We report ionization cross section measurements for electron impact single ionization ( EISI ) of \mathrm { Fe ^ { 11 + } } forming \mathrm { Fe ^ { 12 + } } and electron impact double ionization ( EIDI ) of \mathrm { Fe ^ { 11 + } } forming \mathrm { 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 für 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 \mathrm { Fe ^ { 11 + } } forming \mathrm { Fe ^ { 12 + } } may be underestimated by as much as 25 % at temperatures for which \mathrm { Fe ^ { 11 + } } is abundant in collisional ionization equilibrium . This is likely due to the absence of 3 s 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 \geq 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 2 l electron is followed by autoionization resulting in a net double ionization .