Zheng Chun-Yang

PIC-MC Code to Model Fast Electron Beam Transport Through Dense Matter

A PIC (particle-in-cell)-MC (Monte Carlo) code to model electron beam transport into dense matter is developed. The background target is treated as a cold, stationary fluid and the fast electrons as particles with the relativistic motions. The process is described by a particle-in-cell method with consideration of the influence of both the self-generated electric and magnetic fields as well as collisions between the fast electrons and the target. The collisional part of the code is solved by the Monte Carlo-type method. Furthermore by assuming that the background current balances with the fast electron current, the electric field is given by the Ohms law and the magnetic field is calculated from the Faradays law. Both are solved in a two-dimensional cylindrical geometry. The algorithms implemented in the code are demonstrated and the numerical experiments are performed for monoenergy homogeneous fast electron beam transport in an aluminum target when the fields, collision and angular scattering are switched on and off independently.

Effects of Electron-Ion Collisions on Stimulated Raman Backward Scattering Under Different Electron Densities

Effect of electron-ion collision on stimulated Raman backward scattering (SRBS) spectrum are investigated by numerical simulations. In the given parameters and plasma condition, the growth rates of SRBS are found to strongly depend on the electron density, and the gap in the SRBS spectrum corresponding to the high electron density could be explained by the collisional damping. In the low density region, a much higher Landau damping estimated by the linear theory makes the collisional damping negligible. However, the present results show that, collisions play a even more important role than known in the linear theory.

Three-Dimensional PIC-MC Modeling for Relativistic Electron Beam Transport Through Dense Plasma

We have developed a three dimensional (3D) PIC (particle-in-cell)-MC (Monte Carlo) code in order to simulate an electron beam transported into the dense matter based on our previous two dimensional code. The relativistic motion of fast electrons is treated by the particle-in-cell method under the influence of both a self-generated transverse magnetic field and an axial electric field, as well as collisions. The electric field generated by return current is expressed by Ohms law and the magnetic field is calculated from Faradays law. The slowing down of monoenergy electrons in DT plasma is calculated and discussed.

Quasistatic Magnetic Field Generation by an Intense Ultrashort Laser Pulse in Underdense Plasma

The quasistatic magnetic field created in the interaction of intense ultrashort laser pulses with underdense plasmas has been investigated by two-dimensional particle simulation. The relativistic ponderomotive force and plasma wave excited in self-modulation processes can drive intense electron current mainly in the propagation direction. As a result, an azimuthal, multi-mega Gauss order quasi-static magnetic field can be generated around the laser beam.

Spatiotemporal Evolution of Stimulated Raman Scattering in the Absolute and Convective Regimes

A three-wave interaction (3WI) code is developed to study the stimulated Raman scattering (SRS) in both absolute and convective regimes. In the simulations, the time and spatial evolutions of a plasma wave are described by temporal growth rate and spatial factor, respectively. The spatial factors in different phases and different instability regimes are investigated. It is found that the spatial factor is caused by the finite velocity of the pump wave in the first phase and by damping in the last phase. With inclusion of the spatial factor, the temporal growth rate decreases and the threshold for SRS for a finite frequency mismatch increases. Meanwhile, the effects of wave frequency mismatch on the temporal growth rate are also discussed.

Simulations of Stimulated Raman Scattering in Low-Density Plasmas

Stimulated Raman scattering (SRS) in a low-density plasma slab is investigated by particle-in-cell (PIC) simulations. The backward stimulated Raman scattering (B-SRS) dominates initially and erodes the head of the pump wave, while the forward stimulated Raman scattering (F-SRS) subsequently develops and is located at the rear part of the slab. Two-stage electron acceleration may be more efficient due to the coexistence of these two instabilities. The B-SRS plasma wave with low phase velocities can accelerate the background electrons which may be further boosted to higher energies by the F-SRS plasma wave with high phase velocities. The simulations show that the peaks of the main components in both the frequency and wave number spectra occur at the positions estimated from the phase-matching conditions.

Particle simulation on electron acceleration process by the laser ponderomotive force in inhomogeneous underdense plasma layers

The mechanism of electron ponderomotive acceleration due to increasing group velocity of laser pulse in inhomogeneous underdense plasma layers is studied by two-dimensional relativistic parallel particle-in-cell code. The electrons within the laser pulse move with it and can be strongly accelerated ponderomotively when the duration of laser pulse is much shorter than the duration of optimum condition for acceleration in the wake. The extra energy gain can be attributed to the change of laser group velocity. More high energy electrons are generated in the plasma layer with descending density profile than that with ascending density profile. The process and character of electron acceleration in three kinds of underdense plasma layers are presented and compared.

Scattering of light waves by electron electrostatic waves in laser produced plasmas

The propagation of light waves in an underdense plasma is studied using one-dimensional Vlasov–Maxwell numerical simulation. It is found that the light waves can be scattered by electron plasma waves as well as other heavily and weakly damping electron wave modes, corresponding to stimulated Raman and Brilluoin-like scatterings. The stimulated electron acoustic wave scattering is also observed as a high scattering level. High frequency plasma wave scattering is also observed. These electron electrostatic wave modes are due to a non-thermal electron distribution produced by the wave–particle interactions. The collision effects on stimulated electron acoustic wave and the laser intensity effects on the scattering spectra are also investigated.

Laser plasma instability in indirect-drive inertial confinement fusion

In indirect-drive inertial confinement fusion (ICF), the incident laser beam could excite laser plasma instabilities (LPI) such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and two plasmon decay (TPD) besides gently heat the hohlraum through collisional absorption. These instabilities would largely reduce the X-ray conversion and degrade the drive symmetry of the radiation environment. In addition, when the amplitude of parametric instability increases to a certain level, there would be interplay between different instabilities, which makes LPI complicated and unpredictable. Therefore, LPI has become one of the major challenge in achieving ignition. LPI research during recent few years made great strides in identifying, understanding, and controlling instabilities in the context of laser fusion. This paper reviews the progress in this important field according to laser (L), plasma (P), and instability (I). Prospects for the application of our improved understanding for indirect drive ICF and some exciting research opportunities are also discussed.

Effects of Beam Energy Spread on the Weibel Instability in Fast Ignitor Scenarios

The effects of transverse temperature distribution on the Weibel instability in a laser produced plasma are studied analytically by using a three dimensional waterbag model. It is found that the purely transverse Weibel instability can be stabilized for the case in which the electron beam has a more symmetric transverse temperature distribution. This analytical expectation is supported by our two dimensional particle-in-cell simulations.