Magnet assembly

Abstract

An electromagnet comprising a ferromagnetic yoke which comprises a yoke. Mutually opposing first and second pole pieces are provided. The first pole piece is provided with a planar coil having a first side facing the yoke and a second side facing the yoke. A balancing member is arranged on the second side of the planar coil to counterbalance the attractive force between the planar coil and the yoke. The other pole piece may also be provided with a corresponding balancing member.

1 . An electromagnet comprising a yoke and a pair of mutually facing pole pieces, one or both of which is surrounded by a respective coil, the coil or coils being supported by a supporting member enclosed in a cryostat assembly and also being provided with a respective balancing member which at least partially counterbalances the attractive force between the coil or coils and the yoke. 
   
   
       2 . The electromagnet of  claim 1 , wherein the yoke is a ferromagnetic yoke. 
   
   
       3 . The electromagnet of  claim 2 , wherein the ferromagnetic yoke comprises first and second arms linked by a spine, wherein the first and second arms are provided respectively with said pole pieces which are first and second pole pieces, the first pole piece being provided with said at least one coil in the form of a planar coil having a first side facing the first arm and a second side facing the second arm, the balancing member being a ferromagnetic balancing member arranged on the second side of the planar coil. 
   
   
       4 . The electromagnet of  claim 3 , wherein the second pole piece is provided with a second one of said coils in the form of a planar coil having a first side facing the second arm and a second side facing the first arm, a second of said at least one balancing members being a ferromagnetic balancing member arranged on the second side of the second coil. 
   
   
       5 . The electromagnet of  claim 3 , wherein at least part of either or both of said at least one balancing members has a higher magnetic permeability than that of its respective associated arm and/or a higher magnetic saturation than that of the respective associated arm. 
   
   
       6 . The electromagnet of  claim 3 , wherein either or both of said at least one balancing members counterbalances the attractive force between the respective coil and arm by virtue of its position. 
   
   
       7 . The electromagnet of  claim 3 , wherein either or both of said at least one balancing members is non-ferromagnetically separated from its respective pole piece and yoke arm. 
   
   
       8 . The electromagnet of  claim 3 , wherein the yoke is C-shaped, or H shaped and the arms and spine are straight or curved. 
   
   
       9 . The electromagnet of  claim 1 , wherein the or each balancing member is constituted by part of its associated pole piece. 
   
   
       10 . The electromagnet of  claim 9 , wherein each pole piece is generally annular and extends toward the other pole piece thereby defining an outer wall in which is provided with a circumferential recess, the balancing member being that part of the pole piece extending beyond and at least partially below the second side of the coil. 
   
   
       11 . The electromagnet of  claim 9 , wherein the yoke is provided with a recess, near or in which the associated coil is at least partially situated. 
   
   
       12 . The electromagnet of  claim 9 , wherein the pole piece is constructed as a composite comprising two or more constituent members, the balancing member comprising at least two of said constituent members. 
   
   
       13 . The electromagnet of  claim 12 , wherein at least one of the constituent members has a substantially square or substantially rectangular profile in cross section and wherein at least one corner of the substantially square or substantially rectangular profile is chamfered. 
   
   
       14 . The electromagnet of  claim 13 , wherein the chamfering is on a corner facing the coil. 
   
   
       15 . The electromagnet of  claim 12 , wherein the constituent members are respectively composed of different materials. 
   
   
       16 . The electromagnet of  claim 12 , wherein one or more of the constituent members of the pole piece is substantially annular such that each pole piece has azimuthally-varying non-uniform height for correcting components of field inhomogeneity which are not axially symmetric. 
   
   
       17 . The electromagnet of  claim 11 , wherein the pole piece is generally annular and inside one of the annular pole pieces is situated one or more ferromagnetic or permanent magnet annular field tuning rings. 
   
   
       18 . An electromagnet comprising a ferromagnetic yoke being provided with at least one pair of mutually facing planar coils, each coil in said at least one pair being supported by a supporting member enclosed in a cryostat assembly and having a first side facing the yoke and a second side facing the other coil, wherein a respective ferromagnetic balancing member is arranged on the second side of each coil to counterbalance the attractive force between the coil and the yoke. 
   
   
       19 . The electromagnet of  claim 18 , wherein each coil is situated closer to its associated balancing member than to the yoke. 
   
   
       20 . The electromagnet of  claim 18 , wherein at least part of each balancing member has a higher magnetic permeability than that of the yoke. 
   
   
       21 . The electromagnet of  claim 18 , wherein at least part of each balancing member has a higher saturation magnetisation than that of the yoke. 
   
   
       22 . The electromagnet of  claim 18 , wherein each balancing member is constituted by part of a respective pole piece. 
   
   
       23 . The electromagnet of  claim 22 , wherein each pole piece is generally annular and extends toward the other pole piece thereby defining an outer wall in which is provided with a circumferential recess, near or in which the associated coil is at least partially situated, the balancing member being that part of the pole piece extending beyond and at least partially below the second side of the coil. 
   
   
       24 . The electromagnet of  claim 22 , wherein the yoke is provided with a recess, near or in which the associated coil is at least partially situated. 
   
   
       25 . The electromagnet of  claim 22 , wherein each pole piece is constructed as a composite comprising two or more constituent members, each balancing member comprising at least two of said constituent members. 
   
   
       26 . The electromagnet of  claim 25 , wherein at least one of the constituent members has a substantially square or substantially rectangular profile in cross section and wherein at least one corner of the substantially square or substantially rectangular profile is chamfered. 
   
   
       27 . The electromagnet of  claim 26 , wherein the chamfering is on a corner facing the coil. 
   
   
       28 . The electromagnet of  claim 24 , wherein the constituent members are respectively composed of different materials. 
   
   
       29 . The electromagnet of  claim 24 , wherein one or more of the constituent members of each pole piece is substantially annular such that each pole piece has azimuthally-varying non-uniform height for correcting components of field inhomogeneity which are not axially symmetric. 
   
   
       30 . The electromagnet of  claim 22 , wherein both pole pieces are generally annular and inside one or both generally annular pole pieces is situated one or more ferromagnetic or permanent magnet annular field tuning rings. 
   
   
       31 . The electromagnet of  claim 1 , wherein each coil of said at least one pair is supported on a respective yoke member, the yoke members being joined by a support member, the coils being superconducting coils sharing a common cryo-cooling system. 
   
   
       32 . An electromagnet comprising a ferromagnetic yoke which comprises first and second arms linked by a spine, wherein the first and second arms are provided respectively with mutually opposing first and second pole pieces, the first pole piece being surrounded by a planar coil supported by a supporting member enclosed in a cryostat assembly and also having a first side facing the first arm and a second side facing the second arm, a ferromagnetic balancing member being arranged on the second side of the planar coil to counterbalance the attractive force between the planar coil and the first arm. 
   
   
       33 . An electromagnet comprising a ferromagnetic yoke which comprises first and second arms linked by a spine, wherein the first and second arms are provided with mutually opposing first and second pole pieces, the first pole piece being provided with a planar coil having a first side facing the first arm and a second side facing the second arm, a ferromagnetic balancing member being arrange arranged on the second side of the planar coil, at least part of the balancing member having a higher magnetic permeability than that of the first arm and/or at least part of the balancing member having a higher saturation magnetisation than that of the first arm. 
   
   
       34 . An electromagnet comprising a ferromagnetic yoke which comprises first and second arms linked by a spine, wherein the first and second arms are provided with mutually opposing first and second pole pieces, the first pole piece being provided with a planar coil having a first side facing the first arm and a second side facing the second arm, a ferromagnetic balancing member being arranged on the second side of the planar coil, wherein the ferromagnetic balancing member is non-ferromagnetically separated from the first pole piece and the first arm. 
   
   
       35 . The electromagnet of  claim 32 , wherein the or each coil is situated closer to its associated balancing member than to the yoke. 
   
   
       36 . The electromagnet of  claim 32 , wherein at least part of the or each balancing member has a higher magnetic permeability than that of the yoke. 
   
   
       37 . The electromagnet of  claim 32 , wherein at least part of each balancing member has a higher saturation magnetisation than that of the yoke. 
   
   
       38 . An MRI, NMR or ESR machine comprising an electromagnet which comprises a yoke and a pair of mutually facing pole pieces, one or both of which is surrounded by a respective coil, the coil or coils being supported by a supporting member enclosed in a cryostat assembly and also being provided with a respective balancing member which at least partially counterbalances the attractive force between the coil or coils and the yoke.