Electron self-injection during interaction of tightly focused few-cycle laser pulses with underdense plasma

We study the interaction of short laser pulses tightly focused in a tiny volume proportional to the cube of the pulse wavelength (λ³) with underdense plasma by means of real-geometry particle-in-cell simulations. Underdense plasma irradiated by relatively low-energy λ³ (and λ²) laser pulses is shown to be an efficient source of multi-MeV electrons, ∼50 nC∕J, and coherent hard x rays, despite a strong pulse diffraction. Transverse wave breaking in the vicinity of the laser focus is found to give rise to an immense electron charge loading to the acceleration phase of a laser wake field. A strong blowout regime provoked by the injected electrons resulting in the distribution of accelerated electrons is found for λ³ pulses (further electron acceleration driving by λ² pulses runs in the usual way). With an increase of pulse energy, wiggling and electron-hose instabilities in the λ³ pulse wake are recognized in the blowout regime. For higher-energy λ³ pulses, the injected beams are well modulated and may serve as a good source of coherent x rays.