Mitigation of CSR induced spectral broadening in EEHG FEL

Abstract

Externally seeded schemes have offered a major leap in improving the longitudinal coherence of FELs in the EUV and soft X-ray range. The Echo Enabled Harmonic Generation (EEHG) technique creates bunching at wavelengths of a few nanometers in two stages of longitudinal phase space manipulation, each consisting of an energy modulator and a chicane. Compared to other seeding techniques, EEHG is less sensitive to electron beam phase space distortions occurring during acceleration and compression. However the strong chicane needed in the first stage makes the system vulnerable to Coherent Synchrotron Radiation (CSR), which imprints a long-wavelength energy modulation on the electron bunch after the EEHG process has been initiated. Coupled with the second dispersive section, this modulation creates a position dependent change in the final bunching wavelength. In this work we show that tailoring the second seed laser wavelength can successfully compensate for the varying energy profile induced by CSR.