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Evolution of a non-neutral cold electron–positron plasma slab is investigated. Initially the slab consists of a quasineutral plasma core bounded on both sides by layers containing only positrons (or electrons). Results from a nonperturbative, or mathematically exact, analysis of the governing fluid conservation equations and the Poisson equation show that despite their equal mass and charge magnitude, the electron and positron fronts can expand separately as well as a single fluid, and that nonlinear surface oscillations can be excited on the expansion fronts. [ABSTRACT FROM AUTHOR]

The generation of an isolated relativistic attosecond electron bunch (RAEB) from the interaction of a laser with magnetized subcritical-density plasma is proposed. Particle-in-cell simulations show that a relativistic-intensity right-hand circularly polarized laser pulse can be much shortened and amplified if the magnetic field is along the laser propagation direction and the electron cyclotron frequency is appropriately larger than the laser frequency. In this case the plasma electrons are trapped in and propagate with the laser pulse like a solitary wave. The resulting charge separation field eventually causes the pulse to split into a forward propagating and a backward propagating pulse. Thus, the electrons in the latter are strongly pulled backwards by the ponderomotive force of the tail of the original laser pulse as well as the charge separation force, and eventually acquire the form of a RAEB of several attoseconds duration with a density of more than 5 times the critical density of the pristine plasma. The properties of the RAEB can be controlled by tailoring the initial laser and plasma parameters. [ABSTRACT FROM AUTHOR]

Transport of moderate relativistic micro electron beam in overdense plasma is investigated using numerical simulation and analytical modeling. It is found that if the product of the initial beam current density jb and radius R0 is above a lower limit, which scales like , where ne is the plasma density, the beam will periodically self pinch and relax as it propagates. Otherwise the beam diverges. But if is too large, the pinching behavior becomes quite different. Such micro-beam transport behavior can be attributed to the nonlinear dynamics of the beam electrons in overdense plasma. [ABSTRACT FROM AUTHOR]

Serious wafer bending and residual stress are formed during the growth of the epi-GaN layer on a sapphire substrate due to the difference of the thermal expansion coefficients (TECs) in these two different materials. Using the theoretical analysis and a simulation model with the finite element method to describe the realistic shape for the wafer bending of epi-GaN wafers, we examine the influence of different thicknesses and TECs in the top epi-GaN layer for wafer bending reduction. Furthermore, wafer bending is also found to be worse when the process temperature and the wafer size become higher and larger. Although commercial patterned sapphire substrate can partially solve this issue, the quality of the epi-GaN layer, grown on this patterned substrate, will be impacted. In this work, the new process to reduce the wafer bending and relax the residual stress is proposed. With an additional laser treatment on the sample surface after the growth of the top epi-GaN layer on the sapphire substrate, a slight crack can provide the extra space for the relaxation of the residual stress and will not influence the GaN quality. The wafer bending can be reduced to ~37µm from the original ~45µm in the 2 inch wafer with optimized surface structure design by this treatment. [ABSTRACT FROM AUTHOR]

A relatively simple interfering-pulses-assisted laser wakefield acceleration (IPA-LWFA) scheme is proposed for enhancing the charge of the LWFA electron bunch. Prior to the short intense pump pulse, two long lowintensity auxiliary laser pulses first interact in the plasma and excite a slow electron plasma wave at the beat frequency. The weak but finite-amplitude plasma wave energizes the affected electrons and acts like a slow-moving grating. Particle-in-cell simulations show that electron trapping in the wakefield of the pump laser pulse, which arrives at a later time, can be significantly enhanced. The charge of the IPA-LWFA electron bunch depends mainly on the intensity of the auxiliary pulses and the time delay of the pump laser. [ABSTRACT FROM AUTHOR]

Using fluid theory, the properties of voids in collision-dominated plasmas containing negative ions are studied. The profiles of the charged-particle densities in the void region are obtained. It is also shown that with an increase of the negative-ion concentration, the electric field, the ion drift velocity, the dust charge at the void edge, as well as the void size decrease. [ABSTRACT FROM AUTHOR]

Space weather is a relatively new and important field of research. It is relevant to diverse topics such as radio communication, space travel, diagnostics of ionospheric and space plasmas, detection of pollutants and re-entry objects, prediction of terrestrial weather and global warming. Recently it has been shown that nano- and micrometre-sized electrically charged particulates from interplanetary space and from the Earth's atmosphere can affect the local properties as well as the diagnostics of the interplanetary, magnetospheric, ionospheric and terrestrial complex plasmas. In this report the sources of the charged dust particulates and the effects of the latter on the near-Earth space weather are examined. [ABSTRACT FROM AUTHOR]

A scheme for generating gamma photons from laser interaction with a hollow-tube target filled with near-critical-density plasma is investigated using quantum electrodynamics particle-in-cell simulation. The solid-density tube wall stabilizes the laser propagation in the tube, guides the laser-accelerated relativistic electrons, and its endplate reflects the laser light. As the electrons collide head-on with the reflected laser, a large number of MeV gamma photons are generated through inverse Compton scattering. It is found that bright gamma radiation with power up to 1.4 PW and 37% laser-to-gamma ray energy conversion efficiency can be obtained with 5 × 1022 W cm–2 laser pulse. [ABSTRACT FROM AUTHOR]

A scheme for controlling the divergence and direction of an energetic proton beam driven by intense laser pulse is proposed. Simulations show that a precisely directed and collimated proton bunch can be produced by a sub-picosecond laser pulse interacting with a target consisting of a thin solid-density disk foil with a solenoid coil attached to its back at the desired angle. It is found that two partially overlapping sheath fields are induced. As a result, the accelerated protons are directed parallel to the axis of the solenoid, and their spread angle is also reduced by the overlapping sheath fields. The proton properties can thus be controlled by manipulating the solenoid parameters. Such highly directional and collimated energetic protons are useful in the high-energy-density as well as medical sciences. [ABSTRACT FROM AUTHOR]

Generation of ultra-short betatron x-rays by laser-accelerated electron beams is of great research interest as it has many applications. In this paper, we propose a scheme for obtaining bright betatron x-rays by applying external wiggler magnetic field in the laser wakefield to resonantly drive the betatron oscillations of the accelerated electrons therein. This results in a significant enhancement of the betatron oscillation amplitude and generation of bright x-rays with high photon energy. The scheme is demonstrated using two-dimensional particle-in-cell simulation and discussed using a simple analytical model. [ABSTRACT FROM AUTHOR]