A. Hannah

Influence of deposition pressure and pulsed dc sputtering on pumping properties of Ti–Zr–V nonevaporable getter films

The performance of a UHV vessel can be improved with a new CERN technology nonevaporable getter (NEG) coating, which is already widely used for accelerator vacuum chambers. Better understanding of the processes involved in NEG film deposition, activation, and poisoning should allow optimization and engineering of the film properties, which are necessary for a particular application. Ti–Zr–V NEG films were created by magnetron sputtering from a single Ti–Zr–V target, and the NEG performance and morphology dependence on deposition pressure, sputtering conditions, and substrate surface roughness have been investigated. It was found that the average grain size of the Ti–Zr–V film was 5–6 nm and was broadly independent of the substrate material and deposition conditions. However, film topography and density were shown to depend very much on the substrate surface roughness and deposition conditions. Rough substrates, high working pressures, and the absence of ion bombardment produced open columnar structures, w...

Electron stimulated desorption from bare and nonevaporable getter coated stainless steels

An installation for investigation of the electron stimulated desorption (ESD) from both pumping and nonpumping tubular samples was designed and built. This installation allows studying ESD and sample sticking probability as a function of electron dose up to about 1023 e−/m2, electron energy in the range 10 eV–6.5 keV, and sampling temperature in the range 0–80 °C. Two samples were investigated: bare and Ti–Zr–V coated stainless steels. The ESD yields were measured as a function of electron accumulated dose, electron energy, and different NEG coating activation temperatures. The effect of electron stimulated pumping of CO saturated NEG coating was demonstrated for the first time and is in a good agreement with the effect of photon stimulated NEG activation measured earlier.

Comparison of Ti-Zr-V nonevaporable getter films deposited using alloy or twisted wire sputter-targets

A comparison of the performance of nonevaporable getter (NEG) films deposited using two different types of targets has been made to find the one that has the best pumping properties. For the first time, the NEG coating was deposited using a preformed Ti-Zr-V alloy target. The NEG film characterization and pumping properties have been studied in comparison with a film deposited using the commonly used three-wire twisted target. It was demonstrated that the alloy target produces a NEG coating with uniform composition both laterally and in depth. The composition of the film was found to be the same as the target. Film topography and microstructure with 5 nm grain sizes were found to be the same for both targets. The main result is that the activation temperature of the NEG coating deposited using the Ti-Zr-V alloy target is 160 °C, which is 20 °C lower than for NEG coatings deposited using three twisted wires.

Electron-stimulated desorption from polished and vacuum fired 316LN stainless steel coated with Ti-Zr-Hf-V

In this study, two identical 316LN stainless steel tubular samples, which had previously been polished and vacuum-fired and then used for the electron-stimulated desorption (ESD) experiments, were coated with Ti-Zr-Hf-V with different morphologies: columnar and dense. ESD measurement results after nonevaporable getter (NEG) activation to 150, 180, 250, and 350 °C indicated that the values for the ESD yields are significantly (2–20 times) lower than the data from our previous study with similar coatings on nonvacuum-fired samples. Based on these results, the lowest pressure and best long-term performance in particle accelerators will be achieved with a vacuum-fired vacuum chamber coated with dense Ti-Zr-Hf-V coating activated at 180 °C. This is likely due to the following facts: after NEG activation, the hydrogen concentration inside the NEG was lower than in the bulk stainless steel substrate; the NEG coating created a barrier for gas diffusion from the sample bulk to vacuum; the dense NEG coating performed better as a barrier than the columnar NEG coating.

Electron stimulated desorption from the 316 L stainless steel as a function of impact electron energy

The electron stimulated desorption (ESD) yields of vacuum chamber wall materials are required as one of the important input parameters in the design of vacuum systems for particle accelerators and many other vacuum systems where energetic electrons hit vacuum chamber walls. In the present study, the ESD yields were measured and analyzed (a) as a function of accumulated electron dose, (b) as a function of the total amount of desorbed gas, and (c) as a function of the energy of electrons bombarding. In this study, three separate 316 L stainless steel samples were bombarded with electrons up to an accumulated electron dose of ∼2 × 1023 e−/m2 at three different corresponding electron energies: 50 eV, 500 eV, and 5 keV. Once the required dose was reached, the ESD yield as a function of electron energy was measured between 10 eV and 6.5 keV. The results obtained by both methods were in agreement showing that ESD yield increases with electron energy. Experimental difficulties and problems with interpretation of data relating to such a study are described and discussed in the paper.

Physical vapour deposition of NbTiN thin films for superconducting RF cavities.

The production of superconducting coatings for radio frequency (RF) cavities is a rapidly developing field that should ultimately lead to acceleration gradients greater than those obtained by bulk Nb RF cavities. The use of thin films made from superconductors with thermodynamic critical field, Hc > HC(Nb), allows the possibility of multilayer superconductor – insulator – superconductor (SIS) films and accelerators that could operate at temperatures above 2 K. SIS films theoretically allow increased acceleration gradient due to magnetic shielding of underlying superconducting layers [1] and higher operating temperature can reduce cost [2]. High impulse magnetron sputtering (HiPIMS) and pulsed DC magnetron sputtering processes were used to deposit NbTiN thin films onto Si(100) substrate. The films were characterised using scanning electron microscopy (SEM), x-ray diffraction (XRD), Rutherford back-scattering spectroscopy (RBS) and a four-point probe.

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

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 5...