The C-Band accelerating structures for SPARC photoinjector energy upgrade

Abstract

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.

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

“…Prior to the realization of the final structures, a prototype with a reduced number of cells has been manufactured and highpower tested at KEK (Japan). The maximum gradient that has been reached is 50 MV/m with a breakdown rate (BDR) per unit length of E10 À 6 in units of breakdowns/pulse/meter, as thoroughly reported in [32]. In light of the results achieved at KEK, a conservative estimation of the C-band structure obtainable gradient can be made assuming 50 MV/m as peak electric field in the first cell of the structure, with a uniform RF input pulse as example.…”

“…SPARC_LAB C-band structures have been designed by LNF-INFN laboratories with the support of COMEB for their manufacture. The detailed description of the electromagnetic project of these sections is given in [32]. The final geometrical and electromagnetic parameters have been summarized in Table 1.…”

“…Moreover, also the linac of the Gamma Beam System to be built in Magurele (Romania), in the framework of the ELI-NP project [29], is based on strongly damped high repetition rate C-band structures [30,31]. The SPARC_LAB linac original energy upgrade [32] foresaw the replacement of the last 3 m long low gradient (13 MV/m) S-band TW section with two identical 1.4 m long high gradient ( > ) 35 MV/m C-band TW structures, as schematically represented in Fig. 2.…”

“…The new system will also include a SKIP-type pulse compressor [34], developed by the Institute of High Energy Physics (IHEP) of Beijing (China), that is currently under commissioning at LNF. The electromagnetic, mechanical and thermal design of the C-band structures has been carried out at LNF-INFN laboratories [32] with the support of a private company (COMEB [35]) for their production. Prior to the realization of the final structures, a prototype with a reduced number of cells has been manufactured and highpower tested at KEK (Japan).…”

New technology based on clamping for high gradient radio frequency photogun

“…The residual magnetic field component is about 2.5 × 10 −4 T (at 120 MV=m cathode peak field). Concerning the quadrupole field component induced by the presence of the two holes, it has been evaluated according to the formulas given in [29] which are reported in the Appendix for completeness. The resulting integrated quadrupole gradient as a function of the injection phase is shown in Fig.…”