The first experimental measurements of intense (${\sim }7\times 10^{19}~ {\rm W}~ {\rm cm}^{-2}$) laser-driven terahertz (THz) radiation from a solid target which is preheated by an intense pulse of laser-accelerated protons is reported. The total energy of the THz radiation is found to decrease by approximately a factor of 2 compared to a cold target reference. This is attributed to an increase in the scale length of the preformed plasma, driven by proton heating, at the front surface of the target, where the THz radiation is generated. The results show the importance of controlling the preplasma scale length for THz production.

Collisional effects on the current-filamentation instability (CFI), accounting for the space charge effect (SCE), are investigated kinetically for a relativistic beam propagating in dense plasmas. It is shown that collisions can completely suppress the SCE in low temperature dense plasma, leading to enhancement of the CFI. This kind of decoupling mechanism is quite different from the well-known resistive mechanism [Molvig (1975). Phys. Rev. Lett. 35, 1504]. In particular, we find the present decoupling mechanism can well explain the recent numerical simulation results [Karmakar et al. (2008). Phys. Rev. Lett. doi: 101, 255001]. In the parameter regime related to the laser-solid interaction and fast ignition scenario (FIS), the CFI growth rate with SCE included is enhanced in the low plasma density region through the decoupling mechanism. In the high plasma density region, it is enhanced mainly through the resistive mechanism.

A scheme of generating ultra-intense attosecond pulses in ultra-relativistic laser interaction with under-dense plasmas is proposed. The attosecond pulse emission is caused by an oscillating transverse current sheet formed by an electron density spike composed of trapped electrons in the laser wakefield and the residual transverse momentum of electrons left behind the laser pulse when its front is strongly modulated. As soon as the attosecond pulse emerges, it tends to feed back to further enhance the transverse electron momentum and the transverse current. Consequently, the attosecond pulse is enhanced and developed into a few cycles later until the density spike is depleted out due to the pump laser depletion. To control the formation of the transverse current sheet, a non-uniform plasma slab with an up-ramp density profile in front of a uniform region is adopted, which enables one to obtain attosecond pulses with higher amplitudes than that in a uniform plasma slab.

The effects of laser incidence angle on lateral fast electron transport at front target surface, when a plasma is preformed, irradiated by intense (>1018 W/cm2) laser pulses, are studied by Kα imaging technique and electron spectrometer. A horizontally asymmetric Kα halo, resulting from directional lateral electron transport and energy deposition, is observed for a large incidence angle (70°). Moreover, a group of MeV high energy electrons is emitted along target surface. It is believed that the deformed preplasma and the asymmetrical distribution of self-generated magnetic field, at large incidence angle, play an important role in the directional lateral electron transport.

Based upon the Ammosov-Delone-Krainov ionization model, it is shown that two-color laser interaction with neutral gas generates strong ionization currents, which lead to electromagnetic emission at terahertz frequency when the gas density is at proper values. The emission efficiency depends on the difference of the phases between the fundamental and its second harmonic. The intensity ratio between the two pulses also affects the emission strength. An optimum intensity ratio has been found within our parameter region. The above ionization current theory is in agreement with one-dimensional particle-in-cell simulations with field ionization included.

In this article, we investigate the effects of foam target composition and laser parameters on deuterium ion energy spectra with particle-in-cell simulations. We find that localized electrostatic fields with multi peaks around the surfaces of lamellar layers inside foam target are induced. These fields accelerate deuterium ions from thin foam layers by restricting the flow of hot electrons. This mechanism of ion acceleration called as bulk ion acceleration generates large number of high energy deuterium ions. Deuterons inside foam target are accelerated up to 126 MeV in case of oblique optimal angle of 30° where it is much greater than the normal laser incidence energy of 88 MeV.

It is shown that an intense laser pulse can be focused by a conical channel. This anomalous light focusing can be attributed to a hitherto ignored effect in nonlinear optics, namely that the boundary response depends on the light intensity: the inner cone surface is ionized and the laser pulse is in turn modified by the resulting boundary plasma. The interaction creates a new self-consistently evolving light-plasma boundary, which greatly reduces reflection and enhances forward propagation of the light pulse. The hollow cone can thus be used for attaining extremely high light intensities for applications in strong-field and high energy-density physics and other areas.

The acceleration of a micrometer-sized plasma pellet at 100 critical densities (1023 cm−3) by consecutive application of ultra-short ultra-intense laser pulses is studied using two-dimensional particle-in-cell simulation. It is shown that due to the repeated actions of the laser ponderomotive force, a small dense plasma pellet can be efficiently accelerated, with a considerable fraction of the plasma ions accelerated to high speeds. The proposed scheme can provide a high-density flux of energetic ions, which should be valuable in many practical applications.