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A scheme for neutron production is investigated in which an ultra-intense laser is irradiated into a two-layer (deuterium and aurum) spherical shell target through the cone shaped entrance hole. It is found that the energy conversion efficiency from laser to target can reach as high as 71%, and deuterium ions are heated to a maximum energy of several MeV from the inner layer surface. These ions are accelerated towards the center of the cavity and accumulated finally with a high density up to tens of critical density in several picoseconds. Two different mechanisms account for the efficient yield of the neutrons in the cavity: (1) At the early stage, the neutrons are generated by the high energy deuterium ions based on the "beam-target" approach. (2) At the later stage, the neutrons are generated by the thermonuclear fusion when the most of the deuterium ions reach equilibrium in the cavity. It is also found that a large number of deuterium ions accelerated inward can pass through the target center and the outer Au layer and finally stopped in the CD2 layer. This also causes efficient yield of neutrons inside the CD2 layer due to "beam-target" approach. A postprocessor has been designed to evaluate the neutron yield and the neutron spectrum is obtained. [ABSTRACT FROM AUTHOR]

A novel double cone funnel target design aiming at efficiently guiding and focusing fast electron beams produced in high intensity (>1019W/cm2) laser-solid interactions is investigated via two-dimensional particle-in-cell simulations. The forward-going fast electron beams are shown to be directed and focused to a smaller size in comparison with the incident laser spot size. This plasma funnel attached on the cone target guides and focuses electrons in a manner akin to the control of liquid by a plastic funnel. Such device has the potential to add substantial design flexibility and prevent inefficiencies for important applications such as fast ignition. Two reasons account for the collimation of fast electron beams. First, the sheath electric fields and quasistatic magnetic fields inside the vacuum gap of the double cone provide confinement of the fast electrons in the laser-plasma interaction region. Second, the interface magnetic fields inside the beam collimator further guide and focus the fast electrons during the transport. The application of this technique to cone-guided fast ignition is considered, and it is shown that it can enhance the laser energy deposition in the compressed fuel plasma by a factor of 2 in comparison with the single cone target case. [ABSTRACT FROM AUTHOR]

The energy distribution of the produced high energy electrons in the interaction of ultraintense picosecond laser pulses with high-Z solid targets is shown to be sensitive to the preformed plasma created by the prepulse and the amplified spontaneous emission pedestal. The created preformed plasmas, which are obtained by radiation hydrodynamic simulations for the present heating laser system at ILE, Osaka University, are seen to extend up to 30–100 μm just before the arrival of the main pulse. The dependences of the coupling efficiency of the laser energy to high energy electrons, and the energy spectra of these accelerated electrons, on this preformed plasma, are studied via a two-dimensional particle-in-cell simulation code. It is found that in a small preformed plasma case, J×B heating is dominant and the produced electron temperature agrees well with Haines’ scaling law [Haines et al., Phys. Rev. Lett., 102, 045008 (2009)]. While in a large preformed plasma case, in addition to J×B heating and/or vacuum heating, other acceleration mechanisms, such as stochastic heating, can accelerate electrons to very high energies, carrying a significant fraction of input laser energy. Even after several picoseconds, the number of high energy electrons (0.5 MeV

The evolution of high current electron beams in dense plasmas has been investigated by using two-dimensional particle-in-cell (PIC) simulations with immobile ions. It is shown that electron beams are split into many filaments at the beginning due to the Weibel instability, and then different filamentation beams attract each other and coalesce. The profile of the filaments can be described by formulas. Hammer et al. [Phys. Fluids 13, 1831 (1970)] developed a self-consistent relativistic electron beam model that allows the propagation of relativistic electron fluxes in excess of the Alfvén-Lawson critical-current limit for a fully neutralized beam. The equilibrium solution has been observed in the simulation results, but the electron distribution function assumed by Hammer et al. is different from the simulation results. [ABSTRACT FROM AUTHOR]

An analytical fluid model for vacuum heating during the oblique incidence by an ultrashort ultraintense p-polarized laser on a solid-density plasma is proposed. The steepening of an originally smooth electron density profile as the electrons are pushed inward by the laser is included self-consistently. It is shown that the electrons being pulled out and then returned to the plasma at the interface layer by the wave field can lead to a phenomenon like wave breaking since the front part of the returning electrons always move slower than the trailing part. This can lead to heating of the plasma at the expense of the wave energy. An estimate for the efficiency of laser energy absorption by the vacuum heating is given. It is also found that for the incident laser intensity parameter, aL>0.5, the absorption rate peaks at an incident angle 45°-52° and it reaches a maximum of 30% at aL≈1.5. [ABSTRACT FROM AUTHOR]

The evolution of electron beams in dense plasmas has been investigated by using two-dimensional particle-in-cell simulations with mobile and immobile ions, respectively. It is shown that electron beams are split into many filaments at the beginning due to the Weibel instability, and then different filamentation beams attract each other and coalesce. It is observed that there exist two different current density distributions, the Bennett-like equilibrium and the delta-like equilibrium, which are mainly determined by the total beam current. When the initial beam current exceeds Alfvén current, the system approaches a delta-like state. Otherwise, the system approaches a Bennett-like state. The physical mechanism for this phenomenon is pointed out; also the temporal evolution of particle distribution function is discussed. [ABSTRACT FROM AUTHOR]

Based on the ten-moment Grad system of hydrodynamic equations, a self-consistent fluid model is presented for the generation of quasistatic magnetic fields in relativistic intense laser plasma interaction. In this model, the nondiagonal stress tensor is taken into account and the generalized vorticity is proved to be not conserved, which are different from previous ideal fluid models. In the quasistatic approximation, where the low-frequency phase speed vp is much smaller than the electron thermal speed vte, the axial magnetic field Bz and the azimuthal one Bθ are derived. It is found that the condition vp>vte used as the cold fluid approximation by previous papers is improper, where the derived Bz is incomplete and one magnetization current for Bz associated with the electron thermal motion does not appear. The profiles of both Bz and Bθ are analyzed. Their dependence on the laser intensity is discussed. [ABSTRACT FROM AUTHOR]

The excitation of terahertz radiation by stimulated Raman scattering is considered. Vlasov–Maxwell numerical simulations show that powerful terahertz radiation can be produced by cascade Raman scatterings. [ABSTRACT FROM AUTHOR]

The experiment results on spontaneous magnetic fields given by Najmudin et al. [Phys. Rev. Lett. 87, 215004 (2001)] can be explained by the kinetic model proposed by the present authors [Phys. Plasmas 8, 312 (2001)]. The dependence of the peak spontaneous magnetic field on the laser intensity is discussed. It is found the ratio of the number of thermal electrons to the total number of electrons is an important parameter. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates a self-organization process of a magnetohydrodynamic (MHD) plasma with a finite thermal conductivity, by means of a three-dimensional MHD simulation. Smoothing of the perpendicular component of electric current and the nonuniformity of thermal pressure generated by driven reconnection.