Patent Publication Number: US-2013234815-A1

Title: Persistent switch control system, superconducting magnet apparatus employing the same, and method of controlling persistent switch

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2012-0025224, filed on Mar. 12, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Systems, apparatuses, and methods consistent with exemplary embodiments relate to a persistent switch control system, a superconducting magnet apparatus employing the persistent switch control system, and a method of controlling a persistent switch. 
     2. Description of the Related Art 
     A superconducting apparatus such as a magnetic resonance imaging (MRI) apparatus or a nuclear magnetic resonance (NMR) apparatus uses a superconducting magnet. The superconducting magnet operates when a current is applied to a superconducting coil cooled at an ultralow temperature, such as 4.2K, so that a superconducting phenomenon is generated. The superconducting magnet operates often in a persistent mode and includes a persistent switch to enter the persistent mode. The persistent mode is for allowing a current to flow through a closed loop in a superconducting magnet, and the persistent switch is used to block a current flow to a current lead between a superconducting magnet at a low temperature and a magnet power source at a normal temperature. Such persistent switch provides high field stability in application devices such as NMR apparatuses and MRI apparatuses, and can reduce a low temperature heat load for increasing a liquid helium retention time of a cryostat. 
     Generally, the persistent switch includes a superconducting wire and a switch heater. As heat is applied from the switch heater to the superconducting wire, the superconducting wire has resistivity since the temperature thereof rises above a superconducting transition temperature. When the persistent switch is designed appropriately, the persistent switch has a relatively high resistance. A high resistance of the persistent switch in an open state allows the superconducting magnet to be charged. The conducting wire is cooled and restores superconductivity when the charge is completed, and thus, the persistent switch is closed. 
     SUMMARY 
     Applying heat to a persistent switch to open it causes a heat load on a cooling system of a superconducting magnet apparatus. For example, applying heat to a persistent switch to open it causes consumption of liquid helium. Furthermore, consumption of liquid helium is additionally increased due to heat dispersed from the persistent switch, which is caused by a ramp voltage that is applied to the persistent switch when the superconducting magnet is charged. 
     An aspect of an exemplary embodiment provides a persistent switch control system for reducing a heat load that is generated when a superconducting magnet is charged. 
     An aspect of an exemplary embodiment also provides a superconducting magnet apparatus employing the persistent switch control system. 
     Another aspect of an exemplary embodiment also provides a method of controlling a persistent switch. 
     According to an aspect of an exemplary embodiment, there is provided a persistent switch control system including: a persistent switch which switches between an open state and a closed state of a superconducting coil; and a persistent switch controller which controls the persistent switch, wherein, during a charging mode, a resistance state of the persistent switch is maintained by a ramp heat load that is generated by a ramp voltage applied to the persistent switch. 
     The persistent switch may include: a superconducting wire which constitutes a portion of a superconducting coil; and a switch heater which applies heat to the superconducting wire, wherein the persistent switch controller may supply a power supply voltage to the switch heater when the charging mode starts and may block the power supply voltage supplied to the switch heater when a current is supplied to the superconducting coil. 
     In the charging mode, a ramping rate of the current that is supplied to the superconducting coil may be set so that the ramp heat load, which is generated by the ramp voltage applied between ends of the superconducting wire, maintains a resistance state of the superconducting wire. 
     The supply of current to the superconducting coil may be stopped when the current that is supplied to the superconducting coil reaches a target current. The current that is supplied to the superconducting coil may be adjusted when it is substantially close to the target current. The persistent switch controller may turn on the switch heater before adjusting the current that is supplied to the superconducting coil, and may turn off the switch heater after adjusting the current that is supplied to the superconducting coil. 
     According to another aspect of an exemplary embodiment, there is provided a superconducting magnet apparatus including: a superconducting coil; a power source for a superconducting coil, which supplies a current to the superconducting coil; and a persistent switch control system, which controls an open state and a closed state of the superconducting coil. The persistent switch control system comprises: a persistent switch which switches between the open state and the closed state of the superconducting coil; and a persistent switch controller which controls the persistent switch, wherein, during a charging mode, a resistance state of the persistent switch is maintained by a ramp heat load that is generated by a ramp voltage applied to the persistent switch. 
     The superconducting magnet apparatus may be a magnetic resonance imaging (MRI) apparatus, a nuclear magnetic resonance (NMR) apparatus, or a superconducting magnet apparatus for a maglev car. 
     According to another aspect of an exemplary embodiment, there is provided a method of controlling a persistent switch for switching between an open state and a closed state of a superconducting coil, the method including maintaining a resistance state of the persistent switch by a ramp heat load that is generated by a ramp voltage that is applied to the persistent switch during a charging mode. 
     The persistent switch control system, the superconducting magnet apparatus employing the persistent switch control system, and the method of controlling a persistent switch may reduce a heat load, which is separately applied from the outside for operation of the persistent switch, by using the persistent switch&#39;s own ramp heat load in a charging mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of exemplary embodiments will become more apparent with reference to the attached drawings in which: 
         FIG. 1  schematically illustrates a superconducting magnet apparatus employing a persistent switch control system, according to an exemplary embodiment; 
         FIG. 2  is a flowchart illustrating a method of controlling a persistent switch, according to an exemplary embodiment; and 
         FIG. 3  is a flowchart illustrating an example of an operation of minutely adjusting a current in the method of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, exemplary embodiments will be described in detail with reference to the attached drawings. The same reference numerals in the drawings denote the same element. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
       FIG. 1  schematically illustrates a persistent switch control system  120  according to an exemplary embodiment and a superconducting magnet apparatus  100  employing the persistent switch control system  120 . 
     Referring to  FIG. 1 , the superconducting magnet apparatus  100  includes a superconducting coil  110 , a current lead  150  that transmits a current to the superconducting coil  110 , a power source  160  for a superconducting magnet, which supplies a current to the superconducting coil  110  through the current lead  150 , and a cryostat  190  that maintains the temperature of the superconducting coil  110  in an ultralow temperature state. In addition, the superconducting magnet apparatus  100  includes a persistent switch control system  120 . Furthermore, although not illustrated, the superconducting magnet apparatus  100  may further include a cooler that cools the superconducting coil  110  and maintains an ultralow temperature state of the cryostat  190 . 
     The persistent switch control system  120  includes a persistent switch  121  that is disposed at one side of the superconducting coil  110  and a persistent switch controller  125  that is disposed outside the cryostat  190  and controls the persistent switch  121 . 
     The persistent switch  121  switches between an open state of the superconducting coil  110  and a closed state thereof. That is, the state of the persistent switch  121  may become a resistance state and thereby allow the superconducting coil  110  to be in the open state, and may become a superconducting state and thereby allow the superconducting coil  110  to be in the closed state. For this, the persistent switch  121  includes a superconducting wire  122  and a switch heater  123  that is disclosed adjacent to the superconducting wire  122  and may apply heat to the superconducting wire  122 . It may be understood that the superconducting wire  122  is a portion of the superconducting coil  110  in a persistent mode. From the viewpoint of the power source  160  for a superconducting magnet, the superconducting wire  122  and the superconducting coil  110  are connected to each other in parallel. The superconducting wire  122  may be formed of a non-inductive coil to have a minimum inductance. The superconducting wire  122  is designed to have a ramp heat load that may maintain a temperature for maintaining the resistance state (that is, a state in which the superconducting state is broken). That is, the superconducting wire  122  is designed so that a resistance value thereof in the resistance state has a predetermined ramp heat load. The ramp heat load will be described below. 
     The persistent switch controller  125  may control the persistent switch  121  by controlling a power supply that is applied to the switch heater  123 . In addition, the persistent switch  121  may be controlled by the ramp heat load also in the state where the switch heater  123  has been turned off, and the ramp heat load may be controlled by adjusting a ramping rate of a current that is supplied from the power source  160  for a superconducting magnet. 
     The superconducting coil  110  may be electrically connected to the power source  160  for a superconducting magnet, which is disposed outside the cryostat  190 , through the current lead  150 , and thus may receive a current from the power source  160  in a charging mode. In the superconducting magnet apparatus  100 , a current supply to the superconducting coil  110  is blocked in the persistent mode, and in some cases, the current lead  150  may have a detachable and attachable structure. 
       FIG. 2  is a flowchart illustrating a method of controlling a persistent switch in the superconducting magnet apparatus  100 , according to an exemplary embodiment. The method of controlling a persistent switch will be described with reference to  FIGS. 1 and 2 . 
     The superconducting coil  110  is cooled in the superconducting state during operation of the superconducting magnet apparatus  100 . The superconducting magnet apparatus  100  may have a charging mode in which a current is supplied to the superconducting coil  110  to charge it and a persistent mode in which a current flows in a closed circuit of the superconducting coil  110 . 
     Prior to start of the charging mode, a target current and a ramping rate are set (operation S 10 ). The target current is the amount of current that is used for determining whether charging has been completed. The ramping rate is a change of current via time, in which the current is gradually increased until the target current is reached. The ramping rate is set in consideration of a load that is generated by the inductance of the superconducting coil  110 , and is set so that the superconducting wire  122  may maintain the resistance state by using a ramp heat load thereof in the charging mode. 
     Next, when the charging mode starts, the persistent switch controller  125  applies heat to the superconducting wire  122  to break the superconducting state thereof, by supplying a current to the switch heater  123 , and changes the state of the superconducting wire  122  from the superconducting state to the resistance state (operation S 20 ). For example, the amount H r  of heat per hour, which is generated in the switch heater  123 , may be obtained according to equation 1 below. 
       H r   =I   h   2   ·R   h    (1)
 
     I r  denotes a current supplied to the switch heater  123 , and R h  denotes a resistance value of the switch heater  123 . For example, if R h  is 100Ω and I b  is 30 mA, the amount H r  of heat per hour, which is generated in the switch heater  123 , is 90 mW, and the generated heat may increase the temperature of the superconducting wire  122  to a temperature over 10K by heating the superconducting wire  122 . Thus, the state of the superconducting wire  122  may be changed from the superconducting state to the resistance state. If the state of the superconducting wire  122  is changed from the superconducting state to the resistance state, it may be understood that the persistent switch  121  is substantially in an open state while the superconducting coil  110  is in the superconducting state. 
     If the state of the persistent switch  121  is changed to the open state, the power source  160  for a superconducting magnet supplies a current to the superconducting coil  110  through the current lead  150  (operation S 30 ). Since the superconducting wire  122  is in the resistance state in the charging mode, most of the current flows to the superconducting coil  110  that substantially has no resistance, and thus, the superconducting coil  110  is charged. In this case, the power source  160  for a superconducting magnet slowly increases a current (that is, ramps up a current), which is supplied to the superconducting coil  110 , in consideration of the inductance of the superconducting coil  110 . If a voltage V r  is applied through the current lead  150  from the power source  160  for a superconducting magnet, the increase amount dI/dt of current that is supplied to the superconducting coil  110  may be obtained by the following equation 2. 
     
       
         
           
             
               
                 
                   
                     
                        
                       I 
                     
                     
                        
                       t 
                     
                   
                   = 
                   
                     
                       V 
                       r 
                     
                     L 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     L denotes the inductance of the superconducting coil  110 . The increase amount dI/dt of current is referred to as a ramping rate, and the voltage V r  that is applied to the superconducting coil  110  is referred to as a ramp voltage. 
     Referring to equation 2, it may be understood that a current flowing through the superconducting coil  110  increases by the ramping rate Vr/L if the ramp voltage V r  is applied to the superconducting coil  110 . 
     Since the superconducting wire  122  is connected in parallel to the superconducting coil  110 , the ramp voltage V r  supplied from the power source  160  for a superconducting magnet is applied also between ends of the superconducting wire  122 . Since the superconducting wire  122  is in the resistance state in the charging mode, the superconducting wire  122  generates Joule&#39;s heat due to the ramp voltage V r . A heat load H r  of the superconducting wire  122  (hereinafter, referred to as a ramp heat load), which is generated due to the ramp voltage V r , may be obtained according to equation 3 below. 
     
       
         
           
             
               
                 
                   
                     H 
                     r 
                   
                   = 
                   
                     
                       V 
                       r 
                       2 
                     
                     
                       R 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     R s  denotes the resistance of the superconducting wire  122  in the resistance state. 
     Referring to  FIG. 3 , in the resistance state, the superconducting wire  122  has a ramp heat load H r  that is proportional to the square of the ramp voltage V r  and is inversely proportional to the resistance R s  of the superconducting wire  122 . Conventional superconducting magnet apparatuses minimize a heat load of a persistent switch by enlarging the resistance of a superconducting wire of the persistent switch in the resistance state. However, since the superconducting magnet apparatus  100  according to the current exemplary embodiment uses heat that is generated in the resistance state of the superconducting wire  122 , the resistance R s  of the superconducting wire  122  is set to a small value such that the ramp heat load H r  has a heat load value so that the superconducting wire  122  may be maintained in the resistance state when the switch heater  123  has been turned off. The heat load value to maintain the superconducting wire  122  in the resistance state may be changed according to a thermal environment of the superconducting wire  122 . For example, if the resistance R s  of the superconducting wire  122  is 500Ω and the ramp voltage V r  is 4 V, the ramp heat load H r  becomes 32 mW, and the ramp heat load H r  of 32 mW may maintain the superconducting wire  122  in the resistance state when the switch heater  123  has been turned off. 
     When or immediately after the power source  160  for a superconducting magnet applies the ramp voltage V r  to the superconducting coil  110 , the persistent switch controller  125  blocks a current, which is applied to the switch heater  123 , to change the state of the switch heater  123  to a turn-off state (operation of S 40 ). Since the resistance and the ramping rate (or the ramping voltage) of the superconducting wire  122  according to the current exemplary embodiment is designed to have a ramp heat load H r  so that the resistance state may be maintained in the charging mode, the superconducting wire  122  may maintain the resistance state due to heat, which is generated by the superconducting wire  122  itself, although the state of the switch heater  123  is changed to the turn-off state. Thus, since the persistent switch  121  is still in the open state, the superconducting coil  110  is charged by a current that is supplied from the power source  160  for a superconducting magnet. 
     Next, after a predetermined time has elapsed, the power source  160  for a superconducting magnet determines whether the charging of the superconducting coil  110  has been completed (operation S 50 ). For example, it is possible to determine whether the charging has been completed by determining whether a current that is supplied from the power source  160  for a superconducting magnet has reached a target current. Until a current that is supplied from the power source  160  for a superconducting magnet is close to the target current, the turn-off state of the switch heater  123  and the charging state are maintained. In this case, a heat load that is generated in the switch heater  123  may be minimized since the switch heater  123  is in the turn-off state, and thus, a heat load that is applied to the cryostat  190  may be reduced. 
     If a current that is supplied from the power source  160  for a superconducting magnet reaches a target current, the power source  160  for a superconducting magnet stops the supply of current. A ramp heat load that is generated in the superconducting wire  122  is also reduced when the supply of current from the power source  160  is stopped, and thus, the superconducting wire  122  may be automatically cooled and then may return to the superconducting state. If the superconducting wire  122  returns to the superconducting state, the persistent switch  121  is closed and a current flowing through the superconducting coil  110  is turned toward the persistent switch  121  constituting a closed loop, and thus, the charging mode is ended. 
     Operation S 60  of minutely adjusting a current may be further performed in the operation of stopping the supply of current from the power source  160  for a superconducting magnet.  FIG. 3  illustrates an example of operation S 60  of minutely adjusting a current. Referring to  FIG. 3 , if a current that is supplied from the power source  160  for a superconducting magnet is close to the target current, the power source  160  minutely adjusts the current to make the current reach the target current and then stops the supply of current if the current reaches the target current (operation S 63 ). Since a change of the current in operation S 63  may be relatively large or relatively small at need, the resistance state of the superconducting wire  122  may not be maintained by a ramp heat load alone that is generated in the superconducting wire  122 . Thus, by further performing operation S 61  of turning on the switch heater  123  prior to operation S 63  of minutely adjusting a current and performing operation S 65  of turning off the switch heater  125  again if operation S 63  of minutely adjusting a current is completed, the persistent switch  121  may stably maintain an open state in operation S 63  of minutely adjusting a current. If the supply of current from the power source  160  for a superconducting magnet is stopped and the switch heater  123  is turned off again, the persistent switch  121  is closed and a current flowing through the superconducting coil  110  is turned toward the persistent switch  121  constituting a closed loop, and thus, the charging mode is ended. 
     Since, as stated above, the superconducting magnet apparatus  100  according to the current exemplary embodiment places the switch heater  123  in the turn-off state except in the charging process and the minute current adjustment process, the superconducting magnet apparatus  100  may minimize a heat load that is generated in the switch heater  123 , may reduce a heat load that is applied to a cooler of the superconducting magnet apparatus  100 , and may improve heat efficiency. 
     The superconducting magnet apparatus  100  according to the current exemplary embodiment may be a magnetic resonance imaging (MRI) apparatus, a nuclear magnetic resonance (NMR) apparatus, a superconducting magnet apparatus for a maglev car, or the like. For example, if the superconducting magnet apparatus  100  is an MRI apparatus, the superconducting magnet apparatus  100  may further include a gradient coil or a radio frequency (RF) coil. 
     While exemplary embodiments have been particularly shown and described, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.