Experimental Investigation of Multipacting Suppression by amorphous Carbon Coatings

The presence of electron cloud is considered as the most important limitation concerning the quality of the particle beam in the accelerators, especially with respect to the forthcoming LHC luminosity upgrade. The electron cloud can be mitigated by coating the vacuum beam chambers with thin films of low secondary electron yield (SEY). This technique is applied to two stand-alone main bending dipoles of the SPS, where the RF power is fed through a tungsten wire, stretched inside the vacuum chamber. 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. The first results of a fully coated beam chamber do not exhibit any pressure rise or reflected RF power up to the maximum available input power. Here, reflected power has been observed only once and could not be reproduced. The results of a partially coated beam chamber show a strong reduction of multipacting. The remaining multipacting is suspected to take place outside the dipole. Several proposals for optimization of the test stand have been made. In parallel, electron cloud measurements via the microwave transmission method have been performed during the annual scrubbing run in the SPS. These technique was successfully applied to two consecutive uncoated dipoles. Clear electron cloud signals have been observed while operating with high intensity beams and a 25 ns bunch spacing. For low intensity beams, no electron cloud could be observed. This method has proven to be very useful and the potential of a future implementation for permanent diagnostic purposes has been outlined.