DISTURBANCE EFFECTS CAUSED BY RF POWER LEAKING OUT FROM CAVITIES IN THE PSI RINGCYCLOTRON

While commissioning the PSI high intensity proton beam facility after the shutdown 2010, direct and indirect phenomena of interaction occurred between the RF power leaking out from the cavities and the electrostatic septa at the injection and extraction region in the Ringcyclotron. As an indirect influence RF fields outside the cavities generate plasma clouds in the peripheral area of magnet poles. Accelerated plasma ions sputtered metallic atoms from the vacuum chamber wall, which then covered the insulator surface with an electrically conductive layer. The septum therefore had to be replaced. Directly, RF power dissipated from the 150 MHz flattop cavity was redirected by a beam stopper in such a way, that a linear correlation between the RF pick up signal monitored at the extraction septum (EEC) and the leakage current across the septum insulator could be observed. As an instant mending action the beam stopper, which is not permanently used, has been removed. The long term attempt to minimize these disturbing effects will be an asymmetrical setting of the hydraulic cavity tuning system and an effective RF grounding of build in components.