Terawatt-scale attosecond X-ray pulses from a cascaded superradiant free-electron laser

Intense attosecond X-ray pulses are key for ultrafast nonlinear spectroscopy and diffractive imaging. Here we demonstrate the generation of terawatt-scale isolated attosecond pulses with a two-stage cascaded X-ray free-electron laser. These pulses have a median energy in excess of 100 μJ in the soft X-ray region. The temporal profile is characterized with an angular streaking measurement, revealing a maximum peak power of 1.1 TW. Our data show an increase in the average peak power of attosecond X-ray pulses by one order of magnitude over previous reported results and provide strong evidence of soliton-like superradiant behaviour in the X-ray regime. Terawatt-scale, sub-femtosecond, soft X-ray pulses have been demonstrated at the Linac Coherent Light Source using a superradiant cascaded amplification approach.