Advancing Field Emission Technology for High Power Injectors Operating in GHz and Beyond

As the next generation of electron injectors pushes to achieve higher gradient fields than ever before (>300 MV/m), they are driven to operate at higher frequencies (C-band through W-band). This shrinks the fabrication dimensions of these cavities, making field emission cathodes (FECs) an electron source of choice. Photoemission and thermionic sources are increasingly less suited as the complex laser transport schemes and heating source powering these injectors cannot provide the necessary beam quality and may cause damage to the cathode or the injector itself. Carbon-based FECs have dominated the field emission sources R&D portfolio at DOD and DOE for the past 30 years across various high-power vacuum electronic device activities. Compared to traditional metal cathode technology, carbon-based technology cathodes are able to produce higher charge at low electric fields. Small intrinsic electron momentum and simple fabrication means these can become a leading technology, e.g., in the case of carbon nanotubes, nanoscale emitters make them attractive for producing high brightness beams. Specifically, diamond-based cathodes can handle extreme temperature and mechanical stresses that can occur under high gradient conditions.Most promising is a unique form of diamond, ultra-nano-crystalline diamond (UNCD) due to its material and electrical properties, which include being the most conductive form of diamond due to having the largest amount of grain boundaries. This cathode material allows us to explore new frontiers of cathode physics research, revealing a new field emission mechanism that diverges from classical Fowler Nordheim, termed space charge dominated Fowler Nordheim. This form of Fowler Nordheim is space charge dominated but can surpass the 1D Child Langmuir limit and approaches the 2D limit. This is not space charge limited Fowler Nordheim. This ability to decouple the extracted current from the space charge effects allows for the production of extremely xiii bright beams. This can be achieved by expanding the current cathode testing facilities beyond L band into C band so as to access these high fields and explore the temporal dynamics of a field emission source. This will yield the new physics knowledge needed to construct the world’s first custom-built injector specifically designed for field emission sources. Furthermore, exploring other forms of diamond cathode such as Diamond Field Emitter Arrays (DFEA) yields insight into the applications of transversely shaped beams for advanced accelerator applications such as emittance exchange beam lines. DFEA’s allow for the exploration of additional materials effects on the cathode performance such as the ballast resistance. This ultimately allows the derivation of a comprehensive concept map for the field emission dynamic regimes needed for the design of RF injectors. Previously, the theoretical assumption was that everything operated under classical Fowler Nordheim without any additional contributions from other materials properties or beam effects.