Main Linac Accelerator Physics

Fermilab is actively involved in the Main Linac accelerator physics studies. After the decision to choose cold technology for the ILC in August 2004, Fermilab started carrying out static tuning simulations on the US ColdLC Technology option lattice. Conventional survey and alignment techniques will not be sufficient to meet the required emittance dilution budget for the ILC, and hence new and improved beam based alignment techniques must be understood. In order to develop these new techniques, Fermilab scientists compared the performance of the two different beam based steering algorithms: Dispersion Free Steering (DFS) and the Flat (or one-to-one) steering in terms of the emittance dilution. They performed the comparison of the various lattices with different quad configurations in terms of the emittance dilution. They then parameterized the emittance dilution as a function of number of quads in the lattice design. In the same line, they also compared the results of the TESLA lattice with the US Cold LC lattice.

Main Linac final realistic lattice design: Fermilab is working on finalizing the main linac lattice. This includes working on the specifications of different beamline components, their positions in the linac, specifications of cold and warm drift spaces, matching of the main linac sections with ring-to-main-linac, undulator, beam delivery system lattices, locations of diagnostic sections, and integrating the final lattice in generalized XSIF or/and MAD format. This lattice will be used as the final main linac lattice for the ILC Reference Design Report. The Fermilab based simulation program OPTIM, along with MAD have been used for the matching purposes.

Development of New Simulation Package, CHEF, for Low Emittance Transport Simulation: Fermilab is working on extending the capability of a Fermilab based simulation package CHEF to the requirements of the ILC low emittance transport (LET) simulation. This code can be used to perform the complete start-to-end static as well as dynamic tuning simulations. To this end, the simulation code has already been benchmarked against other simulation codes in terms of tracking, and work is on going to add static tuning algorithms in this package.

Modifications in the Simulations Code LIAR: The baseline design for the ILC calls for the main linac to follow the curvature of the earth, rather than being laser-straight. The simulation code, LIAR, did not have the capability to simulate the curved linac system. In response, Fermilab worked on the implementation of an arbitrary "dispersion free" element, called GKICK (or geometrical kick), which places beamline elements on the earth curvature by changing the reference trajectory. In LIAR, dispersion could not be used as the initial condition and there was no provision for propagating it through the linac. Fermilab also added this feature to the code, and now the matched dispersion condition at the beginning of the linac can be artificially introduced into the initial beam.

Static Tuning: Fermilab performed a simulation study of single bunch emittance dilution for the ILC main linac following the earth curvature. They also performed a study on the implications of curved geometry on the beam based alignment methods. Fermilab compared the emittance dilution performance of the curved and the laser-straight geometry and investigated the sensitivity of these steering algorithms by taking into account the effect of various static misalignments on the emittance dilution performance of the ILC main linac. These studies are particularly important because they provide information on the stringent requirements for the system and also provide an understanding for the sensitivity of an algorithm on various systematic effects. Fermilab continues to study these effects and will develop new and modify existing tuning algorithms.