Design and high order optimization of the Accelerator Test Facility lattices

The Accelerator Test Facility 2 (ATF2) aims to test the novel chromaticity correction scheme which is implemented in the final focus systems of future linear colliders such as the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). The ATF2 nominal and ultralow ${\beta }^{*}$ lattices are designed to vertically focus the beam at the focal point, or usually referred to as interaction point (IP), down to 37 and 23 nm, respectively. The vertical chromaticities of the nominal and ultralow ${\beta }^{*}$ lattices are comparable to those of ILC and CLIC, respectively. When the measured multipole components of the ATF2 magnets are considered in the simulations, the evaluated spot sizes at the IP are well above the design values. In this paper we describe the analysis of the high order aberrations that allows identifying the sources of the observed beam size growth. In order to recover the design spot sizes three solutions are considered, namely final doublet replacement, octupole insertion, and optics modification. Concerning the future linear collider projects, the consequences of magnetic field errors of the focusing quadrupole magnet of the final doublet are also addressed.

Figure 1

Scheme of the ATF2. The beam line on the left represents the extraction beam line (EXT). The beam line on the right represents the FFS as the continuation of the EXT line (the figure is courtesy of Araki).

Figure 2

The ${\beta }_{x,y}$ functions and the ${\eta }_x$ function for the ATF2 nominal lattice throughout the ATF2 final focus line.

Figure 3

Plot of ${\sigma }_y^{*}$ as a function of $\gamma {\epsilon }_x$ for the ATF2 ultralow ${\beta }^{*}$ lattice. The red curve represents the vertical IP beam size considering up to the quadrupole components, the green dashed curve includes the sextupole components, and the blue dot curve takes into account all of the multipoles up to the dodecapole components.

Figure 4

${\sigma }_y^{*}$ evaluated by MAPCLASS as a function of $\gamma {\epsilon }_x$ for the ATF2 ultralow ${\beta }^{*}$ considering the expected multipole components of the PM magnet for QD0FF. The value of $\gamma {\epsilon }_x$ when operating ATF is $5\mu \mathrm{m}$.