Arizona State University, Tempe, USA
LBNL, Berkeley, California, USA
Photocathodes that have a low mean transverse energy (MTE) are crucial to the development of compact X-ray Free Electron Lasers (XFEL) and ultrafast electron diffraction (UED) experiments. For FELs, low MTE cathodes result in a lower requirement for electron energy when lasing at a defined energy, and for a defined electron energy result in lasing at higher energy. For UED experiments, low MTE cathodes give a longer coherence length, allowing measurements on larger unit cell materials. A record low MTE of 5 meV has been recently demonstrated from a Cu (100) surface when measured near the photoemission threshold and cooled down to 30 K with liquid Helium [*]. For UED and XFEL applications that require a high charge density, the low quantum efficiency of Cu (100) near threshold necessitates the use of a high laser fluence to achieve the desired charge density [**]. At high laser fluences the MTE is limited by nonlinear effects, and therefore it is necessary to investigate near photoemission threshold at these high laser fluences. In this paper we report on nonlinear, near-threshold photoemission from a Cu (100) cathode, and its effect on the MTE.
* S. Karkare et al, Phys. Rev. Lett. 125, 054801 (2020)
** J. Bae et al, J. Appl. Phys., 124, 244903 (2018)
Private Address, Bolingbrook, USA
Arizona State University, Tempe, USA
University of Florida, Gainesville, Florida, USA
Euclid Beamlabs, Bolingbrook, USA
LBNL, Berkeley, California, USA
Photocathodes lead in brightness among electron emitters, but transverse momenta are unavoidably nonzero. Ultra-low transverse emittance would enable brighter, higher energy x-ray free-electron lasers (FEL), improved colliders, and more coherent, detailed ultrafast electron diffraction/microscopy (UED/UEM). Although high quantum efficiency (QE) is desired to avoid laser-induced nonlinearities, the state-of-the-art is 100 pC bunches from copper, 0.4 mm-mrad emittance. Advances towards 0.1 mm-mrad require ultra-low emittance, high QE, cryo-compatible materials. We report efforts towards epitaxial growth of cesium antimonide on lattice matched substrates. DFT calculations were performed to downselect from a list of candidate lattice matches. Co-evaporations achieving >3% QE at 532 nm followed by atomic force and Kelvin probe microscopy (AFM and KPFM) show ultra-low 313 pm rms (root mean square) physical and 2.65 mV rms chemical roughness. We simulate roughness-induced mean transverse energy (MTE) to predict <1 meV from roughness effects at 10 MV/m in as-grown optically thick cathodes, promising low emittance via epitaxial growth.
Arizona State University, Tempe, USA
Euclid Beamlabs, Bolingbrook, USA
LBNL, Berkeley, California, USA