Energy-resolved secondary-electron emission of candidate beam screen materials for electron cloud mitigation at the Large Hadron Collider

Energy-resolved secondary electron spectroscopy has been performed on air-exposed standard Cu samples and modified Cu surfaces that are tested and possibly applied to efficiently suppress electron cloud formation in the high-luminosity upgrade of the Large Hadron Collider at CERN. The Cu samples comprise pristine oxygen-free, carbon-coated and laser-structured surfaces, which were characterized prior to and after electron irradiation and rare-gas ion bombardment. Secondary-electron and reflected-electron yields measured with low charge dose of the samples exhibit a universal dependence on the energy of the primary impinging electrons. State-of-the-art models can successfully be used to describe the spectroscopic data. The supplied spectral dependence of electron emission and integrated electron yield as well as the derived parametrization can serve as a basis for forthcoming simulations of electron cloud formation and multipacting.

Figure 1

Energy distribution curves, $N(E)$, of secondary and elastically reflected electrons as a function of their kinetic energy, $E$, and for the indicated primary energy, $E_{\mathrm{p}}$, of incident electrons for Cu OFE samples that had been cleaned by ion bombardment (ci). Raw (smoothed) data appear as dots (solid lines). The spectra are normalized by setting the SE peak height to unity. $N(E)$ data with $E_{\mathrm{p}}\ge 25\mathrm{eV}$ are vertically offset. Inset: definition of secondary-electron (SE) and reflected-electron (RE) contributions to the total number of emitted electrons. SE and RE areas are separated by the Shirley background (blue line). Numerically integrated data, $\int N(E)\mathrm{d}E$, are normalized to unity and presented by the dashed green line.

Figure 2

Fractions $\sigma (E_{\mathrm{p}})$ and $\varrho (E_{\mathrm{p}})$ for (a) ar and (b) ce samples. The same symbols as in (a) are used for ce samples in (b); reversed triangles are chosen for Cu OFE-ci. Solid lines represent fits to the $\varrho (E_{\mathrm{p}})$ data using Eq. (1).

Figure 3

Energy distribution curve, $N(E)$, of a Cu OFE sample cleaned by ${\mathrm{Ar}}^{+}$ bombardment with focus on SE emission in the kinetic energy interval $0\mathrm{eV}\le E\le 50\mathrm{eV}$. (a) $N(E)$ (circles) acquired at $E_{\mathrm{p}}=96\mathrm{eV}$. Fits to the data are depicted as solid [Eq. (2)] and dashed [Eq. (3)] lines. (b) Evolution of fit parameters $\mu$, $\mathrm{\Gamma }$ [Eq. (2)] with $E_{\mathrm{p}}$. (c) Evolution of fit parameters $x$ and $E_0$ [Eq. (3)] with $E_{\mathrm{p}}$ for fixed $\mathrm{\Phi }=4.5\mathrm{eV}$.

Figure 4

Secondary-electron yield ($\delta$) as a function of the primary energy ($E_{\mathrm{p}}$) obtained at CERN for all types of samples studied in this work. (a) $\delta (E_{\mathrm{p}})$ for Cu OFE surfaces. (b) Like (a), for a-C-coated samples. For comparison, ITU data are plotted as open symbols. The uncertainty margins reflect the experimental uncertainties in the measured sample current. (c) Like (a), for Cu LESS samples. The insets to each panel depict the low-energy ($0\mathrm{eV}<E_{\mathrm{p}}<35\mathrm{eV}$) region of $\delta (E_{\mathrm{p}})$.