SPS Dipole Multipactor Test and TEWave Diagnostics

Electron cloud accumulation in particle accelerators can be mitigated by coating the vacuum beam pipe with thin films of low secondary electron yield (SEY) material. The SEY of small coated samples are usually measured in the laboratory. To further test the properties of different coating materials, RF-induced multipacting in a coaxial waveguide configuration can be performed. The technique is applied to two main bending dipoles of the SPS, where the RF power is fed through a tungsten wire stretched along the vacuum chamber (6.4 m). A dipole with a bare stainless steel chamber shows a clear power threshold initiating an abrupt rise in reflected power and pressure. The effect is enhanced at RF frequencies corresponding to electron cyclotron resonances for given magnetic fields. Preliminary results show that the dipole with a carbon coated vacuum chamber does not exhibit any pressure rise or reflected RF power up to the maximum available input power. In the case of a large scale coating production this technique will be a valuable resource for quality control. In parallel, electron cloud measurements via the microwave transmission method have been performed during the annual scrubbing run in the SPS. Clear electron cloud signals have been observed while operating with high intensity beams, confirming that this measurement technique provides a valuable tool for in-situ diagnostic. This paper is divided into two parts, where the first part deals with the multipacting test stand on stand-alone dipoles and the second part deals with the TE wave diagnostics in the CERN SPS. (author)