UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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2004 - 20062

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Author: search.filters.author.Moody, Nathan AndrewSubject: search.filters.subject.Physics, Condensed Matter

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    Fabrication and Measurement of Regenerable Low Workfunction Photocathodes
    (2006-08-03) Moody, Nathan Andrew; O'Shea, Patrick G.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Laser-switched photoemitters are a source of electrons for high current applications such as free electron lasers. Laser-modulated photoemission permits rapid switching of the electron beam, far surpassing what can be achieved using electric-field gated emission. Photoinjector systems consist of a drive laser producing short bunches of photons and an efficient photocathode, which converts photon bunches into electron beam pulses. Development of both technologies is required, but the scope of this project is restricted to improvement of the photocathode. Most high-efficiency photocathodes employ cesium-based surface coatings to reduce work function and enable efficient electron emission in the visible range. Lifetime is severely limited by the loss of this delicate coating, which degrades rapidly in practical vacuum environments. More robust photocathodes exist, but have much lower efficiency, and place unrealistic demands on drive laser power and stability. This research proposes a novel dispenser concept that dramatically extends the lifetime of high efficiency cesium-based cathodes by continuously or periodically restoring the cesium surface monolayer during an in situ rejuvenation process. Sintered tungsten provides an interface between a cesium reservoir and the photoemitting surface. During temperature-controlled rejuvenation, cesium diffuses through and across the sintered tungsten to create and sustain a low-work function photocathode. The prototype dispenser cathode was fabricated and tested for two modes of operation: continuous and periodic near-room temperature rejuvenation. The data are compared with a photoemission model of partially covered surfaces under design for integration with existing beam simulations. Overall performance suggests that this cesium-delivery mechanism can significantly enhance the efficiency and operational lifetime of a wide variety of present and future cesium-based photocathodes. Also reported are surface characterization, ion beam cleaning, and fabrication techniques used to optimize performance of the dispenser photocathode.
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    Fabrication and Measurement of Cesiated Metal Photocathodes
    (2004-11-30) Moody, Nathan Andrew; O'Shea, Patrick G.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A requirement for accelerator applications such as free electron lasers is a high current, high quality electron beam. This is achieved using a photoinjector, where a drive laser modulates the electron emission of a cathode in an electric field. Current photocathodes are plagued with limited efficiency and short lifetime, due to contamination or evaporation of a photosensitive surface layer. An ideal photocathode would have high efficiency in the visible range, long lifetime, and prompt emission. Cathodes with high efficiency typically have limited lifetime, and vice versa. A potential solution is the dispenser cathode, where limited lifetime is overcome by periodic in situ regeneration that restores the photosensitive surface. This project prepares for fabrication of dispenser cathodes by studying properties of cesiated tungsten. A test facility was constructed and used to fabricate and test cesiated tungsten cathodes, whose behavior closely agreed with recently developed photoemission theory at the Naval Research Laboratory.