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 (lambda3) with underdense plasma by means of real-geometry particle-in-cell simulations. Underdense plasma irradiated by relatively low-energy lambda3 (and lambda2) laser pulses is shown to be an efficient source of multi-MeV electrons, approximately 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 lambda3 pulses (further electron acceleration driving by lambda2 pulses runs in the usual way). With an increase of pulse energy, wiggling and electron-hose instabilities in the lambda3 pulse wake are recognized in the blowout regime. For higher-energy lambda3 pulses, the injected beams are well modulated and may serve as a good source of coherent x rays.