Abstract:
In the framework of the HL-LHC project design, the vacuum performance of new surface material needs to be studied in details. In particular, amorphous carbon (a-C) coating is proposed as an anti-multipactor surface with the objective to minimize the beam induced electron cloud heat load deposited on the shielded beam screen and the background to the experiments due to proton scattering onto the residual gas. Since the protons in the HL-LHC final focusing triplets generates synchrotron radiation with ~ 10 eV critical energy and ~ 1016 ph/m/s flux, it is therefore of great importance to study the impact of such photons on a-C coating held at room and cryogenic temperature and compare the results against present LHC material. This paper presents results on quantitative measurements of photon stimulated gas desorption, relative measurements of photo-electron yield, photon specular reflectivity and azimuthal distribution of diffusely scattered photons. All the measurements are done at room temperature in the range of synchrotron radiation critical energy 12.5 ÷ 1250 eV for a tube with a-C coating irradiated at grazing incidence angle and a simple Cu tube used as a reference.