The Influence of an Air Exposure on the Secondary Electron Yield of Copper

Inhaltsangabe:Abstract: The influence of different air exposure times on the secondary electron emission of clean copper surfaces as well as on technical copper surfaces has been studied in the context of the phenomenon of multipacting, which can limit the performance of superconducting radio-frequency (RF) cavities for particle acceleration. The copper samples were prepared by heat treatments and in situ sputter-etching and they were investigated with a dedicated instrument for SEY measurements, by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and by Auger electron spectroscopy (AES). After short air exposures of some seconds the maximum secondary electron yield dmax of clean copper is reduced from 1.3 to less than 1.2, due to the oxidation of the copper surface. Subsequent air exposure increases the secondary electron yield (SEY) until, after about 8 days exposure dmax is higher than 2. Clean copper samples were also exposed to the single gases present in air to find out the reasons for the dramatic increase of the SEY after long lasting air exposures. Only oxygen and water were found to affect secondary electron emission. An oxygen exposure decreases the SEY, while pure water exposure increases the SEY, but no single gas exposure changes dmax more than 0.2. Different methods have been tried in order to reduce the secondary electron yield of technical copper surfaces. For instance a 5 minutes air exposure of copper at 350 °C followed by a 350 °C bakeout reduces dmax to values close to unity. This procedure was applied to the outer, copper plated conductor of the LEP2 power couplers and its influence on pre-conditioning was tested. The results are promising but further tests are needed to confirm a beneficial effect of this treatment. Inhaltsverzeichnis:Table of Contents: Glossaryiv 1.Introduction1 2.Basics3 2.1Secondary electron emission3 2.1.1The energy distribution of the emitted electrons3 2.1.2The secondary electron yield (SEY)4 2.1.3The SEY as a function of the primary electron energy4 2.1.4Influence of adsorbed layers of another species on the SEY5 2.1.5Influence of the work function on the SEY5 2.1.6Influence of the surface structure on the SEY5 2.2Air6 2.3Copper and copper oxidation7 2.4Vacuum basics9 2.4.1Kinetic theory of gases9 2.4.2The mean free path of a gas molecule9 2.4.3The monolayer time W9 2.4.4Gas flow regimes9 2.4.5Pumping speed S and throughput Q10 2.4.6Conductance [¿]