Collisional-radiative calculations

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:

• spontaneous transition:
  • spontaneous radiative decay
  • spontaneous radiative recombination
  • autoionization
  • dielectronic capture
• induced by an impact of free electron:
  • excitation
  • deexcitation
  • ionization (including a removal of few electrons in one collision)
  • three-body recombination
• induced by photon:
  • photoexcitation
  • photodeexcitation
  • photoionization
  • induced radiative recombination
• induced by an impact of atom:
  • excitation
  • deexcitation
  • ionization
  • charge exchange

The rates of these processes depend on electron and atom energy distribution functions (mostly but not necessarily Maxwellian), and particle densities (both electron and atom).

The transport of photons through plasma volume is generally described by complicated radiative transfer equations. More simple methods are often used in practical 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.

References

• Yu. V. Ralchenko and Y. Maron
  Accelerated recombination due to resonant deexcitation of metastable states
  J. Quant. Spectr. Rad. Transfer 71, 609 (2001).
• V. I. Fisher, D. V. Fisher, and Y. Maron
  Radiation transport and density effects in non-equilibrium plasmas
  High Energy Density Phys. 3, 283 (2007).
• E. Stambulchik, V. Bernshtam, L. Weingarten, E. Kroupp, D. Fisher, Y. Maron, U. Zastrau, I. Uschmann, F. Zamponi, E. Förster, A. Sengebusch, H. Reinholz, G. Röpke, and Yu. Ralchenko
  Progress in line-shape modeling of K-shell transitions in warm dense titanium plasmas
  J. Phys. A: Math. Theor. 42, 214056 (2009).