The variation of the secondary electron yield (SEY) of sputter-cleaned OFHC-copper has been studied as a function of air exposure duration at room temperature. After short air exposures of some seconds the maximum SEY (δ MAX ) of clean copper is reduced from 1.3 to less than 1.2, due to the oxidation of the copper surface. Prolonged air exposure increases the SEY steadily until, after about 8 days of atmospheric exposure, δ MAX is higher than 2.Air exposures at higher temperatures have been found to be effective in reducing the SEY of technical copper surfaces. A 5-minute air exposure of copper at 350°C followed by a 350°C bake-out under vacuum reduces δ MAX to about 1.05, which is lower than the value of pure copper and that of Cu 2 O.

The secondary electron yield (SEY) variation of atomically clean metal surfaces due to air exposures and during subsequent heat treatments is described with the example of a sputter-deposited Nb thin film. Corresponding variations of the surface chemical composition have been monitored using AES and SSIMS. On the basis of these results and of previously obtained SEY results on metals and metal oxides the origin of the SEY variations is discussed.The SEY increase, which is generally observed during long lasting air exposures of clean metals, is mainly caused by the adsorption of an airborne carbonaceous contamination layer. The estimated value of about 3 for the maximum SEY of this layer is higher than that of all pure metals.Only in some cases the air-formed oxide can contribute to the air exposure induced SEY increase while many oxides have a lower SEY than their parent metals. From the experimental data it can also be excluded that the SEY increase during air exposures is mainly due to an increased secondary electron escape probability. Paper submitted to Applied Physics A Geneva, SwitzerlandJune 2002 -2- INTRODUCTIONThe secondary electron yield (SEY) of the internal surfaces of ultra high vacuum systems is of great importance for devices the operation of which can be perturbed by the occurrence of a resonant electron multiplication (multipacting) [1]. Examples for such devices are superconducting RF cavities for particle acceleration [2], and the beam vacuum system of CERN's next accelerator, the Large Hadron Collider (LHC) [3].Reliable SEY data for most atomically clean metals have been published already several decades ago [4]. However, despite the fact that for many applications the SEY of air exposed surfaces is more relevant than the SEY of atomically clean surfaces, SEY data of air exposed surfaces are rare and the reasons for the SEY variation during air exposures of atomically clean metals are still discussed controversially. There are mainly two reasons for this.Firstly, it is difficult to define precisely an air exposed surface because of the many parameters influencing the surface properties (e.g. initial sample state, air exposure time, humidity, concentration of contaminants in the ambient air, etc.). For initially clean copper, as an example, the maximum SEY value after air exposure can vary between 1.2 and above 2 for an air exposure lasting a few minutes and a few days, respectively [5].The second reason why SEY data of air exposed surfaces is often questionable is the fact that the SEY of such surfaces is much more sensitive to electron irradiation than the SEY of atomically clean surfaces [6]. Electron doses as low as 10 -6 C mm -2 remove efficiently a part of the surface species and, as a result, reduce the SEY [7]. Therefore, measurements on air exposed metal surfaces provide in some cases the SEY of an electron beam damaged surface, which has a much lower SEY than the as-received surface.Several SEY and surface analysis measurements of differently treated air exposed Nb surfaces h...

The secondary electron yield (SEY) of two different non-evaporable getter (NEG) samples has been measured`as received' and after thermal treatment. The investigated NEGs are TiZr and TiZrV thin ®lm coatings of 1 mm thickness, which are sputter deposited onto copper substrates. The maximum SEY d max of the air exposed TiZr and TiZrV coating decreases from above 2.0 to below 1.1 during a 2 h heat treatment at 250 and 2008C, respectively.Saturating an activated TiZrV surface under vacuum with the gases typically present in ultra-high vacuum systems increases d max by about 0.1. Changes in elemental surface composition during the applied heat treatments were monitored by Auger electron spectroscopy (AES). After activation carbon, oxygen and chlorine were detected on the NEG surfaces. The potential of AES for detecting the surface modi®cations which cause the reduction of SE emission during the applied heat treatments is critically discussed. #

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A summary of the main results obtained so far from the electron cloud studies using strip detectors, pick-ups, COLDEX and a 100 MHz coaxial resonator will be presented. The spatial and energy distributions of the electrons in the cloud measured by the strip detectors will be detailed and compared to the results obtained with a conventional retarding field detector. The evidence of the scrubbing effect and of the NEG coatings as remedies to reduce the electron cloud activity will also be shown.In a second part, the improved hardware of the experiments will be presented together with the program of measurements foreseen for the 2003 SPS run.