Design of main linac emittance tuning bumps for the Compact Linear Collider and the International Linear Collider

The installation of elements in the main linac of future linear colliders can only be done with a limited precision. The inevitable misalignments lead to unacceptable emittance growth. Beam-based alignment, e.g., one-to-one correction, dispersion free steering, or ballistic alignment, is necessary to reduce the emittance growth. In some cases, this is, however, not sufficient. For further reduction of the emittance growth, so-called emittance tuning bumps have to be used. A general strategy for the design of emittance tuning bumps has been developed and tested. Simulations suggest that the method can be conveniently used to understand the weaknesses of existing emittance tuning bumps and to significantly improve their performance in terms of, e.g., emittance reduction capability and convergence speed. An example of an application is the design of ten orthogonal knobs that, according to simulations, can reduce the normalized emittance growth in the Compact Linear Collider (CLIC) main linac from 23.8 to 0.34 nm with convergence within two iterations. Four orthogonal knobs have also been designed for the International Linear Collider (ILC). Simulations show that these knobs converge within a single iteration and reduce normalized emittance growth from 3.8 to 0.05 nm.