SUMMARY OF THE SINGLE-BEAM COLLECTIVE EFFECTS IN THE LHCBerg, J.S.; Bruening, O.; Caspers, F.; D'yachkov, M.*; Morvillo, M.; Ruggiero, F.Single beam collective effects can limit the performance of the LHC and, together with parasitic losses, impose constraints on the design of the components of the LHC vacuum system. Because of the large number of bunches in the LHC, coherent losses and multibunch instability rise times generated by the impedance items can be rather large. For example, the vacuum chambers of the main experiments can lead to large parasitic losses and a continuous feedback from the impedance calculations during their design process is desirable. We first summarise the current status of the impedance model for the LHC and estimate rise times and threshold currents for different instabilities. In a second step, we discuss possibilities of controlling these instabilities, using feedback systems and Landau damping. Abstract Single beam collective effects can limit the performance of the LHC and, together with parasitic losses, impose constraints on the design of the components of the LHC vacuum system. Because of the large number of bunches in the LHC, coherent losses and multibunch instability rise times generated by the impedance items can be rather large. For example, the vacuum chambers of the main experiments can lead to large parasitic losses and a continuous feedback from the impedance calculations during their design process is desirable. We first summarise the current status of the impedance model for the LHC and estimate rise times and threshold currents for different instabilities. In a second step, we discuss possibilities of controlling these instabilities, using feedback systems and Landau damping.

We discuss the results of recent impedance measurements for an LHC dump kicker prototype, performed at CERN using the coaxial wire method. The kicker design includes a vacuum barrier consisting of a ceramic chamber internally coated with a thin metallic layer having good electric contact with the external beam pipe. For the bench test the coated ceramic tube was replaced by a kapton foil with a 0.2 µm copper layer having the same DC resistance of 0.7 Ω. The measurements show that this resistive coating provides a very effective RF screening down to frequencies below 1 MHz, where the skin depth is two orders of magnitude larger than the layer thickness and one could expect full penetration of the electromagnetic fields. We also present simulation results and analytic considerations in agreement with the measurements, showing that the return currents almost entirely flow through the copper layer down to frequencies where the reactive impedance of the kicker elements located behind it becomes comparable to the layer resistance. Finally we discuss the relevance of such coaxial wire measurements to the RF shielding by thin metallic layers in the presence of a higly relativistic proton beam.

An adequate dose of photoelectrons, accelerated by lowintensity proton bunches and hitting the LHC beam screen wall, will substantially reduce secondary emission and avoid the fast build-up of an electron cloud for the nominal LHC beam. The conditioning period of the liner surface can be considerably shortened thanks to secondary electrons, provided heat load and beam stability can be kept under control; for example this may be possible using a special proton beam, including satellite bunches with an intensity of 15-20% of the nominal bunch intensity and a spacing of one or two RF wavelengths. Based on recent measurements of secondary electron emission, on multipacting tests and simulation results, we discuss possible 'beam scrubbing' scenarios in the LHC and present an update of electron cloud effects.

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