Experimental Demonstration of Mode-Coupled and High-Brightness Self-Amplified Spontaneous Emission in an X-Ray...

X-ray free-electron lasers (FELs) are modern research tools with applications in multiple scientific fields. Standard x-ray FEL pulses are produced by the self-amplified spontaneous emission (SASE) mechanism. SASE-FEL pulses have high power, short duration, and excellent transverse coherence but exhibit poor temporal coherence with power and spectral profiles consisting of multiple randomly distributed spikes. Here, we present the demonstration of two modes that enhance the temporal coherence of SASE-FEL radiation: mode-coupled and high-brightness SASE. Both schemes are based on delaying the electron beam with magnetic chicanes placed between the undulator modules of the FEL facility. First, we show the generation of frequency combs with tunable peak separation via the mode-coupled SASE scheme. Second, we present a proof-of-principle demonstration of the high-brightness SASE mechanism, producing FEL pulses with a bandwidth reduction up to a factor of 3 with respect to standard SASE pulses. The demonstration was done at Athos, the soft x-ray beamline of SwissFEL, for photon energies between 500 and 600 eV. Our work will benefit current applications and may open up new research areas requiring frequency combs or narrow bandwidths.