Na-K-Sb photocathodes-synthesis, characterization and application in photoinjector

State-of-the-art and future accelerators such as energy recovery linacs (ERLs), free electron lasers (FELs) and ultra-fast electron diffraction (UED) facilities have increasing demands on the brightness and robustness of the electron source. To meet these requirements, photocathodes are employed in a variety of material systems. Among them, multi-alkali antimonide photocathodes are promising candidates for high-brightness applications due to their high quantum efficiency (QE) and low intrinsic emittance in the visible range.

Na–K–Sb photocathodes exhibit QE values comparable to Cs–K–Sb in the visible range, reaching several percent in the green region, while offering superior thermal robustness—a critical attribute for many applications, especially ERLs. Despite these advantages, the widespread adoption of Na–K–Sb photocathodes has been hindered by poor reproducibility and low success rates, stemming from three fundamental challenges: (1) an exceptionally narrow stoichiometric window for high QE, (2) the extremely low equilibrium vapor pressure of sodium required for the high-QE Na2KSb phase, and (3) insufficient control over key deposition parameters, particularly alkali metal flux rates.

In this work, these challenges are systematically addressed through the design and commissioning of a new preparation chamber and the development of an optimized triple evaporation method. Through detailed spectral response and X-ray photoelectron spectroscopy (XPS) characterization, the reproducible growth of Na2KSb photocathodes under highly controlled conditions is demonstrated for the first time on substrates suitable for electron accelerators. A control variable approach establishes clear correlations between growth parameters, stoichiometry, and photoemissive properties. Comprehensive analysis of QE homogeneity, supported by a growth model, enables rapid and precise tuning of photoemissive performance. Furthermore, an extensive XPS database is compiled from over 35 Na–K–Sb photocathodes based on their stoichiometry. Finally, the successful operation of a superconducting radio-frequency (SRF) photoinjector using a Na2KSb photocathode is reported, achieving first beam in the SEALab SRF photoinjector.