Laser Surface Engineering for the Reduction of Secondary Electron Emission in metals and Ceramics

Electron clouds driven by secondary electron emission limit the performance of particle accelerators. In the large hadron collider (LHC), electron clouds arise when the beam-exposed surface has a secondary electron yield (SEY) greater than unity. Addressing this issue requires an innovative solution which recognises the current constraints of the machine. One attractive solution, which avoids decommissioning the LHC, involves reducing the SEY of the beam-exposed surface in situ by introducing roughness through Laser Engineered Surface Structuring (LESS). Transitioning LESS from laboratory to practical application on the copper beam screens at CERN requires addressing several challenges. The proposed strategy consists of an inch-worm robot coupled to a photonic crystal fibre. Initially, the treatment used 532 nm, picosecond (ps) pulses, but lacked fibre compatibility. Engineering constraints demanded a transition of LESS treatment to IR pulses where the fibre technology was more mature for ultrafast pulses. As such LESS treatment was successfully transitioned to 1030 nm, femtosecond (fs) pulses, while reducing the SEY close to unity. The IR structuring was then developed to structure a hippodrome geometry, requiring out-of-focus processing. This was addressed through the novel use of a diffractive optical element (DOE) in the setup, extending the focal depth of the laser beam. Measurements revealed the SEY was consistently reduced below CERN’s threshold when structuring over a varied working distance with the DOE, facilitating in situ treatment. Next, a model was developed to predict the SEY of surfaces subject to laser irradiation. For the first time, multiscale geometrical features, and realistic experimental data, including SEY data of the laser modified surface composition were included as input parameters. Simulating copper showed that multi-scale geometrical features in conjunction with a modified surface composition culminate in effective SEY reduction. Consequently, high aspect ratio structures are not necessarily required for effective SEY reduction. Additionally, controlling selected variables such as the laser power, beam shape and hatch distance could give rise to a variety of different multiscale surface structures and surface compositions capable of reducing the SEY in many materials and applications, thereby supporting the design of low SEY surfaces. Finally, with this understanding, LESS treatment was applied to a ceramic, ferrite, used in the LHC kicker magnets. For the first time, this material was studied under optical and electron irradiation, deriving parameters for LESS treatment. Using ultrafast, fs, UV pulses the SEY of ferrite was reduced below unity, demonstrating the applicability and versatility of ultrafast laser structuring, and establishing possible future applications.