The effect of ionization on the populations of excited levels of C IV and C V in tokamak edge plasmas

TL;DR

This paper investigates how ionization processes influence the populations of excited states of C IV and C V ions in tokamak edge plasmas, especially during transient impurity influx events, using atomic calculations.

Contribution

It provides the first detailed analysis of ionization effects on excited level populations in tokamak edge plasmas, including new cross section data from distorted-wave calculations.

Findings

Ionization can significantly affect excited level populations during transient events.

Electron collisional ionization from and to excited levels is quantitatively assessed.

Results inform impurity diagnostics in tokamak edge plasmas.

Abstract

The main populating and depopulating mechanisms of the excited energy levels of ions in plasmas with densities <1023-1024 m-3 are electron collisional excitation from the ion's ground state and radiative decay, respectively, with the majority of the electron population being in the ground state of the ionization stage. Electron collisional ionization is predominately expected to take place from one ground state to that of the next higher ionization stage. However, the question arises as to whether, in some cases, ionization can also affect the excited level populations. This would apply particularly to those cases involving transient events such as impurity influxes in a laboratory plasma. An analysis of the importance of ionization in populating the excited levels of ions in plasmas typical of those found in the edge of tokamaks is undertaken for the C IV and C V ionization stages. The emphasis is on those energy levels giving rise to transitions of most use for diagnostic purposes. Carbon is chosen since it is an important contaminant of JET plasmas; it was the dominant low Z impurity before the installation of the ITER-like wall and is still present in the plasma after its installation. Direct electron collisional ionization both from and to excited levels is considered. Distorted-wave Flexible Atomic Code calculations are performed to generate the required ionization cross sections, due to a lack of atomic data in the literature.