The 7-gap-resonator-accelerator for the REX-ISOLDE-linac

The REX-ISOLDE-experiment which is presently being under construction at CERN is intended to investigate exotic, very neutron rich, radioactive nuclei. A linear accelerator will deliver radioactive beams which are produced 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 H-structure (IH) and three 7-gap-resonators for variable final energy. Assuming an acceleration voltage of one 7-gap-resonator to be 1.75 MV at 90 kW rf power, the design velocities of the three resonators were chosen to be 5.4%, 6.0% and 6.6% of the velocity of light. Three downscaled models (1:2.5) were built in order to optimize the shunt impedance and the field distribution for the operating frequency of the amplifiers of 101.28 MHz. The development of the resonators was accompanied by extensive MAFIA calculations. It could be demonstrated that spiral-resonators like 7-gap-resonators can be calculated with MAFIA. Important quantities like frequency, shunt impedance, quality factor and field distribution were compared between simulation and measurement. The first two power type resonators (5.4% and 6.0%) are finished, frequency tuning and low power measurements were done. The Q-values are about 5560 and 5280, respectively, the shunt impedance 71 MΩ/m and 68 MΩ/m, respectively, and are in very good agreement with the model measurements. After preparation for high power tests a beam test for the voltage calibration is planned. In this paper the status of the production of the 7-gap-resonators is reported