Optimisation analysis and improvement of the effective beam sizes in Accelerator Test Facility 2

A lepton linear collider is considered by the accelerator and particle physics communities as an appropriate machine to perform high precision particle physics research in the TeV energy regime. There are two proposals for the future e+e- linear collider: the Compact Linear Collider (CLIC) and the International Linear Collider (ILC), both developed by two wide international collaborations with strong overlap between them. Both designs satisfy the particle physics requirements. At the TeV energy regime the cross sections of many processes of interest are small, therefore large luminosities on the order of 10^{34} cm^{-2}s^{-1} at the interaction point (IP) are required to deliver the required event rates. The luminosity inversely depends on the transverse size of the colliding beams which restricts the beam sizes at the IP to the nanometer level. The strong focusing of the beams occurs in the final focus system (FFS), the most inner part of a linear collider, where the beams are focused at the IP by means of two strong quadrupole magnets called the final doublet (FD). The efficiency of the beam focusing is deteriorated by the chromatic effects of the FD quadrupoles meaning that off-momentum particles are not exactly focused at the focal point, leading to larger spot sizes at the IP. A novel design of the final focus system with a local compensation of the chromatic effects has been proposed in 2001 by P. Raimondi and A. Seryi. This design is being tested in the KEK Accelerator Test Facility 2 (ATF2) in Japan, a scaled down implementation of the linear collider beam delivery system. It has already been demonstrated that the IP vertical beam size in ATF2 decreases from some hundreds of nanometers to about 40 nm when the chromaticity is corrected. Therefore, the local chromaticity correction scheme is considered as a baseline for the ILC and a strong candidate for CLIC. However, for CLIC the expected level of chromaticity is higher by about a factor 5. The parameter describing the focusing strength is the IP value of the optical \beta function (\beta^{*}). In linear approximation, the IP beam size is proportional to the square root of the \beta^{*} value and chromaticity is inversely proportional to the \beta^{*} value. Therefore, decreasing the \beta^{*} value in ATF2 allows to reduce the IP beam size and to increase the level of chromaticity. The main objective of the study described in this thesis is to decrease the \beta^{*} value in the ATF2 in order to investigate the performance of more chromatic optics (close to the level of CLIC) and to study the limits of beam focusing at the IP. Understanding the tradeoffs between lowering \beta^{*} and increasing aberrations is of critical importance for future linear colliders. In Chapter 1, after a reminder of the Standard Model of elementary particles, the main differences of linear colliders with respect to circular colliders are discussed. The physics potential of linear colliders is also summarized. Linear colliders are discussed in more detail in Chapter 2. The most important tools of the linear and nonlinear beam dynamics are described first. Afterwards, the linear collider layout is presented with a special emphasis given to the final focus system. Finally, the two existing designs for a future linear collider are described, together with the test facilities. The Accelerator Test Facility serving for performing the experimental studies of this thesis is described in detail in Chapter 3. The feasibility studies of the low \beta^{*}_{y} optics in the ATF2 are presented in Chapter 4. The results of the nonlinear beamline optimisation for minimising the IP beam size are presented in the first section of this chapter. The impact of the quadrupole fringe fields on CLIC, the ILC and three lattices of the ATF2 are described in the second section. In the third section the contribution of optical aberrations to spot size increase with bunch intensity is investigated. Finally, the developed computer tools for performing the tuning simulations are described in the last section of this chapter. The experimental studies of the new ATF2 optics with the \beta^{*}_{y} value decreased by a factor 2 are described in Chapter 5. Completing this research required 11 visits in the High Energy Accelerator Research Organization (KEK) in Japan and spending 23 weeks working with the ATF2 accelerator. The optics control and implementation was achieved by introducing a new method of beam diagnostics at the IP based on precise beam size measurements and fine, well-controlled changes of the vertical beam waist position. The beam sizes measured after two complete tuning sessions were almost a factor three larger than the design values. Comparison with the nominal \beta^{*}_{y} optics suggests that the beam size growth due to machine imperfections for lower \beta^{*}_{y} values is much stronger than expected in the design. The experimental results are also compared with the tuning simulations. The realistic errors applied to the machine model are not sufficient to reproduce the experimental results. Simulation results get closer to the experiment for larger machine errors. Simulations also show that an accurate orbit correction can help in lowering the IP beam size. The main reasons for observing larger beam sizes than expected are identified: insufficient orbit control and sensitivity to machine drifts, contribution of wakefields combined with the beam orbit jitter, larger multipolar fields, larger magnet alignment errors, instrumentation errors and stability (especially Shintake monitor and beam position monitors). Addressing these issues is recommended for future experiments with low \beta^{*}_{y} optics in the ATF2.