Laser-Plasma Based Seeded Free Electron Laser in the High-Gain Regime

The realization of compact laser-plasma accelerator (LPA) driven free electron lasers (FELs) has been attempted for nearly two decades, yet, difficult to overcome electron beam quality and stability issues prevented FEL lasing until recently. LPA based FEL is now demonstrated in both the self-amplified spontaneous emission and the fully coherent seeded modes, albeit for the latter only in the low-gain regime. Here, we report on the observation of the high-gain regime in the seeded configuration, opening the path to FEL saturation. Exponential FEL signal growth was established through an increase in spectral charge density of the electron beam. Within a 2 m long undulator and with a gain length of 0.55 m, FEL pulse energies of 45 nJ were achieved at wavelengths of around 272 nm, in good agreement with quantitative numerical simulations. The effects of a systematic variation of the seed pulse duration and thus power via pulse chirping was studied in detail, a refinement up to now reserved to radio-frequency accelerator based FELs. These results encourage the future scaling to shorter wavelengths.