C. Y. Zheng

Quasistatic magnetic and electric fields generated in intense laser plasma interaction

A self-consistent kinetic model based on relativistic Vlasov–Maxwell equations is presented for the generation of quasistatic spontaneous fields, i.e., both the quasistatic magnetic (QSM) field and the quasistatic electric (QSE) field, in intense laser plasma interaction. For the circularly polarized laser, QSM field includes two parts, the axial part Bz as well as the azimuthal Bθ; the QSE field Es, corresponding to the space-charge potential, forms a plasma density channel. For the linearly polarized laser, Bz is absent. Equations for Bz, Bθ, and Es are uniformly derived from one self-consistent model under the static-state approximation, which satisfies the conservation law of charge. The profile of the plasma density channel and the dependence of the peak QSM fields on the laser intensity are discussed. The experiment and simulation results are explained by the model. The predicted QSM and QSE fields are also observed in the three-dimensional particle simulation.

Magnetic field generation and relativistic electron dynamics in circularly polarized intense laser interaction with dense plasma

The generation of both axial and azimuthal magnetic field components by the intense laser-dense plasma interaction and relativistic electron dynamics are studied in theoretical analysis and three-dimensional particle-in-cell simulations. For a circularly polarized laser, the cylindrical symmetry can be applied. Interacting with this laser, both axial and the azimuthally magnetic fields are generated and play an essential role in hot electron beam collimation, stabilization, and shaping. It is significantly different from the linearly polarized case, where the azimuthal current and the axial magnetic field are absent, therefore the axial current density gets filamentary early in the ramp plasma and is focused into a single channel in the dense (with a density larger than the critical density nc) plasma later, and the electron and ion density distributions as well as the azimuthally magnetic field profile are broadened in the polarized direction to form an ellipse-like structure in the transverse plane. Ins...

Enhancement of backward Raman scattering by electron-ion collisions

The propagation of light waves in an underdense plasma is studied using one-dimensional Vlasov–Maxwell numerical simulation. It is found that the backward stimulated Raman scattering will be enhanced by electron-ion collisions. With appropriate electron-ion collision rate the Langmuir waves, driven via SRS, can be made to propagate for a long distance.

Electron acceleration in combined intense laser fields and self-consistent quasistatic fields in plasma

The acceleration of plasma electron in intense laser-plasma interaction is investigated analytically and numerically, where the conjunct effect of laser fields and self-consistent spontaneous fields (including quasistatic electric field Esl, azimuthal quasistatic magnetic field Bsθ and the axial one Bsz) is completely considered for the first time. An analytical relativistic electron fluid model using test-particle method has been developed to give an explicit analysis about the effects of each quasistatic fields. The ponderomotive accelerating and scattering effects on electrons are partly offset by Esl, furthermore, Bsθ pinches and Bsz collimates electrons along the laser axis. The dependences of energy gain and scattering angle of electron on its initial radial position, plasma density, and laser intensity are, respectively, studied. The qualities of the relativistic electron beam (REB), such as energy spread, beam divergence, and emitting (scattering) angle, generated by both circularly polarized (CP)...

The transition from plasma gratings to cavitons in laser-plasma interactions

One-dimensional Vlasov–Maxwell simulations of laser-plasma interactions are presented. It is shown that plasma gratings and density cavitons are formed sequentially. There are strong electromagnetic fields in the cavitons and the electromagnetic structures are nearly standing and long-lived. The formation of gratings and cavitons can be explained by a nonlinear second-order differential equation. The electromagnetic fields trapped in cavitons have both subcycle and cycle structures. Plasma whose density is higher than the critical density can be formed around the cavitons. Gratings and high density plasmas can reflect light in a very high level. This may be detrimental to the inertial confinement fusion.

Experimental demonstration of low laser-plasma instabilities in gas-filled spherical hohlraums at laser injection angle designed for ignition target

Octahedral spherical hohlraums with a single laser ring at an injection angle of 55^{∘} are attractive concepts for laser indirect drive due to the potential for achieving the x-ray drive symmetry required for high convergence implosions. Laser-plasma instabilities, however, are a concern given the long laser propagation path in such hohlraums. Significant stimulated Raman scattering has been observed in cylindrical hohlraums with similar laser propagation paths during the ignition campaign on the National Ignition Facility (NIF). In this Rapid Communication, experiments demonstrating low levels of laser-driven plasma instability (LPI) in spherical hohlraums with a laser injection angle of 55^{∘} are reported and compared to that observed with cylindrical hohlraums with injection angles of 28.5^{∘} and 55^{∘}, similar to that of the NIF. Significant LPI is observed with the laser injection of 28.5^{∘} in the cylindrical hohlraum where the propagation path is similar to the 55^{∘} injection angle for the spherical hohlraum. The experiments are performed on the SGIII laser facility with a total 0.35-μm incident energy of 93 kJ in a 3 nsec pulse. These experiments demonstrate the role of hohlraum geometry in LPI and demonstrate the need for systematic experiments for choosing the optimal configuration for ignition studies with indirect drive inertial confinement fusion.

Competition between stimulated Raman scattering and two-plasmon decay in inhomogeneous plasma

We demonstrate competitions between stimulated Raman scattering (SRS) and two-plasmon decay (TPD) in the laser polarization plane in inhomogeneous near quarter-critical density plasma by using linear convective gain analysis and two-dimensional (2D) particle-in-cell (PIC) simulations. Linear theoretical analysis implies that convective SRS occurs in a wider and lower density region than absolute SRS and has a shared occurrence region with convective TPD. This convective SRS prefers a parameter space with the laser intensity larger than the order of 1015u2009W/cm2 and the density scale length about several hundreds microns, which may be common in large scale direct-drive scheme, shock ignition scheme, and hybrid-drive scheme. A convective nature and saturation mechanism under these parameter regions are identified to be Langmuir decay instability and strong pump depletion. The significance of this convective SRS is shown in our 2D PIC simulations that hot electrons are reduced through suppressing the electron ...

Stimulated backward Brillouin scattering in two ion-species plasmas

Stimulated Brillouin back-scattering in mixed carbon and hydrogen plasmas is studied using one-dimensional Vlasov–Maxwell simulation. It is found that both the fast and slow ion acoustic waves can scatter the incident light. Carbon ions can be trapped in the slow ion acoustic wave, and the hydrogen ions can be trapped in both the fast and slow waves. The trapped ions tend to reduce the Landau damping of the ion acoustic waves, and both the fast and slow ion acoustic waves can be excited. From the time-integrated scattering spectra, the scattering peaks of the fast and slow ion acoustic waves can be clearly distinguished.

Simulation of electron beam instabilities in collisionless plasmas

The influence of self-generated magnetic and electric fields on the transport of relativistic electrons in dense plasmas was studied using a particle-in-cell simulation. For conditions relevant to the fast igniter, the laser-driven relativistic electrons may, have significant energy spread along or perpendicular to the propagation direction of the beams. The effect of electron energy spread on the growth rate. the occurrence threshold of both the Weibel-like filamentation instability and the electrostatic two-stream instability and the competition between them were investigated. The Weibel instability results in the formation of a magnetic channel, which may collimate inward fast electrons without significant deviation, and the excitation of a longitudinal electric field due to two-stream instability is destructive to the stability of the magnetic channel. The generation of relativistic electrons by the interaction of a high-intensity laser beam at the vacuum-dense plasma boundary and propagation of the electrons in the dense plasma were studied using a three-dimensional particle-in-cell code. It is shown that the electron velocity spread owing to transverse collective heating saturates the magnetic field and the longitudinal electrostatic field may play a dominant role in limiting the stable propagation of fast electrons into over-dense plasmas.

Surface plasma waves with their harmonics generation from pre-structured targets

Surface plasma waves with their harmonics are generated from pre-structured targets. The harmonics are generated by coherent synchrotron emission or relativistically oscillatingmirror and then resonantly amplified by surface plasma wave excitation. Two dimensional particle-in-cell simulations and the theoretical analysis show that the laser is coupled to the structured target by generating a periodic current. Some of the generatedharmonics have half integer wave numbers but integer frequencies. This interesting phenomenon is controlled by the structure period of the target.