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The use of C-Band structures for electron acceleration and production of high quality beams has been proposed and adopted in several linac projects all over the world. The two main projects that adopted such type of structures are the Japanese Free Electron Laser (FEL) project in Spring-8 and the SwissFEL project at Paul Scherrer Institute (PSI). Also the energy upgrade of the SPARC photo-injector at LNF-INFN (Italy) from 150 to more than 240 MeV will be done by replacing a low gradient S-Band accelerating structure with two C-band structures. The structures are Traveling Wave (TW) and Constant Impedance (CI), have symmetric axial input couplers and have been optimized to work with a SLED RF input pulse. The paper presents the design criteria of the structures, the realization procedure and the low and high power RF test results on a prototype. The high power tests have been carried out by the Frascati INFN Laboratories in close collaboration with the Japanese Laboratory KEK. Experimental results confirmed the feasibility of the operation of the prototype at 50 MV/m with about 10 6 breakdowns per pulse per meter. Such high gradients have not been reached before in C-Band systems and demonstrated the possibility to use C-band accelerators, if needed, at such high field level. The results of the internal inspection of the structure after the high power test are also presented.

In this paper we illustrate the tuning procedure we implemented to tune the traveling wave structures designed and constructed for the SPARC photo-injector energy upgrade. The procedure has been derived from that proposed and adopted for the CLIC structures at CERN (J. Shi et al., Tuning of clic accelerating structure prototypes at Cern, Proc. of LINAC 2010, Tsukuba, Japan, 2010). In the first part of the paper we go through the analytical formulas also illustrating and discussing an algorithm to tune the input and output couplers. A detailed description of the measurements we have done before and after the tuning of the SPARC C-band structure prototype is then illustrated.

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