Approved for public release, distribution is unlimited / Present-day high power microwave devices suffer from a lack of reliable, reproducible cathodes for generating the requisite GW-level electron beam in a vacuum. Standard explosive emission cathode pulse durations have been limited to 10's or 100's of ns due to the expansion of cathode-generated plasma and the ensuing impedance collapse that debilitates microwave output. Traditional thermionic cathodes do not suffer from this drawback of plasma generation, but have not yet been able to provide the required emission current densities explosive emission cathodes are capable of. It is expected that if the plasma could be made cooler and less dense, explosive emission would be more stable. Cesium iodide (CsI) has been found to slow the impedance collapse in many explosive emission cathodes. Herein we will experimentally examine diode impedance collapse, gas production, and cathode conditioning in an effort to perform an evaluation of explosive cathode performance in a typical thermionic electron gun environment. These results will then be used to help demarcate the parameter space over which these CsI-coated carbon fiber cathodes are viable candidates for the electron beam source in next-generation high power microwave devices. / Lieutenant, United States Navy