Optimization of the Beam Size at the Interaction Point of the Accelerator Test Facility 2

A new era of discovery in particle physics has opened in November 2009 with the start-up of the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, in Geneva, Switzerland. The LHC, a circular proton-proton synchrotron, operates at the highest energies particle accelerators ever achieved. Nowadays, an upgrade of the LHC to achieve instantaneous luminosities a factor of five larger than the LHC nominal value, thereby enabling the experiments to enlarge their data sample by one order of magnitude compared with the LHC baseline programme, has been developed, called the High Luminosity LHC (HL-LHC). A Linear Collider could be the next large accelerator after LHC. Instead of protons, electrons and their antiparticles, positrons, will be colliding with each other at very high energies. Just like at the LHC, the physics at the Terascale will be explored in order to clarify questions such as the nature of dark matter in the universe, possible new symmetries and new dimensions of space, and to provide the possibility for unexpected discoveries. In comparison to the LHC, the use of electrons and positrons has the advantage of significantly cleaner events with less background, allowing a higher precision and detailed studies of physics beyond the standard model, as well as an analysis of the Higgs mechanism with higher accuracy. These are some of the reasons that induced the Accelerator Physics community to go towards an electron positron collider. In this context, the High Energy Physics (HEP) community produced a Technical Design Report (TDR) presenting the matured technology, design and construction plan for the International Linear Collider (ILC). The Compact Linear Collider (CLIC) collaboration demonstrated the feasibility of the project. Therefore, the International Committee for Future Accelerators (ICFA) decided to gather the two projects in order to develop the common issues of the two different technologies in the Linear Collider Collaboration (LCC). The two projects have in common the structure of a linear collider that accelerates the particles to high energy and brings them to collision. These two technologies, ILC, with a maximum energy between 250 GeV and 1 TeV, and CLIC, with energies up to 3 TeV represent the new era of the linear collider research and development platform in the accelerator physics scenario. Furthermore, for these accelerators extremely complex detector systems are designed to achieve high accuracies. More than a thousand scientists are involved in the development of these new accelerators and detectors world-wide. This thesis work concentrates on the final-focusing system (FFS) of future linear colliders, like ILC or CLIC. The FFS has the function of squeezing the beams to the IP, down to nanometer transverse sizes. The beam transport and focusing optics in such a system are very challenging. The FFS of CLIC and ILC share the same conceptual design based on the local chromaticity correction scheme (although beam parameters are different, e.g. energy, emittances, beam sizes). On the other hand, CLIC has also an alternative correction scheme, the traditional one. Since the local chromaticity correction scheme was never used in any accelerator before, a small version of this system was built as a demonstrator in order to prove, experimentally, its effectiveness, called ATF2 located at KEK in Japan, near Tokyo. This test facility, operating since early 2010, has achieved many experimental successes, such as achieve the extremely low vertical beam or emittance required for linear accelerators. ATF2 has two main goals: squeeze the beam to 37 nm; and to demonstrate beam stability at IP at nanometer level. In this thesis I focus on the optimization methods to achieve tens of nanometer beam size at the IP in the ATF2 facility, such as mitigation of ground motion effects using the feed-forward control system and experimental studies on ultra-low $\beta_{y}^{*}$ optics. Measurements and simulations results are presented together with a novel proposal to use crystal focusing to reach the same beam size of the traditional focusing scheme (with normal quadrupoles). An introduction of the framework where this study is carried out, in particular the CLIC and ATF2 projects is presented and discussed in Chapter 1. Chapter 2 presents some basic concepts of the transverse accelerator physics and some important tools for optics design with particular emphasis on the FFS optics. The mitigation of ground motion effects using feed-forward control in ATF2 is described in Chapter 3, together with the measurements results achieved in December 2017 and February 2018 beam operations at ATF2 (described in the last section of the Chapter). In Chapter 4 the feasibility studies, the motivation and the measurements results of the ultra-low $\beta_{y}^{*}$ optics in the ATF2 are presented. The results of the nonlinear beamline optimization for minimizing the IP beam size are presented in the first section of this chapter. In the second section the computer tools for performing the tuning simulations are described. The tuning results achieved in December 2017, February 2018 and May 2018 beam operations are discussed in the last section of this chapter. A novel concept of optics design using crystal focusing in the FFS region is investigated through simulation studies in Chapter 5. At first, the implementation of the crystal in MAD-X and MAPCLASS environments is described. A first design of a crystal-based FFS optics is presented and compared to the baseline design. Then, luminosity simulations for CLIC 1.5 TeV were done and described in the last section of the Chapter. Completing this research required 5 visits to the High Energy Accelerator Research Organization (KEK) in Japan and spending 9 weeks working in ATF2. In Chapter 6 all the results obtained during these 9 weeks of operation in December 2017, February 2018 and May 2018 are discussed together with future works.