complete disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention concerns a superconducting magnetic field coil comprising at least one coil section which is wound in layers. 
         [0003]    Magnetic field coils of this type are disclosed e.g. by the NMR magnets of the UltraShield product line of the company Bruker BioSpin GmbH, Rheinstetten, DE. 
         [0004]    Superconducting magnetic field coils are used to generate magnetic fields of high strength (approximately 20 Tesla or more). Magnetic fields of such high strength are used, in particular, in high-resolution nuclear magnetic resonance (NMR) spectroscopy. Superconducting magnetic field coils are cooled during operation, e.g. using liquid helium in a cryostat. 
         [0005]    The magnetic field coils typically have a solenoidal shape and are wound from superconducting wire in layers which are disposed on top of each other and connected in series. The superconducting wire is generally tightly wound within one layer, i.e. neighboring windings of the superconducting wire contact each other. This prevents the windings from being displaced by magnetic forces, in particular, when the magnetic field coil is charged. 
         [0006]    Individual parts of the superconducting wire can only be combined outside of the coil using superconducting connecting elements, so-called joints. One piece of wire is therefore conventionally always used to wind an integer number of layers. For producing the coil, only that portion of a wire piece is used which corresponds to the length of an integer multiple of the “layer length” (this is the wire length within one layer). Any residue of the wire piece is discarded (“waste”). 
         [0007]    Since the production of wires from Nb 3 Sn or NbTi superconductors is usually based on bolts of a weight of typically approximately 30 kg to 300 kg, the available wire piece length is determined to a large degree by the conductor cross-section of the wire. In high-temperature superconductors, such as YBaCuO or BiSrCaCuO, the maximum available wire piece length is currently between approximately 100 m and approximately 2000 m. 
         [0008]    For magnet coils having very short layer lengths compared to the available wire piece lengths (e.g. due to a short layer length with small coil diameter and/or a thin conductor cross-section e.g. with a conductor weight&lt;10 kg/layer), the waste can be neglected compared to the overall production costs. 
         [0009]    For magnetic field coils with layer lengths of the same magnitude as the available lengths of wire pieces (e.g. due to a large coil diameter and/or a large conductor cross-section and therefore high conductor weight/layer, approximately&gt;10 kg/layer), the superconducting wire waste considerably increases the production costs of the magnetic field coil. If e.g. a wire piece of a length corresponding to 1.9 layers is available, the remaining wire of a length that corresponds to 0.9 layers is discarded, e.g. the waste can almost double the material costs for the superconducting wire. 
         [0010]    It is therefore the underlying purpose of the present invention to provide a superconducting magnetic field coil which can be produced at much less cost than conventional magnetic field coils, in particular, when the magnetic field coil has a comparatively large layer length. 
       SUMMARY OF THE INVENTION 
       [0011]    This object is achieved by a superconducting magnetic field coil of the above-mentioned type which is characterized in that, in at least one layer of the coil section N (with N≧2), the superconducting wire sections are wound in parallel, such that the windings of the N wire sections are adjacent to each other and the N wire sections are connected in series. 
         [0012]    In accordance with the invention, the at least one layer is not wound with one single continuous wire section but with N parallel wire sections. While the length of one continuous wire section should correspond to one layer length (or a multiple thereof), the N wire sections must only have a length of approximately (1/N)* layer length (or a multiple thereof). The layer length corresponds to the number of all wire windings in the layer multiplied by the coil periphery (thereby neglecting the axial extension of the coil). In turn, the number of wire windings in one layer substantially corresponds to the quotient between the coil length in the axial direction and the wire width. 
         [0013]    In accordance with the invention, an increased fraction of each available wire piece can be utilized. The waste per wire section in accordance with the invention is less than (1/N)* layer length, while in prior art, the waste could amount up to one layer length (with only one wire section in one layer). In accordance with the invention, a wire piece of 1.9 layer lengths (see above) and N=4 parallel wire sections in each layer can e.g. be divided into seven wire sections of ¼ layer lengths each, leaving a rest of 1.9-1.75=0.15 layer lengths as waste. It must be noted that in accordance with the invention, several wire sections, in particular, wire sections of different layers may belong to one coherent wire piece. 
         [0014]    In accordance with the invention, the windings of the superconducting wire in one layer are uniformly divided to the N wire sections, typically with a fixed, periodic sequence of windings of the individual wire sections along the longitudinal axis of the coil within one layer, thereby preventing wire crossings. Each wire section then has a slope of N wire widths per winding (thereby assuming that the wire type for all wire sections in one layer is the same). 
         [0015]    In accordance with the invention, the N wire sections are connected in series in order to obtain a magnetic field in correspondence with one single, continuous wire in one layer, if required using joints outside of the coil. The series connection may also be effected indirectly, i.e. other coil sections (e.g. from higher or lower layers of from other coil sections) can be connected between the connected wire sections of one layer. 
         [0016]    The inventive division of one layer into several, parallel wire sections is moreover advantageous in that the magnetic dipole moments of the magnetic field coil can be more finely quantized. For shielding, one part of a first coil section (main field coil) and one part of a second coil section (shielding coil) are typically connected in series and commonly bridged with a protective resistance. The two parts should thereby preferably have exactly opposite magnetic dipole moments. Since the contacts may be provided only at the coil edges, conventional compensation is effected in each case over an integer number of complete layers or their dipole moments. The invention also provides access to partial layers and their dipole moments. 
         [0017]    Most of the layers, in particular, all layers of the at least one coil section advantageously have several parallel-wound wire sections. 
         [0018]    Within the scope of the invention, the magnetic field coil or its coil sections typically have a solenoid shape. 
         [0019]    In one preferred embodiment of the inventive magnetic field coil, each winding of the N wire sections of one layer is disposed, in non-positive engagement, next to the other. The windings thereby stabilize each other mechanically and conductor displacement or oscillations due to magnetic or external forces are prevented. This improves the field homogeneity and temporal stability of the magnetic field coil. 
         [0020]    In another preferred embodiment, the wire sections of all layers are connected in series. This facilitates charging and operation of the magnetic field coil. 
         [0021]    In another equally preferred embodiment, at least two coil sections are provided and the wire sections of all coil sections are connected in series. Two important examples for coil sections are a main field coil and a shielding coil. There may also be further or other subdivisions and coil sections. The series connection simplifies charging and operation. 
         [0022]    In a preferred further development of this embodiment, the series connection of the wire sections extends from one wire section of a first coil section to a wire section of a second coil section and back again to a further wire section of the first coil section. Thus, wire sections with correlating properties from different coil sections can be connected e.g. for shielding or stray field flash prevention in case of a quench. 
         [0023]    In one particularly preferred further development of the above-mentioned embodiment, the magnetic field coil comprises coil sections with different winding directions, enabling defined manipulation of the magnetic field, in particular, for homogenization or field weakening. 
         [0024]    In a preferred further development of this design, the magnetic field coil is actively shielded, thereby protecting people and technical equipment in the surroundings of the magnetic field coil from stray fields. 
         [0025]    In another advantageous embodiment, N=2 or N=3 or N=4. This number of parallel wire sections limits the expense for connections and greatly reduces the material costs. Even larger N values are mainly preferred for very expensive superconducting wires or superconducting wires which are available only with very short lengths. 
         [0026]    In one particularly preferred embodiment of the inventive magnetic field coil, the coil areas of the magnetic field coil are bridged by protective ohmic resistances, such that these coil areas generate an approximately vanishing dipole moment during operation, wherein at least one of the coil areas comprises only part of the wire sections of a layer. The inventive fine subdivision of the magnetic field coil, which also provides access to parts of one layer, permits particularly exact compensation or shielding, such that particularly low stray fields can be achieved. The term “during operation” includes both normal operation and a quench, such that even a so-called stray field flash in case of a quench can be effectively prevented. 
         [0027]    In another preferred embodiment, the wire sections of one layer have identical cross-sections, which facilitates winding of the superconducting wire onto the coil body, in particular, when several layers are wound directly on top of each other. 
         [0028]    In another preferred embodiment, at least one coil section comprises several layers, permitting generation of larger field strengths. 
         [0029]    Finally, in another preferred embodiment, the wire sections consist of NbTi and/or Nb 3 Sn and/or MgB 2  and/or high-temperature superconductor (HTSL), in particular YBaCuO and/or BiSrCaCuO conductors. These materials have proven to be useful in practice. 
         [0030]    Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used in accordance with the invention either individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention. 
         [0031]    The invention is illustrated in the drawing and explained in more detail with reference to embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0032]      FIG. 1   a  shows a schematic cross-section through an inventive magnetic field coil with three parallel-wound wire sections per layer, wherein the wire sections are commonly turned during layer change; 
           [0033]      FIG. 1   b  shows a schematic top view of the inventive magnet coil of  FIG. 1   a  at a layer change; 
           [0034]      FIG. 2   a  shows a schematic cross-section through a further inventive magnetic field coil with three parallel-wound wire sections per layer, wherein the wire sections are individually turned during layer change; 
           [0035]      FIG. 2   b  shows a schematic top view of the inventive magnet coil of  FIG. 2   a  at a layer change; 
           [0036]      FIG. 3  shows a schematic cross-section through a further inventive magnetic field coil with changing number of wire sections per layer; 
           [0037]      FIG. 4  shows a schematic cross-section through an inventive magnetic field coil with two coil sections with wiring diagram; 
           [0038]      FIG. 5  shows a schematic cross-section through an inventive magnetic field coil with two layers with wiring diagram; 
           [0039]      FIG. 6  shows a schematic cross-section through an inventive magnetic field coil with four layers with wiring diagram. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0040]      FIG. 1   a  shows a cross-sectional view of part of an inventive magnetic field coil  1 . The magnetic field coil  1  has a solenoidal shape. The axial direction is designated with Z and the radial direction with R. The magnetic field coil  1  has a coil carrier  2  onto which superconducting wire  5  has been wound with numerous windings. Each winding is shown in cross-section by a circle that corresponds to the cross-sectional profile of the wire  5 . Only an upper partial section of the coil carrier  2  is visible. The actual coil carrier  2  has the shape of a circular cylindrical jacket. The magnetic field coil  1  also has a flange  3  on the left-hand side and a flange  4  on the right-hand side, which guide the wire  5 . 
         [0041]    The superconducting wire  5  is wound in several layers.  FIG. 1  shows three layers  11 ,  12 ,  13 , wherein  11  is the radially innermost and  13  the radially outermost layer. 
         [0042]    The inventive particularity of the magnetic field coil  1  consists in that several wire sections A, B, C are wound in each layer  11 ,  12 ,  13 . 
         [0043]    The innermost layer  11  is e.g. wound in parallel from three wire sections A, B, C. The wire section A advances within one winding by three wire widths Br in the z direction, such that one winding of the wire sections B and C can be housed between two neighboring windings of the wire section A. The wire sections B and C are correspondingly wound. Each wire section A, B, C in one layer requires only approximately one third of the length (“layer length”) of a continuous wire which would form one complete layer by itself. 
         [0044]    The wire sections A, B, C within each layer  11 ,  12 ,  13 , or to be more precise, the windings of the wire sections A, B, C directly abut each other. The position of the wire  5  is thereby stabilized through the “gap filling” offset in the z-direction. As viewed in cross-section, one obtains a substantially two-dimensional, hexagonally tight packing. 
         [0045]    In the embodiment of  FIG. 1   a , the wire sections A, B, C are wound together, i.e. the three wire sections A, B, C are always next to each other in the same order and are identically guided, i.e. one strand with the three wire sections A, B, C, is guided to the coil carrier  2  during winding. The wires of the wire sections A, B, C, may thereby be optionally glued to each other. 
         [0046]    The common guidance becomes particularly clear in a layer change, e.g. from layer  11  on the left to layer  12 . In layer  11 , the strand “ABC” is wound to the left. The order of the wire sections A, B, C remains the same during and after layer change. In layer  12 , the strand “ABC” is further wound to the right. The block of the wire sections A, B, C is commonly turned at the same peripheral position. 
         [0047]    The layer change from layer  11  to layer  12  is additionally illustrated in  FIG. 1   b , wherein the wire sections A, B, C are shown as lines for simplification.  FIG. 1   b  shows the winding state during layer change. It shows the Z-position of the wire sections A, B, C, in dependence on the peripheral position U. The wire sections A, B, C are turned (i.e. reverse their winding pitch) at the identical peripheral position. The position of the wire sections relative to each other remains the same. 
         [0048]    It is, however, also possible to set a joint for one (or several) wire sections A, B, C during layer change, e.g. when the wire source for wire section A is insufficient for wire section A of the next layer  12 . A joint would e.g. connect the wire section A of layer  11  to the wire section A in layer  12  at the left-hand edge of the magnetic field coil  1 . The wire section A of layer  11  simply continues as wire section A without joint in layer  12 . 
         [0049]    Each wire section A, B, C, preferably has its own wire source during winding. A wire source with a glued strand for the wire sections A, B, C, may alternatively be provided. 
         [0050]      FIG. 2   a  shows a similar inventive magnetic field coil  21  like in  FIG. 1   a . However, the superconducting wire  5   a  of the magnetic field coil  21  has an approximately rectangular cross-section. The magnetic field coil  21  also has several layers, of which three layers  11 ,  12 ,  13  are shown. 
         [0051]    The wire sections A, B, C of the magnetic field coil  21  of  FIG. 2   a  are guided individually and not together. This becomes clear in the layer change, e.g. between layer  11  and  12  at the left-hand edge. 
         [0052]    When one single wire section A, B, C reaches the left-hand flange  3 , this wire section is individually turned (i.e. the winding pitch is reversed). The wire sections A, B, C are thus individually reflected on the flange  3 , thereby changing the order of the wire sections A, B, C during layer change. While a wire order “ABC” is wound in layer  11  to the left, the sequence in layer  12  is reversed “CBA” and wound to the right. This is explained in detail in  FIG. 2   b . The wire section A of layer  11  is reflected at first, then the wire section. B, and then the wire section C. For this reason, the order of the wire sections is reversed in the next layer above  12 . 
         [0053]    It is thereby also possible to set a joint for one or more wire sections A, B, C during layer change. Each wire section A, B, C has its own wire source during winding. 
         [0054]      FIG. 3  shows a further magnetic field coil  31 , wherein the wire sections A, B are wound in parallel only in the first layer  11 , e.g. from the right to the left. In the next layer above  12 , only one wire section A is returned to the right and in the next wire section above, the other wire section B is returned to the right. 
         [0055]      FIG. 4  shows a magnetic field coil  41  which has a first coil section  42  and a second coil section  43  which is coaxial thereto, with a wiring diagram. The first coil section  42  has two layers  11 ,  12  and the second coil section has four layers  101 ,  102 ,  103 ,  104 . The second coil section  43  is wound in an opposite direction to the first coil section  42 , such that the magnetic fields generated by the second coil section counteract those of the first coil section  42 , thereby shielding the surroundings. 
         [0056]    The current flows in series through all wire sections A, B, D, E and layers  11 ,  12 ,  101 ,  102 ,  103 ,  104  of the magnetic field coil  41  in the following order: the current is fed at connection  47   a  into the wire section A of layer  11  at the left-hand edge. At the right-hand coil edge, the wire section A merges from layer  11  into wire section A of layer  12  (normal layer, joint is possible). At the left-hand edge, the wire section A of layer  12  is connected to the wire section D in layer  101  via a joint  44 . The wire section D of layer  101  merges at the right-hand edge into wire section D of layer  102 . The left-hand edges of the wire section D of layer  102  and of the wire section B of layer  11  are then connected using a joint  45 . The wire section B of layer  11  then merges into wire section B of layer  12  at the right-hand edge. The wire section B of layer  12  is, in turn, connected to wire section E of layer  103  via a joint  46 . The wire section E of layer  103  merges at the right-hand edge into wire section E of layer  104 . There is a second supply point  47   b  at the left-hand edge in the wire section E of layer  104 . 
         [0057]    The wire sections A and B of the layers  11  and  12  are connected to each other in series via the detour through the wire section D. 
         [0058]    This magnetic field coil  41  additionally comprises protective ohmic resistances  48   a ,  48   b . The protective resistance  48   a  commonly bridges the wire sections A of the layers  11 ,  12 , and D of the layers  101 ,  102 . In case of a quench in these wire sections (i.e. in this coil section), the resistance  48   a  may effect an emergency discharge. The wire sections A of the layers  11 ,  12  and D of the layers  101 ,  102  thereby have a magnetic dipole moment of approximately zero during operation. The wire sections B of the layers  11 ,  12  and E of layers  103 ,  104  are likewise bridged and secured by the protective resistance  48   b . In total, these wire sections (or this coil section) also have an approximately vanishing dipole moment, thereby actively shielding the magnetic field coil  41 . 
         [0059]      FIG. 5  shows a circuit of a simple inventive magnetic field coil  51  with only one coil section. 
         [0060]    The current is fed into the wire section A of layer  11  at the left-hand edge. The wire section A is connected at the right-hand edge to the wire section C from the layer above via a joint  52 . The wire section C is, in turn, connected to the wire section B of layer  11  at the left-hand edge via a joint  53 . The wire section B is in turn connected to the wire section D of the layer  12  via joint  54  at the right-hand edge. Another supply point extends from the wire section D at the left-hand edge. 
         [0061]    All wire sections A, B, C, D are thereby also connected in series. 
         [0062]      FIG. 6  shows a circuit of a further inventive magnetic field coil  61  which also comprises a coil section. 
         [0063]    A supply point  47   a  contacts the wire section A of the layer  11  at the left-hand edge. The wire section A merges from the layers  11 , via  12 ,  13 , to  14  into the next upper layer (preferably without joint; joints may however be used at the layer changes at the edge in order to take into account a terminating source wire). A joint  62  connects the wire section A in layer  14  and wire section B in layer  11 . The wire section B merges from layers  11  to  12 ,  13  and  14  (preferably also without joint). The wire section B in layer  14  has a connection  47   b  at the left-hand edge. 
         [0064]    In summary, the invention proposes use, within one layer, of several wire sections which are not connected within the layer, for winding these layer windings in a superconducting magnetic field coil which typically has a solenoidal shape. Each wire section winds only part of the windings of the layer, but each wire section is guided over the entire width of the magnetic field coil. At the transition to neighboring layers, one can set joints to separate wire sections or continue the wire sections to the neighboring layers. The wire sections of one layer are connected in series in order to obtain a magnetic field as for one single continuous wire for one layer. The wire sections of one layer can be separately contacted, e.g. in order to form coil sections with vanishing dipole moment with high precision and bridge them with a protective ohmic resistance.