Patent Publication Number: US-8989828-B2

Title: Superconducting magnet apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to superconducting magnet apparatuses each equipped with a superconducting coil, and more particularly to protection of a superconducting coil during a quench. 
     2. Description of the Related Art 
     Superconducting magnet apparatuses are each equipped with, for example, a superconducting coil, an excitation power supply that supplies current to the superconducting coil, and a persistent current switch that forms a closed circuit for supplying a persistent current. Once a portion of the superconducting coil being energized with the persistent current has suffered a transition into a normal conducting state and developed resistance, the resulting occurrence of joule heat will convert stored magnetic energy into heat energy and increase the temperature of the superconducting coil portion which has transitioned into normal electrical conduction. The periphery of the superconducting coil section which has entered the normal conducting state will also suffer a temperature rise due to heat conduction and make a transition from superconductivity into normal electrical conduction. This transition into normal conduction may eventually extend to the entire superconducting coil in rapid sequence, thus resulting in a so-called quench occurring. When the persistent current is flowing through the superconducting coil and this superconducting coil is holding a large volume of stored magnetic energy, if the large volume of stored magnetic energy is converted into heat energy by the quench, a possible excessive increase in the temperature of the superconducting coil might result in thermal damage to the coil. 
     Consider a case in which the superconducting coil is a high-temperature superconducting coil constructed of a high-temperature superconductor having a critical temperature exceeding 18 K, such as magnesium diboride (MgB 2 ), iron-based superconductor, or oxide superconductor. The critical temperatures of high-temperature superconductors lie in a region that these superconductors have specific heat capacities at least 10 times as great as those of niobium titanium (NbTi), niobium tin (Nb3Sn), and other low-temperature superconductors having critical temperatures below 18 K. Heat conduction due to a quench causes a delay in the propagation of a normal-conducting region. The quench in a high-temperature superconducting coil, therefore, causes a more significant temperature rise than in low-temperature superconducting coils, since stored magnetic energy is consumed locally. 
     For this reason, JP-1993-190325-A and other related technical documents propose methods of protecting a superconducting coil. In these methods, a protective resistor that receives a supply of current upon a quench event and consumes stored magnetic energy is provided to suppress the consumption of the stored magnetic energy in the superconducting coil. Since the amount of energy that the protective resistor consumes is proportional to the square of the value of the current flowing through the resistor, applying a higher current to the protective resistor yields a greater suppression effect against the temperature rise due to the quench in the superconducting coil. JP-1991-278504-A and other related technical documents propose methods of supplying a high current to a protective resistor. That is to say, the protective resistor and a persistent current switch are each connected in parallel to and across a superconducting coil so that when a quench occurs, a section of a closed circuit composed of the protective resistor and the persistent current switch, this section not being a closed circuit composed of the protective resistor and the superconducting coil, will be electrically disconnected. By so doing, the current that has been supplied to the persistent current switch can be bypassed and induced into the protective resistor. In addition, when a superconducting magnet apparatus is to be operated on a persistent current, a current lead needs to be disconnected from the internal superconducting circuit of a cryostat for suppressed entry of heat into the cryostat, so a protective resistor cannot be connected to the outside of the cryostat. In this case, therefore, the protective resistor is to be connected to the inside of the cryostat and this connection makes it necessary to provide large enough an installation space inside the cryostat. JP-1986-20303-A and the like, for example, propose methods in which a normal-conducting wire to perform the function of a protective resistor is wound around a superconducting coil in order to save the space required for protective resistor connection. 
     SUMMARY OF THE INVENTION 
     To induce a high current into a protective resistor so that stored magnetic energy is consumed therein, a heat capacity large enough to avoid thermal damage due to the induction of the high current needs to be imparted to the protective resistor. To this end, a large installation space needs to be provided for the protective resistor. According to JP-1986-20303-A and the like, since a section for supporting the protective resistor can be imparted to the superconducting coil, an installation space for the support section can be saved and that of the protective resistor can be correspondingly increased. Even so, the installation space for the protective resistor is required and the need to provide a large installation space for the resistor remains to be met. It is considered useful if the installation space for the protective resistor can be reduced while at the same time assigning it the function that consumes the stored magnetic energy without causing thermal damage. 
     Accordingly an object of the present invention is to provide a superconducting magnet apparatus adapted to consume stored magnetic energy without causing thermal damage to a protective resistor, even if an installation space for the protective resistor is reduced. 
     In order to solve the foregoing problems, a superconducting magnet apparatus according to an aspect of the present invention includes: a bobbin around which a superconducting coil is wound, the bobbin serving as a protective resistor; a persistent current switch for supplying a persistent current to the superconducting coil; a first closed circuit with the superconducting coil and the persistent current switch connected to each other in series; and a second closed circuit with the superconducting coil and the bobbin connected to each other in series. 
     In accordance with the present invention, since the protective resistor also serves as the bobbin for the superconducting coil, providing a space for the superconducting coil bobbin makes it unnecessary to provide an independent space for the protective resistor. This means that substantially the space provided for the protective resistor separately from the space for the bobbin can be reduced. In other words, a superconducting magnet apparatus adapted to consume stored magnetic energy without causing thermal damage to a protective resistor, even if an installation space for the protective resistor is reduced, can be supplied in accordance with the present invention. Further objects, configurational aspects, and advantages of the invention will be apparent from the detailed description of embodiments that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: 
         FIG. 1  is a circuit diagram of a superconducting magnet apparatus according to a first embodiment of the present invention; 
         FIG. 2A  is a longitudinal sectional view of a fuse and its peripheral members; 
         FIG. 2B  is a transverse sectional view of the fuse and its peripheral members; 
         FIG. 3A  is a front view of a bobbin for a superconducting coil; 
         FIG. 3B  is a longitudinal sectional view of the superconducting coil and its bobbin; 
         FIG. 4  is a circuit diagram of a superconducting magnet apparatus according to a second embodiment of the present invention; 
         FIG. 5  is a circuit diagram of a superconducting magnet apparatus according to a third embodiment of the present invention; 
         FIG. 6A  is a front view of a bobbin for a superconducting coil used in a superconducting magnet apparatus according to a fifth embodiment of the present invention; 
         FIG. 6B  is a longitudinal sectional view of the superconducting coil and its bobbin used in the superconducting magnet apparatus according to the fifth embodiment of the present invention; 
         FIG. 7A  is a longitudinal sectional view of fastening portions and peripheral members thereof; 
         FIG. 7B  is a transverse sectional view of a fastening portion and peripheral members existing when seen from a direction of line B-B in  FIG. 7A ; 
         FIG. 8A  is a front view of a bobbin for a superconducting coil used in a superconducting magnet apparatus according to a sixth embodiment of the present invention; 
         FIG. 8B  is a longitudinal sectional view of the superconducting coil and its bobbin used in the superconducting magnet apparatus according to the sixth embodiment of the present invention; 
         FIG. 9A  is a front view of a bobbin for a superconducting coil used in a superconducting magnet apparatus according to a seventh embodiment of the present invention; and 
         FIG. 9B  is a longitudinal sectional view of the superconducting coil and its bobbin used in the superconducting magnet apparatus according to the seventh embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes embodiments of the present invention in detail referring to the accompanying drawings as appropriate. Elements common to each drawing are each assigned the same reference number or symbol, and overlapped description is omitted herein. 
     First Embodiment 
     Configuration of a Superconducting Magnet Apparatus  1   
       FIG. 1  shows a circuit diagram of a superconducting magnet apparatus  1  according to a first embodiment of the present invention. The superconducting magnet apparatus  1  includes a superconducting coil  3 , a fuse  4 , a persistent current switch  6 , a bobbin  5  around which a superconducting coil is wound, the bobbin  5  functioning as a protective resistor, a circuit breaker  11 , and an excitation power supply  10 . 
     The superconducting coil  3  is provided in singularity or plurality (in an example of  FIG. 1 , two units). The superconducting coil  3  uses a high-temperature superconductor having a critical temperature exceeding 18 K, such as magnesium diboride (MgB 2 ), iron-based superconductor, or oxide superconductor. The plurality of (in the example of  FIG. 1 , two) superconducting coils  3  ( 3   a ,  3   b ) are connected in series. The superconducting coils  3  ( 3   a ,  3   b ) are each constructed of a superconducting wire wound around the bobbin  5 . In each superconducting coil, a peripheral region of one or a plurality of superconducting filaments is shrouded with a cryogenic stabilizer, spatial gaps between the plurality of superconducting filaments are filled in with the cryogenic stabilizer, and the superconducting filaments are bundled together in the cryogenic stabilizer. The superconducting filaments can be, for example, magnesium diboride (MgB 2 ), iron-based superconductors, or oxide superconductors. The cryogenic stabilizer is preferably a material having low electrical resistivity and high thermal conductivity, and can be, for example, silver (Ag), oxygen-free copper (pure copper: Cu), iron (Fe), or the like. The superconducting wire may also be applied to wiring interconnected between the superconducting coil  3 , the fuse  4 , the persistent current switch  6 , and the like. 
     As with the superconducting coil  3 , the persistent current switch  6  uses a high-temperature superconductor having a critical temperature exceeding 18 K. The persistent current switch  6  includes a superconducting wire and a heater. As in the superconducting coil  3 , in the superconducting wire, a peripheral region of one or a plurality of superconducting filaments is shrouded with a cryogenic stabilizer, spatial gaps between the plurality of superconducting filaments are filled in with the cryogenic stabilizer, and the superconducting filaments are bundled together in the cryogenic stabilizer. The heater and superconducting wire of the persistent current switch  6  are thermally connected to each other. When the heater generates heat, the heater heats the superconducting wire and thus enables the superconducting wire to make a transition from a superconducting state into a normal conducting state. The transition of the superconducting wire into the normal conducting state opens (turns off) the persistent current switch  6 . Conversely, stopping the generation of heat in the heater provides cooling by a heat transfer element  2   d  described later herein, hence allows the superconducting wire to return to the superconducting state, and closes (turns back on) the persistent current switch  6 . The cryogenic stabilizer is a metal, such as a copper-nickel alloy or gold-silver alloy, that has higher electrical resistivity than the cryogenic stabilizer (e.g., silver and oxygen-free copper) of the superconducting coils  3  ( 3   a ,  3   b ). In addition, since heating due to magnetic field fluctuations during a quench is likely to make the persistent current switch  6  suffer a transition into normal conduction, in order to prevent thermal damage to the persistent current switch  6 , a resistance value of the protective resistor and a circuit composition are appropriately designed for a sufficient reduction in the amount of energy consumed. The persistent current switch  6 , the fuse  4 , and the superconducting coils  3   a  and  3   b  are interconnected in series, these elements composing a first closed circuit C 1 . Turning on the persistent current switch  6  supplies a persistent current Ip to the first closed circuit C 1 , especially the superconducting coils  3 . 
     As with the superconducting coils  3 , the fuse  4  uses a high-temperature superconductor having a critical temperature exceeding 18 K. The fuse  4  includes a superconducting wire. As in the superconducting coils  3 , in the superconducting wire, a peripheral region of one or a plurality of superconducting filaments is shrouded with a cryogenic stabilizer, spatial gaps between the plurality of superconducting filaments are filled in with the cryogenic stabilizer, and the superconducting filaments are bundled together in the cryogenic stabilizer. The fuse  4  and the superconducting coils  3   a  and  3   b  are interconnected in series. Connection terminals (connections)  14   c  and  14   d  are provided at the opposite ends of the fuse  4 . 
     The bobbin (protective resistor)  5  uses a non-magnetic material, a normal conducting material (electric conductor), and a member having sufficient strength to operate as one element of the bobbin  5 . More specifically, the bobbin (protective resistor)  5  uses a member of aluminum, copper, stainless steel, or the like. The bobbin (protective resistor)  5 , the persistent current switch  6 , and the superconducting coils  3   a  and  3   b  are interconnected in series, these elements composing a second closed circuit C 2 . Connection terminals (connections)  14   a  and  14   b  are provided at two places each distant from the bobbin (protective resistor)  5 . The connection terminal  14   a  connects to the connection terminal  14   d  of the fuse  4  via a superconducting wire  15   a . The connection terminal  14   b  connects to the connection terminal  14   c  of the fuse  4  via a superconducting wire  15   b . Superconducting wires  15   a ,  15   b  may instead be superconductive wiring. The bobbin (protective resistor)  5 , the connection terminal  14   a , the superconducting wire  15   a , the connection terminal  14   d , the fuse  4 , the connection terminal  14   c , the superconducting wire  15   b , and the connection terminal  14   b  also compose a closed circuit independent of the first closed circuit C 1  and the second closed circuit C 2 . 
     The excitation power supply  10  is a direct-current source for supplying a direct current to each superconducting coil  3 . The circuit breaker  11  lets the direct current flow from the excitation power supply  10  into the superconducting coil  3  or interrupts the flow of the direct current. The circuit breaker  11  is connected in series to the excitation power supply  10 . The circuit breaker  11 , the excitation power supply  10 , and the persistent current switch  6  are further interconnected in series, these elements composing a third closed circuit C 3 . In addition, the circuit breaker  11 , the excitation power supply  10 , the superconducting coil  3 , and the fuse  4  are interconnected in series to compose a further, closed circuit, thus enabling magnetic energy to be stored into the superconducting coil  3 . The circuit breaker  11  and the excitation power supply  10  are arranged outside a cryostat  2 , and can be removed from a main body of the superconducting magnet apparatus  1 . 
     The superconducting magnet apparatus  1  additionally includes a quench detector  7 , heater  9 , and a current source (direct-current source)  8 . The quench detector  7  detects the normal conducting state that may occur in part of the superconducting coil  3  ( 3   a ,  3   b ). The quench detector  7  can detect the occurrence of the normal conducting state in part of the superconducting coil  3  ( 3   a ,  3   b ), as, for example a change in differential potential across the superconducting coil  3  ( 3   a ,  3   b ). The quench detector  7 , upon detecting the occurrence of the normal conducting state in part of the superconducting coil  3  ( 3   a ,  3   b ), generates an output of a quench detection signal “Sq” and transmits the signal to the current source  8 . The current source  8  is a direct-current source, and upon receiving the quench detection signal “Sq”, supplies a direct current to a heater  9  to energize this heater. 
     The heater  9  is thermally connected to the fuse  4 . In addition, the heater  9  is preferably in contact with the fuse  4 . A flow of the direct current into the heater  9  activates the heater  9  to generate heat, which in turn heats the fuse  4 . The fuse  4  then rises in temperature and experiences a transition from superconductivity into normal conduction. When the persistent current Ip through the first closed circuit C 1  flows into the fuse  4  which has transitioned into normal conduction, the fuse  4  generates joule heat to heat itself and blow. This opens the first closed circuit C 1  and allows the persistent current Ip to continue to flow, with the result that the persistent current Ip flows through the second closed circuit C 2  into the bobbin (protective resistor)  5 . The bobbin (protective resistor)  5  generates joule heat to heat itself and attenuate the persistent current Ip. When the persistent current Ip is still flowing through the first closed circuit C 1 , the persistent current switch  6  as well as the fuse  4  may transition into normal conduction. The fuse  4  is desirably designed so that even in this case, a temperature of the fuse  4  will readily increase above that of the persistent current switch  6  to heat the fuse to such an extent that it blows. 
     The superconducting magnet apparatus  1  further includes a cryostat  2 . The cryostat  2  includes a refrigerator  2   c  that cools the superconducting coil  3  and the like by depriving these elements of heat, a heat transfer element  2   d  that conducts heat from the superconducting coil  3  and the like to the refrigerator  2   c , a vacuum vessel  2   a  that accommodates the heat transfer element  2   d  and the like and conducts heat insulation under a vacuum, and a radiation shield  2   b  that accommodates the heat transfer element  2   d  and the like and suppresses entry of radiant heat. The radiation shield  2   b  is included in the vacuum vessel  2   a , and the heat transfer element  2   d  is included in the radiation shield  2   b . The refrigerator  2   c  includes a first stage and a second stage, each of which can be cooled down to a different temperature. The second stage, which is able to be cooled down to a temperature lower than that of the first stage, can be cooled down to a level below a critical temperature of a high-temperature superconductor. The second stage is thermally connected to the heat transfer element  2   d , and the heat transfer element  2   d  is cooled below the critical temperature of a high-temperature superconductor. The first stage is thermally connected to the radiation shield  2   b . The first stage cools the radiation shield  2   b , whereby the radiant heat that the radiation shield  2   b  has absorbed can be released from the first stage. 
     The heat transfer element  2   d , thermally connected to the superconducting coil  3  ( 3   a ,  3   b ), the fuse  4 , the persistent current switch  6 , and the superconducting wires that connect these elements, transfers (releases) heat to cool them below the critical temperature of a high-temperature superconductor. Thus the superconducting coil  3  ( 3   a ,  3   b ), the fuse  4 , the persistent current switch  6 , and the superconducting wires that connect these elements can be maintained in a superconducting condition. 
       FIG. 2A  shows a longitudinal sectional view of the fuse  4  and its peripheral members, and  FIG. 2B  shows a transverse sectional view thereof. The fuse  4  and its peripheral members in  FIG. 2B  are shown in more enlarged view than those shown in  FIG. 2A . The heater  9  is in contact with the fuse  4 , and both are thermally connected to each other. This makes the heater  9  heat the fuse  4  and thus enables it to transition from the superconducting state into the normal conducting state. For ease of heating, the fuse  4  and the heater  9  have their periphery covered with a heat insulator  12  so that the heat generated in the heater  9  will not diffuse to the periphery thereof. The heat insulator  12  can be, for example, an adiabatic material such as a resin, or an adiabatic material having a vacuum structure. The fuse  4  has its connections (terminals)  14   c ,  14   d  connected to the superconducting coil  3  and the persistent current switch  6 . The superconducting coil  3  and the persistent current switch  6  are cooled in contact with the heat transfer element  2   s , and the fuse  4  is cooled via the connections (terminals)  14   c ,  14   d  connecting the superconducting coil  3  and the persistent current switch  6 . 
     The fuse  4 , if it blows out, will be replaced with a new fuse  4 . This replacement can be easily performed by disconnecting the connections (terminals)  14   c ,  14   d  from the blown fuse  4  and then removing the fuse  4 , along with the heater  9  and the heat insulator  12 , from the heat transfer element  2   d . In order to allow for such a blowout, the fuse  4  is placed at a position that enables one to easily access a non-blown fuse, for example at an end of the heat transfer element  2   d.    
     The fuse  4  is a so-called superconducting wire, and as shown in  FIG. 2B , the periphery of the superconducting filaments  4   a  in the fuse  4  is shrouded with a cryogenic stabilizer  4   b . The number of superconducting filaments  4   a  is not always limited to two or more and may be one. The plurality of superconducting filaments  4   a  are bundled in the cryogenic stabilizer  4   b.    
       FIG. 3A  is a front view of the bobbin (protective resistor)  5  for the superconducting coil  3  ( 3   a ), and  FIG. 3B  is a longitudinal sectional view of the superconducting coil  3  ( 3   a ) and bobbin (protective resistor)  5  as viewed longitudinally along a plane from a central axis  5   c  of both. Although  FIGS. 3A and 3B  show the superconducting coil  3   a  by way of example, this coil may be the superconducting coil  3   b  and the bobbin (protective resistor)  5  for both of the superconducting coils  3   a  and  3   a  may also serve as the protective resistor. The bobbin (protective resistor)  5  includes a cylinder  5   a  and one pair of flanges  5   b  extended with the cylinder  5   a  put therebetween. The paired flanges  5   b  are provided at the opposite ends (open ends) of the cylinder  5   a . The flanges  5   b  have an inside diameter substantially equal to that of the cylinder  5   a . Outside diameter of the flanges  5   b  is greater than that of the cylinder  5   a . The superconducting coil  3  ( 3   a ) is wound around the cylinder  5   a , between the flanges  5   b . The superconducting wire material of the superconducting coil  3  ( 3   a ) wound around the bobbin (protective resistor)  5  is of a type whose surface includes an electrical insulating layer or covered with an electrical insulating sheet. This structure keeps the superconducting wire of the superconducting coil  3  ( 3   a ) out of electrical contact with the bobbin (protective resistor)  5 . 
     Connection terminals  14   a  and  14   b  are provided at two places that are distant from each other on one of the paired flanges  5   b . The connection terminals  14   a  and  14   b  are provided on outer circumferential sections of the paired flanges  5   b . The connection terminals  14   a  and  14   b  are positioned across the central axis  5   c  of the bobbin (protective resistor)  5  (flange  5   b ). The connection terminals  14   a  and  14   b  are positioned at where a line segment (line) connecting the connection terminals  14   a  and  14   b  intersects with the central axis  5   c  of the bobbin (protective resistor)  5  (flange  5   b ). The superconducting wire  15   a  connecting to the persistent current switch  6  and forming a part of the first closed circuit C 1  (see  FIG. 1 ) is connected to the connection terminal  14   a . The superconducting wire  15   b  connecting to the superconducting coil  3  ( 3   a ) and forming a part of the second closed circuit C 2  (see  FIG. 1 ) is connected to the connection terminal  14   b . Thus, the bobbin (protective resistor)  5  to which the superconducting wires  15   a  and  15   b  connect is also considered to form a part of the second closed circuit C 2  (see  FIG. 1 ). During the quench of the superconducting coil  3 , the persistent current Ip flows into the bobbin (protective resistor)  5  from the connection terminal  14   a , and after flowing along a bifurcated flow route  17  of the current, flows out from the connection terminal  14   b . As the persistent current Ip is flowing through the bobbin (protective resistor)  5 , the bobbin (protective resistor)  5  generates joule heat and consumes the energy stored within the superconducting coil  3 . The bobbin (protective resistor)  5  thus functions as a protective resistor. The bobbin (protective resistor)  5  is formed to be strong enough to support the superconducting coil  3  ( 3   a ) upon which a strong electromagnetic force acts by the flow of the persistent current Ip. An installation space wide enough to meet this physical requirement is ensured for the bobbin (protective resistor)  5 . At the same time, to flow a large current into the protective resistor so that the stored magnetic energy is consumed therein, it is necessary to impart large enough a heat capacity to the protective resistor so as to prevent its thermal damage due to the inflow of the large current. For this reason, a wide installation space also needs to be provided for the protective resistor. The bobbin (protective resistor)  5  also functions as the protective resistor. This means that a sufficient installation space is also already ensured for the bobbin (protective resistor)  5  as the protective resistor. This, in turn, further means that the heat capacity that is necessary and large enough to prevent the bobbin (protective resistor)  5  from suffering thermal damage during the quench can be assigned. Additionally, when the space for the bobbin  5  of the superconducting coil  3  is provided, a space independent of that space is not needed for the protective resistor, which means that an internal configuration of the cryostat  2  (see  FIG. 1 ) is simplified. 
     (Operation of the Superconducting Magnet Apparatus  1 ) 
     Next, operation of the superconducting magnet apparatus  1  is described below. First, as shown in  FIG. 1 , the superconducting coils  3  ( 3   a ,  3   b ), the fuse  4 , and the persistent current switch  6  are cooled below the critical temperature of a high-temperature superconductor by heat-conductive cooling with the heat transfer element  2   d , and thereby maintained in a superconducting state. 
     Next after the persistent current switch  6  has been opened (turned off) for normal conduction, the circuit breaker  11  is closed (turned on) and the current is supplied from the excitation power supply  10  to the superconducting coils  3  ( 3   a ,  3   b ). After this, the persistent current switch  6  is closed (turned on) for superconductivity, the current from the excitation power supply  10  is turned off, and then the circuit breaker  11  is opened (turned off). At this time, although the supply of the current from the excitation power supply  10  to the superconducting coils  3  ( 3   a ,  3   b ) is stopped, current attenuation in the first closed circuit C 1  having the superconducting coils  3  ( 3   a ,  3   b ), fuse  4 , and closed (activated) persistent current switch  6  connected in series, becomes very small, which then resumes the flow of the persistent current Ip and places the superconducting magnet apparatus  1  in persistent-current operation. During persistent-current operation, the superconducting magnet apparatus  1  can form/hold the magnetic fields over extended periods of time, even without power being supplied from the excitation power supply  10 . Since the bobbin (protective resistor)  5  has finite electrical resistance, substantially no current flows into the bobbin (protective resistor)  5  (second closed circuit C 2 ) during persistent-current operation. 
     A description is given below of a case in which, during the persistent-current operation of the superconducting magnet apparatus  1 , part of the superconducting coil  3   a  of the two superconducting coils  3  ( 3   a ,  3   b ) undergoes a transition into the normal conducting state and this normal conduction expands to the peripheral region of that part, that is, the quench event occurs. First, if a normal-conduction transition occurs in part of the superconducting coil  3   a , the quench detector  7 , upon determining that for example, the differential potential across the superconducting coil  3   a  has exceeded a predetermined value, detects the partial normal-conduction transition of the superconducting coil  3   a  and transmits the quench detection signal “Sq” to the direct-current power supply  8 . After receiving the quench detection signal “Sq”, the direct-current power supply  8  supplies the direct current to a heater  13  in contact with the fuse  4 . The heater  13  then heats the fuse  4 , whereby the fuse  4  transitions from the superconducting state into the normal-conducting state and generates joule heat in itself to blow. The cryogenic stabilizer  4   b  (see  FIG. 2B ) of the fuse  4  is formed from a material higher in electrical resistivity and lower in thermal conductivity than the superconductor cryogenic stabilizer material used in the superconducting coils  3  ( 3   a ,  3   b ), and the fuse  4  itself is covered with the heat insulator  12  (see  FIGS. 2A and 2B ). Compared with the normal-conduction transition part of the superconducting coils  3  ( 3   a ,  3   b ), therefore, that of the fuse  4  is large in the amount of heat generated, low in heat diffusion rate, and high in an increase rate of temperature. This means that even if the superconducting coils  3  ( 3   a ,  3   b ) suffer thermal damage, the fuse  4  can be made to blow out before that. 
     When the fuse  4  blows, this section exhibits a very high resistance value and the persistent current Ip is rerouted to the bobbin (protective resistor)  5  having a lower resistance value. A consequential flow of a larger persistent current Ip into the bobbin (protective resistor)  5  correspondingly increases the volume of stored magnetic energy consumed, and the generation of heat in the superconducting coils  3  ( 3   a ,  3   b ) is suppressed, that is, the superconducting coils  3  ( 3   a ,  3   b ) are lowered in maximum temperature. This avoids thermal damage to the superconducting coils  3  ( 3   a ,  3   b ). Additionally, since the fuse  4  is placed at a position readily accessible for replacement, the fuse can be replaced after the attenuation of the persistent current Ip, so the superconducting magnet apparatus  1  can be energized once again. 
     (Operational Effects) 
     As described above, in accordance with the first embodiment, since the bobbin (protective resistor)  5  of the superconducting coils  3  also serves as a protective resistor, if the space for the bobbin  5  of the superconducting coils  3  is provided, a space independent of that space does not need to be provided for the protective resistor ( 5 ). This means that the installation space provided for the protective resistor ( 5 ) separately from the space for the bobbin  5  in conventional technology has been reduced. Briefly, in accordance with the embodiment is provided the superconducting magnet apparatus  1  adapted to consume stored magnetic energy without thermally damaging the protective resistor ( 5 ), even if the installation space for the protective resistor ( 5 ) is reduced. 
     Second Embodiment 
       FIG. 4  shows a circuit diagram of a superconducting magnet apparatus  1  according to a second embodiment of the present invention. The superconducting magnet apparatus  1  of the second embodiment differs from that of the first embodiment in that a second closed circuit C 2  is composed substantially by series connection between a bobbin (protective resistor)  5  and superconducting coils  3   a  and  3   b , and in that a persistent current switch  6  is excluded from the second closed circuit C 2 . The second embodiment provides substantially the same advantageous effects as those of the first embodiment. Additionally in the second embodiment, once a fuse  4  has blown out, a persistent current Ip does not flow into the persistent current switch  6 . Therefore, even if heat due to magnetic field fluctuations during a quench causes the persistent current switch  6  to transition into a normal conducting state, consumption of stored magnetic energy in the persistent current switch  6  is suppressed by the blowout of the fuse  4 . By virtue of this protection function, the heat capacity of the persistent current switch  6  that has been needed for the prevention of thermal damage in the first embodiment can be made smaller than in the first embodiment, and an installation space for the persistent current switch  6  can also be reduced. 
     Third Embodiment 
       FIG. 5  shows a circuit diagram of a superconducting magnet apparatus  1  according to a third embodiment of the present invention. The superconducting magnet apparatus  1  of the third embodiment differs from that of the second embodiment in that a third closed circuit C 3  is composed substantially by series connection between a persistent current switch  6 , a fuse  4 , an excitation power supply  10 , and a circuit breaker  11 , and in that the fuse  4  is added as an element of the third closed circuit C 3 . Series connection between superconducting coils  3   a  and  3   b , a bobbin (protective resistor)  5 , series connection between the persistent current switch  6  and the fuse  4 , and series connection between the excitation power supply  10  and a circuit breaker  11  are each in a parallel connection format. The third embodiment provides substantially the same advantageous effects as those of the first and second embodiments. 
     Fourth Embodiment 
     Next, a superconducting magnet apparatus  1  according to a fourth embodiment is described below. The superconducting magnet apparatus  1  according to the fourth embodiment differs from those of the first to third embodiments in that a low-temperature superconductor that exhibits superconductivity at a critical temperature equal to or less than 18 K is used in each of superconducting coils  3 , a fuse  4 , superconducting filaments in a superconducting wire used in a persistent current switch  6 , and superconducting filaments in a superconducting wire interconnecting each of those elements. In association with this difference, a cryostat  2  has appropriate or sufficient cooling capabilities to maintain superconductivity of the low-temperature superconductor. Niobium titanium (NbTi), niobium tin (Nb 3 Sn), or the like can be used as the low-temperature superconductor having the critical temperature of 18 K or less. The critical temperature of the low-temperature superconductor, compared with the high-temperature superconductors having critical temperatures exceeding 18 K, lies in a region that the low-temperature superconductor has a specific heat capacity at most one-tenth as great as those of niobium titanium (NbTi), niobium tin (Nb3Sn), and other low-temperature superconductors having critical temperatures below 18 K. For this reason, heat conduction due to a quench causes a delay in the propagation of a normal-conducting region. In the superconducting coils  3 , therefore, stored magnetic energy can also be consumed without causing thermal damage, so that the stored magnetic energy to be consumed in a protective resistor ( 5 ) can be lessened. Hence the installation of a protective resistor for the low-temperature superconductors does not require a space as wide as that required for high-temperature superconducting coils. In the fourth embodiment, however, since a bobbin (protective resistor)  5  for the superconducting coils  3  also serves as a protective resistor, if an appropriate space for the bobbin  5  of the superconducting coils  3  is provided, a space independent of that space does not need to be provided for the protective resistor ( 5 ). The installation space for the protective resistor ( 5 ) can be made smaller than in the first embodiment, but even so, it follows that the installation space required has been reduced. In addition, even when the installation space for the protective resistor ( 5 ) is reduced, the stored magnetic energy can be consumed without thermally damaging the protective resistor ( 5 ). 
     Fifth Embodiment 
       FIG. 6A  shows a front view of a bobbin (protective resistor)  5  for a superconducting coil  3  used in a superconducting magnet apparatus according to a fifth embodiment of the present invention, and  FIG. 6B  shows a longitudinal sectional view of the superconducting coil  3  and its bobbin (protective resistor)  5 . The superconducting magnet apparatus  1  according to the fifth embodiment differs from those of the first to fourth embodiments in that the bobbin (protective resistor)  5  includes a cylinder  5   a  and flanges  5   b  electrically insulated from the cylinder  5   a , and in that a plurality of grooves  5   f  are formed on each flange  5   b . In the example of  FIG. 6A , on the flange  5   b  are formed sixteen grooves  5   f  in all, eight reaching an inner edge of the flange  5   b  and eight reaching an outer edge thereof. The grooves  5   f  are carved downward from an upper surface of the flange  5   b  through to a lower surface thereof. While an insulator is buried in the grooves  5   f , nothing may be buried therein. The grooves  5   f  are provided in a radial form extending outward from a central portion of the bobbin (protective resistor)  5  (flange  5   b ). One end of each groove  5   f  reaches either the inner edge or outer edge of the flange  5   b . One of any two adjacent grooves  5   f  reaches the inner edge of the flange  5   b , and the other of the two adjacent grooves  5   f  reaches the outer edge of the flange  5   b . This layout of the grooves  5   f  staggers a current-flow route  17  directed from a connection terminal  14   a , towards a connection terminal  14   b , and thus increases a resistance value of the bobbin (protective resistor)  5  relative to that obtained in the first embodiment. The flange  5   b  and the cylinder  5   a  are fastened together at fastening portions  70   a  by bolts  60  and nuts  61 . 
     As shown in  FIG. 6B , the flange  5   b  and the cylinder  5   a  are isolated from each other by an insulating member (insulating sheet)  5   e . The insulating member (insulating sheet) Se is positioned between the flange  5   b  and the cylinder  5   a . The flange  5   b  and the cylinder  5   a  are electrically insulated from each other. The fastening portions  70   a  are provided on the flange  5   b . Fastening portions  70   b  are provided on the cylinder  5   a . Fastening with the fastening portions  70   a  and  70   b  is accomplished by tightening a bolt  60  and a nut  61 . 
       FIG. 7A  shows a longitudinal sectional view of fastening portions  70   a  and  70   b  and peripheral members thereof, and  FIG. 7B  shows a transverse sectional view of one fastening portion and peripheral members existing when seen from a direction of line B-B in  FIG. 7A . The insulating sheet  5   e  is positioned between the fastening portion  70   a  and the fastening portion  70   b . A through-hole is formed in each of the fastening portion  70   a , the fastening portion  70   b , and the insulating sheet  5   e . An insulating collar  64  of a cylindrical shape extends through the through-hole penetrating the fastening portion  70   a , the fastening portion  70   b , and the insulating sheet  5   e . Insulating washers  63   a  and  63   b  are provided at the opposite ends of the insulating collar  64 . The insulating washers  63   a  and  63   b  have an outside diameter greater than a diameter of the through-hole extending through the fastening portions  70   a  and  70   b . The bolt  60  extends through the insulating collar  64  and the insulating washers  63   a  and  63   b . A spring-lock washer  62  is set on the bolt  60  and then the nut  61  is threadably engaged with the spring-lock washer  62  to obtain secure fastening with the fastening portions  70   a  and  70   b . The bolt  60 , the nut  61 , and the spring-lock washer  62  are each formed of a material, for example stainless steel, that has sufficient mechanical strength required for fastening. The bolt  60 , the nut  61 , and the spring-lock washer  62  are not in direct contact with the fastening portions  70   a  and  70   b , and are in close proximity thereto via the insulating collar  64  and the insulating washers  63   a  and  63   b , so the fastening portions  70   a  and  70   b  do not come into electrical connection. In addition, since the insulating sheet  5   e  is positioned between the fastening portions  70   a  and  70   b , these fastening portions can be electrically insulated from each other to provide firm fastening. This in turn enables electrical insulation of the flange  5   b  and cylinder  5   a  shown in  FIG. 6B . Since the connection terminals  14   a  and  14   b  are provided on the flange  5   b  insulated from the cylinder  5   a , although the current-flow route  17  directed from the connection terminal  14   a  towards the connection terminal  14   b  might go through the flange  5   b , the current-flow route  17  is limited in itself so as not to go through the cylinder  5   a . The resistance value of the bobbin (protective resistor)  5  is therefore increased relative to that obtained in the first embodiment. 
     As described above, since the current-flow route  17  is limited to the inside of the flange  5   b  by the presence of the insulating sheet  5   e  and is staggered within the flange  5   b  by the presence of the grooves  5   f , the resistance value of the bobbin (protective resistor)  5  is high relative to that obtained in the first embodiment. A time required for the current attenuation during the quench is correspondingly reduced. A time required for the quench detector  7  to detect the quench in the superconducting coil  3 , and a time required until the fuse  4  has been blown out are extended according to the particular reduction in the quench detection time required. Consequently, the quench detector  7 , the fuse  4 , and other members and system elements required for the circuit composition are simplified, for example, the capacity of the heater  13  is reduced. 
     Sixth Embodiment 
       FIG. 8A  shows a front view of a bobbin (protective resistor)  5  for a superconducting coil  3  ( 3   a ) used in a superconducting magnet apparatus according to a sixth embodiment of the present invention, and  FIG. 8B  shows a longitudinal sectional view of the superconducting coil  3  ( 3   a ) and its bobbin (protective resistor)  5 . The superconducting magnet apparatus according to the sixth embodiment of the present invention differs from those of the first to fifth embodiments in that two connection terminals,  14   a  and  14   b , are provided on each of one pair of flanges  5   b . This also allows a current-flow route to be staggered. The bobbin (protective resistor)  5  includes a plurality of partial bobbins  21 , namely  21   a ,  21   b ,  21   c ,  21   d ,  21   e , each formed by dividing the bobbin vertically from a cutting plane perpendicular to a central axis  5   c . The bobbin also includes insulating sheets  5   e  each provided between any two of the cutting planes each facing one of adjacent partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e . The bobbin additionally includes electroconductive connecting portions  16   a ,  16   b ,  16   c ,  16   d  each provided on an inner wall of the bobbin (protective resistor)  5  and electrically connecting any two of the adjacent partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e . The two electroconductive connecting portions  16   a ,  16   b ,  16   c ,  16   d  connecting to one partial bobbin  21   b ,  21   c ,  21   d , face each other across the central axis  5   c . More specifically, the two electroconductive connecting portions,  16   a  and  16   b , connecting to the partial bobbin  21   b , face each other across the central axis  5   c . Similarly, the two electroconductive connecting portions,  16   b  and  16   c , connecting to the partial bobbin  21   c , face each other across the central axis  5   c . Likewise, the two electroconductive connecting portions,  16   c  and  16   d , connecting to the partial bobbin  21   d , face each other across the central axis  5   c . The electroconductive connecting portions  16   a ,  16   c  and the electroconductive connecting portions  16   b ,  16   d  are each positioned at where a line segment (line) connecting the electroconductive connecting portion  16   a ,  16   c  and the electroconductive connecting portion  16   b ,  16   d  intersects with the central axis  5   c.    
     The bobbin (protective resistor)  5  is divided into the plurality of partial bobbins  21 , namely  21   a ,  21   b ,  21   c ,  21   d ,  21   e  (in the example of  FIG. 8B , five units). In the example of  FIG. 8B , the partial bobbing  21   a  and  21   b  correspond to the flange  5   b , but are not limited to this correspondence relationship. The partial bobbins  21   a  and  21   e  may form part of the flange  5   b  or include part or all of the cylinder  5   a  in addition to the flange  5   b . In addition, in the example of  FIG. 8B , the cylinder  5   a  is divided into three partial bobbins,  21   b ,  21   c ,  21   d , but not divided only into the three. The cylinder  5   a  may be one partial bobbin,  21   b , or divided into a plural number other than three. 
     The cutting planes between the adjacent partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e  are in close proximity to each other via one insulating sheet  5   e . A fastening portion  70  is provided on each partial bobbin  21   a ,  21   b ,  21   c ,  21   d ,  21   e . Any two of the fastening portions  70  on the adjacent partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e  face each other via one insulating sheet  5   e , and are securely tightened together by a bolt  60  and a nut  61 . This structure with the paired fastening portions  70  tightened together by the bolt  60  and the nut  61  can be the structure described per  FIGS. 7A and 7B , that is, the structure with the fastening portions  70   a  and  70   b  tightened together by one bolt  60  and one nut  61 . That is to say, it suffices if the phrasing “fastening portions  70   a  and  70   b ” is read to mean the “fastening portions  70 ”. Thus, electrical insulation on the cutting planes between the adjacent partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e  can be maintained and at the same time, the two opposed adjacent partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e  can be fastened together. 
     The adjacent partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e  are electrically interconnected via the electroconductive connecting portions  16   a ,  16   b ,  16   c ,  16   d . The electroconductive connecting portions  16   a ,  16   c  are opposed to the electroconductive connecting portions  16   b ,  16   d  across the central axis  5   c . The current-flow route  17  therefore extends from the connection terminal  14   b  through the partial bobbin  21   a , the electroconductive connecting portion  16   a , the partial bobbin  21   b , the electroconductive connecting portion  16   b , the partial bobbin  21   c , the electroconductive connecting portion  16   c , the partial bobbin  21   d , the electroconductive connecting portion  16   d , and the partial bobbin  21   e , in that order, to the connection terminal  14   b . In this way, while staggering, the current-flow route  17  is narrowed down and elongated, whereby the resistance value of the bobbin (protective resistor)  5  can be increased. Since the number of partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e  into which the bobbin is divided is odd (in the example of  FIG. 8B , five units), the connection terminals  14   a  and  14   b  are arranged at a rate of one on each side of the central axis  5   c . If the number of partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e  into which the bobbin is divided is even, the connection terminals  14   a  and  14   b  are arranged only at one side of the central axis  5   c . In addition, an electroconductive material to which strength is easy to impart, for example, stainless steel, can be used as the material of the electroconductive connecting portions  16   a ,  16   b ,  16   c ,  16   d.    
     Seventh Embodiment 
       FIG. 9A  shows a front view of a bobbin (protective resistor)  5  for a superconducting coil  3  ( 3   a ) used in a superconducting magnet apparatus according to a seventh embodiment of the present invention, and  FIG. 9B  shows a longitudinal sectional view of the superconducting coil  3  ( 3   a ) and its bobbin (protective resistor)  5 . In the superconducting magnet apparatus according to the seventh embodiment, the grooves  5   f  in the fifth embodiment and the partial bobbins  21   a ,  21   b ,  21   c ,  21   d ,  21   e , etc. in the sixth embodiment are combined to form a longer current-flow route  17  for further increased resistance value of the bobbin (protective resistor)  5 . The grooves  5   f  are formed on both of one pair of flanges  5   b  (partial bobbins  21   a  and  21   e ). The grooves  5   f  on the partial bobbin  21   e  have a layout pattern matching that obtained by rotating a layout pattern of the grooves  5   f  of the partial bobbin  21   a  through 180 degrees about a central axis  5   c . Thus in the partial bobbin  21   a , the current-flow route  17  starts from a connection terminal  14   a , detours the grooves  5   f  on the partial bobbin  21   a , and reaches an electroconductive connecting portion  16   a . In the partial bobbin  21   e , the current-flow route  17  starts from an electroconductive connecting portion  16   d , staggers along an upper surface of the partial bobbin  21   e  while detouring the grooves  5   f  present thereon, and reaches a connection terminal  14   b . In addition, the current-flow route  17  from the electroconductive connecting portion  16   a  to the electroconductive connecting portion  16   d  extends from the electroconductive connecting portion  16   a , through the partial bobbin  21   b , an electroconductive connecting portion  16   b , the partial bobbin  21   c , an electroconductive connecting portion  16   c , and the partial bobbin  21   d , in that order, to the electroconductive connecting portion  16   d . In this way, while staggering, the current-flow route  17  is elongated, whereby the resistance value of the bobbin (protective resistor)  5  can be increased. 
     The present invention is not limited to the above-described first to seventh embodiments and may include various modifications. For example, the first to seventh embodiments have been described in detail only for clarity of the present invention and are not limited to apparatus configurations including all described constituent elements. In addition, part of the configurations in the first to seventh embodiments may be replaced by any one or more of the other embodiments, and conversely, any one or more of the other embodiments may be added to part of the configurations in the first to seventh embodiments. Furthermore, addition, deletion, and/or replacement of any one or more of the other embodiments may take place for part of the configurations in the first to seventh embodiments.