贺贤土院士从事科学研究工作60周年暨激光聚变相关研究进展
Finding ways to suppress stimulated Raman scattering(SRS) and stimulated Brillouin scattering(SBS) is an important subject in inertial confinement fusion(ICF). Under the typical laser fusion plasma conditions, the effects of transverse magnetic field and laser bandwidth on the suppression of SRS and SBS are studied by utilizing the particle-in-cell(PIC) program. It is found that the transverse magnetic field can significantly inhibit the autoresonance of SRS in the non-uniform plasma. The transverse magnetic field accelerates the electrons trapped in the electron plasma wave(EPW) excited by SRS, which causes the nonlinear damping and reduces the nonlinear frequency shift of the EPW, thus narrowing the autoresonance space and significantly reducing the SRS reflectivity. Based on the characteristics that SRS can be suppressed by a transverse magnetic field and the SBS growth is sensitive to the laser bandwidth, a scheme is proposed that utilizing the transverse magnetic field and the broadband laser to suppress SRS and SBS simultaneously. When the transverse magnetic field is on the order of 10 T and the bandwidth is on the order of 0.001, both of which are easy to achieve in experiments, the total reflectivity of SRS and SBS can be effectively suppressed in the ICF-related plasmas.
This work introduces our recent research works on the propagation of intense laser and transport of relativistic electron beam in inhomogeneous plasmas. First of all, we investigated the propagation of intense laser pulse in plasma with randomly uneven density distribution and found a new nonlinear branched flow regime of intense laser propagation in inhomogeneous plasma. In particular, we identify the important effects played by the laser photoionization and relativistic motion of electrons. In addition, we also investigated the plasma density gradient effect on evolution of electrostatic wave excited by ultra-relativistic electron beam in inhomogeneous plasma. It is found that the local region wavenumber and phase velocity of the excited wave varies with time because of the plasma inhomogeneity. Independent of the positive and negative density gradient, the wavenumber finally increases with time. As a result, Landau damping gradually becomes dominant in the whole region of inhomogeneous plasma, and leads to transfer of the wave energy to background plasma electrons, presenting a novel energy dissipation regime caused by the plasma density gradient.
The influence of plasma effect on the shock wave and hydrodynamic instabilities is a widely concerned problem in the current research of laser fusion. However, due to the limitations of the numerical simulation methods, there is still a lack of research tools on this issue. In this work, a hybrid fluid PIC(particle-in-cell) simulation method is established tentatively. Electrons are described by a massless fluid, and multi-component ions are described by PIC method; The fluid motion is obtained by solving the equations of electro-magnetohydrodynamic, and the electromagnetic fields are obtained by solving Ohm's law and Faraday's law. Aiming at the plasma condition of laser fusion, we use hybrid fluid-PIC simulation to study the shock wave structure and its characteristics in high energy density conditions, and the influence of plasma effect on the evolution of hydrodynamic instabilities. Hybrid fluid-PIC physical modeling provides a new research method for studying the effect of plasma effect on shock wave and hydrodynamic instabilities under high energy density.
The stimulated Brillouin scattering for a bundle beam, which consists of several laser beams, is studied by using the three-dimensional large scale laser plasma interaction code (LAP3D). Several collective scattering, including the ray-retracing, shared ion acoustic wave, shared scattering light, are observed. These phenomena are explained in different parameters.
We report new progress in hybrid-drive (HD) ignition target design with a high-adiabat (>3.0) and high-velocity (>400 km·s-1). First, two-shock indirect-drive (ID) radiation temperature with lower peak 200 eV ablates and pre-compresses the capsule. Later, direct-drive lasers of power 340 TW in flat-top pulse are absorbed near critical surface, combined with the radiation to drive the implosions. The "snowplow" effect in the HD heaps low ID corona density into a high HD plasma density at the radiation ablation front where maximal HD pressure reaches over 500 Mbar. Such high pressure further drives capsule imploding with peak velocity about 424.5 km·s-1 and fuel aidabat about 3.4, and the high-velocity and high-adiabat lower the hotspot pressure required to ignition lower to about 200 Gbar at a low convergent ratio 23 to suppress hydrodynamic instabilities. 2D simulation also predicts the growth factor (GF) at hotspot is very small < 10, beneficial for a robust hotspot and further burn.
The direct drive laser is the key factor in hybrid drive inertial confinement fusion. Its drive asymmetry has a great influence on the ignition performance of nuclear fusion. Using the same laser power, the impacts of the direct drive laser focal spot size on the ignition performance of a hybrid drive model are studied. It is shown that the size of the laser focal spot is a key parameter to influence the ignition performance of the hybrid drive model. When the size of the laser focal spot is 1 500 μm, the neutron yield of the target is close to the one got by the one dimensional implosion. When the 1 400 μm laser focal spot is used, the neutron yield is 40% of the one got by the one dimensional implosion. However, it is failed to ignite for the 1 200 μm laser focal spot. The high drive asymmetry caused by the small laser focal spot can increase the adiabat in the fuel. The fuel compressibility will decrease when the adiabat is increased, which goes against creating the ignition condition. In this way, the ignition wave is weak. Meanwhile, the high drive asymmetry can lead a high perturbation of the fuel areal density. The perturbation growth of the fuel areal density can make the shell asymmetry grow greatly when the ignition wave is formed. Under the conditions of the weak ignition wave and the high fuel areal density perturbation, the fuel spike with a high density is hard to be ignited. Therefore, the positive feedback between the increase of hotspot temperature and the ignition is restrained. Meanwhile, the fuel spike can decrease the hotspot temperature. The fuel bubble can lead the hotspot expand rapidly. All these factors will make the ignition performance decrease when a small laser focal spot is used.
Fully kinetic Particle-in-cell (PIC) simulations are performed to study the structure of collisional plasma shock waves with different Mach numbers. It is found that for low-Mach shocks, the spatial gradient of physical quantities near the shock front is gentle, corresponding to small Knudsen numbers, and the plasma transport properties (e. g. viscosity, heat flux) are well described by the classical transport theory. The simulated shock structure is consistent with that obtained from the numerical solution of the two-fluid equations. With increasing Mach numbers, the spatial gradient of physical quantities near the shock front becomes steep (i. e. the Knudsen number is increased), and the influence of kinetic effects on plasma transport properties become significant. For high-Mach shocks, kinetics effects come into play mainly in the following two aspects: (1) enhanced ion viscosity and heat flux due to the precursor ions and (2) nonlocal transport effects on the electron heat flux. Kinetic effects can significantly influence the shock wave structure by changing the plasma transport properties.
A hybrid particle-in-cell/fluid method is introduced to simulate the transport of intense ion beams in the solid plasmas. A two-dimensional numerical program opic2d-hybrid has been developed and it is used to study the collective behavior of the intense proton beam transport into the polyethylene and solid aluminum targets. It is shown that intense magnetic field is self-generated by the transport of intense proton beam in solid targets. This magnetic field is of benefit to pinch the proton beam. Moreover, due to the production of substantial free-electrons by the target heating and ionization, the stopping power of solid target become weaken, thereby lengthening the proton beam range. On the contrary, the increase of target temperature will reduce the resistivity and thus inhibit the generation of magnetic field. Furthermore, the target ionization leads to much stronger ion-ion scattering effect, thus resulting in the diffusion of protons in transverse space. These effects compete with each other and determine the transport behavior of intense proton beam. At the last, the physical factors contributing to the generation of magnetic field are also analyzed. Some means to increase the strength of magnetic field have been proposed in order to realize the pinch transport of intense proton beams in solid targets.
We study the transport and energy deposition of superthermal electrons in high-density plasma, starting from the basic physics of relativistic particle collisions, considering relativistic Coulomb collisions and the collective effect, combined with the background plasma electron return and temperature equation, developed a hybrid model of the electronic relativistic Fokker-Planck equation. The finite volume algorithm of the Fokker-Planck equation in the rectangular-momentum spherical coordinate system is constructed, and the numerical algorithm and numerical simulation program are verified by calculating the energy deposition and magnetic field generation process of the monoenergetic electron beam in the high-density plasma. For evaluating of the preheat effects of superthermal electrons during the implosion process, the energy deposition processes and energy distributions in the implosion target under the single-energy and bi-Maxwellian energy spectrum are calculated.
Accurate electron heat conduction model is essential for understanding energy transport and dissipation during inertial confinement fusion and celestial burst process, and guiding the design of fusion capsule and experiment design of laboratory astrophysics. We successfully couple the non-local electron heat conduction model to the radiation hydrodynamics code, the simulation results of which in two analytical tests are basically consistent with the results calculated by the Fokker-Planck code. Applying the coupled code to the interaction between nanosecond laser and solid targets, we find that the non-local effect is mainly manifest in the time-dependent flux limited effect of the coronal critical surface in the early stage and the gradually enhanced preheating effect of the ablation surface in the late stage. These results are of great significance to clearly understand the laser energy deposition and transmission, and the generation and development of hydrodynamic instability in inertial confinement fusion, especially in direct drive.
The programming and development of the Monte Carlo code MCNRC for neutron calculation driven by intense laser with pitcher-catcher scheme are described in this study. We describe the physical models and databases utilized in the MCNRC's ion transport and neutron production processes, and we compare simulation results to experimental or other simulated data to demonstrate the validity of MCNRC. The program is applied to three neutron sources that are based on various nuclear reactions, including the proton-lithium reaction, the deuterium-lithium reaction, and the lithium-proton reaction. The results show that MCNRC has preferable simulation capabilities for these neutron sources.
The difference between quantum plasma and classical plasma is mainly reflected in the following two aspects: 1) the statistical equilibrium state of the system changes from the classical Maxwell distribution to the Fermi-Dirac distribution; 2) the single-particle quantum wave effect of electrons cannot be avoided. Corresponding to the Vlasov equation in the classical plasma, the kinetic equation of the quantum plasma is the Wigner equation, but the numerical solution of the Wigner equation is more complicated than the Vlasov equation. In this paper, we propose a new method based on flux balance and Fourier spectrum methods. The hybrid method is used to solve the Wigner-Poisson equations. This method adopts different time advancing algorithms in the coordinate and velocity spaces. Compared with the general discrete Euler method, it can significantly improve the accuracy of nonlinear simulation results. This paper investigates the behavioral changes of some common electrostatic kinetic instabilities in quantum plasmas through this method; verifies the reliability of the code through linear eigensolutions, and then simulates some nonlinear phenomena, including Nonlinear Landau damping and nonlinear saturation for two-stream instability, etc.
In this paper, the basic framework of PIMC method is firstly described in detail. In particular, four techniques developed for PIMC simulations to deal with the Fermi sign problem are reviewed. EOS data of various WDM systems obtained with PIMC simulations are then summarized, with which the advantages and disadvantages of PIMC method are illustrated. Finally, some future prospects of PIMC method are discussed.
贺贤土院士从事科学研究工作60周年暨激光聚变相关研究进展
