Coupled CH-Cavity Development for the FAIR Proton Injector

The availability of commercial 325 MHz, 2.5 MW klystrons initiated the development of linac resonators which fit to that power level source: this will lead to a significant overall linac cost reduction. This is achieved if individual cavity modules can be rf coupled efficiently. At IAP Frankfurt a coupling cell for CH – cavity modules was designed, which at the same time can house a transverse focusing lens as well as the rf power coupler for the whole unit. A half scale model of the coupled CH modules 3 and 4 of the FAIR proton injector is under construction at IAP. Design and fabrication of a full scale high power cavity is planned for in 2007. The Proposed Linac Design The beam dynamics calculations lead to a limited number of gaps per acceleration section which have to be followed by a transversely focusing quadrupole lens. Especially at the low energy end each acceleration section would need quite modest rf power levels only, when realized as an individual cavity. By rf coupling up to two of these sections and by integrating the quadrupole lens within the central coupling cell it is possible to match all cavities to the available amplifier power. Each 2.5 MW amplifier will drive one cavity via one rf power coupler in this approach. The coupled CH-cavity concept The original design of the CH-cavity made use of large end half drift tubes to make the end cell resonant and, at the same time, to host the magnetic lens needed for beam focusing [1]. By putting together two CHcavities of that type and by replacing the inner end walls by a radial stem support for the big drift tube, one is approaching the coupled CH-cavity geometry as one can see from Fig.1 [2]. Taking the coupling tank diameter as well as the drift tube outer diameter as variables one finally gets the resonant coupling of both CH-cavities. The field distributions in the coupling cell were investigated by MWS field simulations and are shown in Fig. 2. The large coupling drift tube is capable to house a magnetic quadrupole triplet and/or diagnostics instrumentation as well as a cooled beam collimator. A robust radial stem is well suited for tube adjustment. Moreover, it allows comfortable access to feed all installations within the coupling tube. Cavity Model In order to test this new coupling concept and to validate the simulations performed with Microwave studio it was decided to build a half-scale model of the second resonator of the p-injector. This model consists in two tanks with 13 and 14 gaps respectively, coupled by a 151 mm long intetank section which includes the focusing lens. The field distribution is plotted in Fig.3 Fig.1: Sketch of a coupled cavity CH linac Fig.2: Coupling section between CH cavities with electric and magnetic field distribution, respectively. Fig.3:Axial electric field disribution for resonator 2 of the GSI proton Injector The outer cylinder will be entirely in aluminum while allthe stems and the lenses are made of massive brass. Afterthe fabrication end, foreseen for early spring 2007, anintensive RF measurements campain will start to investi-gate all the properties of innovative coupling concept References[1] G.Clemente et al.,Proceed. of the PAC 05 Conference,Knoxville, TN, USA, p. 883 885.[2] U.Ratzinger et al, Proceed. of the LINAC 06 Confer-ence, Knoxville, TN, USA, p. 526 530. ___________________________________________ *Work supported by EU Contract Nr . RII3-CT-2003-506395u.ratzinger@iap.uni-frankfurt.deFAIR-ACCELERATORS-07