  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.
