Design, realization and test of C-band accelerating structures for the SPARC_LAB linac energy upgrade

Abstract

The energy upgrade of the SPARC_LAB photo-injector at LNF-INFN (Frascati, Italy) has been originally conceived replacing one low gradient (13 MV/m) 3 m long SLAC type S-band traveling wave (TW) section with two 1.4 m long C-band accelerating sections. Due to the higher gradients reached by such structures, a higher energy beam can be obtained within the same accelerator footprint length. The use of C-band structures for electron acceleration has been adopted in a few FEL linacs in the world, among others, the Japanese Free Electron Laser at SPring-8 and the SwissFEL at Paul Scherrer Institute (PSI). The C-band sections are traveling wave, constant impedance structures with symmetric input and output axial couplers. Their design has been optimized for the operation with a SLED RF pulse compressor. In this paper we briefly review their design criteria and we focus on the construction, tuning, low and high-power RF tests. We also illustrate the design and realization of the dedicated low level RF system that has been done in collaboration with PSI in the framework of the EU TIARA project. Preliminary experimental results appear to confirm the operation of such structures with accelerating gradients larger than 35 MV/m.

Realization and high power test of damped C -band accelerating structures

“…2 The tuning has been done acting on four small deformation tuners using the same procedure already illustrated and adopted in Refs. [7,16] and following the procedure illustrated in Ref. [16].…”

“…C-band accelerating structures have been already designed and fabricated for different projects [3][4][5][6][7][8][9]. Those implemented for SPARC_LAB, SwissFel, and SXFEL in Shanghai Institute of Applied Physics [7][8][9] are iris-loaded structures, without damping, because they operate in single-bunch mode.…”

“…C-band accelerating structures have been already designed and fabricated for different projects [3][4][5][6][7][8][9]. Those implemented for SPARC_LAB, SwissFel, and SXFEL in Shanghai Institute of Applied Physics [7][8][9] are iris-loaded structures, without damping, because they operate in single-bunch mode. The Spring 8 ones [3][4][5][6] are, indeed, choke mode-based damped structures, but those we developed for the ELI-NP structures [10] exhibit a higher shunt impedance and can operate at a higher repetition rate with an higher average dissipated power.…”

6D phase space electron beam analysis and machine sensitivity studies for ELI-NP GBS

“…2. The downstream C-band RF linac operates at 5.712 GHz, with the accelerating structures designed and developed at LNF, [5], where the accelerating gradient can be set up to a maximum of 33 MV/m, allowing enough margin for the off crest minimization of the energy spread in the 75-750 MeV energy range [1,6]. The C-band linac is divided in two main sections as shown in Fig.…”

“…bandwidth lower than 0.5% and energy tunability in the range 0.2-19. 5 MeV. As pointed in detail in [1], to design a linac able to provide the requested electron beam quality the following factor of merit η, has to be maximized:…”