The invention relates to a magnet structure for a superconducting isochronous cyclotron for use in particle therapy. The cyclotron according to the invention is using two sets of three or more superconducting sector coil elements for generating an azimuthally varying magnetic field across the acceleration region. In this way, high-field (e.g. above 4 T) isochronous cyclotrons are provided which do not suffer the problem of a low flutter amplitude.
