Pumping and electron-stimulated desorption properties of a dual-layer nonevaporable getter

Abstract

The ASTeC Vacuum Science Group has an ongoing study for the improvement of the nonevaporable getter (NEG) coatings currently used in many accelerators around the world. The main advantages of using NEG coatings are evenly distributed pumping speed, low thermal outgassing rates, and low photon-stimulated gas desorption and electron-stimulated gas desorption (ESD). Previously, it was shown that the dense NEG coating provides lower ESD compared to that of a columnar film, but its pumping properties are reduced. This paper describes the results for a dual layer of NEG where a dense layer was deposited first and a columnar layer was then deposited on top. In such a dual-layer NEG coating, the dense layer acted as a barrier for hydrogen diffusion and the columnar layer further reduced the ESD of hydrogen and provided improved pumping properties. An alloy target of Ti-Zr-Hf-V was used to deposit a 0.5 μm-thick layer of the dense NEG followed by a 1 μm-thick columnar structure of NEG onto the inner surface of a 50-cm long stainless steel tube. The composition and the structure of the dual layer were determined by energy dispersive x-ray spectroscopy and scanning electron microscopy, respectively. The NEG vacuum performance was tested using the ESD and pumping properties, which were measured on a dedicated in-house designed facility.