Efficient direct electron acceleration in the plasma channel with injection through the breaking of plasma waves generated by parametric instabilities was demonstrated experimentally and reproduced in the 2D3V PIC simulations. The electron bunch was produced using the specific plasma profile containing arbitrary sharp, ∼0.5λ, gradient at the vicinity of 0.1-0.5 critical density and a long tail of a tenuous preplasma. Such a preplasma profile was formed by an additional nanosecond laser pulse with intensity of 5×10 12 W cm −2 . In the case of optimal preplasma parameters femtosecond laser pulse with an intensity of 5×10 18 W cm −2 and an energy of 50 mJ generates a collimated electron bunch having divergence of 50 mrad, exponential spectrum with the slope of ∼2 MeV and charge of tens of pC. The charge was confirmed measuring neutron yield from Be(g, n) photonuclear reaction with threshold of 1.7 MeV. By the contrast, a ring-like electron beam with divergency of 300 mrad and significantly lower charge is generated if the prepulse intensity drops to 5×10 11 W cm −2 . The 2D PIC simulations confirmed beamed electron's acceleration in the plasma channel (so-called direct laser acceleration). This channel is formed in a long tail of teneous preplasma by the laser pulse specularly reflected from the arbitrary sharp gradient. The ring-like electron beam was attributed to the longer gradient case enlarging divergence of the reflected laser beam, preventing channel's formation and electron acceleration by the so-called vacuum laser acceleration, or VLA. We also showed that injected electrons appeared from the wave breaking of plasma waves of hybrid SRS-TPD instability for the both gradients. Electrons received an initial momentum from this breaking to be effectively injected into the plasma channel.

The efficient injection of electrons into a propagating relativistic laser pulse with normalized vector potential a 0 ∼ 2 is demonstrated numerically and experimentally in a thin plasma layer with density 0.15–0.3 of the critical value. The injection is due to the wavebreaking of parametric plasma waves. The trapped particles gain multi-MeV (up to 20 MeV) energies by the direct laser acceleration in the plasma channel formed by the laser pulse in the lower density plasma tail. Numerical calculations were supported by experiments with micron-scale films pre-evaporated by an additional nanosecond laser pulse and a TW femtosecond laser facility. The experimentally observed bunch of electrons with energy above 1.6 MeV had a divergence of ∼0.05 rad and charge of ∼50 pC measured with photoneutron Be(g,n) reaction.

Generation of terahertz (THz) radiation in the interaction of laser pulse with intensity ∼5 × 1018 W cm−2 with a controlled preplasma, created by an additional laser pulse interacting with a 16 μm film target, was studied. The mechanism of generation of THz radiation in the frequency range 1–5 THz was found to be coherent transition radiation of accelerated electrons transversing the rear plasma-vacuum boundary. Angular distribution of the THz radiation changes with the delay between main pulse and prepulse due to different regimes of electron acceleration, while THz radiation spectrum reflects the spatial size of the preplasma cloud and may be used for diagnostics purposes. THz radiation energy reaches ∼0.1 mJ in 1–5 THz spectral range, corresponding to 0.2% conversion efficiency, and increases linearly with laser pulse energy.