Abstract:
A current lead ( 1 ) for connecting a superconducting load system ( 5 ), in particular, a magnet coil, to a current feed point ( 3   a ) that is at a higher temperature than the load system ( 5 ) comprises a flat, elongated carrier ( 6 ) and a plurality of mechanical and electrical parallel high-temperature superconductors (HTSC) ( 10 ), wherein the HTSCs ( 10 ) are disposed side by side on the carrier ( 6 ). The carrier ( 6 ) is made of stainless steel, and a plurality of HTSCs ( 10 ) are each disposed side by side on two opposite carrier ( 6 ) sides of the carrier. The carrier ( 6 ) is constituted in the shape of a plate with cut-outs ( 15; 15   a - 15   d ). The current lead has a high current capacity and low thermal conductivity and provides improved emergency conduction properties in case of failure of the superconductivity in the HTSC.

Description:
[0001]    This application claims Paris Convention priority of DE 10 2009 028 413.3 filed Aug. 10, 2009 the complete disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a current lead for connecting a superconducting load system, in particular, a magnet coil, to a current feed point that is at a higher temperature than the load system, comprising
       a flat, elongated carrier and   a plurality of mechanical and electrically parallel high-temperature superconductors (HTSC),
 
wherein the HTSCs are disposed side by side on the carrier.
       
 
         [0005]    The generic prior art is disclosed in DE 10 2005 058 029 A1. 
         [0006]    Superconducting load systems are deployed wherever (usually strong) electrical currents must flow with low electrical resistance. One important superconducting load system is superconducting magnet coils, which are used to produce strong magnetic fields, for example, for NMR spectrometers or MRI tomographs. The superconducting load system must be kept at a cryogenic temperature for operation so that the superconducting material in the load system can be kept below its critical temperature Tc. The superconducting load system must be refrigerated with cryocoolers for this purpose; the necessary cooling power or the energy required to provide the cooling power is a considerable cost factor. 
         [0007]    Electric current is fed into the superconducting load system during start-up or also during operation. For this purpose, current leads are used. Current leads should therefore provide a current path with good electrical conductivity. Current leads connect a current feed point to the superconducting load system at the cryogenic temperature. 
         [0008]    Because the current sources (such as the local power supply network) usually provide their connections at room temperature, the current lead always provides a thermal conduction path to the superconducting load. The associated heat input increases the cooling power requirement of the superconducting load. 
         [0009]    Normally, graduated cooling is provided for the power supply. The first end of the current lead, nearest to the current feed point, or the corresponding first connection element is kept at a medium low temperature (for example, 50 to 90 K) by a first cryocooler or a first stage of a cryocooler. The second end of the current lead near to the superconducting load or a corresponding second connection element is kept at the temperature intended for the superconducting load (usually 1 to 30 K) by a second cryocooler or a second stage of a cryocooler. This graduated cooling already considerably reduces the cooling power requirement. 
         [0010]    The current lead disclosed in DE 10 2005 058 029 A1 is an FRP (fiberglass reinforced plastic) carrier comprising two identical plates, between which strip-shaped, mutually parallel high-temperature superconductors (HTSC) are disposed. The HTSCs extend in the longitudinal direction of the FRP carrier. The ends of the HTSCs are connected electrically-conductively each with a connection element. The strip-shaped HTSCs each comprise a normally conducting current path. 
         [0011]    The disadvantage of this current lead is the relatively low superconducting current capacity relative to the dimensions and, furthermore, if the superconductivity is no longer ensured in the HTSC (quench), the emergency conducting properties are weak. 
         [0012]    It is the object of this invention to provide an HTSC current lead with a high current capacity and low thermal conductivity that provides improved emergency conduction properties in case of failure of the superconductivity in the HTSC. 
       SUMMARY OF THE INVENTION 
       [0013]    This object is solved by a current lead of the type stated above characterized in that,
       the carrier is made of stainless steel,   a plurality of HTSCs are each disposed side by side on two opposite carrier sides of the carrier and   the carrier is constituted in the shape of a plate with cut-outs.       
 
         [0017]    The invention proposes a redesign of HTSC current leads with which the properties of current conduction are greatly improved. The inventive current lead has a plurality of high-temperature superconductors (HTSCs or HTSC wires) whose critical temperature is T c ≧30K. The superconducting current capacity relevant for normal operation is determined by the HTSC. The carrier is used as a support for the (typically strip-shaped) HTSCs and provides a mechanical base frame. The thermal conductive properties are also determined by the carrier. 
         [0018]    According to the invention, the carrier is made of stainless steel. If the superconductivity is no longer ensured on one or more of the HTSCs (in case of quenching), the carrier made of stainless steel maintains a current path of electrical current capacity due it its ohmic electrical conductivity so that the local heat development at the location of origin of the quench can be better controlled, thus being able to protect the HTSC from damage due to overvoltage and overheating. A normally conducting current path in each individual HTSC that is the main source of heat input through the HTSCs in prior art, can be dimensioned to be considerably narrower in the invention because it is load-relieved by the current path in the carrier in case of quenching. In an extreme case, the normally conducting current path is actually unnecessary and can be eliminated. 
         [0019]    At the same time, stainless steel has relatively low thermal conductivity (with respect to its electrical conductivity), so that this already ensures a heat input into the superconducting load that is quite low. The heat flow that is input through the refrigerated user system via the inventive current lead is further minimized by cut-outs in the carrier. The cut-outs themselves reduce the thermal conduction paths available in the current lead while generating bottlenecks for thermal conduction in the carrier around the cut-outs. The inventive measures result in a reduction of the costs for cooling a connected superconducting load system. The cut-outs additionally reduce the weight and the material requirement for the carrier. 
         [0020]    Due to the inventive two-sided use of the carrier by mounting HTSCs, a large number of HTSCs can be fitted in the current lead. The useful superconducting current capacity of the inventive current lead is thus correspondingly large. 
         [0021]    In the invention, the carrier is included in the functionality of the current lead to a greater extent and the properties resulting from the structure of the carrier are considered to a greater degree in the design of the current lead, permitting simple improvement of the properties of the current lead. 
         [0022]    The plate-shaped carrier with cut-outs is essentially flat, elongated and cuboidal. The outer width, outer height, and outer length of the plate-shaped carrier describe the (smallest) enveloping cuboid into which the plate-shaped carrier would just about fit. The region not occupied by the carrier material in this cuboid (that is, the space defined by the extent of the outer width, outer height, and outer length) are the cut-outs of the carrier. In the invention, the ratio of outer width to outer height is typically 3:1 or more and the ratio of outer length to outer width is typically 4:1 or more. 
         [0023]    In a preferred embodiment of the inventive current lead, at least 40%, preferably at least 60% of the space defined by the outer length, the outer width and the outer height of the carrier is occupied by cut-outs. Because of this proportion of cut-outs and the concomitant reduction in material, a reduction in heat flow of an at least similar proportion can be achieved, compared with a plate-shaped (flat and elongated, cuboidal) carrier that does not have cut-outs. 
         [0024]    An embodiment is also preferred in which one or more sections of the length of the carrier are provided in which at least 70%, preferably at least 85% of the cross-sectional area defined by the outer width and outer height of the carrier is occupied by cut-outs. The section or sections that comply with the said conditions with respect to their cross-sectional area preferably occupy at least 10%, in particular preferably at least 25%, of the outer length of the carrier. According to this embodiment, one or more “bottlenecks” are provided in the carrier, in which the cross-sectional area contributing to thermal conduction (perpendicular to the longitudinal extent) is reduced locally as compared to the cross-sectional area defined by the outer height and outer width of the carrier. The bottleneck or bottlenecks typically determine and limit the thermal conduction of the carrier so that other requirements (such as the mechanical stability) can be prioritized in other aspects of design of the carrier. 
         [0025]    An embodiment is also preferred in which the carrier is constituted in the shape of a meander. The meander shape is achieved by cut-outs that are formed starting from one of the upright sides of the carrier, transverse with respect to its longitudinal direction, over the full height of the carrier, and alternating between the upright sides in the longitudinal direction. In particular, the cut-outs can extend in the direction of the opposite upright side beyond the center of the carrier. The meander shape makes the thermal conduction path considerably longer than the outer length of the carrier and the thermal resistance is correspondingly higher. 
         [0026]    In an advantageous embodiment, at least some of the cut-outs are constituted as slots in the longitudinal direction of the carrier, in each of which, one HTSC is disposed. The longitudinal slots facilitate positioning of the HTSC. The side walls of the elongated slots protect the HTSC from mechanical damage. 
         [0027]    An especially preferred embodiment of the inventive current lead is characterized in that the carrier is made of a U-section that is open on one upright side of the carrier. 
         [0028]    In other words, the carrier is based on a U-section. A U-section advantageously provides large areas on the leg sides (outer sides) that can be used as a support for the HTSCs, with a small carrier material cross-section. 
         [0029]    In a further variant of this embodiment, the outer sides of the U-section each have meander-shaped slots. In this way, the thermal conduction path is effectively lengthened as compared with the outer length of the U-section. 
         [0030]    A further variant of the embodiment described above is also preferred in which slots that are transverse to the longitudinal direction are provided on the base side of the U-section, distributed over the longitudinal direction of the carrier. These slots reduce the cross-section of the U-section locally occupied by the carrier material which, in turn, reduces the heat flow along the U-section. 
         [0031]    An embodiment is also especially preferred in which the current lead has two connection elements, in particular made of copper, that are soldered onto the carrier in the region of the longitudinal ends of the carrier. Through the connection elements, the current is conducted into the HTSC; the current lead is usually also cooled through the connection elements. Copper as the material for the connection elements provides high electrical conductivity at low cost. To solder the connection elements onto the carrier, solders containing silver are usually used that are well suited to the copper connection elements. Solders of type L-Ag72 are preferably used. The connection elements are fitted to the carrier, for example, by butt-joining at the end faces. A flat recess can also be provided in the connection elements, resulting in an approximately stepped contact surface with the carrier. A transverse slot is also conceivable that is milled into the connection elements and finds a protrusion on the outer side of the carrier as a mating element, or vice versa. 
         [0032]    In one embodiment of the inventive device, the HTSCs are constituted in a strip-shape and, in particular, are disposed flat on the carrier side. This configuration affords the HTSCs protection because a surface that may be subject to mechanical loads is kept small. Moreover, this arrangement provides a large contact surface between the HTSCs and the carrier. Alternatively, HTSCs of the round conductor type can also be used. 
         [0033]    An embodiment is also preferred in which the HTSCs contain
       Bi-2212 or Bi-2223, in particular, in either case, of type PIT (powder in tube) with an Ag or Ag/Au coating   or YBCO, in particular, of type “coated conductor.”   or MgB 2 .       
 
         [0037]    These materials have proven their worth in practice and can be very well included in line with invention. 
         [0038]    Further advantages of the invention can be derived from the description and the drawing. Equally, the characteristics stated above and the characteristics stated below according to the invention can be used singly or multiply in any combination. The embodiments shown and described are not intended as an exhaustive list but as examples to illustrate the invention. 
         [0039]    The invention is shown in the drawing and is explained in more detail using the examples. The figures show: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0040]      FIG. 1 : a schematic diagram of an inventive current lead that is connected to a current source and a superconducting load; 
           [0041]      FIG. 2   a : a schematic diagram of an inventive current lead in plan view; 
           [0042]      FIG. 2   b : a schematic diagram of an inventive current lead in the plan view of  FIG. 2   a  but without HTSCs; 
           [0043]      FIG. 2   c : a schematic longitudinal section diagram of the current lead of  FIG. 2   a  along intersecting plane A-A according to  2   a;    
           [0044]      FIG. 2   d : a schematic transverse section diagram of the current lead of  FIG. 2   a  along intersecting plane B-B according to  2   c;    
           [0045]      FIG. 2   e : a schematic transverse section diagram of the current lead of  FIG. 2   a  along intersecting plane C-C according to  2   c;    
           [0046]      FIG. 3 : a schematic longitudinal section diagram of a butting soldered connection between a carrier and a connection element for the invention; 
           [0047]      FIG. 4 : a schematic longitudinal section diagram of a stepped soldered connection between a carrier and a connection element for the invention; 
           [0048]      FIG. 5   a : a schematic perspective view of a further inventive current lead with a meander-shaped carrier and longitudinal slots, shown without HTSCs; 
           [0049]      FIG. 5   b : the current lead of  FIG. 5   a , shown with HTSCs; 
           [0050]      FIG. 6 : a schematic perspective view of a carrier for an inventive current lead 
           [0051]      FIG. 7 : a schematic perspective view of a further carrier for an inventive current lead, based on a U-section, with comb-like outer sides; 
           [0052]      FIG. 8   a : a schematic perspective view of a further inventive current lead, based on a U-section with meander-shaped slots in the outer sides, shown without HTSCs; 
           [0053]      FIG. 8   b : the current lead of  FIG. 8   b , shown with HTSCs. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0054]      FIG. 1  schematically shows a configuration in which a superconducting load  5 , in this case a superconducting magnet, is connected to a current feed point  3   a  through an inventive current lead  1 . 
         [0055]    From the current feed point  3   a , which is at room temperature in this case and is provided on a current source  3 , a first conductor  2 , for example, a copper conductor, goes to a first end  1   a  of the current lead  1 ; there, a first connection element (not shown in greater detail) is provided. In the region of the first end  1   a , a connection  5   a  is provided on a first cryocooler (not shown), which cools the first end  1   a  to a medium low temperature T 1  of approx. 50 to 90 K (max. 35 K for MgB2 in the HTSC). The temperature T 1  should be below the critical temperature of the HTSC in the current lead  1 . 
         [0056]    At a second end  1   b  of the current lead  1 , there is a connection  5   b  to a second cryocooler (not shown) that cools the second end  1   b  to a cryogenic temperature T 2  of approx. 1 to 30 K; the temperature T 2  should be at least approximately as cold as the operating temperature of the superconducting load  5 . Instead of the first and second cryocooler, it is also possible to use one cryocooler with a first and a second cooling stage (not shown). 
         [0057]    A connection element constituted at the second end  1   b  (not shown) is connected to the superconducting load  5  by a second conductor, for example, an HTSC cable or low-temperature superconductor cable. 
         [0058]    The  FIGS. 2   a  to  2   e  illustrate a first embodiment of an inventive current lead  1 . 
         [0059]    As can be seen from the plan view  FIG. 2   a  and from the longitudinal section  FIG. 2   c  (cf. intersecting plane A-A in  FIG. 2   a ), the current lead  1  includes a carrier  6  made of stainless steel (“flat steel support”), in particular, made of steel DIN 1.4306, DIN 1.4404, DIN 1.4571, DIN 1.4435, AISI 316L, AISI 304L, or AISI 316LN, which is soldered to connection elements  7  at its longitudinal ends. The connection elements  7  have drill-holes  8  for mounting current contacts and potential taps  9 . 
         [0060]    Both on the top side and the underside of the carrier  6 , a plurality (in this case five) of high-temperature superconductors (HTSC)  10  are disposed that are connected by means of a soldered connection both with the carrier  6  and with the connection elements  7 . The solder used is preferably SnAg4 solder. Solders can also be used with a melting point &lt;200° C. (for example, solders from Fusion GmbH, type solder paste SSX 430-830). 
         [0061]    The carrier  6  is used as a support for HTSC  10  and takes up mechanical loads that act on the current lead  1 . It must be noted that the carrier  6  and the HTSC  10  should have similar heat expansion properties because otherwise damage could occur due to differing expansions caused by temperature fluctuations, in particular, during initial cooling of the current lead. 
         [0062]    As can be seen from  FIG. 2   b , which shows the plan view of  FIG. 2   a  with the HTSCs omitted, and  FIG. 2   c , the carrier  6  has a plurality of cut-outs  15  that extend perpendicularly with respect to the longitudinal direction (the longitudinal direction extends from left to right in  FIGS. 2   a,    2   b , and  2   c ) of the carrier  6 . In the embodiments shown, a total of seventeen cut-outs  15  are distributed over the outer length AL of the carrier  6 . 
         [0063]      FIG. 2   e  shows a transverse section through the current lead  1  at the height of a cut-out  15  (cf. intersecting plane C-C in  FIG. 2   c ). The outer contours of the carrier  6 , with an outer height AH and an outer width AB, define a cross-sectional area that is occupied by the carrier material at lateral webs only; approx. 80% of the defined cross-sectional area is occupied by the cut-out  15 . In an associated section  20  along the longitudinal direction of the carrier  6 , that is, over the longitudinal extent of each cut-out  15  (cf.  FIGS. 2   b  and  2   c ), thermal conduction is therefore heavily restricted because it can only occur through the “bottlenecks” of the two webs. The cut-outs  15  are continuous in the vertical direction. Most of the HTSCs  10  bridge the cut-out  15  completely; the edge HTSCs are partly lying on the webs. 
         [0064]      FIG. 2   d  shows a transverse cross-section through the current lead  1  between two cut-outs (cf. intersecting plane B-B in  FIG. 2   c ). In this region, the entire cross-sectional area defined by the outer height AH and the outer width AB is occupied by the carrier material. In this region, the HTSCs  10  lie directly on the carrier  6  and are soldered onto the surface of the carrier  6  in this embodiment. 
         [0065]      FIG. 3  illustrates, in a longitudinal section view, a first possibility for mounting a connection element  7  on a carrier  6  as part of the invention. The carrier  6  and the connection element  7  are soldered together at their face ends (butting ends). 
         [0066]    The soldered connection  16  is (irrespective of the shape of the soldered connection) preferably implemented with solders containing silver, in particular, solder type L-Ag72. The connection element  7  is (irrespective of the shape of the soldered connection) preferably made of Ag, Cu, or Au, or of alloys of the said materials; optionally, a connection element  8  can be coated with one of the materials Ni, Ag, or Au. 
         [0067]    An alternative mounting of a connection element  7  is depicted in  FIG. 4 . In this case, the carrier  6  and the connection element  7  each form, in the longitudinal section, approximately L-shaped edges so that an approximately S-shaped (stepped) contact surface is provided between the two components. The soldered connection  16   a  extends over the entire S-shaped contact surface and is noticeably larger than the contact surface in case of contacting on the end face only. 
         [0068]    A second embodiment of the inventive current lead  1  is shown in a schematic perspective view in  FIG. 5   a  (without HTSC) and  FIG. 5   b  (with HTSC). 
         [0069]    The carrier  6  of the current lead  1  has a meander-shaped structure. Alternating cut-outs  15  each project from the two opposite upright edges  70   a ,  70   b  into the space defined by the outer length AL, the outer height AH, and the outer width AB of the carrier  6 . The cut-outs  15  extend over the entire outer height AH (that is, the cut-outs are continuous in the vertical direction in  FIG. 5   a ). The carrier  6  therefore extends in sections that are parallel to the longitudinal extent (cf. section  20 ) and in sections that are transverse with respect to the longitudinal extent of the carrier  6 ; the carrier  6  therefore extends in a meandering path. In the case of a completely meander-shaped carrier  6 , as is described here, the length of this meandering path is approx. three times longer than the outer length AL of the carrier  6 , which provides a correspondingly higher thermal resistance. 
         [0070]    In the sections of the carrier  6  that extend transversely with respect to the longitudinal extent of the carrier, further cut-outs, that is, longitudinal slots  11 , are constituted in the top side and the underside of the carrier  6  (e.g. by milling). 
         [0071]    As seen in  FIG. 5   b , in this case, the longitudinal slots  11  have a depth that approximately corresponds to the height of the HTSCs  10  that are disposed on the carrier  6 . The HTSCs  10  are carried in the longitudinal slots  11 , which facilitates manufacturing, and the HTSCs  10  are mechanically stabilized and protected by the side walls of the longitudinal slots  11 . The longitudinal slots  11  also reduce the thermal conductivity of the carrier  6  along the meandering path. 
         [0072]      FIG. 6  shows a schematic perspective view of the carrier  6  for an inventive current lead that is comparable with the carrier  6  from  FIG. 2   b . The essentially flat, elongated, cuboidal carrier  6  has a plurality of cut-outs  15  that are distributed over the longitudinal extent, cut through the full height of the carrier, and leave lateral webs (to ensure mechanical integrity). 
         [0073]      FIG. 7  shows a perspective view of a further carrier  6  for an inventive current lead. 
         [0074]    The carrier  6  is based on a U-section that has its base at the upright side  70   a , and is open at the opposite upright side  70   b . The legs of the U-section constitute the top side and underside (outer sides)  90 ,  91  of the carrier  6  on which the HTSCs are disposed (not shown). 
         [0075]    The outer sides  90 ,  91  are provided, starting from the upright side  70   b  facing away from the base, with slot-like cut-outs  15   a  extending in the transverse direction. The slot-like cut-outs  15   a  reduce the thermal conductivity of the carrier  6  in the longitudinal direction considerably. The open region in the interior of the U-section of the carrier  6  forms the greatest cut-out  15   b  in the space (volume) defined by the outer width AB, outer height AH, and the outer length AL of the carrier  6  of  FIG. 7 . 
         [0076]    In the case of the carrier  6  based on a U-section, occupancy of the space defined by the outer width AB, the outer height AH, and the outer length AH by carrier material is especially low, that is, this defined space is mainly occupied by the cut-outs  15   a ,  15   b  (more than 90% in this case) and thermal conduction is therefore correspondingly low. 
         [0077]    The outer height AH is the height range over which the high-grade steel carrier  6  extends (seen over the entire longitudinal extent of the carrier  8 ). The outer width AB is the width range over which the high-grade steel carrier  6  extends (seen over the entire longitudinal extent of the carrier  6 ). The outer length AL is the length range over which the high-grade carrier  6  extends (seen over the entire vertical extent and longitudinal extent of the carrier  6 ). The outer height AH, outer width AB, and outer length AL are measured at right angles. A flat, elongated cuboid is constituted by the outer length AL, the outer width AB, and the outer height AH lying against the outer contours of the carrier  6  (“defined space”), into which the carrier  6  just about fits. 
         [0078]    A further embodiment of an inventive current lead  1  is shown in  FIG. 8   a  (without HTSCs) and  FIG. 8   b  (with HTSCs). 
         [0079]    The current lead  1  again has a carrier  6  based on a U-section (similar to the carrier in  FIG. 7 ). The legs of the U-section constitute the top side and underside (outer sides)  90 ,  91  of the carrier  6  on which the HTSCs  10  are disposed. 
         [0080]    The outer sides  90 ,  91  each have a cut-out  15   c , in the shape of meander-shaped slots. This reduces the thermal conduction in the carrier  6  in the longitudinal direction (toward the outer length AL). 
         [0081]    Furthermore, gap-like cut-outs (slots)  15   d  are provided that extend transversely with respect to the longitudinal direction in the base-side upright side  70   a  of the U-section-like carrier  6 . This reduces the thermal conduction in the longitudinal direction in the region of the base, too. 
         [0082]    The largest cut-out  15   b  is constituted by the free region in the interior of the U-section of the carrier  6 . 
         [0083]    Note that on an inventive current lead for protecting the HTSC and for electrical insulation, the HTSC tape conductor region can be encapsulated in epoxy resin, such as Stycast 1266, and/or wrapped in Kapton tape. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           1  Current lead 
           2  Conductor 
           3  Current source 
           3   a  Current feed point 
           4  Conductor 
           5  Superconducting load system 
           5   a  Connection for cryocooler 
           5   b  Connection for cryocooler 
           6  Carrier 
           7  Connection element 
           8  Drill-hole 
           9  Potential tap 
           10  HTSC 
           11  Longitudinal slot 
           15  Cut-out 
           15   a - 15   d  Cut-out 
           16 ,  16   a  Soldered connection 
           20  Section 
           70   a ,  70   b  Upright side 
           90  Outer side 
           91  Outer side 
         AB Outer width 
         AH Outer height 
         AL Outer length