Practical Limitations of Low Mean Transverse Energy Metallic Photocathodes

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.