  We report ionization cross section measurements for electron impact single
ionization (EISI) of Fe^11+$ forming Fe^12+ and electron impact double
ionization (EIDI) of Fe^11+ forming Fe^13+. The measurements cover the
center-of-mass energy range from approximately 230 eV to 2300 eV. The
experiment was performed using the heavy ion storage ring TSR located at the
Max-Planck-Institut fur Kernphysik in Heidelberg, Germany. The storage ring
approach allows nearly all metastable levels to relax to the ground state
before data collection begins. We find that the cross section for single
ionization is 30% smaller than was previously measured in a single pass
experiment using an ion beam with an unknown metastable fraction. We also find
some significant differences between our experimental cross section for single
ionization and recent distorted wave (DW) calculations. The DW Maxwellian EISI
rate coefficient for Fe^11+ forming Fe^12+ may be underestimated by as much as
25% at temperatures for which Fe^11+ is abundant in collisional ionization
equilibrium. This is likely due to the absence of 3s excitation-autoionization
(EA) in the calculations. However, a precise measurement of the cross section
due to this EA channel was not possible because this process is not
distinguishable experimentally from electron impact excitation of an n=3
electron to levels of n > 44 followed by field ionization in the charge state
analyzer after the interaction region. Our experimental results also indicate
that the double ionization cross section is dominated by the indirect process
in which direct single ionization of an inner shell 2l electron is followed by
autoionization resulting in a net double ionization.
