Atomic data are important for the optical diagnostics of plasmas encountered in most applications. The low density plasmas in fusion devices are not in local thermodynamic equilibrium (LTE), therefore a huge amount of atomic data must be used, which describe the plasma properties in each point by a “zero dimension” Collisional-Radiative (C-R) model.
This model takes account of the main collisional and radiative processes by means of the corresponding transition probabilities and cross-sections integrated over the prevailing distributions, often taken as maxwellian. Atomic data are entering the right hand side (RHS) of the statistical equations constituting the C-R model [17]. “Zero dimension” refers to the fact that the level populations expressed by the C-R model and obtained by resolving locally the set of the statistical equations are only valid in the considered point. Transport of the species is neglected in this case, an assumption valid only for homogeneous plasmas.
Moreover, in case that each of the left hand side (LHS) part of the statistical equations is set to zero the code is valid only for stationary plasmas. It is possible to introduce the transport effect of all ion species by an additional transport code, which takes into account the particle transfer. Such a code constitutes a type of integrated numerical experiment including plasma physics, atomic and molecular processes and plasma-wall interaction. Although we are in the process of implementing some of our atomic models into various codes, we are primarily considering to use codes of simplified hydrodynamic, plasma in cell (PIC) or hybrid type
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together with a C-R model containing a reduced number of ionization stages and excited levels, whenever the temperature is sufficiently low. In order to reduce the computational effort in such a scheme, we have to keep the total number of the considered levels to a strict minimum. Such global models can be also useful in electric propulsion, reentry and plasma reactors studies.
A collisional-radiative model, taking into account the atomic properties of all the plasma constituents, allows for calculation of the parameters which are necessary for the local optical diagnostics and for validation of the model itself. Such a model, a prerequisite for the detailed optical study of rare gas plasmas, once coupled with a convenient transport code, taking into account the geometry of the plasma and the circulation of the present species, may lead to a realistic multi-dimensional model contributing to the study of various confined plasma devices. Over the last 25 years, the Kern Forschung Zentrum in Juelich has developed, supported and applied such a transport code, the 3D Monte Carlo code EIRENE [8] with Linear and Nonlinear Applications. This code has gained a near world-wide monopoly for applications related to particle (and radiation) transport in fusion devices. Code characteristics, maturity and in particular its parallel performance are currently being re-assessed within dedicated EU projects. Special attention was paid to performance of Monte Carlo transport algorithms in iterative mode to cope with cases of non-linearity. Very few results seem to exist for this type of algorithm (dealing with statistical Monte Carlo noise) in computational science, despite the fact that such iterative schemes are increasingly important in many applications in science in general.
5. CONCLUSION
The typical examples given in Section 3 illustrate the fact that the CbA approximation calculations compared with results obtained by ab initio Aij calculations, produce results which are shown to be satisfactory for most of the applications, especially for diagnostics and modeling of Tokamak plasmas near the edge. It is evident that the results can be improved whenever experimental Aij values are available, therefore experiments are needed to improve the recommended Aij sets.
ACKNOWLEDGMENT
Two of us (KK and CB) gratefully acknowledge the unknown referee for the proposed style improvements.
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