RF screening by thin resistive layers

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 \mu\m copper layer having the same DC resistance of 0.7 Ømega\m. 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.