Fine tuning non-uniform space charge-limited emission in high power microwave devices

Summary form only given. Two very well understood geometries for space charge-limited flow are the 1-D planar diode (as described by Child and Langmuir) and the Pierce diode (for laminar flow in a 2-D geometry). Many interesting and useful diodes, however, fall into neither of these two categories. Even the seemingly simple problem of space charge-limited emission from a finite-width emission strip in a planar diode geometry has yet to be fully described analytically. In the present work, simulations of 2-D space charge-limited emission in pseudo-planar geometries are used to provide insight regarding the non-uniformities introduced in the current density emitted from the cathode - in particular, near the edges of the emission site. These insights are shown to be a valuable predictor of performance in two separate experimental projects: - Simulations show that the current density emitted from a space charge-limited cathode can be tailored by modifying the local vacuum electric field (no space charge) at specific locations on the cathode surface. Such an idea was implemented by using "miniature" focusing electrodes to locally reduce the emitted current density inside the Air Force Research Laboratory's Magnetically Insulated Line Oscillator (MILO). Design simulations and experimental data will be presented detailing this effort to help mitigate plasma production and beam interception inside the device. Another effect seen in simulation is the ability of many small emission patches to provide a total current equivalent to a uniformly emitting surface even when the total area of the emission patches is a mere fraction of the area of the uniformly emitting surface. In AFRL's cathode development program, a variety of cathode materials have been characterized under HPM device-relevant conditions. While these cathodes have widely differing geometries at the scale of their surface structure, they were each tested in identical diode geometries with the same bulk emission area. Data that details the non-uniform current density drawn from these various cathodes will be presented and compared to the reproducibility of the total current drawn from each emitter.