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Manuscript Title: IONMIX: a code for computing the equation of state and radiative properties of LTE and non-LTE plasmas.
Authors: J.J. MacFarlane
Program title: IONMIX
Catalogue identifier: ABJT_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 56(1989)259
Programming language: Fortran.
Computer: VAX8600.
Operating system: VAX/VMS.
RAM: 156K words
Word size: 32
Peripherals: disc.
Keywords: Lte and non-lte, Plasma physics, Equations of state, Atomic process, Transport, Opacities, Semi-classical atomic Physics.
Classification: 19.1, 19.11.

Nature of problem:
The thermodynamic and radiative properties of hot plasmas are often required in studying a wide variety of physical phenomena. Examples of these include hydrodynamic and radiative energy transport in astrophysical and fusion plasmas. The physical conditions of plasmas considered in this paper range from a relatively high density regime, where three-body collisions dominate all atomic processes, to a low density regime where two-body radiative processes become important. In the collisionally dominated regime, the plasma is said to be in local thermodynamic equilibrium (LTE). Ocassionally, problems arise in which portions of a fluid can migrate back and forth through the LTE and non- LTE regimes. One example of this is the evolution of the background gas as it responds to an inertial confinement fusion (ICF) pellet explosion. In such problems, the effects of both two-body and three-body atomic processes must be considered simultaneously when calculating the ionization and excitation populations, as well as the plasma's radiative properties. The IONMIX code does precisely this in computing the equation of state and multigroup opacities needed to describe the absorption, emission, and transport of radiation for both LTE and non- LTE plasmas.

Solution method:
The steady-state ionization and excitation populations are calculated using detailed balance arguments. The atomic processes considered are: collisional ionization and recombination, radiative recombination, dielectronic recombination, collisional excitation and deexcitation, and radiative decay. At relatively low densities, two-body recombination is balanced by collisional ionization. At higher densities, three-body collisional recombination is dominant, in which case the LTE populations are computed using the Saha equation. Ground state ionization energies are taken from the calculations of Carlson et al., and the excitation energy levels are approximated using a modified Bohr model. After the populations are computed, the specific energy, average charge state, and pressure are readily evaluated. Thermal derivatives, such as the heat capacity, and the density derivative of the energy are computed numerically. Contributions from bremsstrahlung, photoionization, bound-bound transitions (lines), Thomson scattering, and plasma waves are included in calculating the absorption, emission, and scattering coefficients. The coefficients are calculated at a large number (^ several hundred) of well-placed photon energies, and numerically integrated to determine the Planck and Rosseland group opacities, mean opacities, and plasma cooling rates.

The IONMIX code assumes that contributions to the internal energy arising from interparticle potentials is small compared to thermal energy. This dictates that the ion densities be <= 10**20 (T/<Z>)**3 cm**-3, where T is the temperature in eV, and <Z> is the mean charge state. In addition, temperatures must be high enough ( >= 10**4 K) that molecular effects, such as vibrational and rotational contributions, can be ignored. Also, the radiation energy density is assumed to be small so that ionization and excitation occur collisionally. Maximum values for the number of gases (10), atomic number of each gas species (54), opacity groups (50), temperatures (20), and densities (20) are constrained by array sizes. However, with rather simple modifications to the code, these limits can be changed.

Unusual features:
The IONMIX code is written in FORTRAN 77 with the sole exception that namelist input is used. Two VAX system subroutines are called to provide the time and date of the calculation. Otherwise, the code is transportable to other mainframe computers.

Running time:
The CPU time required to compute all thermodynamic and radiative properties is about 0.2 seconds/(gas species)/(atomic number) element at each temperature, density point.