Laser Fabrication of High-K Dielectrics for High Current Cathodes.

Abstract

Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. Recent work at the University of Michigan has focused on the electric field distribution at a triple point of a general geometry, as well as the electron orbits in its immediate vicinity. We calculate the orbit of the first generation electrons, the seed electrons. It is found that, despite the mathematically divergent electric field at the triple point, significant electron yield most likely results from secondary electron emission when the seed electrons strike the dielectric. The analysis gives the voltage scale in which this electron multiplication may occur. It also provides an explanation on why certain dielectric angles are more favorable to electron generation over others, as observed in previous experiments.

To leverage triple point emission, we fabricate metal-oxide junction (MOJ) cathodes consisting of dielectric “islands” over stainless steel substrates. The two dielectrics used are hafnium oxide (HfO2) for its high dielectric constant, or magnesium oxide for its high secondary electron emission coefficient. The coatings are deposited by ablation-plasma-ion lithography using a KrF laser (0-600 mJ @ 248 nm) and fluence ranging from 3-40 J/cm2. Ablation plasma plumes are characterized by optical emission spectroscopy to determine temperature and ionization state. Composition and morphology of deposited films are analyzed by Scanning Electron Microscopy coupled with X-ray Energy Dispersive Spectroscopy, as well as X-ray Diffraction.

Cathodes are tested on the Michigan Electron Long-Beam Accelerator (MELBA), with a relativistic magnetron, at parameters V=-300 kV, I=1-15 kA, and pulse-lengths of 0.3-0.5 μs. Six variations of the MOJ cathode are tested, and are compared against five baseline cases. It is found that particulate formed during the ablation process improves the electron emission properties of the cathodes by forming additional triple points. Due to extensive electron back-bombardment during magnetron operation, secondary electron emission also appears to play a significant role. Cathodes exhibit increases in current densities of up to 80 A/cm2, and up to 15% improvement in current start up time, as compared to polished stainless steel cathodes.