Shurvinton, C.L. Dunning, D.J. Thompson, N. JACoW Geneva, Switzerland 978-3-95450-182-3 10.18429/JACoW-IPAC2017-WEPAB089 English 2783-2786 WEPAB089 Contribution to a conference proceedings 2017 2017-05 http://dx.doi.org/10.18429/JACoW-IPAC2017-WEPAB089 http://jacow.org/ipac2017/papers/wepab089.pdf Ultrashort pulse operation in FELs is a highly desirable capability for imaging matter on ultrafast timescales. This paper presents a design study for a proof-­of-­principle demonstration of the mode-locked afterburner (ML-AB) scheme on the FEL test facility CLARA. A start-to-end simulation has been constructed using the time-­dependent three-­dimensional FEL code GENESIS 1.3 to evaluate the performance of the scheme. The ability to produce pulses of several femtoseconds in duration with peak powers of the order of 100 MW at 100 nm wavelength is predicted.­ Such pulses have duration of 2 fs (6 optical cycles), a factor of ~5 shorter than the FEL cooperation length. Potential routes for further optimisation and alternative operating modes are explored.