ASU Electronic Theses and Dissertations
This collection includes most of the ASU Theses and Dissertations from 2011 to present. ASU Theses and Dissertations are available in downloadable PDF format; however, a small percentage of items are under embargo. Information about the dissertations/theses includes degree information, committee members, an abstract, supporting data or media.
In addition to the electronic theses found in the ASU Digital Repository, ASU Theses and Dissertations can be found in the ASU Library Catalog.
Dissertations and Theses granted by Arizona State University are archived and made available through a joint effort of the ASU Graduate College and the ASU Libraries. For more information or questions about this collection contact or visit the Digital Repository ETD Library Guide or contact the ASU Graduate College at gradformat@asu.edu.
Displaying 1 - 2 of 2
Filtering by
- All Subjects: Physics
- Creators: Karkare, Siddharth
Description
Producing a brighter electron beams requires the smallest possible emittance from the cathode with the highest possible current. Several materials like ordered surface, single-crystalline metal surfaces, ordered surface, epitaxially grown high quantum efficiency alkali-antimonides, topologically non-trivial Dirac semimetals, and nano-structured confined emission photocathodes show promise of achieving ultra-low emittance with large currents. This work investigates the various limitations to obtain the smallest possible emittance from photocathodes, and demonstrates the performance of a novel electron gun that can utilize these photocathodes under optimal photoemission conditions. Chapter 2 discusses the combined effect of physical roughness and work function variation which contributes to the emittance. This is particularly seen in polycrystalline materials and is an explanation for their higher than expected emittance performance when operated at the photoemission threshold. A computation method is described for estimating the simultaneous contribution of both types of roughness on the mean transverse energy. This work motivates the need for implementing ordered surface, single-crystalline or epitaxially grown photocathodes. Chapter 3 investigates the effects of coulomb interactions on electron beams from theoretically low emittance, low total energy spread nanoscale photoemission sources specifically for electron microscopy applications. This computation work emphasizes the key role that image charge effects have on such cold, dense electron beams. Contrary to initial expectations, the primary limiter to beam brightness for theoretically ultra-low emittance photocathodes is the saturation current. Chapters 4 and 5 describe the development and commissioning of a high accelerating gradient, cryogenically cooled electron gun and photoemission diagnostics beamline within the Arizona State University Photoemission and Bright Beams research lab. This accelerator is unique in it's capability to utilize photocathodes mounted on holders typically used in commercial surface chemistry tools, has the necessary features and tools for operating in the optimal regime for many advanced photocathodes. A Pinhole Scan technique has been implemented on the beamline, and has shown a full 4-dimensional phase space measurement demonstrating the ability to measure beam brightness in this gun. This gun will allow for the demonstration of ultra-high brightness from next-generation ultra-low emittance photocathodes.
ContributorsGevorkyan, Gevork Samvelovich (Author) / Karkare, Siddharth (Thesis advisor) / Padmore, Howard (Committee member) / Alarcon, Ricardo (Committee member) / Kaindl, Robert (Committee member) / Graves, William (Committee member) / Arizona State University (Publisher)
Created2023
Description
The performance of accelerator applications like X-ray free electron lasers (XFELs)and ultrafast electron diffraction (UED) and microscopy (UEM) experiments is limited
by the brightness of electron beams generated by photoinjectors. In order to
maximize the brightness of an electron beam it is essential that electrons are emitted
from photocathodes with the smallest possible mean transverse energy (MTE).
Metallic photocathodes hold the record for the smallest MTE ever measured at 5
meV from a Cu(100) single crystal photocathode operated near the photoemission
threshold and cooled to 30 K. However such photocathodes have two major limitations:
poor surface stability, and a low quantum efficiency (QE) which leads to
MTE degrading non-linear photoemission effects when extracting large charge densities.
This thesis investigates the efficacy of using a graphene protective layer in order
to improve the stability of a Cu(110) single crystalline surface. The contribution to
MTE from non-linear photoemission effects is measured from a Cu(110) single crystal
photocathode at a variety of excess energies, laser fluences, and laser pulse lengths.
To conclude this thesis, the design and research capabilities of the Photocathode and
Bright Beams Lab (PBBL) are presented. Such a lab is required to develop cathode
technology to mitigate the practical limitations of metallic photocathodes.
ContributorsKnill, Christopher John (Author) / Karkare, Siddharth (Thesis advisor) / Drucker, Jeffery (Committee member) / Kaindl, Robert (Committee member) / Teitelbaum, Samuel (Committee member) / Arizona State University (Publisher)
Created2023