Development of the RFQ Cooler SHIRaC: beam transport and nuclearization

Abstract

The development of the new RFQ Cooler, called SHIRaC, was carried out. As a part of SPIRAL 2 facility, SHIRaC aims to handle and cool typical SPIRAL 2 beams with large emittances (up to 80 pi.mm.mrad) and high currents (up to 1 uA). Its purposes are to enhance as much as possible the beam quality (transverse geometric emittance of less than 3 pi.mm.mrad and longitudinal energy spread close to 1 eV) and to transmit more than 60 % of ions. Numerical simulations and experimental studies have shown that the required beam quality can be reached only in term of the emittance. The energy spread is very far from expected values. It is sensitive to the space charge and the buffer gas diffusion and more importantly to the RF field derivative effect. The latter arises at the RFQ exit and increases with the RF parameters (the frequency and the amplitude of the RF voltage). Studies allowing to enhance the cooled beam quality, mainly the energy spread reduction, are presented and discussed along this paper. They consist in implementing a miniature RFQ at the RFQ exit. Using this method, it becomes possible to improve the cooled beam quality and to reach 1 eV of longitudinal energy spread and around 1.75 π.mm.mrad of transverse geometric emittance for beam currents going up to 1 uA. The transport of the cooled beam from SHIRaC towards a HRS has been done with an electrostatic quadrupole triplet. Simulations and first experimental tests showed that more than 95 % of cooled beams can reach the HRS. Finally, developments related to the nuclearization protection methods aiming to avoid the escape of any nuclear matter from the SHIRaC beamline are studied.