The Development of Coupled CH Structure for FAIR

The primary proton beam for the production of the intense antiproton beam required by FAIR will be provided by a dedicated 70 MeV, 35 mA, 325 MHz Injector. This will be the first linac based on coupled H-mode cavities [1]. A half scale model of the coupled CH modules 3 and 4 of the FAIR proton injector has been constructed and tested. Fabrication of a full scale high power cavity is scheduled for 2009. The Scaled Model of Resonator II The second resonator of the FAIR proton injector, i.e. the coupled third and fourth tanks, consists of 29 gaps in total, 13 in the first CH resonator and 14 in the second one resulting in a total inner lenght of around 2.9 meters. The coupling is ensured by an intertank section housing the magnetic triplet needed for the beam focusing. The main properties are summerized in Tab.1. Tab. 1: The main properties of the second resonator of the FAIR proton injector. Energy Range [MeV] 11.7-24.3 Frequency [MHz] 325.244 Q-value 15300 Kilpatrick 2.0 Effective Shunt Impedance [MΩ/m] 60 Length [m] 2.9 The complexity of such a cavity requires a deeper experimental investigation on the mode behaviour and, for that reason, a 1:2 scaled model has been built at IAP [2] to test the main RF parameters such as resonance frequency, Q-value, shunt impedance, field distribution and coupling strength. Fig.1 shows the model: the outer cylinder is made of aluminum while the stems and the drift tubes are produced in braze. Eleven plungers are mounted along the cavity (45° with respect to the stem planes) to control the resonance frequency and to adjust the field distribution along the cavity axis. Experimental Results The cavity production has been finished in summer 2007 and was followed by several experimental tests. The resonance frequency was 652.64 MHz with an error of 0.3% with respect the design value of 650.444 MHz: minor adjstuments of the plunger position were required in order to opmitimize the field distribution along the cavity. An example of a rather flat field distribution is shown in Fig.2. Fig.1: On top, a view of the cavity; bottom, the cavity on the test bench assembled at Frankfurt University. Fig.2: Measured electric field distribution on the beam axis In correspondance of that optimized configuration for the plungers, the measured shunt impedance reached around 95 % of the ideal value obtained with ideal copper conductivity. This confirms the validity of the simulation method used to investigate the structure.