Flat Field Emitter Based on Ultrananocrystalline Diamond (UNCD) Film for SRF Technology

Statement of the problem: Finding simple solutions to electron injection into superconducting radio frequency (SRF) accelerators is a long standing problem. While energies of 30-50 MeV are achievable by SRF technology, finding an electron source, able to survive under MW electric loads and provide an average current of 1-10 mA, is important. Meeting these requirements would open a novel way to supply the U.S. high demand for medical isotopes, such as Tc-99m, using accelerator technology. The natural way to simplify and integrate SRF architecture with the electron source would be to place the source directly into the SRF cavity. It is impossible to do using photo- and thermionic cathodes. Thus, a cold and dark cathode compatible with operating temperatures of a few kelvins is highly desirable. How the problem is being addressed: We propose a simple, robust and scalable field emission cathode (FEC) fabrication technology. Our material of choice is ultrananocrystalline diamond (UNCD) in the form of a thin film. UNCD has been proven to be a highly emissive material being stable under heavy electrical and heat loads. Thus, it is suitable for high repetition rate/CW applications. We have preliminary tested a planar UNCD FEC in a normal conducting 1.3 GHz electron gun. Electron emission from the UNCD planar surface with excellent emittance, energy spread, and stability was confirmed. A peak current of ~100 mA was achieved. At high repetition rate/CW operation, this current serves as an average current estimate for SRF applications. What will be done in Phase I: In Phase I, we will create predictive models of UNCD FEC performance in SRF injectors for the electron- ion collider at Brookhaven National Lab and for Mo-99 production linac facility at Niowave Inc. Then we will design custom cathode plugs with deposited UNCD emitters on top. We will attempt synthesis of a layered hybrid superconducting system boron-doped UNCD films on Nb substrates. If successful, their superconducting transition temperature will be measured. Applications and benefits: The proposed UNCD FEC technology can greatly benefit scientific programs within DOE, such as eRHIC at Brookhaven National Lab. Commercial applications include (1) electron accelerators for rare isotope production vital for medical diagnostics to replace obsolete nuclear reactor technology and (2) compact tunable (from vacuum UV to X-ray) bright inverse Compton scattering sources, vital for biochemistry research and the semiconductor industry (UV lithography). Key words: field emission; ultrananocrystalline diamond; superconducting accelerator; rare isotopes Summary for members of congress: We plan to develop a new kind of robust planar field emission cathode based on synthetic polycrystalline diamond films that can significantly simplify RF gun architectures to drive high energy accelerator systems. This technology will become a method-of-choice for industrial and scientific applications thanks to simple and low-cost production cycle and avoidance of most of thermionic technology disadvantages.