Selective laser processing of particle accelerator beam screen surfaces for electron cloud mitigation†

Abstract

Laser-induced surface roughening is a technique that facilitates the reduction of secondary electron emission (SEE) from materials, which is crucial for mitigating electron cloud (EC) formation in particle accelerators, that operate with positively charged species, such as the large hadron collider (LHC). This study focuses on the development of a selective laser surface treatment of the inner copper surface of beam screens (BS) within superconducting (SC) magnets. Several technical challenges linked to laser processing exist including the reduction of treatment time and the control of ablation depth. Based on the found correlations between laser treatment parameters and materials properties, and considering all technical constraints for execution of such a process in SC magnets, a tailored laser processing strategy is developed, which includes creation of a rough Cu surface with trenches of 15–20 μm depth and an initial secondary electron yield maximum of 1.4–1.5, only in the most relevant regions of the BS. Resulting material properties are characterized such as the surface resistance and related beam impedance, as well as the SEE at both room temperature and cryogenic conditions. The efficiency to mitigate EC formation and thus improve beam quality is demonstrated via EC simulations and electron-induced conditioning experiments. This study also explores under which circumstances the risk of particulate detachment from the surface, which could lead to critical beam interaction, can be minimized.