  Accurate cross section data for electron impact ionization (EII) are needed
in order to interpret the spectra of collisionally ionized plasmas both in
astrophysics and in the laboratory. Models and spectroscopic diagnostics of
such plasmas rely on accurate ionization balance calculations, which depend, in
turn, on the underlying rates for EII and electron-ion recombination. EII
measurements have been carried out using the TSR storage ring located at the
Max-Planck-Institut fuer Kernphysik in Heidelberg, Germany. Storage ring
measurements are largely free of metastable contamination, resulting in
unambiguous EII data, unlike what is encountered with other experimental
geometries. As it is impractical to perform experiments for every ion, theory
must provide the bulk of the necessary EII data. In order to guide theory, TSR
experiments have focused on providing at least one measurement for every
isoelectronic sequence. EII data have been measured for ions from 13
isoelectronic sequences: Li-like silicon and chlorine, Be-like sulfur, B-like
magnesium, and F-like through K-like iron. These experimental results provide
an important benchmark for EII theory.
