Epitaxial Alkali Antimonide Photocathode Growth and Characterization

Photocathodes excel at emitting bright electron beams, but transverse momenta are unavoidably nonzero. Ultra-low transverse emittance ??,? would enable brighter, higher energy x-ray free electron lasers (FEL), better colliders, and more coherent, detailed ultrafast electron diffraction/microscopy (UED/UEM), unlocking new nanoscale discoveries. Maintaining high quantum efficiency (QE) avoids laser-induced nonlinearities. State- of-the-art is 100 pC bunches from copper (??,? 0.4 ?m). Advances towards 0.1 ?m require ultra- low emittance, high QE, cryo-compatible materials. This proposal minimizes physical and chemical roughness of high QE alkali antimonides to reduce emittance. Single crystal epitaxial growth is planned through novel substrate lattice-matching and ion-beam-assisted molecular beam epitaxy (IBA-MBE). Such cathodes are anticipated to achieve excellent electron transport favorable for high QE and low emittance due to the single or large grain crystal structure with few grain boundaries and minimal crystal defects. It is anticipated that when operated cryogenically at high field, Cs3Sb intrinsic emittance can approach a record 0.1 ?m/mm (5 meV mean transverse energy). Our objective was true epitaxial growth toward large grain or single crystal alkali antimonide photocathodes. We achieved (1) epitaxial growth on a new lattice- matched substrate with stunning 0.157 nm rms physical roughness and 4% QE in Cs3Sb, (2) the first-ever chemical roughness maps of an active alkali antimonide. (3) a near-zero calculated contribution to emittance from the measured roughness, even at higher field (≥10 MV/m), and (4) a feasible Phase II design for IBA-MBE. Based on these completed tasks and our innovative solutions we now have a clear path to product development and prototyping in Phase II. In Phase II, cryogenic 20 K gun test at 10 MV/m field and high laser fluence will demonstrate ultra-low emittance of epitaxial cathodes. Single crystal IBA-MBE will measure crystallinity, stoichiometry, roughness, and band structure. A Monte Carlo model of emission will inform analysis. We will assess compatibility with superconducting RF photoinjectors, and open a production capability. Phase II will conclude with demonstrated ability to produce a new class of ultra-low emittance electron sources, well prepared to commercialize in Phase III. The potential market for high QE, low emittance photocathodes is significant. Such sources are of broad interest to light sources and accelerators, including SRF facilities such as SLAC's LCLS-II HE, Fermilab's IARC, and RF linac based FELs for industrial applications, as well as ultrafast UED/UEM.