Metal-Oxide-Junction, Triple-Point Cathodes for High Current Vacuum Electron Devices

Previous experiments at the University of Michigan have explored the mechanism of electron emission from triple points (vacuum-conductor-dielectric interface) for application to high current cathodes. Recent experiments have fabricated metal-oxide junction (MOJ) cathodes consisting of hafnium oxide (HfO₂) coatings over metal (#304 stainless steel) substrates. High dielectric constant HfO₂ coatings are deposited by ablation-plasma-ion lithography (APIL) using a KrF laser at 248 nm and 50 J/cm² fluence. Experiments were performed on the Michigan Electron Long-Beam Accelerator (MELBA), with a relativistic magnetron, at parameters V=-300 kV, currents 1-15 kA, and pulse-lengths of 0.3-0.5 microseconds. Preliminary experiments tested three cathode configurations: 1) stainless steel cathode - no coating 2) stainless steel cathode - totally coated with HfO₂ (approximately 0.5 microns thick), and 3) stainless steel cathode - coated with patterned arrays of HfO₂ islands Experimental data show initial peak currents for the third case, HfO₂ islands, reached an average of 6 kA, which was 60% larger than either of the other two cases. Current turn-on and rise time are also significantly faster for the patterned arrays of HfO₂. Future experiments will explore the effect of another innovative cathode consisting of HfO₂ dielectric islands exposed to vacuum between two conductors, (cathode and a top metal layer). A semi-analytic theory is developed to assess electron multiplication in the immediate vicinity of a triple point.