Subangstrom Free-Electron Laser Operation with Undulator Deflection Parameter Reaching the Subunity Regime.

X-ray free-electron lasers (XFELs) enable the exploration of matter with atomic-scale spatiotemporal resolution. Generating radiation wavelengths below one angstrom is critical for applications such as crystallography and Mössbauer studies. Until now, achieving the subangstrom regime has required electron beam energies ≥8  GeV and undulator K parameters ≥1.5. In this Letter, we demonstrate the generation of high-power subangstrom FEL radiation using a 6.2 GeV electron beam and a K parameter around unity and below. We show that exceptional electron beam brilliance-characterized by high peak current and low slice emittance-compensates for the reduced undulator coupling, enabling robust lasing. The demonstration was carried out at SwissFEL, where the subangstrom regime has already been successfully employed in user experiments. Our Letter paves the way for more compact and cost-effective accelerator designs through the optimization of electron beam quality.