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A reliable plasma diagnostics requires both accurate measurements and solid theoretical support in interpretation of the experimental results. Since our primary experimental tool is spectroscopy (in other words, study of the light emitted by atoms or ions), our theorists try to understand what are the plasma conditions that bring about the measured light emission. Here they are developing a few directions in theoretical plasma spectroscopy:
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This type of theoretical work explains how many photons leave the working volume during plasma evolution. This in turn depends on (i) probability of photon emission by an atom, (ii) how many atoms present in the upper (initial) state of radiative transition, and (iii) how many emitted photons are lost during their travel to the plasma boundary.
The first of these problems is solved with pure atomic theory. The plasma environment does affect the atom properties; however, in many cases its influence on photon emission probability is rather small and can therefore be neglected. Obviously, an access to and/or possibility to produce the reliable transition probabilities is of highest importance in the CR calculations.
In order to determine the number of atoms in specific states, one has to know what the kinetic processes affecting the atom states are and how they depend on plasma conditions. The kinetic processes, which are typically accounted for in CR calculations, are:
The transport of photons through plasma volume is generally described by complicated radiative trasnfer equations. More simple methods are often used in practial calculations, for example, the so-called escape factor approximation.
We apply collisional-radiative modeling to study behavior of very diverse plasmas, such as those in Plasma Opening Switches, Z-pinches, X-ray lasers, etc.
Line spectra are affected by the motion of plasma particles (electrons, ions, and neutral species). Therefore, by measuring the spectral line shapes, it is possible to deduce such fundamental plasma parameters as temperature and density, as well as more involved characteristics like velocity distributions, presence of electric and magnetic fields etc.
This is the simplest mechanism of line broadening, caused by the thermal motion of the light emitting particles.
Line spectra are affected by both the external electric and magnetic fields and the field distributions formed by the plasma particles (ions and electrons). There are limit cases, when one of these dominates, but evidently, there are many conditions between these limit cases.
In the low-density plasmas, for example, a non-neutral regions can exist, where the plasma particle-motion effects are negligible comparing to strong macro (external) electric fields, thus making emission spectra be affected mainly by the macro fields.
In the opposite case, the high-density regime, the plasma screening is almost entirely effective, and the main source of line broadening is the micro fields generated by the charges of the chaotically moving particles.
Space inhomogenities in the plasma parameters cause the formations and propagation of waves in the plasma. This brings to consideration another possible source of fields in plasma: the plasma waves. Turbulent fields can also develop in the plasma. The turbulent fields in plasma received recently wide attention by researchers, both theoreticians and experimentalists.
The data derived from the spectral measurements, can be very complicated and, usually, there is no straightforward way to infer the plasma parameters directly. Thus, we need to be able to calculate the spectra either analythically or using computer simulations and then compare them to the observed data in order to determine plasma properties.
Some insight in physics of Z-pinches and capillary discharges is provided by theoretical model which couples the electric circuit equation, two-temperature radiation magnetohydrodynamics (RMHD), and collisional-radiative (CR) kinetics of quantum states. The CR calculation of ionization composition in each plasma mass cell provides correct description of transport phenomena in non-equilibrium media. Direct account of unisotrpic radiation field in atomic kinetics allows for rigorous treatment of fine effects like lasing or pumping by external radiation source. At present, self-consistent calculations may by performed in 0-D approach only. The 1-D and 2-D versions of the model operate with some simplifications.
