DOE OSTI2012
The Accelerator Test Facility (ATF) [1] at KEK was originally constructed as a Damping Ring (DR) demonstration facility for the next generation of electron linear colliders. It consists of a 1.3 GeV s-band linac and a 136m circumference damping ring which can routinely generate 1.6 nC electron bunches with very small vertical emittances (<10pm.rad). Completed in December of 2008, a new extraction line and final focus optics was built, this is referred to as ATF2 [2]. The location of ATF in-relation to the KEK site can be seen in Figure 1. ATF is a general-purpose facility for accelerator physics related R and D activities, with approximately 50% of the available beam time currently devoted to the ATF2 program. The ATF2 extraction line (EXT) and final focus system (FFS) extends about 90 meters from the DR extraction point, a sketch of this system can be seen in figure 2, along with the location of key instrumentation. The primary purpose of the ATF2 beam line is to demonstrate a new style of magnetic focusing optics intended for use at future linear colliders, such as ILC or CLIC based on a local chromaticity correction scheme. The primary goal of the collaboration is to demonstrate the ability of this optics to focus an electron beam of similar emittance to the ILC using a FFS of the same design chromaticity. A comparison of some of the ATF2 design parameters with those of the ILC and CLIC are shown in table 1. With the 1.3 GeV beam energy of ATF2, the target spot size to prove the operability of this optics for ILC is 37nm. This is referred to as 'Goal 1'. In addition, Goal 1 requires the demonstration of stable operations with such a beam size at the Interaction Point (IP). 'Goal 2' is to demonstrate nm-level stability with respect to the IP beam monitors, such as is required for stable beam collisions at ILC or CLIC. A further goal is to attempt to increase the chromaticity of the final focus optics, reducing the IP beta functions to a level where the system more closely resembles the CLIC FFS, and also for some of the newly proposed ILC parameters. It is anticipated that the difficulty of successfully tuning the FFS to deliver design IP beam spot sizes scales with the chromaticity of the system. The design of the achromatic focusing system involves fine cancelation of first, second and third order aberrations at the IP. One needs more accurate cancelation the tighter one attempts to focus the beam, also including more higher order terms. By gradually pushing the system to smaller IP beta functions and higher chromaticity we hope to learn what the limits of such a design are when applied practically. This will give confidence in the design philosophy behind future high-luminosity colliders and in the simulations used to predict their behaviour.
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