Abstract
The secondary emission yield (SEY) properties of colaminated Cu samples for LHC beam screens are correlated to the surface chemical composition determined by x-ray photoelectron spectroscopy. The surface of the as-received samples is characterized by the presence of significant quantities of contaminating adsorbates and by the maximum of the SEY curve (${\ensuremath{\delta}}_{\mathrm{max}}$) being as high as 2.1. After extended electron scrubbing at kinetic energy of 10 and 500 eV, the ${\ensuremath{\delta}}_{\mathrm{max}}$ value drops to the ultimate values of 1.35 and 1.1, respectively. In both cases the surface oxidized phases are significantly reduced, whereas only in the sample scrubbed at 500 eV the formation of a graphitic-like C layer is observed. We find that the electron scrubbing of technical Cu surfaces can be described as occurring in two steps: the first step consists in the electron-induced desorption of weakly bound contaminants that occurs indifferently at 10 and at 500 eV and corresponds to a partial decrease of ${\ensuremath{\delta}}_{\mathrm{max}}$; the second step, activated by more energetic electrons and becoming evident at high doses, increases the number of graphitic-like C-C bonds via the dissociation of adsorbates already contaminating the as-received surface or accumulating on this surface during irradiation. Our results demonstrate how the kinetic energy of impinging electrons is a crucial parameter when conditioning the surfaces of Cu and other metals by means of electron-induced chemical processing.
Highlights
A wide range of applications [1,2,3] use or are dependent on the capability of a given material to emit electrons after electron bombardment
We have shown that the secondary emission yield (SEY) of colaminated Cu samples for LHC beam screens can be decreased by electron scrubbing
Lowering the initial max of 2.1 to values below 1.4 requires the formation of a graphitic-like film. This occurs via electron beam-induced reactions in the C-containing contaminating layer covering the as-received sample, and/or via electron beam-induced dissociation of adsorbates coming from the residual gas of the vacuum chamber or released by the eÀ gun filament, whose fragments partly desorb and partly organize in graphitic domains
Summary
A wide range of applications [1,2,3] use or are dependent on the capability of a given material to emit electrons after electron bombardment. 2.2 radiation as in electron microscopy [29], extreme ultraviolet lithography [3,30], or synchrotron radiation beam lines [31] Both the graphitization of the preexisting contaminating layer and the growth of a graphitic film due to the cracking of the residual gas molecules occur with a different efficiency depending on the kinetic energy of the electrons used to scrub the surface. The majority of the electrons forming the e-cloud in the LHC, which have energy below 20 eV [20], do not contribute in lowering max below the value of 1.3 desired for machine stability at design operation [13,14,15,18] The relevance of this issue for accelerator wall conditioning motivates deep investigations of the effects of the electron kinetic energy on the surface chemistry of technical metal surfaces.
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