Study and development of a radiofrequency Cooler with buffer gas for a very high intensity radioactive beams. ; Etude et développement d'un refroidisseur radiofréquence à gaz tampon pour des faisceaux radioactifs de très hautes intensités

The low energy facility DESIR/SPIRAL2 is a second generation installation of radioactive beams. The flows of radioactive ions will require purification at isobaric level of the isotopes. This separation will be made by a high resolution separator (HRS) developed at CENBG. To have the nominal performance the HRS requires a low émittance beam. The only universal technique which can lead to a low émittance beam is the RFQ Cooler with buffer gas. The goal of this Cooler is to reduce the beam émittance to less than 1 π.mm.mrad and the longitudinal spread energy about 1 eV, using the very high intensity beams (i≈1µA). Therefore, the space charge effect is significant to cooling beam degradation. The compensation of this effect requires the high RF voltage and high frequency respectively a few kV and a few MHz. The latter points are what distinguish this Cooler with those who are existing. The RFQ Cooler prototype examined in this thesis, commonly called "SHIRaC". It was developed so as to transmit at least 60% of ions at very high intensity. The numerical simulations related to the definition of SHIRaC led to find the operating parameters in terms of pressure RFQ, the guiding field and the electrodes voltage of injection and extractions cells. They also allowed choosing and optimizing an electrostatic extraction triplet which adapts the cooling beam to the HRS. At an intensity of 1µA, the optimum cooling results of 133Cs+ ions are variants: either minimum longitudinal spread energy of 1.15 eV for a transmission of 21 % or longitudinal spread energy of 4.67 eV for a transmission of 60 %. The émittance is about 2.2 π.mm.mrad. The degradation of longitudinal spread energy is due to contribution of space charge and longitudinal effects. Outside the RFQ, when these two effects act, only the reduction of the second effect is possible. To achieve this reduction we had replace the three electrodes lens of extraction cell by a two electrodes lens. Through the use of this new lens the longitudinal spread energy is reduced of ...