• calcolo strutturale
• certificazione ASME
• FEM
• ottimizzazione

The ILC (International Linear Collider) is a proposed new major particle accelerator. It consists of two 20 km long linear accelerators colliding electrons and positrons at an energy exceeding 500 GeV. Achieving this collision energy while keeping reasonable accelerator dimensions requires the use of high electric field superconducting cavities as the main acceleration element. These cavities are operated at 1.3 GHz inside an appropriate container (He vessel) at temperatures as low as 1.4 K using superfluid Helium as the refrigerating medium.
The purpose of this thesis, in the context of the ILC R&D activities currently in progress at Fermilab (Fermi National Accelerator Laboratory), is the mechanical study of an ILC superconducting cavity and Helium vessel prototype. The main goals of these studies are the determination of the limiting working conditions of the whole He vessel assembly, the simulation of the manufacturing process of the cavity end-caps and the assessment of the Helium vessel's efficiency. In addition this thesis studies the requirements to certify the compliance with the ASME Code of the whole cavity/vessel assembly.
Several Finite Elements Analyses were performed by the candidate himself in order to perform the studies listed above and described in detail in Chapters 4 through 8. In particular the candidate has developed an improved procedure to obtain more accurate results with lower computational times. These procedures will be accurately described in the following chapters.
After an introduction that briefly describes the Fermilab and in particular the Technical Division (where all the activities concerning with this thesis were developed), the first part of this thesis (Chapters 2 and 3) explains some of the main aspects of modern particle accelerators. Moreover it describes the most important particle accelerators working at the moment and the basic features of the ILC project.
Chapter 4 describes all the activities that were done to certify the compliance of the Helium vessel and the cavity to the ASME code standard. After briefly recalling to the main contents of the the ASME Code (Sections II and VIII - Division II), the procedure used for finding all relevant stresses and comparing the obtained results with the maximum values allowed are explained. This part also includes the buckling verification of the cavity.
In Chapter 5 the manufacturing process of the cavity end-caps, whose function is to link the Helium vessel with the cavity, is studied. The present configuration of the dies is described and the manufacturing process is simulated in order to explain the origin of some defects found on real parts. Finally a new design of the dies is proposed and the resulting deformed piece is compared with the design requirements.
Chapter 6 describes a finite elements analysis to assess the efficiency and the stiffness of the Helium vessel. Furthermore the results of the optimization of the Helium vessel (in order to increase the value of the efficiency) are reported.
The same stiffness analysis is used in Chapter 7 for the Blade-Tuner study. After a description of this tuner and of its function, the preliminary analyses done to confirm the results provided by the vendor are described and then its limiting load conditions are found.
Chapter 8 shows a study of the resistance of all the welds present in between the cavity and the end-cap and between the end-caps and the He vessel for a smaller superconducting cavity operating at 3.9 GHz.
Finally Chapter 9 briefly describes some R&D activities in progress at INFN (Section of Pisa) and Fermilab that could produce significant cost reductions of the Helium vessel design.
All the finite elements analyses contained and described in this thesis made possible the certification of the whole superconducting cavity-Helium vessel assembly at Fermilab. Furthermore they gave several useful indications to the Fermilab staff to improve the performance of the Helium vessel by modifying some design parameters or refining the manufacturing processes.