THE 7-GAP-RESONATOR-ACCELERATOR FOR THE REX-ISOLDE-EXPERIMENT AT CERN

The Radioactive Beam Experiment at ISOLDE (REX-ISOLDE-Experiment) whic h presently is being developed and under construction at CERN serves t o investigate exotic, very neutron rich, radioactive nuclei [1] (Radio active beam EXperiment at ISOLDE: Coloumb Excitation and Neutron Trans fer Reactions of Exotic Nuclei, Proposal to the ISOLDE commitee, CERNS IC94-25). A linear accelerator delivers radioactive ions which are pro duced by the isotope separator ISOLDE with energies between 0.85 and 2 .2 MeV/u. The Linac will consist of a RFQ-accelerator, an interdigital I-I-Structure (IH) and three 7-gap-resonators with variable energy. W hile the LMU Munich is responsible for the frontpart of the accelerato r, the backpart is being built by the MPI [2] (H. Podlech, Master Thes is, MPI-H-V21-1997, Heidelburg, 1997). After estimation of the voltage of one resonator to 1.75 MV at 90 kW, the design velocities were fixe d to 5,4%, 6.0% and 6.6% of the velocity of light. Three downscaled mo dels (1:2.5) were built in order to optimize the shuntimpedance and th e field-distribution at the operation frequency of the amplifiers of 1 01.28 MHz. The optimization of all low power resonators is now success fully finished. Extensive beam dynamic calculations were made in order to optimize the transmission of the beam up to the target. It turned out that final energies between 0.85 and 2.2 MeV/u with nearly 100% tr ansmission can be achieved. The acceptance in the x-plane is 1.2 pi mm mrad (norm.) and in the gamma-plane 3.0 pi mm mrad (norm.). The bunch length of the fully accelerated beam (2.2 MeV/u) is 2.4 ns at the targ et. The development of the resonators was accompanied by extensive MAF IA calculations. It could be demonstrated that spiral-resonators like 7-gap-resonators can be calculated with MAFIA with an accuracy of 1% i n comparison with experimental results. Presently, the tanks and the h alf shells of the power type resonators are manufactured in the worksh ops of the MPI. (C) 1998 Elsevier Science B.V.