Study and optimisation of undulator-based polarised positron sources for linear colliders

High-energy accelerators play an important role in physics research. The International Linear Collider (ILC) and Compact Linear Collider (CLIC) are proposed future accelerators which will require of order 1014 positrons per second to fulfil their luminosity requirements. In addition, polarisation of the positron beam will increase the scope of the physics studies that can be performed using a linear collider. Production of large quantities of polarised positrons is one of the major challenges for the research, development and design of any future linear collider. A polarised positron source based on gamma rays produced by a high energy electron beam in a helical undulator has been selected as the baseline option for ILC. The design is relatively mature, and previous studies have shown that it should be capable of producing the required positron beam intensity and polarisation. We review the design using a range of analytical and simulation tools. We also consider, in more detail than in previous studies, two options for the design of a photon collimator that could be used to improve the positron polarisation. Although a Compton source is presently the baseline choice for the positron source for CLIC, an undulator-based scheme remains an option. We discuss the possibility of an undulator-based polarised positron source for CLIC Stage 1 (500 GeV centre of mass collision energy), and consider options for an undulator-based positron source in the upgrade to Stage 2 (3 TeV centre of mass collision energy). For both ILC and CLIC, energy deposition from the gamma rays striking the positron production target is a concern. For ILC, the energy density can be reduced by rotating the target at high speed; however, this raises concerns about the eddy currents induced by the strong field of the matching device. We report the results of experiments at Daresbury Laboratory to understand the eddy current effects, and to validate models for predicting the effects in the final system