High-current energy-recovering electron linacs

The use of energy recovery provides a potentially powerful new paradigm for generation of the charged particle beams used in synchrotron radiation sources, high-energy electron cooling devices, electron-ion colliders, and other applications in photon science and nuclear and high-energy physics. Energy-recovering electron linear accelerators (called energy-recovering linacs, or ERLs) share many characteristics with ordinary linacs, as their six-dimensional beam phase space is largely determined by electron source properties. However, in common with classic storage rings, ERLs possess a high average-current-carrying capability enabled by the energy recovery process, and thus promise similar efficiencies. We discuss the concept of energy recovery and its technical challenges and describe the Jefferson Lab (JLab) Infrared Demonstration Free-Electron Laser (IR Demo FEL), originally driven by a 35-48-Mev, 5-mA superconducting radiofrequency (srf) ERL, which provided the most substantial demonstration of energy recovery to date: a beam of 250 kW average power. We present an overview of envisioned ERL applications and a development path to achieving the required performance. We use experimental data obtained at the JLab IR Demo FEL and recent experimental results from CEBAF-ER-a GeV-scale, comparatively low-current energy-recovery demonstration at JLab-to evaluate the feasibility of the new applications of high-current ERLs, as well as ERLs' limitations and ultimate performance.