Control of bandwidth and signal-to-noise ratio for hard-x-ray self-seeded free-electron lasers

Hard-x-ray self-seeded (HXRSS) free-electron lasers (FELs) can provide nearly fully coherent radiation pulses in the hard-x-ray domain with extremely high spectral density, which opens up new possibilities for a wide range of scientific applications such as resonant inelastic x-ray scattering, nuclear resonance scattering, and x-ray Raman spectroscopy. Spectral bandwidth and signal-to-noise ratio (SNR) are two important parameters for seeded FELs. Our theoretical and experimental studies demonstrate that the bandwidth and SNR of HXRSS FELs are closely tied to the electron beam energy profile and current profile, respectively. Nearly Fourier-transform-limited bandwidth can be realized by flattening the energy profile. For beams with asymmetric current profiles, a higher SNR can be obtained by tuning the current peak towards the head. With proper manipulation of the longitudinal phase space of the beam, decreased bandwidth and improved SNR may further enable more demanding applications requiring higher spectral resolution.