Patent Publication Number: US-2007101742-A1

Title: A cooling system for superconducting magnets

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates generally to a cooling system for a superconductor, more particularly, to a cooling system having a reclamation circuit for retaining boil-off coolant within the superconductor cooling system.  
      A superconducting magnet generally uses liquid cryogens to keep its superconducting coils cold for superconducting operations. A closed cryogenic cooling system typically includes a cryogen liquid tank and a cryocooler recondensing unit. During normal superconducting operations, such a closed cryogenic system provides cooling to balance the heat load of the superconducting magnet so that no cryogen is lost. However, during a power outage or other system failure when the cooling power is lost, the superconducting magnet heat load tends to increase the pressure and the temperature of the cryogen. In an attempt to retain the pressure and temperature within a typical closed cryogenic cooling system, gaseous cryogen is released or boiled-off into the atmosphere through a pressure relief valve. As such, the boiled-off cryogen is lost to the atmosphere.  
      The cryogen from the system that is vented needs to be replaced when the system failure is corrected and the system returns to normal operation. Refilling the closed cryogenic cooling system usually requires that a cryogenic service perform a service call to refill the system. Such cryogenic services add service costs to the system, especially in areas where there is no established cryogenic service network. Furthermore, in systems such as an MRI scanner having a superconducting magnet, waiting for a cryogenic service to perform a service call increases the down-time of the scanner.  
      It would therefore be desirable to have a system and capable of reclaiming and storing boiled-off cryogen within a closed cryogenic cooling system.  
     BRIEF DESCRIPTION OF THE INVENTION  
      The present invention provides a cooling system for a superconducting magnet that overcomes the aforementioned drawbacks. A closed cryogenic cooling system includes a cryogen liquid tank, a cryogen storage tank, and a cryocooler recondensing unit. The cryocooler recondensing unit provides cooling to balance the magnet heat load during normal operation. During a power outage or other failure of the cryocooler recondensing unit, boiled-off gas is reclaimed in the storage tank. Upon re-initialization of the system, the stored boil-off gas is re-introduced into the system for cooling the superconducting magnet.  
      Therefore, in accordance with one aspect of the invention, a magnet assembly includes a magnet and a cooling system in thermal contact with the magnet. A tank is fluidly connected to the cooling system and configured to receive and store boil-off fluid emitted from the cooling system.  
      In accordance with another aspect of the invention, a superconductor system includes a superconducting magnet and a refrigerant in thermal contact with the superconducting magnet and configured to cool the superconducting magnet. A cooling system is included that is configured to condense the refrigerant from a gaseous state to a liquid state. A storage tank is fluidly connected to the cooling system and configured to store discharged refrigerant released from the cooling system.  
      In accordance with a further aspect of the invention, an MRI apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. A cryogenic cooling system is included and is in thermal contact with the magnet. A cryogen reclamation circuit is included and is fluidly connected to the cryogenic cooling system, and configured to store boiled-off cryogen released by the cryogenic cooling system.  
      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 diagram of a closed-loop magnet cooling system in accordance with the present invention.  
       FIG. 2  is a schematic block diagram of an MR imaging system for use with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIG. 1 , a preferred closed-loop magnet cooling system  10  according to the present invention is shown. Magnet  12  is actively cooled to support superconducting operation by a cooling system  14  having a plurality of coolant tubes thermally connected to coils (not shown) of the magnet  12 . In this regard, in a preferred embodiment, magnet  12  is a superconducting magnet having an arrangement of coils that create a magnetic field when current is passed therethrough. Cooling system  14  passes refrigerant  16 , such as helium, nitrogen, neon or the like, past the coils of the magnet  12  such that the temperature of magnet  12  is below a superconducting critical temperature. Preferably, refrigerant  16  is a cryogen.  
      Cooling system  14  includes a liquid mass collector  18  positioned such that liquid refrigerant  16  flows thereinto by gravity. Preferably, liquid mass collector  18  is positioned below magnet  12 . The cooling system  14  includes cooling tubes or risers  20  that surround magnet  12  and allow thermal communication between magnet  12  and refrigerant  16 . Risers  20  are in flow communication with the liquid mass collector  18 . In this manner, refrigerant  16  may flow from the liquid mass collector  18  into the risers  20 .  
      A gas collector  22  is fluidly connected to risers  20  and collects gaseous refrigerant. During normal operating conditions, refrigerant  16  is maintained at a constant boiling-point temperature. In this manner, the refrigerant is distributed throughout the risers  20  to cool magnet  12 . As boiling refrigerant passes through the risers  20 , gaseous refrigerant rises from the risers  20  into the gas collector  22 .  
      A liquefaction cup  24  is fluidly connected to the gas collector  22  and receives gaseous refrigerant. A cryocooler recondenser or cold head  26  is attached to the liquefaction cup  24  and condenses the gaseous refrigerant in the liquefaction cup  24  to liquid refrigerant. The liquid refrigerant falls to the bottom  27  of the liquefaction cup  24  and, with the aid of gravity, results in the liquid refrigerant flowing into the liquid mass collector  18 .  
      The cryocooler recondenser  26 , in general, produces more cooling than the total heat load of the magnet  12 . A heater  28  is fluidly connected to the liquid mass collector  18  and adds heat to the cooling system  14  to increase the pressure and temperature of the refrigerant  16  to a normal operating pressure and temperature. In a preferred embodiment, heater  28  is a pressure sensor controlled heater.  
      During normal operating conditions, cooling system  14  maintains an equilibrium of the refrigerant  16  in the system. That is, gaseous refrigerant that evaporates from the liquid refrigerant is re-condensed back into liquid refrigerant. However, during a failure condition such as a power outage or a cryocooler recondenser failure, the cooling system  14  cannot effectively re-condense the gaseous refrigerant into liquid refrigerant, and the equilibrium is lost. In this situation, the heat load of magnet  12  vaporizes the liquid refrigerant into gaseous refrigerant, and pressure within the cooling system begins to rise. The cooling system  14  contains a sufficient amount of liquid refrigerant to last a required ride-through time to allow restoration of the cooling system  14  from the failure condition. The ride-through time allows the cooling system  14  to continue to cool the magnet  12  using the liquid refrigerant remaining in the system until the supply of liquid refrigerant becomes exhausted. The ride-through time may allow, for example, the liquid refrigerant to continue to cool the magnet  12  from a half day to a full day.  
      During the ride-through time, the gaseous refrigerant is not re-condensed into liquid refrigerant, and the refrigerant  16  stays at its saturation line where gas and liquid coexist. A pressure build-up in the system caused by the failure of the cryocooler recondenser to re-condense the gaseous refrigerant causes the temperature of the refrigerant  16  to increase. To extend the time of the superconducting operation of the cooling system  14  during the failure condition, the pressure within the cooling system  14  is kept low.  
      Still referring to  FIG. 1 , a refrigerant reclamation circuit  30  is attached to the cooling system  14  to allow the gaseous refrigerant within the cooling system  14  to be released thereinto during the failure condition or other operational condition when it is desired to reclaim the refrigerant. A gas tank  32  is fluidly connected to the cooling system  14  and stores gaseous refrigerant flowing thereinto. A gas connection line  34  is fluidly connected to the gas tank  32  at a first end  36  and is fluidly connected to an arcuate loop  38 , e.g., U-shaped, at a second end  40 . Insulation  42  is attached to the gas tank  32  and to the gas connection line  34 . In this manner, the gaseous refrigerant boiled-off from the cooling system  14  is kept cold, and an overall pressure rise is slowed. Gas tank  32  is sufficiently sized and reinforced to contain the gaseous refrigerant at ambient or room temperature and pressure. However, a safety relief valve  44  may be attached to gas tank  32  to allow the pressure in the gas tank  32  to vent gaseous refrigerant to the outside air if the pressure within the gas tank  32  rises above a desired level, for example,  30  bars. Relief valve  44  or an additional fill valve  46  (shown in phantom) may be used to fill the closed-loop magnet cooling system  10  with refrigerant. The closed-loop magnet cooling system  10  may be filled with gaseous refrigerant following construction or repair of the closed-loop magnet cooling system  10  or in the event of a pressure release through safety relief valve  44 .  
      The arcuate loop  38  is fluidly connected to a top portion  48  of the liquefaction cup  24  such that gaseous refrigerant is communicated between the arcuate loop  38  and the liquefaction cup  24 . The arcuate loop  38  reduces natural convection between the cooling system  14  and the gas tank  32  during normal operating conditions. During the failure condition, however, a rise in pressure in the cooling system  14  causes gaseous refrigerant to pass from the liquefaction cup  24  and into the gas tank  32  through the arcuate loop  38  and gas connection line  34 . An extended failure condition period will cause all the liquid refrigerant to convert to its gaseous state.  
      In an alternative embodiment, a flow valve assembly may be used to reduce natural convection between the cooling system  14  and the gas tank  32 . For example, a pair of anti-parallel valves may be used to control convection and flow between the cooling system  14  and the gas tank  32 .  
      Upon restoration and initialization of the cooling system  14  to operating conditions, the cryocooler recondenser  26  begins to condense the gaseous refrigerant existing in the cooling system  14 . As the gaseous refrigerant condenses to liquid refrigerant, the temperature and pressure within cooling system  14  begin to fall. As the pressure within the liquefaction cup  24  falls, a higher pressure in the reclamation circuit  30  causes the gaseous refrigerant stored therein to flow from the gas tank  32  to the liquefaction cup  24  through the arcuate loop  38  and gas connection line  34 . The gaseous refrigerant stored in the gas tank  32  continues to flow into the liquefaction cup  24  until a pressure equilibrium is established between the cooling system  14  and the reclamation circuit  30 . Thereafter, the cooling system  14  functions to maintain an equilibrium of the refrigerant  16  within the cooling system  14  as described above.  
      Referring now to  FIG. 2 , it is contemplated that the closed-loop magnet cooling system  10  may be particularly applicable, but not limited to actively cooling superconducting coils of an MR imaging system  50 . As is well known, operation of the MR imaging system  50  is controlled from an operator console  52  which includes a keyboard or other input device  53 , a control panel  54 , and a display  56  or screen. The console  52  communicates through a link  58  with a separate computer system  60  that enables an operator to control the production and display of images on the screen  56 . The computer system  60  includes a number of modules which communicate with each other through a backplane  60   a.  These include an image processor module  62 , a CPU module  64  and a memory module  66 , known in the art as a frame buffer for storing image data arrays. The computer system  60  is linked to disk storage  68  and tape drive  70  for storage of image data and programs, and communicates with a separate system control  72  through a high speed serial link  74 . The input device  53  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  72  includes a set of modules connected together by a backplane  72   a.  These include a CPU module  76  and a pulse generator module  78  which connects to the operator console  52  through a serial link  80 . It is through link  80  that the system control  72  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  78  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  78  connects to a set of gradient amplifiers  82 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  78  can also receive subject data from a physiological acquisition controller  84  that receives signals from a number of different sensors connected to the subject, such as ECG signals from electrodes attached to the subject. And finally, the pulse generator module  78  connects to a scan room interface circuit  86  which receives signals from various sensors associated with the condition of the subject and the magnet system. It is also through the scan room interface circuit  86  that a subject positioning system  88  receives commands to move the subject to the desired position for the scan.  
      The gradient waveforms produced by the pulse generator module  78  are applied to the gradient amplifier system  82  having Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  90  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  90  forms part of a magnet assembly  92  which includes a polarizing magnet  94  and a whole-body RF coil  96 . A transceiver module  98  in the system control  72  produces pulses which are amplified by an RF amplifier  100  and coupled to the RF coil  96  by a transmit/receive switch  102 . The resulting signals emitted by the excited nuclei in the subject may be sensed by the same RF coil  96  and coupled through the transmit/receive switch  102  to a preamplifier  104 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  98 . The transmit/receive switch  102  is controlled by a signal from the pulse generator module  78  to electrically connect the RF amplifier  100  to the coil  96  during the transmit mode and to connect the preamplifier  104  to the coil  96  during the receive mode. The transmit/receive switch  102  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  96  are digitized by the transceiver module  98  and transferred to a memory module  106  in the system control  72 . A scan is complete when an array of raw k-space data has been acquired in the memory module  106 . 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  108  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  74  to the computer system  60  where it is stored in memory, such as disk storage  68 . In response to commands received from the operator console  52 , this image data may be archived in long term storage, such as on the tape drive  70 , or it may be further processed by the image processor  62  and conveyed to the operator console  52  and presented on the display  56 .  
      Not only may the closed-loop magnet cooling system of the present invention be used in the MR imaging system as described above, it may be used to cool other coils of the MR imaging system for which active cooling is used. Further, the present invention is not limited to actively cooling superconducting magnetic coils used in an MR imaging system. It is contemplated that the closed-loop magnet cooling system  10  may be used in any actively cooled superconducting magnetic coil system. Additionally, the invention may be embodied in any coil cooling system where it is desirable to reclaim boiled off refrigerant as opposed to releasing such refrigerant to atmosphere.  
      A closed-loop magnet cooling system according to the present invention includes the advantage of reducing service costs associated with cryogen refilling services when a failure condition appears. Instead of releasing gaseous refrigerant outside the system, the present invention includes storing the released gaseous refrigerant. In this manner, the stored refrigerant is used when the system becomes operable. As such, cryogen refilling service calls are reduced. Furthermore, where a cryogen refilling service network is not established, as in a developing country, for example, the closed-loop magnet cooling system according to the present invention allows a much sooner system restart.  
      Therefore, in accordance with one embodiment of the invention, a magnet assembly includes a magnet and a cooling system in thermal contact with the magnet. A tank is fluidly connected to the cooling system and configured to receive and store boil-off fluid emitted from the cooling system.  
      In accordance with another embodiment of the invention, a superconductor system includes a superconducting magnet and a refrigerant in thermal contact with the superconducting magnet and configured to cool the superconducting magnet. A cooling system is included that is configured to condense the refrigerant from a gaseous state to a liquid state. A storage tank is fluidly connected to the cooling system and configured to store discharged refrigerant released from the cooling system.  
      In accordance with a further embodiment of the invention, an MRI apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. A cryogenic cooling system is included and is in thermal contact with the magnet. A cryogen reclamation circuit is included and is fluidly connected to the cryogenic cooling system and configured to store boiled-off cryogen released by the cryogenic cooling system.  
      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.