Enhanced single-stage laser-driven electron acceleration by self-controlled ionization injection.

We report on overall enhancement of a single-stage laser wakefield acceleration (LWFA) using the ionization injection in a mixture of 0.3% nitrogen gas in 99.7% helium gas. Upon the interaction of 30-TW, 30-fs laser pulses with a gas jet of the above gas mixture, >300 MeV electron beams were generated at a helium plasma densities of 3.3-8.5 × 10(18) cm(-3). Compared with the uncontrolled electron self-injection in pure helium gas jet, the ionization injection process due to the presence of ultra-low nitrogen concentrations appears to be self-controlled; it has led to the generation of electron beams with higher energies, higher charge, lower density threshold for trapping, and a narrower energy spread without dark current (low energy electrons) or multiple bunches. It is foreseen that further optimization of such a scheme is expected to bring the electron beam energy-spread down to 1%, making them suitable for driving ultra-compact free-electron lasers.

Lateral movement of a laser-accelerated proton source on the target's rear surface.

The spatial dependence of proton acceleration at the rear surface of a target that is irradiated by high-contrast and ultraintense laser pulses is investigated. Lateral movement of the proton acceleration position at the rear surface is observed; this is tested by a two-pinhole measurement which results in the observation of protons with a narrow energy band. This drifting is only observed when relativistic-intensity laser pulses irradiate targets with a small preplasma at oblique incidence, as is confirmed by two-dimensional particle-in-cell simulations. This scenario of proton acceleration by the fast-moving sheath field leads to energy selection of the accelerated protons as a function of observing position.

Self-compression by femtosecond pulse filamentation: experiments versus numerical simulations.

We analyze pulse self-compression in femtosecond filaments, both experimentally and numerically. We experimentally demonstrate the compression of 45 fs pulses down to a duration of 7.4 fs at millijoule pulse energies. This sixfold compression in a self-generated filament does not require any means for dispersion compensation and is highly efficient. We compare our results to numerical simulations, providing a complete propagation model that accounts for full dispersion, pressure variations, Kerr nonlinearity and plasma generation in multiphoton and tunnel regimes. The equations are numerically integrated and allow for a quantitative comparison with the experiment. Our experiments and numerical simulations reveal a characteristic spectrotemporal structure of the self-compressed pulses, consisting of a compressible blue wing and an incompressible red pedestal. We explain the underlying mechanism that leads to this structure and examine the scalability of filament self-compression with respect to pulse energy and gas pressure.