Abstract
For future linear colliders, a nanometer-scale beam size at the interaction point (IP) is one of the most challenging technical aspects. To explore the feasibility of a final focus system with a high chromaticity level, comparable to that of the Compact Linear Collider, the ultralow ${\ensuremath{\beta}}^{*}$ optics has been proposed and tuned at the KEK Accelerator Test Facility 2. In this paper, the recent experimental results are presented, which demonstrate the capability of achieving and stabilizing a vertical average beam size of 60 nm and below at the virtual IP. The observed vertical beam size is about 20 nm above the numerical predictions in the presence of static and dynamic imperfections. We interpret this discrepancy as beam size growth due to multipole fields, beam jitters and wakefield effects, and diagnostic errors.
Highlights
A nanometer-scale beam size at the interaction point is a fundamental ingredient to achieve the design luminosity in future energy-frontier linear colliders
To address the practical feasibility of this new scheme envisioned for future linear colliders, the Accelerator Test Facility 2 (ATF2) project has been launched at KEK, aiming to achieve a nanometer beam size and provide nanometer beam stability at the virtual interaction point (IP) [7,8,9,10]
ATF2 tuning mainly consists of extraction line (EXT) tuning, optics matching, and final focus system (FFS) tuning
Summary
A nanometer-scale beam size at the interaction point is a fundamental ingredient to achieve the design luminosity in future energy-frontier linear colliders. To demagnify the beam to the required spot sizes, a novel local chromaticity correction-based final focus system has been proposed and considered for the baseline designs of the International Linear Collider (ILC) and Compact Linear Collider (CLIC) [1,2,3,4,5,6]. To explore the feasibility of a FFS with a higher chromaticity, comparable to that of the CLIC FFS, the ultralow βà optics has been proposed that has a 4 times smaller IP vertical β function than the nominal and a target vertical beam size of 23 nm [11,12,13,14,15,16]. The ultralow βà optics intends to explore the uncharted chromatic territory and push the limits of ATF2
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