Atomic physics and ionization balance of high-Z Ions: Critical ingredients for characterizing and understanding high-temperature plasmas

Abstract

A correct description of the ionization balance of high-Z ions, i.e., the fractional abundance of different charge states of a particular element, is critical for interpreting diagnostic line ratios, for describing radiation flow within and out of the plasma, and for assessing the energy balance of high-temperature plasmas. Despite the fundamental nature of this atomic physics parameter, the ionization balance as a function of plasma electron density and electron temperature is generally rather poorly known both experimentally and theoretically. This is the case irregardless of the type of plasma, i.e., low-density astrophysical and coronal plasmas, intermediate density magnetically confined fusion plasmas, or high energy density (HED) laboratory plasmas. We illustrate these points by reviewing existing measurements of the ionization balance of high-Z elements in high-temperature plasmas and discussing the difficulties associated with ionization balance calculations. We focus on the two high-Z elements tungsten (Z=74) and gold (Z=79), which play important roles in magnetic fusion and HED plasmas and for which we present calculations of the ionization energies for each charge state. We show that the accuracy of the treatment of dielectronic recombination is currently a limiting factor in proper modeling of the ionization balance and propose “differential” measurements that determine the amount of dielectronic recombination concurrently with the ionization balance as the next step for advancing ionization balance modeling. As an example of the differential measurement approach, we present recent experiments carried out at the Livermore electron beam ion trap, which investigated the ionization balance of gold in the presence of beam-resonant dielectronic recombination. These plasmas limit the immense complexity of dielectronic recombination resonances so that both approximate and reasonably complete detailed calculations can be compared to the measurements. This represents a path forward for constructing and selecting those approximations that produce the most reliable ionization balance models.