Abstract:
A high temperature superconducting (HTS) magnet coil disposed within a cryostat is configured with a thermo-siphon cooling system containing a liquid cryogen. The cooling system is configured to indirectly conduction cool the HTS magnet coil by nucleate boiling of the liquid cryogen that is circulated by the thermo-siphon in a cooling tube attached to a heat exchanger bonded to the outside surface of the HTS magnet coil. A supply dewar is configured with a re-condenser cryocooler coldhead to recondense boiloff vapors generated during the nucleate boiling process.

Description:
BACKGROUND 
     The invention relates generally to superconducting magnets, and more particularly to a high temperature superconducting (HTS) magnet implemented via indirect thermal conduction cooling of the HTS coil using a heat exchanger shell that is bonded to a thermo-siphon cooling coil filled with boiling liquid cryogen. 
     Superconducting magnets are typically immersed in liquid cryogen to implement direct cooling of the superconducting magnet coils. The direct cooling is generally implemented by pool boiling of the liquid cryogen in contact with the magnet coils. The cryogen is typically contained in a large and heavy vessel surrounding the magnet. The liquid cryogen inventory required to fill the vessel and cool the magnet by direct boiling is large and expensive. 
     It would be advantageous to provide a superconducting magnet that overcomes the size, weight and cost constraints associated with typical superconducting magnets described above. 
     BRIEF DESCRIPTION 
     Briefly, in accordance with one embodiment, a high temperature superconducting (HTS) magnet comprises: 
     a HTS magnet coil disposed within a cryostat; 
     a thermo-siphon cooling system comprising a liquid cryogen, the cooling system configured to indirectly conduction cool the HTS coil by nucleate boiling of the liquid cryogen that is circulated by thermo-siphon in a cooling tube attached to a heat exchanger bonded to the outside surface of the HTS magnet coil; and 
     a supply dewar comprising a re-condenser cryocooler coldhead configured to recondense boiloff vapors generated during the nucleate boiling process. 
     According to another embodiment, a high temperature superconducting (HTS) magnet comprises: 
     a HTS magnet coil comprising a heat exchanger bonded thereto; 
     a cooling system configured to indirectly conduction cool the HTS coil by nucleate boiling of a liquid cryogen that is circulated to the heat exchanger; and 
     a cryocooler coldhead configured to recondense boiloff vapors generated during the nucleate boiling process to generate the boiling liquid cryogen. 
     According to yet another embodiment of the invention, a method of cooling a high temperature superconducting (HTS) magnet comprises: 
     providing a HTS magnet coil comprising a heat exchanger bonded thereto; 
     subjecting the heat exchanger to a boiling liquid cryogen to indirectly cool the HTS magnet coil via thermal conduction between the HTS magnet coil and the heat exchanger; and 
     recondensing boiloff vapors generated via the thermal conduction process to generate a continuous supply of the boiling liquid cryogen. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a pictorial diagram illustrating a high temperature superconducting (HTS) magnet assembly according to one embodiment of the invention; 
         FIG. 2  is a sectional view illustrating cooling portions of the HTS magnet assembly depicted in  FIG. 1  in more detail; 
         FIG. 3  is a sectional view illustrating power leads of the HTS magnet assembly depicted in  FIG. 1  in more detail; and 
         FIG. 4  illustrates the HTS magnet assembly depicted in  FIGS. 1 and 2  installed in a vacuum insulated cryostat according to one embodiment of the invention. 
     
    
    
     While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
     DETAILED DESCRIPTION 
       FIG. 1  is a pictorial diagram illustrating a high temperature superconducting (HTS) magnet assembly  10  according to one embodiment of the invention. In the illustrated example arrangement, the magnet assembly  10  includes an HTS coil  12  having a copper heat exchanger shell  14  that may be a copper foil bonded to the outer surface of the HTS coil  12 . The present invention is not so limited however, and it shall be understood that thermally conductive materials known to those skilled in the heat transfer art, other than copper, can be employed in accordance with the principles described herein. Other materials can include, without limitation, aluminum, aluminum alloys, or copper alloys. The copper heat exchanger shell  14  is also bonded to a thermo-siphon cooling coil  16  described in more detail below with reference to  FIGS. 2 and 3 . 
     The thermo-siphon cooling coil  16  is filled with a suitable boiling liquid cryogen such as Neon, for example, to provide indirect thermal conduction cooling of the HTS coil  12  via the copper heat exchanger shell  14 . Other cryogens that can be used include, without limitation, nitrogen, hydrogen, and helium. The evaporated cryogen (e.g. Neon) is re-condensed remotely outside a coil cryostat  28  via a cryocooler coldhead  18  that is coupled to a supply dewar  20  having an inventory of cryogen. The size of inventory depends on the time of operation required to ride through power interruptions and outages, and may typically vary from 1 to 10 liters, according to some aspects of the invention. The HTS coil  12  and surrounding cryostat  28  advantageously provide a very lightweight compact superconducting magnet structure that is capable of withstanding high shock and vibration loads, with sufficient storage of cryogens to safely operate through power interruptions and outages. 
     The magnet assembly  10  also includes coil power leads  22 , described in more detail below with reference to  FIG. 3 . The coil power leads  22  are heat stationed to the cryogen cooling tubes  32  and  34  depicted in  FIGS. 2 and 3  outside the coil cryostat  28  through suitable high thermal conductivity electrical insulators  24  such as beryllium oxide ceramic insulators located in the diagonal space of the transverse blocks shown in  FIG. 3  that connect the leads  22  to the tubes  32 ,  34 ). Other materials can include, without limitation, quartz crystals. The HTS coil  12  is thermally insulated and mechanically suspended inside the cryostat  28  by a composite thermal support  26 , attached to the cryostat  28  at one end and to the HTS coil  12  at the other end. In a particular embodiment, the composite thermal support  26  comprises a thin cantilever fiber reinforced composite shell  26 . 
       FIG. 2  is a sectional view illustrating cooling portions of the HTS magnet assembly  10  depicted in  FIG. 1  in more detail. The HTS magnet assembly  10  is depicted in its normal operating position with a single-stage cryocooler  18  at the top of the assembly  10 . The cryocooler  18  is attached to a liquid cryogen dewar  20  at its lower end. The cryocooler includes a cryogen recondenser  19  at its lower portion that is disposed internal to the cryogen dewar  20 . A supply inventory of liquid cryogen  30  is contained within the liquid cryogen dewar  20 , below the cryogen recondenser  19 . Cryogen supply and return cooling tubes  32 ,  34  forming the end portions of a cryogen cooling coil  16  that forms a gravity driven thermo-siphon cooling system of liquid cryogen  30 , are bonded to the cryogen dewar  20 , and deliver the liquid cryogen  30  to the cooling coil  16  such that the HTS coil  12  is conduction cooled indirectly by nucleate boiling of the liquid cryogen  30 . For the illustrated example arrangement, the HTS coil  12  is thermally insulated and mechanically suspended inside the cryostat  28  by a thin cantilever fiber reinforced composite shell  26 , attached to the cryostat  28  at one end and to the coil  12  at the other end, as stated above. 
       FIG. 3  is a sectional perspective view illustrating power leads  22  of the HTS magnet assembly  10  depicted in  FIG. 1  in more detail. For the illustrated example arrangement, the power leads  22  of the HTS coil  12  are heat stationed to the cryogen cooling tubes  32 ,  34  outside the coil cryostat  28 . The HTS coil  12  with the cooling tubes  16 ,  32 ,  34  and power leads  22  are all suspended inside cryostat  28  by a thin cantilever fiber reinforced composite shell  26 , attached to the cryostat  28  at one end and to the coil  12  at the other end, as stated above. 
       FIG. 4  schematically illustrates the HTS magnet assembly  10  depicted in  FIGS. 1 and 2  installed in a cryostat  28  that is vacuum insulated  29 , according to one embodiment of the invention. In the illustrated embodiment, the HTS magnet assembly  10  can be seen to include an HTS coil  12  having a copper over-wrap heat exchanger  14  attached or bonded thereto. The HTS coil  12  is thermally insulated and mechanically suspended inside the vacuum insulated coil cryostat  28  by a composite thermal support  26  that is attached to the cryostat  28  at one end and to the Cu over-wrap heat exchanger  14  at its other end. A cryogen cooling tube  16  in contact with the copper over-wrap heat exchanger  14  operates to provide indirect thermal conduction cooling of the HTS coil  12  via a boiling liquid cryogen such as Neon contained within the cryogen cooling tube  16 . The evaporated cryogen (e.g. Neon) is re-condensed remotely outside the coil cryostat  28  by a cryocooler coldhead  18  discussed above with reference to  FIGS. 1 and 2 . 
     In summary explanation, a compact, high temperature superconducting (HTS) magnet has been described in accordance with particular embodiments that comprise an epoxy impregnated HTS coil in a vacuum insulated cryostat, a thermo-siphon cooling system of liquid cryogen, and a supply dewar with a re-condenser cryocooler coldhead. The HTS coil is conduction cooled indirectly by nucleate boiling of liquid cryogen that circulates by a gravity driven thermo-siphon in a cooling tube attached to a copper foil heat exchanger bonded to the outside surface of the HTS coil. The liquid cryogen is supplied to the heat exchanger from an external supply dewar and enters the cooling tube where it cools the HTS coil by boiling heat transfer; and the boiloff vapor returns to the supply dewar where it is re-condensed by a single stage cryocooler coldhead. 
     Advantages provided by the HTS magnet assembly  10  include 1) elimination of liquid cryogen filled vessels that are large and heavy, as well as 2) a reduction in the liquid cryogen inventory required to cool the HTS magnet by direct boiling, thus eliminating the large and expensive liquid cryogen inventory generally associated with known HTS magnet structures. Further, the HTS magnet assembly is very lightweight and compact, providing a structure that is capable of withstanding high shock and vibration loads, with sufficient storage of cryogens to safely operate through power interruptions and outages, as stated above. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.