Preliminary physical design and simulation study of an ERL for electron cooling at EicC

Abstract: To meet the requirements of the electron cooling system for the China Electron–Ion Collider regarding high bunch charge, high repetition rate, long pulse length, low emittance, and low energy spread in the electron beam source, this paper proposes a front-end physics design scheme based on an energy recovery linac. Beam dynamics simulations and optimization studies are conducted, focusing on two core challenges: strong space charge effects and high-order nonlinear coupling. The injector employs a synergistic configuration comprising a 162.5 MHz quarter-wave superconducting RF photocathode electron gun, a 650 MHz buncher cavity, a two-cell 650MHz booster cavity, and a two-cell 1.95 GHz third harmonic cavity. A genetic algorithm is applied for global optimization of parameters such as laser spot size, pulse length, cavity phase and gradient, and solenoid magnetic field. Four typical merger section configurations are evaluated comparatively, revealing the physical mechanism by which the synergy between second-order path length coefficient and longitudinal charge density gradient leads to emittance growth in the merger section. The main accelerator section adopts a three-cavity module design. The 180° bending section in the return beamline utilizes a symmetric multi-magnet configuration to suppress high-order aberrations, while the path length adjustment section is used solely for phase matching. Results indicate that at the injector exit, the beam energy reaches 3.5 MeV, with a normalized emittance of 1.4 mm·mrad and a relative energy spread of 0.46‰. Among the merger configurations, the multi-magnet small-angle bending scheme exhibits the minimal emittance growth. At the main accelerator exit, the beam energy is 10.4 MeV, with an emittance of 2.5 mm·mrad and an energy spread of 0.47‰. Phase adjustment in the return beamline yields a theoretical energy recovery efficiency approaching 100%. Global simulations demonstrate that the beam parameters at the cooling section entrance meet the design objectives. This study demonstrates the feasibility of the physical design of an ERL under high-charge, long-bunch parameters, providing critical insights for the development of the ERL-based electron cooler for the future Electron-ion Collider in China.