A particle therapy system comprising: a gantry that is configured to rotate; and a synchrocyclotron that is mounted on the gantry for rotation at least partly around a position of a patient to be treated, the synchrocyclotron to output a particle beam to the position of the patient to be treated; wherein the synchrocyclotron is a variable-energy synchrocyclotron comprising: a cavity in which particles are accelerated to produce the particle beam; a cryostat containing a set of superconducting coils (40, 42; 709, 710); leads to supply current to the set of superconducting coils, the current being variable to produce a variable magnetic field in the cavity that scales with a magnitude of the current, the variable magnetic field for varying an energy of the particle beam; and a voltage source to provide a radio frequency voltage to the cavity to accelerate the particles in the cavity, the RF voltage being sweepable over a range of frequencies that changes with variations to the energy of the particle beam, wherein movement of the cryostat is controllable based on a rotational position of the particle accelerator as measured by a rotation position of the gantry on which the particle accelerator is mounted. a gantry that is configured to rotate; and a synchrocyclotron that is mounted on the gantry for rotation at least partly around a position of a patient to be treated, the synchrocyclotron to output a particle beam to the position of the patient to be treated; wherein the synchrocyclotron is a variable-energy synchrocyclotron comprising: a cavity in which particles are accelerated to produce the particle beam; a cryostat containing a set of superconducting coils (40, 42; 709, 710); leads to supply current to the set of superconducting coils, the current being variable to produce a variable magnetic field in the cavity that scales with a magnitude of the current, the variable magnetic field for varying an energy of the particle beam; and a voltage source to provide a radio frequency voltage to the cavity to accelerate the particles in the cavity, the RF voltage being sweepable over a range of frequencies that changes with variations to the energy of the particle beam, wherein movement of the cryostat is controllable based on a rotational position of the particle accelerator as measured by a rotation position of the gantry on which the particle accelerator is mounted. a cavity in which particles are accelerated to produce the particle beam; a cryostat containing a set of superconducting coils (40, 42; 709, 710); leads to supply current to the set of superconducting coils, the current being variable to produce a variable magnetic field in the cavity that scales with a magnitude of the current, the variable magnetic field for varying an energy of the particle beam; and a voltage source to provide a radio frequency voltage to the cavity to accelerate the particles in the cavity, the RF voltage being sweepable over a range of frequencies that changes with variations to the energy of the particle beam, wherein movement of the cryostat is controllable based on a rotational position of the particle accelerator as measured by a rotation position of the gantry on which the particle accelerator is mounted.