Abstract: A superconducting or non-superconducting compact isochronic cyclotron in which the particle beam is sectorally focused. The cyclotron comprising a solenoid forming a magnetic circuit which is energized by at least one pair of circular main coils surrounding the solenoid poles. The magnetic circuit includes at least three pairs of sectors known as "hills", where the air gap is reduced, separated by sector-shaped spaces known as "valleys", where the air gap is larger. The air gap between the hills has a substantially elliptical curved profile which completely closes off the radial end of the hills at the center plane.