Cristoforo Benvenuti

Vacuum properties of TiZrV non-evaporable getter films

Sputter-deposited thin films of TiZrV are fully activated after 24 h “in situ” heating at 180°C. This activation temperature is the lowest of some 18 different getter coatings studied so far, and it allows the use of the getter thin film technology with aluminium alloy vacuum chambers, which cannot be baked at temperatures higher than 200°C. An updated review is given of the most recent results obtained on TiZrV coatings, covering the following topics: influence of the elemental composition and crystal structure on activation temperature, discharge gas trapping and degassing, dependence of pumping speed and surface saturation capacity on film morphology, ageing consequent to activation–air-venting cycles and ultimate pressures. Furthermore, the results obtained when exposing a coated particle beam chamber to synchrotron radiation in a real accelerator environment (ESRF Grenoble) are presented and discussed.

Decreasing surface outgassing by thin film getter coatings

Abstract The UHV behaviour of stainless steel vacuum chambers, coated ex situ by sputtering with a thin film of a getter material, has been investigated. The purpose of this study was to ascertain if the getter film could be activated after air exposure by in situ baking, so as to transform the vacuum chamber from a gas source into a pump. Many elements and alloys have been tested, all of which could be activated by baking at temperatures acceptable for stainless steel components, i.e. lower than 400 °C. In one case (equiatomic TiZr alloy) an activation temperature of 200–250 °C has been measured. This investigation has been carried out using Electron Stimulated Desorption, pumping speed and ultimate pressure measurements.

Nonevaporable getter films for ultrahigh vacuum applications

The vacuum behavior of stainless steel vacuum chambers, ex situ sputter coated with a thin film (∼1 μm) of getter material, has been studied to determine if after air exposure the getter film could be activated by a bakeout so as to transform the coated vacuum chamber into a pump. The materials studied so far are Ti, Zr, Hf, and some of their binary alloys. They all display an activation temperature lower than 400 °C, i.e., within the reach of the baking temperature of stainless steel vacuum chambers. The lowest activation temperature of 200–250 °C, measured for an equiatomic alloy of Ti and Zr, allows extension of this method to chambers made of copper and aluminum alloys. The experimental results, described here in detail, indicate that the values of the activation temperature obtained using electron stimulated desorption, pumping speed, and Auger spectroscopy measurements are self-consistent.

Niobium films for superconducting accelerating cavities

Superconducting accelerating cavities made of Nb‐coated copper were produced. Niobium films of a thickness ranging from 1.4 to 4 μm were deposited onto the inside of 3‐GHz cavities and 500‐MHz frequency by bias diode sputtering. A maximum accelerating field of 8.6 MV m−1 was reached without quench which is attributed to the large thermal conductivity of copper at liquid helium temperatures.

Study of the surface resistance of superconducting niobium films at 1.5 GHz

Abstract A systematic study of superconducting properties of niobium films sputtered on the inner wall of radiofrequency cavities is presented. The measured quantities include in particular the response to 1.5 GHz microwaves, the critical temperature, the penetration depth and the magnetic penetration field. In addition to films grown in different gas discharges (Xe, Kr, Ar and Ar/Ne mixtures) and to films grown on substrates prepared under different conditions, the study also includes bulk niobium cavities. The surface resistance is analysed in terms of its dependence on temperature, on RF field and, when relevant, on the density of trapped fluxons. A simple parameterisation is found to give a good fit to the data. Once allowance for the presence of impurities and defects is made by means of a single parameter, the electron mean free path, good agreement with BCS theory is observed. The fluxon-induced losses are studied in detail and their dependence on RF field, on temperature and on the density of trapped fluxons is analysed. The residual resistance is observed to be essentially uncorrelated with the other variables, suggesting that it is dominantly extragranular. In occasions very low residual resistances, in the nΩ range, have been maintained over a broad range of RF field, indicating the absence of significant fundamental limitations specific to the film technology in practical applications such as the production of accelerating cavities for particle accelerators.

The Large Hadron Collider (LHC) in the LEP tunnel

After the remarkable start-up of LEP, the installation of a Large Hadron Collider, LHC, in the LEP tunnel will open a new era for the High Energy Physics. This report summarizes the main LHC parameters and subsytems and describes the more recent studies and developments.

Pumping characteristics of the St707 nonevaporable getter (Zr 70 V 24.6‐Fe 5.4 wt %)

The room temperature pumping speeds of the St707 nonevaporable getter (NEG) have been measured both for individual gases and for gas mixtures as a function of the quantities of gas pumped. The interesting feature of this NEG consists in its moderately low activation temperature. Therefore particular attention has been devoted to defining the optimum temperature and duration of the activation process to obtain the highest possible pumping speed in a given practical situation. It has been found that heating at 400 °C for about 1 h, or at 350 °C for one day, results in pumping speeds of about 1000 l s−1 m−1 for H2, 2000 l s−1 m−1 for CO, and 450 l s−1 m−1 for N2, values very close to those obtained after activation at the higher temperature of 740 °C. The St707 NEG is therefore particularly suitable for passive activation during bakeout of stainless steel vacuum systems, avoiding the need for electrical insulation and feedthroughs which are mandatory when activation is carried out by resistive heating.

A NEW PUMPING APPROACH FOR THE LARGE ELECTRON POSITRON COLLIDER (LEP)

Abstract Storing particles inside an electron-positron collider brings about the need for low pressures to reduce interactions with residual gas molecules. In order to provide electron beams with a lifetime of 20 h, an average pressure of about 3×10 −9 Torr is needed for the Larger Electron Positron Collider (LEP), the construction of which has started in the proximity of CERN. Maintaining such a low pressure in spite of the small chamber cross-section and of the large degassing induced by synchrotron light requires an evenly distributed pumping speed. Traditionally, the problem is solved by means of integrated sputter ion pumps which are inserted all along the vacuum chamber and which make use of the field of the bending magnets. More recently a different linear pump, based on the use of a non-evaporable getter (NEG), was proposed. Laboratory measurements as well as real testing in an existing e + e − collider (PETRA at DESY), showed the feasibility and the advantages of this solution. The results which lead to the choice of NEG as main pump for LEP are presented and discussed.

Influence of the substrate coating temperature on the vacuum properties of Ti–Zr–V non-evaporable getter films

Abstract Non-evaporable thin film getters of various compositions have been produced by sputtering. Among about 20 materials which have been studied, the lowest activation temperature (about 180°C) has been displayed by a Ti–Zr–V coating obtained from a cathode made of intertwisted elemental wires. In order to optimize the vacuum properties of this film various production parameters, including the substrate temperature during coating, have been varied. The films have been characterized by pumping speed measurement, secondary electron microscopy, and X-ray diffraction. It has been found that the substrate coating temperature affects significantly the activation temperature, the pumping speed and the gas surface capacity. The highest pumping speed values, obtained for substrate coating temperatures of 250°C and 300°C, are clearly correlated with the increased surface roughness and porosity of the Ti–Zr–V film.

Influence of the elemental composition and crystal structure on the vacuum properties of Ti–Zr–V nonevaporable getter films

Nonevaporable thin film getters based on the elements of the fourth and fifth columns of the periodic table were deposited by sputtering. Among the some 20 alloys studied to date, the lowest activation temperature (about 180 °C for a 24 h heating) was found in the Ti–Zr–V system with a well-defined composition range. Characterization of the activation behavior of such Ti–Zr–V films is presented. The evolution of the surface chemical composition during activation is monitored by Auger electron spectroscopy and the functional properties are evaluated by pumping speed measurements. The pumping speed characteristics are quite similar to those already measured for commercially available nonevaporable getter materials, except for the much lower saturation coverage for CO. This inconvenience, which is due to the smooth surface structure of these films, can be counteracted by increasing the roughness of the substrate.