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Title: Anode materials for high power microwave devices
Author: Gortat, Daniel Piotr
ISNI:       0000 0004 8508 1793
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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Anode Materials for High Power Microwave Devices Abstract Daniel Piotr Gortat The term directed energy relates to devices capable of generating highly focused energy sources, including lasers, particle beams and microwaves. Extensive research is continuingly conducted into such devices’ frequency spectrum in order to increase their power levels to develop new applications. However, one area of the directed energy technology has received significantly less attention due to their power usage and development costs - high power microwaves (HPM). In recent years the military research laboratories have demonstrated that HPMs can significantly upset or even destroy electronic devices within military and commercial systems prompting applications for nonlethal directed energy weaponry. Cathodes and anodes are the two critical components and the engine of the HPM which makes this application possible. The decay of these components during HPMs operation undermines its operational efficiency and is what is stopping the transition of HPMs from engineering and manufacturing development to deployment as operational weapons in the field. Significant progress has been made in the development of the electron-emitting cathode towards new materials to counter its operational wear, however, anodes were left as consumables and underdeveloped. This research is undertaken in order to increase the operational efficiency of HPM devices by developing novel anodes with minimal outgassing and secondary electron emission properties, anodes which do not erode during usage nor limit the lowest achievable pressure in an HPM device. To accomplish this, two methods have been identified: laser surface melting for outgassing reduction and laser surface structuring for secondary electron emission reduction. Localised laser surface melting allows controlled grain nucleation, grain size increase and shaping, to create a hydrogen atom diffusion barrier between the surface layer and the bulk of the treated metal with an added benefit of evacuating the hydrogen atoms from the laser surface melt layer. The tests showed that anodes of 304 stainless steel processed by a continuous wave Yb fibre laser with a wavelength of 1.064 μm and subjected to 50 keV electron bombardment showed a reduction in outgassing by approximately a factor of 4 compared to that from untreated stainless steel. This is attributed to a reduction in the number of grain boundaries which serve as trapping sites for hydrogen in stainless steel through which, when heated, rate of hydrogen atom diffusion to the surface of the anode is reduced where it recombines with another hydrogen atom into a hydrogen gas molecule for outgassing. The current manufacturing method for HPM anodes is baking or vacuum annealing. Laser treated anodes do not require post-processing to preserve the benefits of the treatment and are excellent candidates for use in high power microwave devices. For secondary electron emission reduction, laser surface structuring via pulsed laser irradiation was implemented to create secondary electron traps in the areas of the anode which are directly exposed to the electron beam. Pulsed laser irradiation is a high precision surface structuring tool which allowed to design two types of traps: laser-induced periodic surface structures and high aspect-ratio holes. Both increase the surface roughness of the anode increasing the number of sites for the secondary electron absorption. Laser-induced periodic surface structuring stopped the secondary electron emission (secondary electron yield = (secondary electrons)/(primary electrons) < 1) with the highest secondary electron yield measured to be 0.84 for treated 316 SS, which is lower that C-based materials. Maximum secondary electron emission reduction with high-aspect ratio holes achieved was 0.98 on 304 SS with 0.52 % porosity, percentage of holes per surface area. Increasing the porosity reduces the secondary election yield further. The work conducted led to a novel one-step manufacturing technique for treatment of metals for vacuum applications. Commercial applications include but are not limited to: laser surface melting of steel chambers for ultra/extreme high vacuum systems (particle accelerators, etc.), analytical techniques (XPS, SIMS, SEM, FIB, etc.). Additionally, a single crystal grain refinement and a security marking techniques have been established for polycrystalline metals. Single crystals were achieved after three consecutive passes with constant laser parameters throughout the length of the laser raster at 19.17 kJ/cm2 average energy density by a continuous wave non-polarised fibre laser with a wavelength of 1.064 μm at 300 K, 0.1 % oxygen environment. The depth of localised grain nucleation was measured to be ~20 m for a single pass, making hidden messages in the bulk of the material possible after mechanically removing the immediate surface melt. The patterns are undetectable by conventional optical microscopy but can be viewed with differential interference contrast microscopy or grain imaging tools (FIB, EBSD), this way establishing a metal security marking technique. Several areas have been outlined for future investigations of the benefits of laser surface melting: reduction of hydrogen embrittlement for mechanical failures in metals for increased strength of structural metals, electrical conductivity increase, substrates for chemical vapour deposition growth of graphene and single crystal grain refinement in high thermal conductivity metals, e.g. copper. Laser drilling high aspect-ratio holes has been chosen for secondary electron emission reduction of metal anodes in HPMs due to the evidence of laser-induced periodic surface structuring method nucleating the surface grains and disrupting the laser surface melt treatment. Laser drilling for high aspect-ratio holes is being added to the patent on Reduced Hydrogen Outgassing and Secondary Electron Emission HPM Anode Manufacture. The identified direction for further work includes: conical and reverse conical boreholes for reducing secondary election emission during an angular e-beam impact on the anode and microstructural transformation after laser drilling.
Supervisor: Sparkes, Martin ; O'Neill, William Sponsor: U.S. Air Force Office of Scientific Research (AFOSR) ; Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Grain boundaries ; Laser processing ; Metals and alloys ; Vacuum electronics ; Hydrogen ; Outgassing ; High Power Microwaves ; Secondary electron yield ; empirical model ; microporous surface ; Monte Carlo simulation ; Secondary electron emission ; Laser surface melting ; microstructure single crystal security marking