Patent ID: 8598971

Claim:
A magnetic field control apparatus comprising: a conductive vacuum duct through which a charged particle beam passes; and a plurality of magnetic field correcting plates disposed on the vacuum duct in areas at which magnetic poles of a dipole magnet for bending the charged particle beam are disposed, wherein: the magnetic field correcting plate is disposed for each of four areas defined by dividing a cross section of the vacuum duct, the cross section being perpendicular to a direction in which the charged particle beam travels, the cross section being divided by a symmetrical surface having each of both magnetic poles of the dipole magnet defined as a mirror image and a plane which extends perpendicularly to the symmetrical surface and through which a center of gravity of the charged particle beam passes, and at least one of the four areas of the vacuum duct includes a plurality of magnetic field correcting plates; the magnetic field correcting plates are formed of a material having an electric resistivity lower than that of the vacuum duct; and a magnetic field in the vacuum duct is controlled by superimposing a magnetic field generated by an eddy current induced in the magnetic field correcting plates over a magnetic field generated by an eddy current of the vacuum duct.