Nguyen, D.C. Dunham, M. Lou, W. Mayes, C.E. Stupakov, G. JACoW Publishing Geneva, Switzerland 2673-7035 978-3-95450-224-0 10.18429/JACoW-FLS2023-TU2A2 English 70-73 undulator FEL electron simulation photon Contribution to a conference proceedings 2024 2024-01 https://doi.org/10.18429/JACoW-FLS2023-TU2A2 http://jacow.org/fls2023/papers/tu2a2.pdf SASE FELs operating in the soft X-ray region exhibit multiple temporal and spectral spikes with an overall spectral bandwidth of about 1.5 times the FEL rho parameter. While many ideas have been proposed to achieve fully coherent X-ray FELs, only monochromatic seeding, either harmonic seeding or SASE self-seeding, has been experimentally demonstrated to narrow the output spectra of soft X-ray FELs. In this paper, we study a different method that relies on the Slippage-dominated Tapered Undulator (STU) SASE concept to produce a single longitudinal mode in a soft X-ray FEL driven by ~10-fs, 16-pC electron bunches. We pre-sent numerical simulation results that demonstrate single-mode generation and narrow-lined spectra without seeding in a STU-SASE FEL at 6.67 nm.