Can Low-Energy Electrons Affect High-Energy Physics Accelerators?

Present and future accelerators' performances may be limited by the electron cloud (EC) effect. The EC formation and evolution are determined by the wall-surface properties of the accelerator vacuum chamber. We present measurements of the total secondary electron yield (SEY) and the related energy distribution curves of the secondary electrons as a function of incident-electron energy. Particular attention has been paid to the emission process due to very low-energy primary electrons ($<20\mathrm{e}\mathrm{V}$). It is shown that the SEY approaches unity and the reflected electron component is predominant in the limit of zero primary incident electron energy. Motivated by these measurements, we have used state-of-the-art EC simulation codes to predict how these results may impact the production of the electron cloud in the Large Hadron Collider, under construction at CERN, and the related surface heat load.

Figure 1

Measured EDCs of a fully scrubbed Cu surface at about 10 K for different primary electron energies at normal incidence.

Figure 2

Total SEY ($\delta$) at normal incidence and the contribution of secondary and reflected electrons from a fully scrubbed Cu surface at about 10 K as a function of $E_0$.

Figure 3

Simulated average heat load in an LHC dipole magnet as a function of proton bunch population at 0.45 TeV, calculated by extrapolating the best fit to the data of Fig. 2 (shown in the inset) for a SEY with ${\delta }_{\mathrm{m}\mathrm{a}\mathrm{x}}^{*}=1.7$ and $E_{\max }=240\mathrm{e}\mathrm{V}$, considering the elastic reflection (dashed line) or ignoring it (full line).

Figure 4

Simulated average EC density in a field-free section of the GSI-SIS18 as a function of ${\delta }_{\max }$, comparing the cases with (dashed line) and without (full line) elastic reflection.