Quasi-isochronous triple-bend achromat with periodic stable optics and negligible coherent-synchrotron-radiation effects

Abstract

In modern high-brightness electron accelerators, the goal of preserving the beam quality from the source has long been pursued to ensure the fulfillment of their scientific missions. However, when delivering the electron beam through a beam transfer line, which may consist of bending magnets, the performance of a high-brightness electron beam may be degraded by the coherent synchrotron radiation (CSR) and the residual longitudinal dispersion. It is of critical importance to preserve the transverse emittance and control the bunch length variation as well as maintain the longitudinal phase space distribution in such a beam transfer line. For a multipass beam transfer line, the achromatic cell designs with stable optics would bring great convenience. In this study, for the periodic stable symmetric triple-bend achromat designs with identical dipoles, it is discovered that the CSR-induced emittance growth and the longitudinal dispersion up to high orders can be minimized simultaneously when a certain transfer matrix relation is satisfied; to be specific, the matrix entries between the first two dipoles should follow $[{m}_{11},{m}_{21}({\mathrm{m}}^{\ensuremath{-}1})]\ensuremath{\approx}(\ensuremath{-}2,0)$. Numerical optimizations demonstrate that the normalized transverse emittance, the bunch length, and the longitudinal phase space distribution are well preserved. In addition, such a design strategy shows the potential merit of effectively mitigating the microbunching instability.