In order to meet the ambitions of exploring the fundaments of particle physics, the 2020 update of the European Strategy for Particle Physics emphasised the importance of an intensified and well-coordinated programme of accelerator R&D, supporting the design and delivery of future particle accelerators in a timely, affordable and sustainable way. Accelerating particles will always require a large amount of energy, hence achieving a minimal energy consumption is an unavoidable challenge and duty for future colliders. Indeed, the updated European Strategy stated that the energy efficiency of present and future accelerators is and should remain an area requiring constant attention, and that a detailed plan for the saving and re-use of energy should be part of the approval process for any major project. Integrating the Energy Recovery Linac (ERL) technique intrinsically in the design of especially high-energy and high-luminosity lepton accelerators is a prominent high-impact avenue to enable powerful particle beams with the goal of reaching an as low as reasonable achievable energy consumption. The objective of the ERL R&D Roadmap is to demonstrate during this decade the capability of the high-power ERL technique such as to unlock a portfolio of impactful applications in particle physics and industry.
