• extraction
• slow extraction
• hadrontherapy
• ion therapy
• particle accelerator
• synchrotron
• kicker
• sextupole
• third order resonance
• CNAO
• RF-knockout

The National Centre for Oncological Hadrontherapy CNAO was established on 2001 by the Italian Health Ministry. At present the centre is under commissioning in Pavia and will treat the first patients in one year.
The CNAO is the first Italian Centre for the treatment of patients affected by tumors by means of ions beams: treatments with an active scanning of both protons and carbon ions will be possible.
Protons are required to have kinetic energy of about 220 MeV, while about 4.8 GeV, i.e. 400 MeV/u, are necessary for the carbon ions beam. A synchrotron will provide such energies.
The extraction of the beam from an hadrontherapy synchrotron must be very slow, because the extracted beam (called spill) must have low and constant intensity and must be long, in order to facilitate the measurement and control of the radiation dose delivered to patients. The extraction interval must be about 1 s long: as a consequence a multi-turn extraction is needed. The slow extraction is based on the third order resonance and a sextupole magnet is used to excite the resonance. The beam will be driven into the resonance by a betatron core, which accelerates the beam. The possibility of using the RF-knockout system as an alternative way to drive the beam into the resonance is also under consideration. RF-knockout method has some advantages: in particular the start and stop time of the extraction are very fast. This characteristics is useful for some special treatments needs, such as the synchronization with the breathing of the patient.
The aim of this thesis is to verify the feasibility of the RF-knockout extraction method and the optimization of the corresponding performances with the already present hardware (and minimum upgrades) of CNAO synchrotron.
The RF-knockout extraction system involves the use of a kicker, which perturbs the beam horizontally. The frequency of the signal given to the kicker must match the horizontal betatron frequency.
The particles momentum spread determines a tune spread, due to the horizontal chromaticity.
With a tune spread, the RF frequency must vary to cover all the particle tunes.
Two types of signals have been studied in order to obtain a constant spill with a minimum ripple:
- a carrier wave with a frequency modulation;
- a noise at a given range of frequencies.
A dedicated simulation program has been written in C++ language to track the particles in the synchrotron with the RF-knockout.
The program performs a six-dimensional tracking. Any number of particles can be tracked for any number of turns in the synchrotron. The RF signal can be defined by the user. The synchrotron lattice and the initial particle coordinates are the input of the program.
The frequency modulated signal is commonly used in other hadrontherapy synchrotrons, as the ones in Japan. With that method the extracted beam has a ripple at the frequency of the modulating signal. The noise signal does not produce an extracted beam with ripple, so it is preferable.
The intensity of the extracted beam decays exponentially with time and the characteristic time depends on the kick amplitude. The amplitude of the kick must be increased to have a constant spill. An amplitude modulation function has been found to have a constant spill in the first 70% of the extraction.
The simulation has shown that, in the most demanding conditions, i.e. with a carbon ion beam at maximum energy and minimum emittance, a relatively fast extraction (1 s) can be obtained with a perturbation of less than 10^-6 rad. To have a slower extraction, an extraction of the proton beam or an extraction of the carbon ion beam at less energy, the perturbation should be smaller.