Report on outstanding open questions and prioritized R&D guidelines for Energy Recovery Linacs

Energy Recovery linacs (ERLs) are a class of electron accelerators which could achieve extremely high beam intensities at moderate operating costs by recovering the beam energy, i.e. the energy of the accelerated particles, after their experimental use. The future pathway goes towards multi MW facilities, calling for the establishment of the technology required for stable high current operation. Some smaller ERLs are already in existence. Further research is needed on the following topics: ERL operation, beam dynamics, injectors, Superconducting Radiofrequency (SRF) systems and ERL applications.