Progress and R/D challenges for FCC-ee SRF

Abstract The FCC-ee machines present a huge challenge for the RF systems, which need to be adapted very diverse beam conditions going from moderate energy and high current Z machine low ttbar. This inverse scaling results naturally fixed budget synchrotron radiation, SRF cavities compensate. A global solution was elaborated FCC Conceptual Design Report (Abada in Eur Phys J Spec Top 228):261–623, 2019), is referred here as baseline. Recently, further studies have led new optimized baseline, still based on traditional elliptical cavities. In parallel, novel concept, named Slotted Waveguide ELLiptical (SWELL), proposed with potential of greatly simplified logistics reduced costs. Under several aspects, all these changes call enhanced performance systems. vigorous R&D program has therefore continued since publication CDR, aim pushing demonstrating feasibility more advanced baseline and, recently, SWELL option. progress challenges this ambitious were presented dedicated sessions at week 2022 (FCC website, 2022, https://indico.cern.ch/event/1064327/timetable/ ) are summarized paper.