We report on the completion of the first version of a new-generation simulation code, FLASH. The FLASH code solves the fully compressible, reactive hydrodynamic equations and allows for the use of adaptive mesh refinement. It also contains state-of-the-art modules for the equations of state and thermonuclear reaction networks. The FLASH code was developed to study the problems of nuclear flashes on the surfaces of neutron stars and white dwarfs, as well as in the interior of white dwarfs. We expect, however, that the FLASH code will be useful for solving a wide variety of other problems. This first version of the code has been subjected to a large variety of test cases and is currently being used for production simulations of X-ray bursts, Rayleigh-Taylor and Richtmyer-Meshkov instabilities, and thermonuclear flame fronts. The FLASH code is portable and already runs on a wide variety of massively parallel machines, including some of the largest machines now extant.

https://iopscience.iop.org/article/10.1086/317361/pdf

Flash: An adaptive mesh hydrodynamics code for modeling astrophysical thermonuclear flashes

FLASH: Adaptive Mesh Hydrodynamics Code for Modeling Astrophysical Thermonuclear Flashes

The processes that lead to charging of dust grains in a plasma are briefly reviewed. Whereas for single grains the results have been long known, the reduction of the average charge on a grain by 'Debye screening' has only recently been discovered. This reduction can be important in the Jovian ring and in the rings of Uranus. The emerging field of gravitoelectrodynamics which deals with the motion of charged grains in a planetary magnetosphere is then reviewed. Important mechanisms for distributing grains in radial distance are due to stochastic fluctuations of the grain charge and a systematic variation due to motion through plasma gradients. The electrostatic levitation model for the formation of spokes is discussed, and it is shown that the radial transport of dust contained in the spokes may be responsible for the rich radial structure in Saturn's rings. Finally, collective effects in dusty plasmas are discussed which affect various waves, such as density waves in planetary rings and low-frequency plasma waves. The possibility of charged grains forming a Coulomb lattice is briefly described.

Dusty plasmas in the solar system

Neutron Stars 1: Equation of State and Structure

Complex (dusty) plasmas: current status, open issues, perspectives

We compute the internal energy of the classical one-component plasma, for values of the Coulomb coupling parameter $\ensuremath{\Gamma}$ between 1 and 300, by using converged Monte Carlo chains and a more accurate potential approximation than that used by Hansen. The liquid data are fitted to a simple, very accurate four-parameter formula from which the Helmholtz free energy is derived. The solid data are likewise fitted to a one-parameter formula. The intersection of the two free-energy curves gives an estimate of the fluid-solid transition at $\ensuremath{\Gamma}=168\ifmmode\pm\else\textpm\fi{}4$.

Improved equation of state for the classical one-component plasma

We calculate the internal energy of the classical one-component plasma using a Monte Carlo technique for 128, 250, 432, 686, and 1024 particles for $1l\ensuremath{\Gamma}l300$ in order to determine the effect of a differing number of particles on the thermodynamics. By fitting the internal energy to a function of $\ensuremath{\Gamma}$ and $N$ (the particle number), we find the free energy for both the liquid and solid for an infinite number of particles.

N dependence in the classical one-component plasma Monte Carlo calculations

A general quantum-mechanical transport equation is used to derive a kinetic equation for an electron gas which in the classical limit is not subject to the usual short-range divergence and is exact to first order in the plasma parameter. The method is based on a direct analogy with the well-known equilibrium theory of the electron gas. No arbitrary separations or cutoffs are necessary. The resulting collision integral is similar to that of Weinstock and of Frieman and Book, but the Boltzmann and Fokker-Planck terms are evaluated for the static screened Coulomb potential instead of the bare Coulomb potential. It is shown that the equation of Guernsey, although convergent, does not contain all first-order contributions in the plasma parameter, and that the equations of Weinstock and of Frieman and Book must be carefully interpreted to achieve correct results. Numerical results, given in the classical limit for the dc electrical conductivity, explicitly exhibit the dominant and nondominant terms.

Convergent Kinetic Equation for a Classical Plasma

We evaluate the low-frequency component electric microfield distribution at a charged point, i.e., the field distribution due to ions, possibly of different species, which interact through an electron-screened potential. The method employed is an adaptation of the adjustable-parameter exponential (APEX) approximation previously developed for the high-frequency component and involves a noninteracting-quasiparticle representation of the electron-screened ions designed to yield the correct second moment of the microfield distribution. The APEX results are compared to Monte Carlo simulations, and we find good agreement.

Low-frequency electric microfield distributions in plasmas.

The Monte Carlo data of Hansen for the internal energy of the classical one-component plasma in the fluid state is found to satisfy accurately a simple functional form, U/NkT = aGAMMA + bGAMMA/sup 1;4/ + c, for GAMMA > 1. The fluid static energy is very close to the bcc lattice energy of the solid, and the fluid thermal energy varies as T/sup 3;4/. Simple and accurate expressions for other thermodynamic functions for the plasma fluid are given. (AIP)

H. E. DeWitt

Papers

Improved equation of state for the classical one-component plasma

N dependence in the classical one-component plasma Monte Carlo calculations

Convergent Kinetic Equation for a Classical Plasma

Low-frequency electric microfield distributions in plasmas.

Asymptotic form of the classical one-component plasma fluid equation of state