Patent Publication Number: US-7718897-B2

Title: Low AC loss superconductor for a superconducting magnet and method of making same

Description:
BACKGROUND OF THE INVENTION 
   The invention relates generally to superconducting magnet systems and more particularly to superconducting magnets operating in an alternating current (AC) environment. 
   In one example, an MR system includes a cold mass comprises a superconducting magnet, a magnet coil support structure, and a helium vessel. Liquid helium contained in the helium vessel provides cooling for the superconducting magnet and maintains the superconducting magnet at a low temperature for superconducting operations, as will be understood by those skilled in the art. The liquid helium maintains the superconducting magnet approximately and/or substantially at the liquid helium temperature of 4.2 Kelvin (K). For thermal isolation, the helium vessel that contains the liquid helium in one example comprises a pressure vessel inside a vacuum vessel. 
   An MR superconducting magnet typically includes several coils, a set of primary coils that produce a uniform B 0  field at the imaging volume, and a set of bucking coils that limit the fringe field of the magnet. These coils are wound with superconductors such as NbTi or Nb3Sn conductors. The magnet is cooled down to liquid helium temperature (4.2 K) so that the conductors are operated at their superconducting state. The heat loads of the magnet, such as that produced by the radiation and conduction from the environment, are removed by either the boil-off of liquid helium in an “open system” or by a 4 K cryocooler in a “closed system”. The magnet is typically placed in a cryostat to minimize its heat loads since the replacement of liquid helium is expensive and since the cooling power of a cryocooler is limited. If the coils are exposed to an AC field such as an AC field generated by gradient coils of the MR system, AC losses are generated in the superconductors. That is, when superconducting coils are exposed to an AC field, hysteresis loss and eddy currents are induced therein that contribute to AC losses, which can raise the conductor temperatures and potentially cause a quench. The AC losses also add to the total heat load for the refrigeration system. A rise in heat load requires additional cryogenic refrigeration power, which increases operating costs. 
   It would therefore be desirable to have an apparatus configured to reduce AC losses caused by hysteresis loss and eddy currents induced in superconducting magnet coils. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention provides a superconductor for reducing AC losses in superconducting coils that overcomes the aforementioned drawbacks. 
   In accordance with one aspect of the invention, a low AC loss electrical conductor includes a radially central electrically conductive core and a first layer radially surrounding the conductive core and comprising a plurality of superconductor filaments. A second layer radially surrounds the first layer and forms a resistive shell thereabout. The electrical conductor also includes an insulation coating radially enclosing the second layer. 
   In accordance with another aspect of the invention, a method of constructing a conductor includes forming an electrically conductive core and winding superconducting filaments about the electrically conductive core. The method also includes forming a resistive shell enclosing the superconducting filaments and placing insulation about the resistive shell. 
   In accordance with yet another aspect of the invention, a superconducting cable includes a plurality of superconductors arranged in an insulative jacket. Each superconducting bundle has a plurality of superconducting strands wound about a central core, a shell surrounding the plurality of superconducting strands, and an insulation sleeve surrounding the shell. 
   Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a schematic block diagram of an MR imaging system that can benefit from incorporation of the present invention. 
       FIG. 2  is a cross-section view of a superconductor in accordance with the present invention. 
       FIG. 3  is a perspective cutaway view of a plurality of the superconductors of  FIG. 2  bundled together. 
       FIG. 4  is a perspective cutaway view of a plurality of superconducting strands in accordance with the present invention. 
       FIG. 5  is a cross-section view of a superconducting strands of  FIG. 4  along line  5 - 5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , superconducting magnet system  10  in an example comprises a superconducting magnet systems operating in an alternating current (AC) environment. Exemplary superconducting magnet systems comprise a transformer, a generator, a motor, superconducting magnet energy storage (SMES), and/or a magnetic resonance (MR) system. Although a conventional MR magnet operates in a DC mode, some MR magnets may operate under an AC magnetic field from the gradient coils when the gradient leakage field to the magnet is high. Such an AC magnetic field generates AC losses in the magnet. An illustrative discussion of exemplary details of a magnetic resonance and/or magnetic resonance imaging (MRI) apparatus and/or system are presented, for explanatory purposes. 
   The operation of the MR system is controlled from an operator console  12  which includes a keyboard or other input device  13 , a control panel  14 , and a display screen  16 . The console  12  communicates through a link  18  with a separate computer system  20  that enables an operator to control the production and display of images on the display screen  16 . The computer system  20  includes a number of modules which communicate with each other through a backplane  20   a . These include an image processor module  22 , a CPU module  24  and a memory module  26 , known in the art as a frame buffer for storing image data arrays. The computer system  20  is linked to disk storage  28  and tape drive  30  for storage of image data and programs, and communicates with a separate system control  32  through a high speed serial link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription. 
   The system control  32  includes a set of modules connected together by a backplane  32   a . These include a CPU module  36  and a pulse generator module  38  which connects to the operator console  12  through a serial link  40 . It is through link  40  that the system control  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module  38  connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  38  can also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module  38  connects to a scan room interface circuit  46  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan. 
   The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a magnet assembly  52  which includes a polarizing magnet  54  and a whole-body RF coil  56 . A transceiver module  58  in the system control  32  produces pulses which are amplified by an RF amplifier  60  and coupled to the RF coil  56  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil  56  and coupled through the transmit/receive switch  62  to a preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the coil  56  during the transmit mode and to connect the preamplifier  64  to the coil  56  during the receive mode. The transmit/receive switch  62  can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. 
   The MR signals picked up by the RF coil  56  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control  32 . A scan is complete when an array of raw k-space data has been acquired in the memory module  66 . This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor  68  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  34  to the computer system  20  where it is stored in memory, such as disk storage  28 . In response to commands received from the operator console  12 , this image data may be archived in long term storage, such as on the tape drive  30 , or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on the display  16 . 
     FIG. 2  shows a cross-section of a superconducting electrical conductor  70  having low AC losses in an AC field. Conductor  70  has a copper core  72  radially surrounded by a first layer  74  having a plurality of superconducting filaments  76  forming a superconductor filament bundle disposed in a copper matrix  78 . The filaments  76  are preferably constructed of niobium-titanium (NbTi); however, one skilled in the art will appreciate that the filaments  76  may be constructed of other superconducting materials. The filaments  76  are twisted longitudinally within the first layer  74  about the copper core  72 . Preferably, the twist pitch of the plurality of superconducting filaments  76  is greater than or equal to 100 mm. The filaments  76  are also, preferably, a few microns in diameter such that hysteresis losses are reduced due to a high AC field magnitude. 
   When the conductor  70  is exposed to increasing AC frequencies, electrical conduction therein begins to concentrate in the first layer  74 . As such, the electrical conduction skin depth of the conductor  70  decreases. The skin depth can be calculated by: 
                   t   =       ρ     π   ⁢           ⁢   f   ⁢           ⁢   μ           ,           (     Eqn   .           ⁢   1     )               
where ρ is the resistivity, μ is the permeability, and ƒ is the frequency. Electrical resistance between the filaments  76  and the copper matrix  78  is proportionally related to AC losses. That is, the lower the electrical resistance between the filaments  76  and the copper matrix  78 , the lower the skin depth of the electrical conduction. As the skin depth decreases, the eddy currents induced in the electrical conduction decreases. As such, the AC losses caused by induced eddy currents are also reduced.
 
   An outer shell  80  surrounds the first layer  74 , and an insulation coating  82  surrounds the outer shell  80 . The outer shell  80 , if conductive, can produce significant AC losses, especially at high frequencies. The outer shell  80  is, therefore, preferably constructed of resistive materials, such as CuNi or CuMn, to reduce induced eddy currents and AC losses. The insulation coating  82  insulates one conductor  70  from another or the conductor  70  from itself when wrapped on top of itself. In this manner, each conductor  70  or portion thereof acts individually in generating AC losses. 
     FIG. 3  shows a bundle  84  of conductors  70  of  FIG. 2  inside an insulative sleeve  86 . At higher frequencies, as the diameter of each conductor  70  is reduced, AC losses caused by eddy currents induced therein are also reduced. Each conductor  70  in the bundle  84  is in parallel with the other conductors  70 , and the number of conductors  70  in the bundle  84  may be increased such that the bundle  84  carries a desired current. Preferably, a portion of the insulation coating  82  of each conductor  70  is removed, and a bridge  88  is soldered to each conductor  70  at the site of the removed insulation coating  82  such that the conductors  70  are electrically connected to one another. A number of bridges  88  are periodically soldered to the conductors  70  along a length of the bundle  84 . Electrically connecting the conductors  70  together in this manner enhances current transfer and sharing therebetween and improves stability and quench performance of the bundle  84 . 
     FIG. 4  shows a bundle  90  of insulated single-filament superconducting strands  92 . The strands  92  are wound together in a Litz-type arrangement and bundled together via an insulation jacket  94 . The strands  92  are preferably fully transposed inside the insulation jacket  94 . To reduce AC losses generated in each strand  92 , the diameter thereof may be reduced, for example, to less than 0.15 mm. The strands  92  conduct current in parallel such that the bundle  90  carries a desired current. 
   A number of bridges  96  is periodically soldered to the strands  92  along a length of the bundle  90 . In this manner, a portion of an insulation coating  98  of each strand  92  is removed, and a bridge  96  is soldered to each strand  92  at the site of the removed insulation coating  98  such that the strands  92  are electrically connected to one another. The bridges  96  enhance current transfer and sharing between the strands  92  and improves stability and quench performance of the bundle  90 . 
     FIG. 5  shows a cross-section of one single-filament superconducting strand  92  along line  5 - 5  of  FIG. 4 . Strand  92  has a single superconducting filament  100  surrounded by a copper stabilizer  102 . The strand  92  is preferably constructed of NbTi; however, one skilled in the art will appreciate that the strand  92  may be constructed of other superconducting materials. To insulate one strand  92  from another, the stabilizer  102  is surrounded by insulation coating. 
   A superconducting coil constructed of the conductor  70  and/or bundles  84 ,  90  as described above reduces AC losses generated by induced eddy currents. AC losses are reduced in an MR superconducting magnet under a gradient pulsing AC field and in superconducting armature or field coils of a high speed generator or motor. 
   Therefore, a low AC loss electrical conductor is disclosed and includes a radially central electrically conductive core and a first layer radially surrounding the conductive core and comprising a plurality of superconductor filaments. A second layer radially surrounds the first layer and forms a resistive shell thereabout. The electrical conductor also includes an insulation coating radially enclosing the second layer. 
   A method of constructing a conductor is also presented and includes forming an electrically conductive core and winding superconducting filaments about the electrically conductive core. The method also includes forming a resistive shell enclosing the superconducting filaments and placing insulation about the resistive shell. 
   The present invention is also embodied in a superconducting cable that includes a plurality of superconductors arranged in an insulative jacket. Each superconducting bundle has a plurality of superconducting strands wound about a central core, a shell surrounding the plurality of superconducting strands, and an insulation sleeve surrounding the shell. 
   The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.