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Manuscript Title: MIXERG: an equation of state and opacity computer code.
Authors: R.R. Peterson, G.A. Moses
Program title: MIXERG
Catalogue identifier: AAHO_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 28(1983)405
Programming language: Fortran.
Computer: UNIVAC 1110.
Operating system: UNIVAC 1110 EXEC VIII.
RAM: 64K words
Word size: 36
Peripherals: disc.
Keywords: Laser physics, Semi-classical atomic, Inertial confinement, Ionization, Plasma physics, Group opacities.
Classification: 15, 19.7.

Nature of problem:
The calculation of group opacities and equations-of-state of gases is important to many areas of applied physics. Specifically, to correctly predict the propagation of heat and shock waves through gases, one must have optical data, ionization states and internal energy densities for the gas. MIXERG is a computer code which has been written to provide low cost data for mixtures of up to five gases. The optical data consists of Rosseland and Planck mean opacities or mean free paths, in single group and in multigroup form.

Solution method:
The ionization state of the mixture of gases is the first calculation done for each combination of plasma density and plasma temperature. This is done using the Saha model when the density is high and the temperature is low and is done using the Coronal model otherwise. Care is taken to insure that the transition between models is smooth. With the ionization state as a function of temperature, the plasma internal energy density, the heat capacity and the temperature derivative of the ionization state may be calculated. Once the state of the gas has been determined, MIXERG uses a semiclassical model to find the absorption coefficient of the plasma as a function of photon energy where photo- ionization, inverse bremsstrahlung, Thomson scattering, absorption by plasma waves and atomic line absorption are considered as absorption mechanisms. This absorption coefficient is integrated with a specially designed method to give the opacities of the plasma. The output data is stored in a manner which is easily readable by hydrodynamic codes such as FIRE.

Restrictions:
The ionization models used are only valid when the density is low enough that the electron wave functions are single atom wave functions. This means that the plasma density must be significantly less than the solid density. These models are also invalid if the plasma is not in local thermodynamic equilibrium. This sets a lower limit on the density. There are also limitations due to the absorption model. The most important inaccuracies occur at photon energies below 1 eV. This is particularly due to neglect of molecular states which may be dominant at these low energies but also comes from inaccuracies in the models used at these energies. This means that low energy group opacities and low radiation temperature single group opacities may be inaccurate.

Unusual features:
The MIXERG code is written in standard FORTRAN except for the manner in which the COMMON blocks are used. The COMMON blocks are listed only at the beginning of the program, where they are equated to INCLUDE statements. Thereafter, the INCLUDE statements are used to represent the COMMON blocks. The use of INCLUDE statements abbreviates the listings of a program that uses the same COMMON blocks in many subroutines, because an INCLUDE statement occupies only one line, whereas a COMMON block might occupy many lines. INCLUDE statements only one line, whereas a COMMON block might occupy many lines. INCLUDE statements only have meaning to a UNIVAC compiler, so the user may wish to replace them with the respective COMMON blocks.

Running time:
On the UNIVAC 1110 at the Madison Academic Computer Center at the University of Wisconsin, MIXERG requires 0.4 s of CPU time for each combination of plasma density, plasma temperature and radiation temperature.