Laser-induced surface structuring for electron cloud mitigation in particle accelerators

The formation of electron clouds by secondary electron multiplication in the beam pipes of particle accelerators can lead to reduced performance during operation. Surface roughening using ultrashort pulse lasers efficiently reduces the secondary electron yield (SEY) of a surface. In this study, a solution for the suppression of electron clouds by laser structuring the inner copper walls of the Large Hadron Collider (LHC) beam tubes was developed, fulfilling the technical constraints and surface property requirements. For this purpose, fundamental dependencies between the laser processing parameters and the surface properties such as the modification depth, the surface chemical composition, the particle redeposition, and finally the SEY were investigated on a laboratory scale. A dedicated setup able to perform the modification treatment in situ, directly in the beam pipe hosted by the LHC magnet, was commissioned and the operation parameters were optimized. The device consists of a picosecond laser source, a beam coupling system, a 15 m long hollow-core fiber, and a robot that travels inside the beam tube. Treatment at low accumulated laser fluence in nitrogen flux resulted in "optimal surface properties", and specifically, a low modification depth (≈ 15 μm), low particle redeposition, a Cu2O-dominated surface and a SEY maximum of 1.4 after cleaning, which reduces to 1 upon electron irradiation at both room and cryogenic temperatures. A selective longitudinal scan scheme was developed to process the 10 m long beam pipes installed in the cryogenic magnet assemblies of the LHC with the highest effectiveness. A 3.1 m long laser-processed vacuum chamber was installed in the LHC to validate the method with respect to particle detachment.