Spatio-temporal couplings for controlling group velocity in longitudinally pumped seeded soft X-ray lasers

Controlling the group velocity of an ultrashort laser pulse by means of spatio-temporal couplings has been proposed to overcome the inherent limitations in laser–plasma interactions. Here we report how this method improves the performance of a seeded soft X-ray laser (SXRL), which intrinsically suffers from group velocity mismatch between the infrared pump beam, used to generate the plasma amplifier, and the soft X-ray seed beam. The energy extraction was measured to be three times higher when the pump group velocity varied from 0.55c to 1.05c. We also demonstrate that the SXRL pulse duration is governed by the pump beam velocity. By compensating the natural group velocity mismatch, the SXRL pulse duration can be kept constant along its propagation, resulting in energetic pulses (up to 2 μJ) as short as 350 fs. Such achievements constitute an experimental demonstration of the so-called ‘flying focus effect’ in an application by controlling the group velocity of a high-intensity laser pulse propagating in a plasma.