Magnetic Cusp Configuration of the SPL Plasma Generator

Abstract

The Superconducting Proton Linac (SPL) is a novel linear accelerator concept currently studied at CERN. As part of this study, a new Cs‐free, RF‐driven external antenna H− plasma generator has been developed to withstand an average thermal load of 6 kW. The magnetic configuration of the new plasma generator includes a dodecapole cusp field and a filter field separating the plasma heating and H− production regions. Ferrites surrounding the RF antenna serve in enhancing the coupling of the RF to the plasma. Due to the space requirements of the plasma chamber cooling circuit, the cusp magnets are pushed outwards compared to Linac4 and the cusp field strength in the plasma region is reduced by 40% when N‐S magnetized magnets are used. The cusp field strength and plasma confinement can be improved by replacing the N‐S magnets with offset Halbach elements of which each consists of three magnetic sub‐elements with different magnetization direction. A design challenge is the dissipation of RF power induced by eddy currents in the cusp and filter magnets which may lead to overheating and demagnetization. In view of this, a copper magnet cage has been developed that shields the cusp magnets from the radiation of the RF antenna.