Design and optimization of radio-frequency pulse compressor systems for high brightness linacs

Linac driven free electron lasers (FELs) are useful devices for studying matter and the demand for new FEL facilities is increasing constantly. The main issues of such machines are the size and the costs. Active research is going to improve the performance of the devices, reduce building and operational costs, and make them more compact. In this framework, the INFN project EURPRAXIA@SPARC_LAB, is a proposal to upgrade the SPARC_LAB test facility (at LNF, Frascati) to a soft X-ray user facility based on plasma acceleration and high-gradient X-band accelerating structures [71]. Also the European project CompactLight-XLS [57] aims to design a compactFEL for users, in the hard X-ray range using the X band technology with a new C-band high-brightness photoinjector and innovative short-period undulators. The X band module layout is composed by one klystron feeding, in parallel, four X band accelerating structures through a pulse compressor. This thesis work is focused on the design of this last device. We have performed both the rf and thermo-mechanical design. For the realization of this device we have proposed to use the innovative brazing-free technology developed at LNF-INFN and based on the use of special gaskets with a strong reduction of the cost, realization time and risk of failure due to the brazing process itself. In chapter 1, traveling wave structures are briefly introduced with their main parameters. In chapter 2, the EURPRAXIA@SPARC_LAB and CompactLight-XLS projects are briefly described with their applications, goals, and main components. In chapter 3, the use of the RF pulse compressor systems in LINAC is illustrated and the different adopted solutions are briefly summarized with particular details on the Barrel Open Cavity (BOC) solution. In chapter 4, the advantages of the new brazeless technology recently developed at LNF-INFN, for the realization of rf guns are briefly summarized and the opportunities to fabricate BOC pulse compressors systems with this new approach are put in evidence. Chapter 5, is the core chapter of the thesis in which I describe the work done to completely design the BOC cavity for the EURPRAXIA@SPARC_LAB X bandLINAC. The work flow involves the following main steps: • Analytical and numerical analysis and design of the BOC • Electromagnetic design and simulations • Sensitivity study to evaluate the tolerable mechanical errors and possible tuning • Thermo-mechanical analysis and design of the cooling system. • Final mechanical design of the structure Chapter 6 is focused on the application of pulse compressors to feed standing wave structures such as RF guns. A brief introduction on RF-guns and it’s common feeding scheme is reported. An alternative feeding scheme for the C-band gun proposed for the CompactLight-XLS project has been proposed. The scheme foresees the use of a BOC pulse compressor and avoids the use of a circulator (or isolator). Parameters of the pulse compressor has been optimized, to reach desired gun’s cathode peak field and it’s optimal performance. The electromagnetic design of the C band BOC is then illustrated. The conclusions and perspectives are reported in the last chapter 7.