Ion Beam Bending Magnet for a Ribbon-Shaped Ion Beam

An ion beam bending magnet having an ion beam inlet, an ion beam outlet and a curved path through the magnet for an ion beam, said curved path having a path length between said beam inlet and said beam outlet, said curved path bending monotonically in an ion beam bending plane and having a curved central beam axis, said magnet being adapted to accommodate and bend along said curved beam path, a ribbon-shaped ion beam having a cross-sectional profile with a major dimension normal to said bending plane;

• the magnet comprising
• a ferromagnetic yoke surrounding said curved beam path along said path length, said ferromagnetic yoke defining a uniform cross-sectional internal profile along said path length in respective planes normal to said curved central beam axis, said uniform cross-sectional internal profile being defined in Cartesian (x,y) co-ordinates having an origin at said central beam axis, an x-axis extending in said ion beam bending plane and a y-axis normal to said bending plane;
• and
• electrical windings associated with said ferromagnetic yoke, arranged to generate a magnetic bending field within said yoke across said curved path to bend a ribbon-shaped beam of ions having a desired mass/charge ratio along said curved path;
• wherein said uniform cross-sectional internal profile has sides including four angled sides respectively lying on lines defined by the four equations:
• a1x+b1y=a1b1, 1)
• a2x−b1y=a2b1, 2)
• a3x+b2y=−a3b2, 3)
• a4x−b2y=−a4b2;
• 4)
• where a1, a2, a3, a4, b1 and b2 are positive numbers,
• wherein each of said angled sides lying on the lines defined by equations 1) and 4) extends on the respective said line up to y=y′, and each of said angled sides lying on the lines defined by equations 2) and 3) extends on the respective said line down to y=−y″, where both y′ and y″ are positive, y′ is less than or equal to the smaller of a1 and a4, and y″ is less than or equal to the smaller of a2 and a3.