Surface effects for electron cloud
The ability of a low secondary electron yield coating to mitigate detrimental electron cloud effects potentially affecting accelerators’ performances has been convincingly validated. The interference of such coatings with other properties required to accelerator constructive materials (i.e. vacuum compatibility, magnetic permeability, high surface conductivity, etc.) is of great concern and has recently attracted a lot of interest and studies. For instance, the severe impedance budget constraint requires the highest conductivity in the surface layers within the skin depth (typically some μm) characteristic of the e.m. interaction. It is therefore of uttermost importance to define the minimum thickness one overlayer should have in order to be an eective electron cloud mitigator and minimize its impact to surface conductivity.
To this purpose, XPS and secondary electron spectroscopy have been simultaneously applied to the prototypical system formed by increasing coverages of amorphous Carbon (a-C) deposited on atomically clean Cu. XPS has been successfully used to qualify and quantify the a-C thickness, rendering possible a detailed coverage dependent study. A significantly thin a-C layer, of about 5 to 7 nm, is surprisingly enough to lower the secondary emission properties of the whole system below 1. This observation opens up the possibility to develop, on industrial scale, thin enough electron cloud mitigators that will not affect impedance issues.
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