Magnetic fields cause the ions to move in a circular orbit and they gain energy, or accelerate, as they move through the alternating electric field on each orbit.  As energy is gained, the orbits increase in radius, and the ions spiral outward from the center of the accelerator.  A set of conventional main coils produces the base dipole magnetic field.  Ten sets of concentric trim coils mounted to the two pole faces produce an isochronous condition for various ions by adjusting the radial contour of the of the average field.  Focusing forces are produced by a three-sector magnetic field configuration of the pole tips.  Three hills and three valleys produce pronounced azimuthal variations in the field strength.  Harmonic coils in the valleys are utilized to cancel undesirable asymmetries or introduce desired harmonics for efficient beam extraction.