Speaker
Description
The Beam Screen Test Experiment (BESTEX) at the Karlsruhe Research Accelerator (KARA) is built as a specialized platform to study synchrotron-radiation-induced surface effects that are important for accelerator vacuum systems, i.e. for mitigating electron cloud formation and controlling desorption processes in future high-intensity storage rings like the FCC-ee. The setup allows for in-situ irradiation of beam screen materials and vacuum chambers under realistic synchrotron radiation conditions, enabling systematic studies of the evolution of the material characteristics and the photon-induced conditioning behaviour during accelerator operation.
While the original setup was designed for photon-stimulated desorption (PSD) measurements, the present campaign transferred the experiment to a setup for Photoelectron Yield (PEY) measurements. Four LHC-type beam screen samples, including degreased OFE copper and copper substrates coated with amorphous carbon (a-C) of varying thicknesses and configurations, are under investigation. The PEY measurements were acquired at KARA under synchrotron radiation exposure by tracking the drain current caused by photoemission as a function of the total photon dose that the samples had been exposed to. A specific arrangement of multi-sample switching and biasing allowed for the sequential measurement of each sample during beam operation, while reducing the interference between adjacent samples.
All the surfaces exhibited conditioning induced by the synchrotron radiation, with the PEY gradually decreasing for higher photon doses. The PEY values of the bare Cu OFE sample (LHC standard) were the highest, generally ranging between ~0.12 and 0.14, and a clear surface conditioning occurred to 0.17. In contrast, all a-C coated samples showed significantly reduced electron emission and better operational stability. After only slight conditioning, the 15 nm a-C/Cu coatings had PEY values of approximately 0.09–0.10, whereas the 100 nm a-C/Ti/Cu configuration delivered the best overall performance, with the PEY dropping from around 0.07 to about 0.065 during irradiation, accompanied by smooth and stable conditioning behaviour. The measurements also showed good reproducibility between independently coated 15 nm a-C prepared samples.
These results give experimentally relevant input for assessing surface treatments with low electron emission aimed at reducing the electron cloud in future accelerator beam screens. The ongoing BESTEX program also serves as a preparation platform for future PSD studies, using FCC-ee type vacuum chambers under synchrotron radiation exposure at KARA.