No abstract

Laser interaction with a nanobrush target plasma is investigated at the SILEX-I laser facility [X. F. Wei et al., J. Phys. Conf. Ser. 112, 032010 (2008)] with a laser of intensity 7.9×1018 W/cm2. Highly collimated fast electron beams with yields of more than three times higher than that from the planar target can be produced. Two-dimensional particle-in-cell simulation confirms that a layered surface structure can increase the efficiency of laser energy absorption, and the resulting fast electrons are tightly collimated and guided by the plasma layers to a cross section of about the laser spot size.

Hot electrons generated by short-pulse-laser interaction with nanolayered target (NT) are investigated using two-dimensional particle-in-cell simulation. Compared to the planar target, the NT leads to more efficient conversion of laser energy to the kinetic energy of the accelerated electrons. However, the energy absorption by the NT decreases at both too-low and too-high laser intensities. At lower laser intensities it is because of the weaker electric and magnetic fields generated by the hot-electron jets and smaller relativistic skin depth. At higher laser intensities it is because of the damage or destruction of the layered structure by the laser field. On the other hand, the dependence of the conversion efficiency and hot-electron number on the duration of the (short) laser pulse and the nanolayer length is weak. Control of the hot-electron characteristics by tailoring the parameters of the laser and the NT is discussed.

Channeling by a train of laser pulses into homogeneous and inhomogeneous plasmas is studied using particle-in-cell simulation. When the pulse duration and the interval between the successive pulses are appropriate, the laser pulse train can channel into the plasma deeper than a single long-pulse laser of similar peak intensity and total energy. The increased penetration distance can be attributed to the repeated actions of the ponderomotive force, the continuous between-pulse channel lengthening by the inertially evacuating ions, and the suppression of laser-driven plasma instabilities by the intermittent laser-energy cut-offs.

The properties of the nonlinear frequency shift (NFS) especially the fluid NFS from the harmonic generation of the ion-acoustic wave (IAW) in multi-ion species plasmas have been researched by Vlasov simulation. The pictures of the nonlinear frequency shift from harmonic generation and particles trapping are shown to explain the mechanism of NFS qualitatively. The theoretical model of the fluid NFS from harmonic generation in multi-ion species plasmas is given and the results of Vlasov simulation are consistent to the theoretical result of multi-ion species plasmas. When the wave number kλDe is small, such as kλDe = 0.1, the fluid NFS dominates in the total NFS and will reach as large as nearly 15% when the wave amplitude |eφ/Te| ∼ 0.1, which indicates that in the condition of small kλDe, the fluid NFS dominates in the saturation of stimulated Brillouin scattering especially when the nonlinear IAW amplitude is large.

Backward stimulated Raman scattering (BSRS) with Langmuir decay instability (LDI) and anti-Langmuir decay instability (ALDI or anti-LDI) has been researched by Vlasov simulation. The decay productions of anti-LDI in LDI cascade and their evolution with time are demonstrated for the first time. The BSRS reflectivity will be decreased largely through LDI cascade and ALDI in the small wave-number region. Different mechanisms to saturate BSRS in CH (or H) and C plasmas have been demonstrated. The dominant saturation mechanism of BSRS in CH (or H) plasmas is LDI cascade and ALDI. However, in C plasmas, due to very weak Landau damping of ion acoustic waves, LDI cascade will promote stimulated Brillouin scattering (SBS) excitation, then SBS will compete with BSRS and saturate BSRS in the later stage. The proportion of the hot electrons is decreased largely through LDI cascade and ALDI. These results give an effective mechanism to suppress BSRS and hot electron generation in the small wave-number region, which are of important significance in the inertial confinement fusion.

By irradiating a flat Al target with femtosecond laser pulses at moderate intensities of ∼10(17)  W/cm(2), we obtained stable collimated quasimonoenergetic electrons in the specular direction but deviated somewhat toward the target normal. An associated local minimum located on the other side of the specular direction seems to indicate that the peak actually results from the deflection of the collimated electrons from their initial ejection direction. We have proposed a two-step model in which some laser-accelerated electrons are able to leave the plasma in a narrow phase-locked window of the moving wave interference pattern, and are then steered toward the target normal by the ponderomotive force of the interference field. The periodic repetition of the electron emission leads to a pulse train of collimated quasimonoenergetic electrons with subcycle duration.

Using conventional methods, a laser pulse can be focused down to around 6-8 microm, but further reduction of the spot size has proven to be difficult. Here it is shown by particle-in-cell simulation that with a hollow cone an intense laser pulse can be reduced to a tiny, highly localized, spot of around 1 microm radius, accompanied by much enhanced light intensity. The pulse shaping and focusing effect is due to a nonlinear laser-plasma interaction on the inner surface of the cone. When a thin foil is attached to the tip of the cone, the cone-focused light pulse compresses and accelerates the ions in its path and can punch through the thin target, creating highly localized energetic ion bunches of high density.