Abstract: The present inventions is related to a superconducting or non-superconducting isochronous sector-focused cyclotron, comprising an electromagnet with an upper pole and a lower pole that constitute the magnetic circuit, the poles being made of at least three pair of sectors (3,4) called “hills” where the vertical gap between said sectors is small, these hill-sectors being separated by sector-formed spaces called “valleys” (5) where the vertical gap is large, said cyclotron being energized by at least one pair of main coils (6), characterised in that at least one pair of upper and lower hills is significantly longer than the remaining pair of hill sectors in order to have at least one pair of extended hill sectors (3) and at least one pair of non-extended hill sectors (4) in that a groove (7) or a “plateau” (7′) which follows the shape of the extracted orbit is present in said pair of extended hill sectors (3) in order to produce a dip (200) in the magnetic field.