Lattices for medical synchrotrons employing active scanning

Medical synchrotrons fall naturally into two categories; those using passive scattering techniques to spread the beam over the irradiation field and those using active scanning techniques to perform a high-precision conformal treatment of the tumour. The latter category is the newer generation that requires slower, more stable beam spills, achieved by the use of a third-integer extraction. These machines work below transition and for the stability of the stack, waiting outside the resonance, the chromaticity should be negative. This imposes strict constraints on the lattice for the application of the Hardt condition that aligns the separatrices of different momenta. This in turn affects the lattice between the electrostatic and magnetic septa, which should be, as close as possible, an achromatic transfer. It is also important that the extraction separatrices grow in a balanced way to economise the machine aperture. Since the separatrices occupy the same space at all energies, the good-field region cannot be relaxed at higher energies as is normal in machines dominated by adiabatic damping. A sample lattice is presented to illustrate these points.