  Recent work at Fermilab in collaboration with the Advanced Photon Source and
members of other national labs, designed an experiment to study the
relationship between the RF repetition rate and the average current per RF
pulse. While existing models anticipate a direct relationship between these two
parameters we observed an inverse relationship. We believe this is a result of
damage to the barium coating on the cathode surface caused by a change in
back-bombardment power that is unaccounted for in the existing theories. These
observations shed new light on the challenges and fundamental limitations
associated with scaling an un-gated thermionic cathode RF gun to high average
current machines.
