Abstract: A superconducting magnet system with a quench protection circuit includes a superconducting magnet having a plurality of series-connected superconducting coils, a persistent current switch connected in parallel with the superconducting coils, and first and second quench protection circuits connected in parallel with the superconducting coils and the persistent current switch. The first quench protection circuit includes anti-parallel diodes and parallelly connected resistive heaters and shunt resistors, and the resistive heaters and the shunt resistors are each connected in series with the diodes. The second quench protection circuit has a diode and a metal of copper connected with the diode, and a turn-on voltage of the diode in the second quench protection circuit is lower than a turn-on voltage of the diodes in the first quench protection circuit.

Abstract: A superconducting magnet system includes resistive heaters and bidirectional diodes connected in series with each other installed in a superconducting coil protection circuit, and a diode is installed in a persistent current switch protection circuit.

Abstract: A system and method of controlling movement of magnetic material with at least first and second high temperature superconductors at spaced locations. A plurality of solenoids are associated with the superconductors to induce a persistent currents in preselected high temperature superconductors establishing a plurality of magnetic fields in response to pulsed currents introduced to one or more of the solenoids. Control mechanism in communication with said solenoids and/or said high temperature superconductors are used to demagnetize selected ones of the high temperature superconductors to reduce the magnetic fields substantially to zero. Magnetic material is moved between magnetic fields by establishing the presence thereof and thereafter reducing magnetic fields substantially to zero and establishing magnetic fields in other superconductors arranged in a predetermined configuration.

Abstract: A flux pump of the rectifier type having high-temperature superconductors with switches, which can be controlled by way of a heater, in particular with trigger pulses D? and self-stabilization in time with the rectifier is disclosed. An arrangement of the flux pump and a superconductor electromagnet, which may be supplied using the flux pump, may be integrated in a common cryostat housing.

Abstract: A superconducting shielded core reactor (SSCR) operates as a passive device for limiting excessive AC current in a circuit operating at a high power level under a fault condition such as shorting. The SSCR includes a ferromagnetic core which may be either closed or open (with an air gap) and extends into and through a superconducting tube or superconducting rings arranged in a stacked array. First and second series connected copper coils each disposed about a portion of the iron core are connected to the circuit to be protected and are respectively wound inside and outside of the superconducting tube or rings. A large impedance is inserted into the circuit by the core when the shielding capability of the superconducting arrangement is exceeded by the applied magnetic field generated by the two coils under a fault condition to limit the AC current in the circuit. The proposed SSCR also affords reduced AC loss compared to conventional SSCRs under continuous normal operation.

Abstract: A device includes a flux pump and an electromagnet to be fed thereby for the production of highly constant magnetic fields of high field strength. In a flux pump of the rectifier type, switches used are MOSFETs. These MOSFETs include HTc superconductor material at operating temperatures. Preferably, the flux pump and a coil of the electromagnet are arranged in one and the same cryostat housing.

Abstract: A magnet arrangement comprising a superconducting magnet coil system (M) which has an ohmic resistance (R) of zero or more during operation, and a flux pump (P) which comprises at least one superconducting switch and at least two superconducting secondary coils (M1,M2), is characterized in that at least one superconducting current path is provided, wherein the superconducting magnet coil system (M) or parts thereof is/are connected in series with at least two secondary coils (M1, M2), and wherein at least one secondary coil (M2) can be superconductingly bridged through closing of a superconducting switch (S1), and at least two primary coils (C1, C2) are provided which can each be fed independently of each other with a current (I1, I2) and which are each inductively coupled with at least one of the secondary coils (M1, M2). The flux pump is suitable for the stabilization of the magnetic field of the magnet coil system (M) during long-term operation.

Abstract: A superconducting magnetic energy storage (SMES) device including a first coil made of superconducting material, a cooling mechanism for cooling the first coil to superconducting temperatures, a second coil inductively coupled to the first coil for inputting emergy to, and/or outputting energy from, the first coil, and a switch for switching the first coil between a superconducting condition and a non-superconducting condition. The first coil is arranged as a closed loop electric circuit having no connecting device mechanically connected to it for inputting or outputting energy. The switch includes a third coil for the application or removal of a magnetic field for switching the first coil between its non-superconducting and superconducting conditions. A method inputs energy to and/or outputs energy from the first coil and a power supply system utilizes the device and method.

Abstract: Apparatus and method for controlling movement of an object through a medium using a magnetic field by combining the magnetic field of a primary electromagnetic coil having a central axis with the magnetic fields of a plurality of secondary electromagnetic coils having axes generally parallel to the central axis of the primary coil and positioned within the primary coil to produce an operational magnetic field by which movement of the object in a medium is controlled outside of the primary coil.

Abstract: Remote monitoring of superconducting magnet systems of various types, manufacturers, vintages, and so forth, via a magnet selector interface providing for configuring the monitoring system to the particular magnet system of interest. The technique provides for scalable analogue to digital conversion with integrated excitation circuitry for the input and output of magnet system sensors. Devices, such as remote terminal units and other data-logging technology may be adapted to remotely monitor primary indicators and secondary indicators of magnet system performance and related boil-off of helium. The technique provides earlier warning of impending failures in the magnet system, and thus facilitates predictive maintenance, reduces maintenance costs, reduces MRI downtime, reduces helium loss, and the like.

Abstract: An electrical connector for use in a power module includes a first end portion for forming an electrical connection with a substrate, a second end portion, and a compliant portion situated between the first end portion and the second end portion. The compliant portion includes a compressed position and a decompressed position. The first end portion is configured for forming an electrical connection with a substrate if the compliant portion is in the compressed position.

Abstract: A superconducting fault current controller/current controller employs a superconducting-shielded core reactor (SSCR) with a variable impedance in a secondary circuit to control current in a primary circuit such as an electrical distribution system. In a second embodiment, a variable current source is employed in a secondary circuit of an SSCR to control current in the primary circuit. In a third embodiment, both a variable impedance in one secondary circuit and a variable current source in a second circuit of an SSCR are employed for separate and independent control of current in the primary circuit.

Abstract: A superconducting magnet electrical circuit is provided for quench protection. A superconducting coil assemblage is provided including a plurality of spatially separated main and secondary magnet coil portions. The main magnet coil portions are connected in series to form at least one main coil series circuit. The secondary magnet coil portions are also connected in a series to form at least one secondary coil series circuit. At least one temperature limiting circuit is also provided. The temperature limiting circuit may be a quench heater circuit or a quench resistor circuit. The temperature limiting circuit has a plurality of quench heaters or a plurality of quench resistors connected with the superconducting coil assemblage. A superconductive switch is coupled with the superconducting coil assemblage.

Abstract: A superconducting magnet circuit includes, besides a closed circuit formed of a plurality of unit superconducting coils 2a and 2b connected in series and a first persistent-current switch 4, a second persistent-current switch 5 which forms a closed circuit by connecting the ends of an arbitrary unit superconducting coil 2a, and is therefore made up of the two closed circuits, whereby their mutual induction suppresses the decay of the magnet apparatus' central magnetic field to provide extreme stability.

Abstract: Self-quenching switching elements are connected in series in a semiconductor switching device. Snubber circuits including a diode, a capacitor, and a non-linear circuit are connected in parallel with the respective semiconductor switching elements. In the snubber circuit, the diode and the capacitor are connected in series, and the non-linear circuit is connected in parallel with the capacitor. The non-linear circuit includes an impedance element, a Zener diode, and a controlling semiconductor element and draws current through the controlling semiconductor element when applied voltage exceeds the Zener voltage of the Zener diode. The Zener voltage is larger than a divided voltage applied to the semiconductor element during a fault in a power system.

Abstract: A superconducting magnet assembly comprises a superconducting magnet (1) which, under working conditions, generates a magnetic field in a working volume, the superconducting magnet being connected in parallel with a superconducting switch (3), the switch and magnet being adapted to be connected in parallel to a power source (4) whereby under working conditions with the switch (3) open, the magnet (1) can be energised by the power source to generate a desired magnetic field in the working volume following which the switch (3) is closed, characterised in that the assembly further comprises a resistor (5) connected in series with the switch (3), the resistor (5) and switch (3) being connected in parallel to each of the magnet (1) and the power source (4).

Abstract: A current-limiting device contains at least one conductor track including high Tc superconductor material, on a support body. The conductor track is provided with a cover layer including an insulating plastic material, which includes a filler which increases the thermal conductivity. The thickness of the cover layer is greater than the thickness of the conductor track.

Abstract: Apparatus and method for controlling movement of an object through a medium using a magnetic field by combining the magnetic field of a primary electromagnetic coil having a central axis with the magnetic fields of a plurality of secondary electromagnetic coils having axes generally parallel to the central axis of the primary coil and positioned within the primary coil to produce an operational magnetic field by which movement of the object in a medium is controlled outside of the primary coil.

Abstract: A flux pump of the rectifier type having high-temperature superconductors with switches, which can be controlled by way of a heater, in particular with trigger pulses D′ and self-stabilization in time with the rectifier is disclosed. An arrangement of the flux pump and a superconductor electromagnet, which may be supplied using the flux pump, may be integrated in a common cryostat housing.

Abstract: A device includes a flux pump and an electromagnet to be fed thereby for the production of highly constant magnetic fields of high field strength. In a flux pump of the rectifier type, switches used are MOSFETs. These MOSFETs include HTc superconductor material at operating temperatures. Preferably, the flux pump and a coil of the electromagnet are arranged in one and the same cryostat housing.

Abstract: A system of navigating a magnetic medical device within that part of a patient located within an operating region of the system, the system comprising magnets, and preferably electromagnets, arranged to provide a magnetic field sufficient to navigate the magnetic medical device within the operating region. There are preferably three magnetic coils arranged in mutually perpendicular planes such that their axes intersect in the operating region. The magnetic coils are sized and arranged so that a patient can easily access the operating region to allow virtually any portion of the patient to be positioned within the operating region. The openness of the magnetic system allows access to the operating region by a bi-planer imaging system.

Abstract: A current limiting device comprises a first superconducting element for connection in an electrical circuit. The superconducting element has a critical current density. A second superconducting element detects the onset of a fault condition. A coil is responsive to the second superconducting element to vary the relationship between the critical current density and the current carried by the first superconducting element so that the element transforms into a resistive state.

Abstract: A quench protection circuit for the series connected but spatially separated magnet coils of a superconducting magnet utilizes a parallel circuit of multiple heaters and reverse connected diodes shunting portions of the series connected magnets coils with the heaters physically positioned in thermal contact with other coils in the series circuit to thermally transfer a heat quenching action in any coil to all coils, along with a shunting current through the diodes to shunt peak currents upon coil quenching to avoid undesired spot temperature rise and enable controlled quenching of the entire magnet upon quenching of any coil.

Abstract: A method and apparatus for ramping current in an electromagnet in which a coil is used to generate the magnetic field provides rapid changes in the generated magnetic field. The method allows a change in current in the coil to be accomplished more rapidly than by applying a step change in voltage, when superconducting coils subject to quenching are used or when nonsuperconducting coils subject to other physical limitations are used. The method requires that both the current I(t) through the coil and the first derivative of the current vary with respect to time t during the ramping period, so that magnitude of the derivative of the current is higher when the magnitude of applied current is lower, lower when the magnitude of the applied current is higher. One variation of this method supplies (or removes) a constant amount of power from the magnetic field of the magnet, while another variation compensates for both self-generated eddy current losses and self-generated high field effects.

Abstract: Plural units each composed of a metallic frame and a dielectric substrate mounted on the frame are stacked on one another, thereby forming a unitary case. A superconducting circuit such as a resonator is formed on an upper surface of the substrate, and a ground plane, made of a metallic material, preferably, a superconducting material, is formed on a lower surface of the substrate. The ground plane is exposed to a center opening of the frame, so that the ground plane of one substrate faces the superconducting circuit of another substrate which is stacked underneath the one substrate with a space therebetween. Thus, a plurality of the superconducting circuits are mounted in a compact unitary case.

Abstract: A control method allowing stable operation of a refrigerator conduction cooling type superconducting coil employing an oxide high temperature superconductor is provided. Thermal resistance between a refrigerator and a superconducting coil connected to a cooling stage of the refrigerator is obtained. From the obtained thermal resistance and the rated cooling capacity of the refrigerator, an effective cooling curve representing the relation between the temperature and calorific value is obtained. Operation of the superconducting coil which is energized while being cooled by the refrigerator is controlled such that the calorific value of the superconducting coil at a prescribed temperature does not exceed the effective cooling curve.

Abstract: In a superconducting short circuit current limiter for an AC power supply net including an iron core, a primary coil and a secondary short-circuited superconductive coil, both being magnetically coupled with the iron core but being arranged at a distance from each other, the iron core has gaps at opposite sides of the secondary coil for thermically uncoupling the secondary coil from the rest of the magnetic core and a tertiary coil is arranged closely adjacent the primary coil in a magnetically strongly coupled relationship therewith, and has a heat capacity sufficient to accommodate the joulean heat generated when a fault or short circuit occurs in the power supply net, and the iron core has an area of reduced cross-section which is so selected that it becomes magnetic flux saturated when the current in the primary coil exceeds a predetermined reaction current level.

Abstract: A current limiting device including at least one superconductor and at least one nonsuperconducting shunt resistor in parallel with each superconductor. Each superconductor has a first main superconductor face in contact with a main shunt resistor face of a shunt resistor so as to form a compound conductor generally in the form of a meandering band having a band width greater than approximately 3.5 times the superconductor thickness. At least one insulator has opposing faces each in contact with a compound conductor. Multiple compound conductors are arranged such that current flows in opposite directions through adjacent compound conductors.

Abstract: A control method permits the operation of multiple superconducting magnetic coils so as to move a magnetic object to precisely specified locations within the body under command of a physician-operator observing the motion with live fluoroscopic imaging fused with more detailed preoperative imaging of some other kind. A computer contains the preoperative images and the fluoroscopic images, as well as the means to effect changes in the coil currents so as to accomplish the desired magnetic object motion and positioning. The control method operates the coils in pairs on opposite sides of the body in a manner to minimize the necessary current changes, thus avoiding the quenching of the superconducting coils. Combinations of these pairs can execute motion of the magnetic object in any direction in an impulsive manner and with high precision. The method should function well and provide advantages with coils which are not superconducting as well.

Abstract: A superconductive-magnet electrical circuit includes a superconductive-coil assemblage having first, second, third, and fourth coil portions sequentially coupled together in series. A bipolar current-bypass electrical-circuit element (such as a pair of diodes opposingly coupled together in parallel) has:an impedance less than generally one-thousandth that of the coil portions; a first terminal coupled in parallel with the first and second coil portions; and a second terminal coupled in parallel with the third and fourth coil portions. A localized quench in one coil portion is quickly shared with the other coil portions to reduce damage from the quench. A resistor has a lead coupled in parallel with the second and third coil portions to limit quench voltages.

Abstract: The present invention pertains to an electrical fault limiter. The fault limiter comprises a first magnetic core. There is also a second magnetic core opposing the first magnetic core. The fault limiter additionally comprises a third magnetic core adjacent the first magnetic core. Also, there is a fourth magnetic core opposing the third magnetic core and adjacent the second magnetic core. The electrical fault limiter also moreover comprises a rotor disposed between the first and second magnetic cores, and the third and fourth magnetic cores. The rotor is rotatable about a rotor axis. The first and second magnetic cores are disposed on a first side of the rotor axis and the third and fourth magnetic cores are disposed on a second side of the rotor axis. Each core has a first arm, a second arm and a body to which the first and second arms are connected. Each body has a superconducting bias coil disposed about it. Each arm has a conduction mode coil disposed about it.

Abstract: An electrical fault limiter having first, second, third, and fourth magnetic cores. The first magnetic core opposes the second magnetic core and is adjacent to the third magnetic core. The fourth magnetic core opposes the third magnetic core and is adjacent to the second magnetic core. A rotor is disposed between the first and second magnetic cores, and the third and fourth magnetic cores. The rotor is rotatable about a rotor axis. The first and second magnetic cores are disposed on a first side of the rotor axis and the third and fourth magnetic cores are disposed on a second side of the rotor axis. Each core has a first arm, a second arm and a body to which the first and second arms are connected. Each body has a superconduction bias coil disposed about it. Each arm has a conduction mode coil disposed about it.

Abstract: An inductive superconducting current storage (2), characterized in that it comprises an inner coil (4) wound from superconducting material and an outer coil (6) wound from superconducting material and disposed around the inner coil (4) in spaced manner therefrom, said inner coil (4) and said outer coil (6) in operation having current flowing therethrough in opposite directions so that the same magnetic flux as in the inner space (14) of the inner coil (4), but of opposite direction, is present in the annular space (12) between inner coil (4) and outer coil (6).

Abstract: A current limiting device using a superconductor limits a current flowing throu A c5 V pV:a current limiting coil, which mangetically couples to the superconductor. A control coil is provided to magnetically couple with the current limiting coil when the superconductor is switched to a normal conduction state. A variable control impedance is connected to the control coil to adjust a control current flowing through the control coil. The current limiting impedance value of the current limiting coil is adjusted by the value of the control impedance to obtain an adjustable value of the current flowing through the current limiting coil.

Abstract: The present invention provides a connection for feeding electricity to a transposed superconductive coil, the coil being placed in a cryostat and comprising a plurality of identical superconductive conductors, the free ends of which are distributed uniformly with cylindrical symmetry, the connection being constituted by two current leads each composed of a plurality of identical lead conductors each comprising a first end and a second end, the connection being characterized in that the number of the conductors is identical in each lead and equal to the number of the superconductive conductors in the coil, in that the conductors of the leads are disposed regularly and in parallel with the cylindrical symmetry about the axis of said coil, in that the leads are disposed coaxially and in that the first end of each lead conductor is connected to a respective superconductive conductor.

Abstract: A fault current limiter incorporating a high temperature superconductor material. During a fault condition, the superconductor material is allowed to operate above its critical temperature, providing sharply increased resistance. The fault current limiter can be sized and selectively cooled to provide a desired range of fault current limiting capability.

Abstract: A stable superconducting switch suitable for use in a conduction-cooled superconducting magnet includes a tape wound in a coil with the tape including an Nb.sub.3 Sn conductor sandwiched between stabilizing layers of copper, bronze or brass and wound in layers, with groups of layers separated by a partial layer of electrically conductive material.

Abstract: The present invention relates to a superconductive coil assembly which may be used particularly for a current limiter, the coil assembly comprising a plurality of coaxial cylindrical coils electrically connected in series, each coil comprising two windings of superconductive material electrically connected in parallel, arranged coaxially and wound in opposite directions, wherein in each coil, the windings are separated by a material which has a surface resistance in the radial direction lying in the range 10.sup.-5 .OMEGA..m.sup.2 to 10.sup.-2 .OMEGA..m.sup.2 at the temperature of the cryostat, and in which the strands which constitute the superconductive wires may themselves be covered with a layer of the same material.

Abstract: A persistent switch system for protecting a large superconducting magnet coil during quench while providing the necessary different resistances needed for operation of the system in coil charging, discharging, persistent and quench modes while minimizing the heat load on the system refrigerator. The system includes a circuit having a combined persistent switch and shunt heater coil using two superconductor coils in parallel with a normal resistive thermal shunt coil thermally linked to the superconducting magnet coil. Switch superconducting coils can be driven normal to provide the switch resistance needed for efficient charging/discharging or the resistance needed to drive sufficient current through the shunt resistor coil during a quench. The quench operation uses shunt coil heating to enhance normal zone growth in the main coil, limiting the maximum quench temperatures reached.

Abstract: A variable impedance device comprises a coil (10) and an associated magnetic circuit (16) having a gap receiving a plate (20) of superconducting material which acts as a shield or barrier so that, while the plate (20) is in its superconducting state, magnetic flux is caused to traverse the gap via pathways substantially greater in length than the width of the gap. If a large current is passed through the coil (10), the plate (20) becomes resistive thereby presenting a substantially increased impedance to the current. The device may be used in current limiting applications, or other applications where a variable impedance is required.

Abstract: Electrical energy is transferred or switched by selectively holding off the coupling of a magnetic field to a secondary inductive element (a coil) through a path which contains a high temperature superconductive element (HTS) which is capable of holding off the field when in its superconductive state notwithstanding that it is a high energy magnetic field. The HTS operates to hold off the magnetic field in accordance with the flux exclusion effect. When the HTS element is driven normal by heating with a laser pulse, the flux passes through the element and couples the field to the secondary, which may be connected to a load. A primary coil of superconducting material around the secondary coil can provide superconducting magnetic energy storage. The primary field is held off by HTS elements in the flux path to opposite ends of the secondary coil. These elements may be driven normal by laser pulses to transfer the stored magnetic energy to a load.

Abstract: A device having cores of metal oxide ceramic (for example, Y-Ba-Cu-O) for limiting a short circuit current in power supply systems. The concept provides that a choke core, when operated at a rated current, is superconductive and its shielding currents keep the resulting inductance in the choke at a low level. In the event of an overload, the winding of the choke generates a correspondingly high magnetic field in the core which puts the core into the normally conducting state. This causes the shielding currents to disappear in connection with a rise in the resulting inductance, thus limiting the current. In order to realize a particularly high inductance in the normally conductive case, the superconductive choke core may be made hollow and may be filled at least in part with a ferromagnetic material.

Abstract: A superconductive load bearing support without a mechanical contact and vibration damping for cryogenic instruments in space. The levitation support and vibration damping is accomplished by the use of superconducting magnets and the "Meissner" effect. The assembly allows for transfer of vibration energy away from the cryogenic instrument which then can be damped by the use of either an electronic circuit or conventional vibration damping apparatus.

Abstract: A superconducting magnet apparatus has two or more divided superconducting coils successively arrayed in series and a magnetic shield. The divided superconducting coils arrayed in the axial direction are successively electrically connected in series, and protective diodes are connected between two connection points symmetrically-located with respect to the center of the serially-connected superconducting coil array as a reference.

Abstract: An apparatus for producing an electrical impulse comprising a tube made of superconducting material; a source of magnetic flux is mounted about one end of the tube; a means for intercepting the flux is mounted along the tube; and a means for changing the temperature of the superconductor is mounted about the tube. As the tube is progressively made superconducting, the magnetic field is trapped within the tube and in so doing creates an electrical impulse in the means for intercepting. A reversal of the superconducting state produces a second pulse.

Abstract: A superconducting magnet coil is protected from local damage due to quenching by inductively driven heaters formed by heater strips embedded in the coil and connected in closed loops. A cold bypass diode shunting the coil winding commutates current out of the magnet when the voltage drop across the region of the coil which has gone normal exceeds the forward bias of the diode. This change in coil current induces sufficient current in the heater loops to cause the entire coil to go normal for uniform dissipation of stored energy throughout the magnet.

Abstract: A levitating support and positioning system (10) is provided for orienting an electromagnetic energy reflecting assembly (40). System (10) includes a reflective member (60) supported by an annular ring (50) having a plurality of superconductors (70) disposed thereon. Ring (50) is levitated above a base surface (20) by means of a plurality of electromagnetic assemblies (30), each of the electromagnetic assemblies (30) corresponding to a respective one of the plurality of superconductive elements (70), whereby the magnetic fields generated by the electromagnetic assemblies (30) are repelled by the respective superconductive elements. The orientation of the support ring (50), and the reflector therewith, is adjusted by changing the relative magnetic field strength between each of the electromagnetic assemblies (30), allowing the reflector to be directed in both elevation and azimuth.

Abstract: An electrical switch structure which employs superconductive material. A magnetizable core is encompassed by a body of superconductive material. The body of superconductive material has a superconductive state and a normal resistive state and can be placed in either state. The magnetizable core is also encompassed by at least one electrically conductive winding through which electrical current flows to create a magnetic flux within the magnetizable core. When the body of superconductive material is in its superconductive state current is induced therein by the magnetic flux in the magnetizable core. Current flow in the body of superconductive creates a magnetic flux in the magnetizable core which cancels the magnetic flux which is created by current flow in the electrically conductive winding.

Abstract: An improvement of a current limiting device using a superconductive coil. A trigger coil and current limiting coils are formed as a superconductive coil. A switching element which is actuated when a current limiting operation by the current limiting coil occurs, is connected in series to the trigger coil to immediately suppress heat generation by the trigger coil. When a quenching is detected by a detector, the trigger coil is disconnected and an additionally provided recovery coil is connected to speed up the recovery operation. A plurality of trigger coils connected in series may be used.

Abstract: A superconducting magnet device which is capable of protecting at the time of quenching a superconducting magnet divided into a plurality of sections comprises a superconducting magnet circuit which is formed by sequentially connecting in series coil pairs 1a, 1b, and 2a, 2b which are positioned symmetrically with respect to a ferromagnetic shield 6, and by connecting in parallel to the superconducting coils 1a, 1b and 2a, 2b superconducting coil protecting elements 3a and 3b which protect the superconducting coils 1a, 1b and 2a, 2b when the voltage at the ends of the coils has exceeded a predetermined value. If quenching occurs at the superconducting coils 1a, 1b and 2a, 2b, generation of an unbalanced electromagnetic force, a local electromagnetic force, etc., against the ferromagnetic shield 6 is prevented, so that the support structure for the superconducting coils and the ferromagnetic shield can be simplified.