The technique of Energy Recovery Linacs ERL lies at the basis of the LHeC and FCC-eh proposals for electron and positron acceleration. Fulls details about ERLs, included in the most recent Accelerator R&D Roadmap for particle physics, can be found here. A demonstrator for the high current multiturn ERL technology required for the LHeC and FCC-eh, PERLE, is under construction at IJCLab at Orsay.
