An electromagnet for steering a beam of charged particles comprising a ferromagnetic frame and a plurality of electrically conducting coils wound upon the frame for producing a magnetic field in and around the frame 112. The frame includes a plurality of limbs 114 connected at vertices 116 and arranged in a closed shape around a beam line space centred on a longitudinal axis 111; each limb extends along a limb axis joining adjacent vertices 116. The coils are wound onto each of the limbs to form a plurality of windings 130 that define an inner profile around the beam line space. The inner frame profile may be defined in part by portions of the limbs, wherein a first limb portion extends away from the limb axis towards the longitudinal axis and a second limb portion extends towards the limb axis and away from the longitudinal axis. The windings 130 may also include bevelled portions adjacent to the ends of the limbs. The ferromagnetic frame may comprise steel and it may possess four limbs.
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