High-gradient structures and rf systems for high brightness-electron linacs

Free electron lasers (FELs) driven by linacs have demonstrated to be a reliable tool for studying matter. The demand of new FEL facilities is increasing and the main issues of this kind of machines are costs and required space. In this framework, the INFN project named EuPRAXIA@SPARC_LAB, is a proposal to upgrade the SPARC_LAB test facility (in Frascati, Italy) to a soft X-ray user facility based on plasma acceleration and high-gradient X-band accelerating structures. Also the European project CompactLight aims to design a compact FEL for users, in the hard X-ray range. Its main pillars are a new concept high-brightness photoinjector, high-gradient X-band accelerating sections and innovative short-period undulators. In this thesis work, the rf designs of the X-band linacs for both the mentioned projects have been performed. In Chapter 1, there is a brief description of what a linac is, its main applications, characteristics and issues. Moreover, the state of the art of X-band technology for accelerators is briefly summarized. In Chapter 2, main concepts about the characteristics and design criteria of traveling wave structures for electron linacs are described: main parameters of a traveling wave structure, constant impedance and constant gradient structures, the SLED pulse compressor system and rf power coupling design approaches. In Chapter 3, the main concepts about rf vacuum breakdown, together with the main parameters that have been introduced through the years to predict its probability, are summarized. Chapter 4 is dedicated to describe the main concepts about wakefields and beam instabilities: longitudinal and transverse wake functions and potentials, asymptotic solutions, the simplified single-bunch beam breakup two-particle model. In Chapter 5, the work flow for the design of electron linacs with traveling wave structures is explained. In particular, the designs of the EuPRAXIA@SPARC_LAB and CompactLight linacs are described in detail. The work flow involves the following main steps: • calculation of the minimum average iris radius of the structure based on two-particle model and asymptotic solution of wakefields; • electromagnetic design of the regular cell, which goal is to maximize the rf efficiency and, at the same time, reduce the breakdown probability; • analytical and numerical design and optimization of the accelerating structures, finding the optimal length and tapering as the best compromise between rf efficiency and breakdown probability; • sensitivity study of the cells due to mechanical errors; • design of the rf power couplers, which goal is to minimize the power reflection at the input port and the multipolar components of the fields that can reduce the beam quality; • design of an rf module that is repeated the number of times needed to reach the desired energy; • design of high repetition rate schemes for FEL applications; • calculation of the wake function for beam dynamics simulations; • thermal analysis and preliminary design of the structure cooling system. Chapter 6 is finally dedicated to conclusions and outlook.