- TitleEnhanced field emission from metallic surfaces and nanowires / Arti Dangwal
- Institutional NoteZugl.: Wuppertal, Univ., Diss., 2007
- Document typeDissertation (PhD)
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Metallic surfaces free from enhanced field emission (EFE) are prerequisite for getting the optimum performance of high-voltage vacuum devices such as superconducting niobium radiofrequency cavities for e⁺e⁻ accelerators. With the advancement of surface preparation and cleaning techniques, high performance of superconducting multi-cell cavities with surface fields typically up to 50 MV/m have been achieved. However, the onset of field emission at higher fields still imposes the limitation for future linear accelerators like the European X-ray Free Electron Laser and International Linear Collider. Therefore, systematic field emission investigations were performed on broad area copper and niobium cathodes by means of dc field emission scanning microscope, which has been modernized recently for fast scans. Dry ice cleaning is found to suppress EFE from polycrystalline Cu and Nb and single crystal Nb surfaces more efficiently than the conventionally used high pressure water rinsing. The cleaning effects on the emitting sites were investigated up to the fields of 250 MV/m. The number density of emitters at given fields was drastically reduced by dry ice cleaning. Fowler-Nordheim parameters are partially discussed with respect to the morphology and impurity content of the emitters localized by means of a high resolution scanning electron microscope equipped with energy dispersive x-ray spectroscopy. The microscopy results prove the effective removal of field-emitting particulates down to 400 nm as well as the partial smoothing of surface protrusions by the use of this technique. Measurements on high purity single crystal and large grain Nb samples showed the effects of surface preparation on EFE, with its onset observed at high fields (120 – 200 MV/m), due to very smooth surfaces. A low temperature (~ 150 °C) heat treatment in high vacuum for 14 hours on a selected large grain Nb sample gives the evidence for the grain boundary assisted field emission at very high fields above 250 MV/m. An interesting correlation between sizes of all investigated emitters derived from SEM images with respect to their respective onset fields has been found, which might facilitate the quality control of superconducting radio-frequency cavities for linear accelerators. Electron field emission from nanostructures has attracted wide attention in vacuum micro/nano-electronics. Besides various nanostructures, metallic nanowires have been investigated for this purpose. Copper, nickel and gold nanowires of varying wire lengths, diameters and number densities were deposited electrochemically in to the pores of etched ion-track membrane. Electric field maps on Ni nanowire cathodes show up to 10 % of deposited wires as emitters. Thin gold coating on Cu and Ni nanowires has improved the cathode emission properties in terms increased emitter number density and better emission stability. Stable Fowler-Nordheim-like emission was obtained on average up to the currents of ~ 8 µA for individual emission site on Au coated Ni nanowire sample. Au nanowire cathodes yielded up to 40% of deposited nanowires as emitters. A controlled field enhancement ß with the small spread factor (1.23) of individual emitting sites was achieved for thin Au nanowire cathode. The emission current density up to 78 mA/cm² was obtained from agglomerated Au nanowire cathode without any current saturation. Linear dependence of ß on electrode spacing d has been established for all Au nanowire cathodes.
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