Emittance compensation in a return arc of an energy-recovery linac

Dilution of the electron beam emittance through a return arc is one of the critical problems in the design of an energy-recovery linac for a high-power FEL and synchrotron light sources. This study reports that the emittance dilution can be compensated by matching a beam envelope to the CSR-induced dispersion. In linear regime, this optimized design of an achromatic cell is obtained from a simple matrix calculation.