• S. Gammino, L. Celona, G. Ciavola, F. Maimone, D. Mascali
  INFN/LNS, Catania

During the last 10 years it was demonstrated that slight variations of microwave frequency used in ECRIS strongly influence their performances either for extracted current and for beam brightness and stability. Theoretical investigations put in evidence that such frequency tuning is linked to the electromagnetic field structure inside the resonant cavity. On this basis, we carried out PIC simulations, showing that the frequency tuning has a global influence on plasma properties and on beam brightness. Such analysis allowed the design of the optimum setup for plasma chamber dimensions and microwave injection, to achieve higher currents and better emittances. The magnetic field is based on the use of steep gradient but the cryogenics issues are simplified; the extraction system is designed to minimize the aberrations. The overall dimensions of the MISHA source (Multicharged Ion Source for HAdrontherapy) have been chosen as a compromise between the ideal size for microwave to plasma interaction, the need to get long ion confinement time and the request of getting a compact ECRIS. The description of the source design will be given, along with the expected performances.

• D. Mascali, L. Celona, G. Ciavola, S. Gammino, F. Maimone
  INFN/LNS, Catania

The trend of ECRIS to higher frequencies and magnetic fields is driven by the need to have higher beam currents and higher charge states for nuclear physics accelerators. Anyway, because of the limits imposed by the magnets’ and microwaves generator’s technology, any further increase of performances requires a detailed investigation of the plasma dynamics. The experiments have shown that the current, the charge states and even the beam shape change by slightly varying the microwave frequency (frequency tuning effect - FTE). Moreover, for last generation ECRIS, electron energies up to 2 MeV have been detected, depending mainly on the magnetic field structure and gradient distribution over the ECR surface. The plasma dynamics have been studied by means of single particle and PIC simulations: they explain the FTE in terms of the wave field distribution over the ECR surface and the existence of high energy electrons due to diffusion in the velocity space above the stochastic barrier. Other methods used to improve the ECRIS performances, e.g. the two frequency heating with an adequate phase relation between the two waves, can be exploited by means of the simulations.