Triple-point cathodes for high current vacuum electron devices

Abstract

Summary form only given. Recent 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. Metal-oxide junction (MOJ) cathodes have been developed and tested; these consist of hafnium oxide (HfO2) or magnesium oxide (MgO) coatings over metal (#304 stainless steel) substrates. To fabricate MOJ cathodes, high dielectric constant HfO2 coatings (or high secondary electron emission coefficient MgO coatings) are deposited by pulsed laser deposition (PLD) using a KrF laser at 248 nm and 50 J/cm2 fluence. Cathodes were tested 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. Six variations of the MOJ cathode were tested, and were compared against five baseline cases. It was 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. MOJ 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. An analytic theory is developed to assess electron multiplication in the immediate vicinity of a triple point, and predict the dielectric angles most susceptible to breakdown. These predictions are shown to agree with previous experiments. Latest experiments are exploring stacked- washer, triple-point cathodes consisting of insulating (BN) washers alternating with oxygen-free copper (OFC) washers. The stacked-washer, triple-point cathode utilizes the theory to predict dielectric angles favorable for electron emission.