Patent Publication Number: US-2009219121-A1

Title: Superconducting magnet current adjustment by flux pumping

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods and equipment for adjusting current flowing in superconducting magnets. The invention is particularly applicable to superconducting magnets employed in imaging systems such as magnetic resonance imaging (MRI) systems. 
     2. Description of the Prior Art 
     Once a superconducting magnet is installed ready for use, it must be energized. Electrical current must be introduced into the coil windings. A superconducting switch is usually provided across the coil windings. When this switch is superconducting, a closed superconducting current path is provided through the coil windings. Once the current has been established in the coil windings, the current will continue to flow with only gradual reduction in current magnitude. A superconducting magnet is typically energized (‘ramped’) by connecting a low voltage, high current power supply across the superconducting switch at suitable input terminals. The superconducting switch is temporarily held in a non-superconducting state to allow current to be introduced into the magnet windings. Present MRI magnets typically carry currents of about 400-500 A. This method of energization requires a suitable power supply to be available when the procedure is due to occur. 
     Although electrical current flows substantially unimpeded in the superconducting magnet, the magnitude of the current flowing will gradually diminish due to imperfections such as non-zero resistance in wire joints. At regular intervals after the initial energization, typically once per year in present systems, further electrical current will need to be supplied into the magnet to restore the current to its initial value. Typically, this current re-establishment is achieved by reconnecting the low voltage, high current power supply across the superconducting switch, temporarily placing the superconducting switch in a non-superconducting state, for example, by applying heat to the superconducting switch, and increasing the current into the magnet by a process very similar to the original energization. This operation is colloquially known as ‘bumping’ the magnet back to its initial field value. Such operation requires reconnection of the power supply and appropriate operation of the superconducting switch. Usually, a service technician is sent to the site of the magnet to perform these operations. 
     It would be beneficial if such current re-establishment (‘bumping’) could be carried out without the need to reattach the power supply; and/or without the need for a service technician to attend. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and apparatus for current re-establishment in superconducting magnets, and removes the need for an external power supply to provide a current source for the current re-establishment (‘bump’) procedure. According to an embodiment of the present invention, a gradient winding and gradient winding power supply, typically conventionally provided in any superconducting magnet-based imaging system, are used to ‘bump’ the magnet. 
     The present invention encompasses a superconducting magnet arrangement and a method for adjusting a current flowing in the main or basic field magnet windings of a superconducting magnet arrangement, wherein the basic field magnetic windings have a first switch connected between first and second ends of the basic field magnet windings, the first switch being controllable between two states, of which a first state is relatively conductive and a second state is relatively non-conductive, and induction coil having a first end connected to the first end of the basic field magnet windings and a second end connected through a second switch, with the second switch being controllable between two states, a first of which is relative conductive and a second of which is relative non-conductive, and the second switch being connected to the second end of the basic field magnet windings, and a gradient coil that is capable of magnetically coupling with the induction coil. 
     In accordance with the present invention, the second switch is controlled into its second state so as to change the magnitude and/or direction of a current flowing in the gradient windings, thereby producing a change in the magnitude and/or direction of magnetic flux that couples with the induction coil. Additionally, the first and second switches are controlled into their first conductive states so as to change the magnitude and/or direction of the current through the gradient windings, so as to induce a current in the gradient coil, which serves to maintain a residual magnetic flux in the induction coil. Additionally, the first switch is controlled into its second state such that the induced current flows through both the induction coil and the basic field magnet windings, to maintain the residual magnetic flux within both the induction coil and the basic field magnet windings. 
     The first switch also is controlled into its first state and the second switch is controlled into its second state, so as to leave a changed level of current flowing in the basic field magnet windings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows an axial half-cross section of coils in a superconducting magnet. 
         FIG. 2  illustrates an example of conventional actively shielded gradient winding interconnection in a magnet arrangement including an induction coil according to an aspect of the present invention. 
         FIG. 3  shows an implementation of the present invention, where a switching arrangement is used to cause current flowing in each gradient shield winding to flow in the same direction as the current flowing in the corresponding gradient winding. 
         FIG. 4  shows an idealized circuit diagram of a magnet and gradient winding flux pumping arrangement according to the present invention. 
         FIGS. 5A-5I  show stages in a method according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides methods and apparatus for ‘bumping’ a superconducting magnet without the need for an external power supply for the purpose. In addition, certain embodiments of the invention allow remote ‘bumping’ simply by controlling parts of the installed magnet system. The remote ‘bumping’ may be initiated by telephone or over the internet. Alternatively, a simple user-operated control may be provided to initiate ‘bumping’. Alternatively, a regular ‘bumping’ cycle may be set to operate at fixed time intervals. Alternatively, a ‘bumping’ cycle may be initiated in response to a measurement indicating a certain level of current degradation in the magnet coils. 
     The various embodiments of the present invention provide at least some of the following advantages: 
     reduced service cost due to reduced equipment requirement, as no magnet power supply is required for ‘bumping’;
 
magnet ‘bumping’ is performed by equipment already available on site, typically gradient windings and gradient power amplifier; and
 
reduced requirement for site visits by maintenance technicians, due to the possibility of remotely- or automatically-controlled ‘bumping’; or user-initiated ‘bumping’.
 
     The present invention achieves current re-establishment (‘bumping’) by use of a magnetic flux pump. While it is believed that a magnetic flux pump will be familiar to those skilled in the art, a brief description of the operation of a flux pump is provided here for reference. 
     Magnetic flux pumping is a method for varying a current in a superconducting circuit by changing the magnetic flux within the superconducting circuit using a sequence of steps of applying and removing external flux. Such operation can be explained by writing Faraday&#39;s law of induction for a closed circuit of resistance, R, and inductance, L. 
     
       
         
           
             
               
                 
                   
                     
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     A superconducting circuit has zero resistance, so setting R=0 and integrating gives: 
       φ+ LI=k    
     where k is a constant representing the total flux in the circuit. In words, the total of the flux LI produced by current flowing in the circuit and any externally applied flux φ is constant. Flux pumping, described below in its specific application to the present invention, enables the constant k to be changed, so changing the current I flowing in the superconducting magnet of inductance L. This equation demonstrates that a superconducting circuit reacts to any change in external flux, φ, by an opposing change in the current, I, in order to maintain a constant value of k. 
     According to an embodiment of the present invention, an external source of magnetic flux φ is provided by causing a change in magnitude and/or direction of an electric current in an existing gradient winding within a superconducting magnet-based imaging system using a corresponding gradient power amplifier. 
     Simply causing a change in current through a gradient winding circuit will not be sufficient to induce current re-establishment (‘bumping’) in a typical superconducting magnet-based imaging system, due to the symmetry of typical systems. An increase in externally applied flux φ at one end of the magnet would be balanced by an equal decrease in externally applied flux φ at the other end of the magnet, resulting in no overall change of the current through the magnet. 
     According to an aspect of the present invention, a separate superconducting coil is connected to the main magnet circuit.  FIG. 1  schematically illustrates possible arrangements of coils in a superconducting magnet  10 .  FIG. 1  represents an axial half-cross section symmetrical about axis A-A, with a nominal plane of symmetry X-X. Conventionally, the axial direction is referred to as the Z-direction, a vertical radial direction is referred to as the X-direction, and a radial direction perpendicular to the X-direction is referred to as the Y-direction. The coils include primary magnet coils  12  which generate the main magnet field; shield coils  14  which reduce the stray magnetic field outside of the coil arrangement; a gradient winding having gradient windings  16   a  generates magnetic field gradients as required for imaging; a gradient shield winding comprising gradient shield windings  16   b  reduces the stray magnetic field outside of the gradient winding; and an induction coil  18 , according to an aspect of the present invention. The induction coil  18  is asymmetrically installed into the magnet coil arrangement so that the field generated by each gradient winding  16   a  and gradient shield winding  16   b  does not cancel itself out in the induction coil  18 . Induction coil  18  is not connected in series with the main magnet windings, but is connected to them, as illustrated in  FIG. 2 . 
     Typically, in use, gradient windings  16   a ,  16   b  towards one axial end of the magnet provide increased magnetic flux density, while gradient windings  16   a ,  16   b  towards the other axial end of the magnet provide reduced magnetic flux density, in order to provide the required magnetic field gradient for imaging. By placing the induction coil  18  near one end, and generating an increased magnetic flux density by the adjacent gradient windings  16   a ,  16   b , the required flux pumping for current re-establishment (‘bumping’) may be achieved, as described below. 
     In normal operation, the gradient windings  16   a , and gradient shield windings  16   b  are connected such that a magnetic gradient can be created in the X, Y &amp; Z directions. This is normally achieved by having an actively shielded Maxwell pair of coils in the Z axis and two orthogonal sets of Golay pairs in the X &amp; Y axes. The gradient windings  16   a  shown in  FIG. 1  are the Z gradient windings and gradient shield windings  16   b  are the corresponding shield windings. Each gradient shield winding  16   b  produces a magnetic field of reduced flux density and opposite polarity to that of the accompanying gradient winding  16   a . The gradient shield windings  16   b  typically reduce the magnetic flux density generated by the gradient windings  16   a  that crosses the main magnet circuit  12 , so reducing the interaction between the primary magnet coils  12  and the gradient windings  16   a.    
     The inductance of the induction coil  18  may, for example, be of several tens of millihenry. Present MRI magnets typically have a main coil comprising main magnet windings  12  of total inductance of several henries. 
       FIG. 2  illustrates an example of conventional actively shielded gradient winding interconnection in a magnet arrangement including an induction coil  18  according to an aspect of the present invention. The switch S 1  is part of magnet circuit C 1  and is a superconducting switch with a normally open state and a superconducting ‘closed’ state. The switch S 2  is part of induction circuit C 2  and may be a non-superconducting switch with a normal high impedance ‘open’ state and a normal low impedance ‘closed’ state. The switch S 2  may be a solid state device and may be located within a cryostat containing the superconducting magnet coils  12  at a higher temperature location within the cryostat, e.g. within the turret, to allow the switch to function correctly. Alternatively, the switch S 2  may be a superconducting switch, similar to switch S 1 . During bumping, current I 2  will flow through the induction coil  18 , while the main magnet circuit C 1  experiences only small changes in the total magnet current I 1 . 
     As shown, the current in each gradient winding  16   a  is provided by gradient power amplifier GPA  22 , and flows in the opposite direction from the current in the accompanying gradient shield winding  16   b . This results in a limited magnetic coupling  20  between the gradient winding  16   a  and the main magnet windings  12 , and between the gradient winding  16   a  and the induction coil  18 . In normal magnet operation, a low level of magnetic coupling  20  is preferred, to avoid any interference with the main magnetic field. However, for the purposes of the present invention, a high level of magnetic coupling  20  between the gradient windings  16   a , and the gradient shield windings  16   b ; and the induction coil  18  would be preferred. 
     One or more switches S 2  are included in the induction circuit C 2  to provide multiple current paths within the induction coil/magnet circuit. This allows current to be accumulated in the magnet circuit by flux pumping. 
     According to certain embodiments of the present invention, a switching arrangement is provided to allow current in the gradient winding  16   a  and gradient shield winding  16   b  to be redirected between the separate gradient windings and gradient shield windings to allow increased magnetic coupling  20  between the induction coil  18  and the gradient windings  16   a , and gradient shield windings  16   b . This causes more external magnetic flux φ to cross the induction coil circuit C 2 , which can be used for flux pumping. 
     To increase the external magnetic flux φ from the gradient windings  16   a , and gradient shield windings  16   b  that is experienced by the induction coil  18 , the gradient windings  16   a  and gradient shield windings  16   b  may have an additional switch arrangement to either cancel the behavior of the gradient shield windings  16   b  or to reverse the current direction in the gradient shield windings, so reinforcing the flux from the gradient windings  16   a . Such switch arrangement may be provided by any suitable switching device, such as a mechanical switch or solid state device. 
       FIG. 3  shows such an implementation, where a switching arrangement is being used to cause current flowing in each gradient shield winding  16   b  to flow in the same direction as the current flowing in the corresponding gradient winding  16   a . A greater level of magnetic coupling  20  is achieved than in the case of  FIG. 2 . This causes an increased change in external magnetic flux φ 1 , φ 2  to be produced at each end of the magnet axis as a result of current flowing in the gradient windings  16   a  and the gradient shield windings  16   b . However, due to the symmetry of the main magnet windings  12  and the gradient windings  16 , no overall change in current occurs in the main magnet windings  12 . On the other hand, as the induction coil  18  is asymmetrically placed, a significant change in externally applied magnetic flux φ 2  crossing the induction coil  18  is observed. This change in flux may be translated into a change in the current I 2  in the induction coil  18 , which may in turn be employed by the present invention to adjust the current I 1  in the magnet circuit C 1  as will be discussed in detail below. 
     A flux pumping procedure can then be applied to the magnet by use of the described switching arrangements to allow external magnetic flux φ from the gradient windings  16   a  and gradient shield windings  16   b  to be accumulated as current I 2  within the induction coil  18  and then transferred to the magnet circuit C 1 . 
     According to an embodiment, the apparatus of the present invention comprises a superconducting solenoidal magnet  10  with a coaxially located gradient coil  16   a , and gradient shield coil  16   b ; an associated gradient power amplifier  22  and an asymmetrically positioned induction coil  18 , with switches S 1  and S 2  controlling current flow in the main magnet windings  12  and the induction coil  18 . 
     The superconducting magnet  10  is provided with a switching arrangement such as switches S 1  and S 2  to redirect current induced during flux pumping of the induction coil  18  by the gradient windings  16   a  and gradient shield windings  16   b , so as to allow current induced in the induction coil  18  to be accumulated within the main magnet windings  12 . 
     Both of the switches S 1  and S 2  and the induction coil  18  must be capable of taking the full magnet current I 1 . 
       FIG. 4  shows an idealized circuit diagram of a magnet and gradient coil flux pumping arrangement according to the present invention. Gradient winding  16   a  and gradient shield winding  16   b  are connected so as to generate magnetic fields in a same direction. Switches S 1  and S 2  are shown closed. The gradient winding  16   a  and the gradient shield winding  16   b  will, for simplicity of the following description of the principle of the invention, be taken to couple only with the induction coil  18  and not with the main magnet windings  12 . This will of course not be the physical reality, but the gradient coil will couple more strongly with the induction coil than the main magnet winding due to the asymmetrical positioning of the induction coil. The switching arrangement provided for the gradient windings may be used to disconnect gradient windings at one end of the magnet, ensuring that only those gradient windings with best coupling to the induction coil  18  are used, as shown in  FIG. 4 . 
     Operation of the flux pumping sequence according to an embodiment of the present invention will now be described with reference to  FIGS. 5A-5I . In the following description, L 12  and L 18  are inductances of the main magnet coils  12  and the induction coil  18  respectively, while k 1  represents the initial total flux in the main magnet coils  12 . 
     Initially, as shown in  FIG. 5A , switch S 1  is closed, allowing an initial current I 1  to flow in circuit C 1 , through the main magnet windings  12 . Switch S 2  is open. Current is provided to gradient winding  16   a  and gradient shield winding  16   b  by gradient power amplifier  22 . This current causes an externally applied flux φ of value φ 2  to cross the induction coil  18  in open circuit C 2 . 
     In this arrangement, the current flowing in induction coil  18  is zero, so the total flux in induction coil  18  is φ 2 . Assuming that no externally applied flux crosses the main magnet coils, the total flux in the main magnet coils  12  is: 
         L   12   ·I   1   =k   1 . 
     In a next step, as illustrated in  FIG. 5B , switch S 1  remains closed while switch S 2  is closed, completing circuit C 2 . The current through gradient winding  16   a  and gradient shield winding  16   b  provided by gradient power amplifier  22  is turned off. The externally applied flux φ falls to zero. This induces a current Ib in circuit C 2  to preserve the flux φ 2  in induction coil  18 , such that: 
         L   18   ·I   b =φ2. 
     In a next step, as illustrated in  FIG. 5C , switch S 1  is opened while switch S 2  remains closed. The total flux in the circuit comprising the main magnet coils  12  and the induction coil  18  is now L 12 ·I 1 +φ 2 . An increased current I+ now flows through the induction coil  18  and the main magnet windings  12  to preserve the total flux such that: 
       ( L   12   +L   18 )· I+=L   12   ·I   1 +φ2. 
     The increase in current is approximately φ 2 /L 12 . 
     In a next step, as illustrated in  FIG. 5D , switch S 1  is closed while switch S 2  is opened. Current ceases to flow in induction coil  18 , but increased current I+ now flows in circuit C 1 . The current flowing in main magnet windings  12  has been increased by approximately φ 2 /L 12  as compared to the situation in  FIG. 5A . An increase in magnetic flux in the main magnet windings  12  is preserved in circuit C 1  by increased current I+ flowing through main magnet windings  12 ; such that to a first approximation: 
         L   12   ·I+=k   1 +φ2. 
     The current in the main magnet coils has evidently been increased by this flux pumping operation. Further flux pumping cycles may be performed as follows. 
     In a next step, as illustrated in  FIG. 5E , switch S 1  remains closed and switch S 2  remains open, while current through gradient winding  16   a  and gradient shield winding  16   b  is again provided by gradient power amplifier  22 . The current flowing in the gradient winding  16   a  and the gradient shield winding  16   b  again induces a magnetic flux φ 2  in the induction coil  18  of circuit C 2 . As no current flows in induction coil  18 , the total flux in induction coil  18  is again φ 2 . 
     In a next step, as illustrated in  FIG. 5F , switch S 1  remains closed while switch S 2  is closed, completing circuit C 2 . The current through gradient winding  16   a  and gradient shield winding  16   b  provided by gradient power amplifier  22  is turned off. This induces a current Ib in circuit C 2  to preserve the flux φ 2  in induction coil  18 , such that: 
         L   18   ·I   b =φ 2 . 
     In a next step, as illustrated in  FIG. 5G , switch S 1  is opened while switch S 2  remains closed. The total flux in the circuit comprising the main magnet coils  12  and the induction coil  18  is now (L 12 ·I++φ 2 ). A further increased current I++ now flows through the induction coil  18  and the main magnet windings  12  to preserve the total flux, such that: 
       ( L   12   +L   18 )· I++=L   12   ·I++φ 2. 
     In a next step, as illustrated in  FIG. 5H , switch S 1  is closed while switch S 2  is opened. Current ceases to flow in induction coil  18 , but further increased current I++ now flows in circuit C 1 . The current flowing in main magnet windings  12  has been changed by magnitude approximately φ 2 /L 12  as compared to the increased current I+ in  FIG. 5D . The total magnetic flux is preserved in circuit C 1  by current I++ flowing through main magnet windings  12 ; such that to a first approximation: 
         L   12   ·I++=k   1 +2·φ2. 
     In a next step, as illustrated in  FIG. 5I , switch S 1  remains closed and switch S 2  remains open, while current through gradient winding  16   a  and gradient shield winding  16   b  is again provided by gradient power amplifier  22 . The current flowing in the gradient winding  16   a  and gradient shield winding  16   b  again induces a magnetic flux φ 2  in the induction coil  18  of circuit C 2 . 
     The above sequence of steps may be repeated as required to further increase the current flowing in the main magnet windings  12  by a further amount approximately equal to φ 2 /L 12 . A limit may be reached when the flux in main magnet windings  12  reaches about the same level (φ 2 ) as that generated in the induction coil by the gradient winding  16   a  and gradient shield winding  16   b  when current is provided by the gradient power amplifier  22 ; that is, when the current in the main magnet coils reaches the value of φ 2 /L 18 . 
     Depending on the circuit components used, each cycle of the described flux pumping may be completed in a few seconds. 
     Alternatively, by applying the current in the gradient winding  16   a  and the gradient shield winding  16   b  in the opposite direction, the current in the main magnet windings  12  may be progressively reduced in steps approximately equal to φ 2 /L 12 , in a corresponding fashion. 
     As may be observed from considering the above description, it is not necessary to connect any special equipment to perform current re-establishment (‘bumping’). All that is required is to perform a certain sequence of switching of the connections of the gradient windings and gradient shield windings, the circuit switches S 1  and S 2  and switching on and off of the current through the gradient windings. Such operation may be readily automated by those skilled in the art, and arrangements may be made to have such current re-establishment performed at predetermined time intervals or in response to the detection of a reduced current in the main magnet windings  12 , or in response to user initiation by operation of a simple control. It may be possible to arrange a superconducting magnet-based imaging system such that current re-establishment (‘bumping’) is performed before each imaging operation. 
     This invention encompasses a method by which the current in a superconducting magnet for an imaging system can be varied by using the associated gradient coil as flux pump. Apparatus for performing such current variation is also described. 
     In order to facilitate flux pumping by the gradient coil, a separate superconducting induction coil  18  is preferably provided, situated asymmetrically within the magnet coil arrangement. In a solenoidal magnet arrangement, the induction coil  18  is preferably coaxial with the other magnet coils but is offset axially to allow coupling between itself and the gradient windings  16   a ,  16   b . It is envisaged that, within a solenoidal magnet, the induction coil  18  may be wound on a common former with the main magnet windings  12 . 
     An induction coil switch S 2  is wired in series with the induction coil  18 . The switch S 2  may be a superconducting switch and having a normal ‘open’ state and a superconducting ‘closed’ state. Alternatively, the induction coil switch S 2  may be a solid state device located at a relatively warm location in the magnet arrangement, e.g. within a turret assembly of a cryostat housing the superconducting magnet. Induction coil switch S 2  may also be any other type of controlled non-superconducting switch. 
     A switching arrangement controlling switches S 1  and S 2  allows the current in the superconducting magnet circuit C 1  to be altered such that externally applied magnetic flux φ can be trapped within an associated coil  18  and the resulting reaction current Ib can be accumulated within the main magnet circuit C 1 . 
     A switching arrangement for the gradient windings and the gradient shield windings can be provided to allow the gradient coil current to be redirected in such a fashion that the magnetic flux coupling between the induction coil  18  and gradient windings  16   a  and the gradient shield windings  16   b  is increased. Typically, this is by causing gradient shield winding  16   b  to carry current in a same direction as an associated gradient winding  16   a.    
     A gradient coil current pulsing scheme, achieved by control of a gradient power amplifier  22  connected to supply current to the gradient coil, is timed to interact with the switching of the controlled switches S 1 , S 2  within the magnet arrangement to cause current to be accumulated or diminished within the main magnet windings  12 , enabling current re-establishment (‘bumping’), or controlled reduction of current in the main magnet windings  12 . 
     In addition to avoiding the need for a power supply to be provided for current re-establishment, the present invention also avoids the need to reconnect current leads to the magnet coils. Typically, current leads are connected to the current coils only when required for introducing or removing current, and are removed at other times to reduce heat influx. 
     Preferably, in order to provide improved efficiency of energy transfer, the time constant of the part of the gradient coil  16  used for flux pumping is adapted to match as closely as possible to that of the part of the induction coil  18  which is used for magnet energization. 
     The described current re-establishment (bumping may be usefully employed in order to set a desired initial magnet operating current at installation, and to restore the initial magnet operating current after decay events. Indeed, the invention could be used for the complete energization (ramping up) or de-energization (ramping down) process of a superconducting magnet. Field decay during normal magnet operation may preferably be compensated for by an automated ‘bumping’ process as described. In addition, high decay magnets presently considered unfit for use could be accepted, if the present invention is employed, since bumping can occur on a regular basis, such as daily or even more frequently if required. 
     As is well known, it is simpler to make non-superconducting joints than superconducting joints, for example by simply soldering superconducting wires together. Such non-superconducting joints have hitherto been considered unacceptable, due to the resulting current degradation. The invention allows the possibility of using non-superconducting joints within the magnet circuit, since the current degradation caused by non-superconducting joints may be readily compensated by the current reestablishment method of the present invention. Such method may be applied to magnets of any strength, and current degradation may be compensated by the methods according to the present invention. Hence, manufacturing costs of superconducting magnets may be reduced. 
     The invention should be equally applicable to both low temperature and high temperature superconducting magnets. 
     The present invention has been described with reference to gradient windings and gradient shield windings operating to provide the flux for flux pumping. The use of gradient windings and gradient shield windings is preferred, partly because the associated gradient power supplies  22  are capable of delivering currents of the same order of magnitude as those flowing in the main magnet windings  12 . On the other hand, it is not presently considered practical to use any of the magnet coils themselves in the place of the described gradient windings and gradient shield windings for flux pumping, as the resultant forces are considered excessive. In embodiments which use gradient windings for flux pumping, the gradient shield windings may be simply turned off by a switching arrangement, rather than used in support of the gradient windings. 
     While the present invention has been particularly described with reference to solenoidal magnets, the present invention may be applied to other magnet arrangements, such as open, biplanar, disc or asymmetric magnets, as will be apparent to those skilled in the art. 
     In magnets which employ active shield coils  14 , these are typically connected in series with the main magnet windings  12  in the magnet circuit C 1 , although active shield coils  14 , are not mentioned in the description of  FIGS. 2-5I  of the present application, for the sake of simplicity of description. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.