Synchrocyclotron for extracting beams of various energies

A synchrocyclotron for extracting charged particles accelerated to an extraction energy comprised between a low energy and a high energy, the synchrocyclotron comprising:

• a first main coil and second main coil centered on a common central axis and arranged parallel to one another on either side of a median plane normal to the central axis and defining a symmetry plane of the cyclotron, the first and second main coils being configured for generating a main magnetic field when activated by a source of electric power;
• a dee configured for creating an RF-oscillating electric field of varying frequencies for accelerating the charged particles;
• a first field shaping unit and second field shaping unit for shaping the main magnetic field and guiding the charged particles along successive orbits of increasing average radii centered on the central axis, the first and second field shaping units being arranged within the first and second main coils on either side of the median plane and separated from one another by a gap, wherein the first and second field shaping units comprise hill sectors and valley sectors alternatively distributed around the central axis with a symmetry of at least three for shaping the main magnetic field;
• a first instability coil unit and a second instability coil unit arranged on either side of the median plane and configured for creating a field bump localized in a z-component of the main magnetic field;
• and
• a controlling unit configured for adjusting the amplitude of the field bump at various levels such that, for all values of an average instability onset radius between a low radius and a high radius, a value of the amplitude of the field bump at the average instability onset radius is equal to a value of an offset amplitude at the average instability onset radius, and lower than the values of the offset amplitude for all values of the average radius smaller than the average instability onset radius;
• wherein
• the z-component of the main magnetic field is controlled such that the radial tune of the successive orbits is not equal to 1 and is comprised within 1±0.1 for all values of an average radius between the low radius and the high radius, the low radius corresponding to average radial positions of the charged particles at the low energy and the high radius corresponding to average radial positions of the charged particles at the high energy;
• the first and second instability coil units are configured for creating the field bump within an azimuthal sector of azimuthal angle, with an amplitude increasing radially between a first field bump amplitude value at the low radius and a second field bump amplitude value at the high radius;
• and
• the offset amplitude is a minimal amplitude of the field bump at the average instability onset radius required for sufficiently offsetting the center of an orbit of an average instability onset radius along which the charged particles are guided, such that a combination of an amplitude of the harmonic 2 and a radial gradient of the amplitude of the harmonic 2 on the orbit is produced by the main magnetic field of symmetry on an offset orbit, and the offset amplitude is large enough to generate a resonance instability of the successive orbits of average radius greater than R≥Ri.

A method for extracting charged particles out of a synchrocyclotron at an extraction energy between a low energy and a high energy, the method comprising the steps of:

• providing a synchrocyclotron configured such that the charged particles reach the extraction energy at a corresponding average instability onset radius of an orbit between a low radius and a high radius, the average instability onset radius corresponding to respective average radial positions relative to a central axis of the charged particles at the low and high energies, wherein a radial tune of successive orbits is not equal to 1 and is within 1±0.1 for all values of an average radius between the low radius and the high radius;
• selecting a value of the extraction energy of the charged particles to be extracted;
• determining a value of an offset amplitude required for offsetting the center of an orbit of average radius of the charged particles at the extraction energy;
• generating a resonance instability of the successive orbits of average radius greater than or equal to the average instability onset radius;
• adjusting a magnitude of the field bump such that the amplitude of the field bump equals a value of an offset amplitude at the average instability onset radius and is lower than the values of the offset amplitude for all values of the average radius smaller than the average radius;
• and
• extracting a beam from the synchrocyclotron through an exit port.