First-principles calculations of K-shell x-ray absorption spectra for warm dense ammonia

The x-ray absorption spectroscopy is a powerful tool for the detection of thermodynamic conditions and atomic structures on warm dense matter.Here,we perform first-principles molecular dynamics and x-ray absorption spectrum calculations for warm dense ammonia,which is one of the major constituents of Uranus and Neptune.The nitrogen K-shell x-ray absorption spectrum(XAS)is determined along the Hugoniot curve,and it is found that the XAS is a good indicator of the prevailing thermodynamic conditions.The atomic structures at these conditions are ascertained.Results indicate that the ammonia could dissociate to NH_(x)(x=0,1,or 2)fragments and form nitrogen clusters,and the ratios of these products change with varying conditions.The contributions to the XAS from these products show quite different characteristics,inducing the significant change of XAS along the Hugoniot curve.Further model simulations imply that the distribution of the peak position of atomic XAS is the dominant factor affecting the total XAS.

Simulation of the Weakly Nonlinear Rayleigh–Taylor Instability in Spherical Geometry

The Rayleigh-Taylor instability at the weakly nonlinear(WN)stage in spherical geometry is studied by numerical simulation.The mode coupling processes are revealed.The results are consistent with the WN model based on parameter expansion,while higher order effects are found to be non-negligible.For Legendre mode perturbation Pn(cos B),the nonlinear saturation amplitude(NSA)of the fundamental mode decreases with the mode number n.When n is large,the spherical NSA is lower than the corresponding planar one.However,for large n,the planar NSA can be recovered by applying Fourier transformation to the bubble/spike near the equator and calculating the NSA of the converted trigonometric harmonic.

Interface Width Effect on the Weakly Nonlinear Rayleigh–Taylor Instability in Spherical Geometry

Interface width effect on the spherical Rayleigh–Taylor instability in the weakly nonlinear regime is studied by numerical simulations.For Legendre perturbation mode Pn with wave number Kn and interface half-width L,the commonly adopted empirical linear growth rate formulaγnem(L)=γn/√1+KnL is found to be sufficient in spherical geometry.At the weakly nonlinear stage,the interface width affects the mode coupling processes.The development of the mode P2n is substantially influenced by the interface width.Moreover,the nonlinear saturation amplitude increases with the interface width.

Review of the current status of fast ignition research atthe IAPCM

We review the present status and future prospects of fast ignition(FI) research of the theoretical group at the IAPCM(Institute of Applied Physics and Computational Mathematics, Beijing) as a part of the inertial confinement fusion project. Since the approval of the FI project at the IAPCM, we have devoted our efforts to improving the integrated codes for FI and designing advanced targets together with the experimental group. Recent FI experiments [K. U. Akli et al., Phys. Rev. E 86, 065402(2012)] showed that the petawatt laser beam energy was not efficiently converted into the compressed core because of the beam divergence of relativistic electron beams. The coupling efficiency can be improved in three ways:(1) using a cone–wire-in-shell advanced target to enhance the transport efficiency,(2) using external magnetic fields to collimate fast electrons, and(3) reducing the prepulse level of the petawatt laser beam. The integrated codes for FI, named ICFI, including a radiation hydrodynamic code, a particle-in-cell(PIC) simulation code,and a hybrid fluid–PIC code, have been developed to design this advanced target at the IAPCM. The Shenguang-II upgraded laser facility has been constructed for FI research; it consists of eight beams(in total 24 kJ/3ω, 3 ns) for implosion compression, and a heating laser beam(0.5–1 kJ, 3–5 ps) for generating the relativistic electron beam. A fully integrated FI experiment is scheduled for the 2014 project.

Preface The 8th International Conference on Computational Physics (ICCP8)

The International Conference on Computational Physics(ICCP)is a series of the conference organized by the Institute of Applied Physics and Computational Mathematics(IAPCM).The 7th of this series was help in Beijing from May 17 to 20,2010,and the 8th ICCP8 was held in Hong Kong on 7-11 January,2013.The ICCP8 is organized by the IAPCM and Hong Kong Baptist University(HKBU).The conference aims to bring together researchers,scientists,engineers,and students to exchange and stimulate ideas from different disciplines,to discuss the practical challenges encountered and the solutions adopted,and to learn about the recent developments in frontiers of theory and numerical methods in computational physics.

First-Principles Calculations of Shocked Fluid Helium in Partially Ionized Region

Quantum molecular dynamic simulations have been employed to study the equation of state(EOS)of fluid helium under shock compressions.The principal Hugoniot is determined from EOS,where corrections from atomic ionization are added onto the calculated data.Our simulation results indicate that principal Hugoniot shows good agreementwith gas gun and laser driven experiments,andmaximum compression ratio of 5.16 is reached at 106 GPa.