Measurements of PSD and photoelectrons on NEG coated vacuum chambers exposed to synchrotron radiation at the Taiwan Light Source beamline

Abstract

A 1.5 GeV Taiwan Light Source (TLS) BL19B beamline was built to measure the photon stimulated desorption (PSD)-yield (η_PSD) and the associated photoelectron yield (PEY, η_e). Three stainless steel vacuum tubes 0.5 m in length were installed in the beamline for synchrotron radiation exposure. They were coated with non-evaporable getter (NEG) films with different structures, including conductive NEG (TiZrVAg) to reduce RF surface resistance, dual NEG (dense-TiZrV/columnar-TiZrV) for higher pumping performance, and triple-layer NEG (TiZrVN/dense-TiZrV/columnar-TiZrV) in which the nitride-layer (TiZrVN) acted as a hydrogen barrier in the chamber. Both η_PSD and η_e were measured simultaneously under two conditions, namely, “non-activated NEG”, baking only the tube at 80 °C without NEG-activation and “activated NEG” where the NEG was activated after at 180 °C. The intrinsic η_PSD comprises two parts, the η_c is measured via the throughput or conductance method for non-activated NEG, hydrocarbons, and noble gases with a sticking probability of α = 0; and the η_α for the activated NEG which absorbs the gases with α > 0. The results of PSD reveal a much lower intrinsic η_PSD for activated NEG than that of the non-activated NEG by a factor of 10–100 for most of the gases including H₂, CO, CO₂, CH₄, H₂O, and C₂H₆. The PEY (η_e) measurements for the NEG-tube showed higher values (>0.033 electrons/photon) for the non-activated NEG and lower values (<0.022 electrons/photon) for the activated NEG. Comparison showed that the triple-layer NEG tube possessed both the lowest η_PSD and η_e of all the NEG tubes. The η_e was also lower after activation of the conductive NEG tube. The simultaneous measurements of both η_PSD and η_e for the NEG-coated tubes at the beamline in this experiment are reproducible and practically convincing.