Emission uniformity and shot-to-shot variation in cold field emission cathodes

High-power microwave tubes require currents and voltages generally in excess of 1 kA and 100 kV. In the past, these system requirements led to the use of single shot machines, with repetition rates well under 1 Hz. With advances in pulsed power, the Air Force Research Laboratory recently began to investigate the performance of field emission diodes at repetition rate operation. Greater numbers of shots allowed better accuracy in measurements and the application of better statistics to experimental data. In this paper, we report on new measurements that, with better experimental accuracy, show the statistical correlation between emission uniformity and the shot-to-shot variation in diode current. We report on a comparison to particle-in-cell simulations. These comparisons show the importance of randomly occurring nonemission regions on the cathode surface in dictating the spread in current data. These results imply that uniformity, in addition to playing an important role in any electron interaction with radiation, also affects the current stability for any device using these cathodes. Finally, these experiments show that for repetition rate machines, shot-to-shot variation quantified in terms of Gaussian distributions characterized by a standard deviation and skewness, provide a diagnostic capable of inferring beam uniformity in situations where direct uniformity diagnostics prove extremely difficult or impractical.