Advanced photoinjectors, which are critical to many next generation accelerators,open the door to new ways of material probing, both as injectors for free electronlasers and for ultra-fast electron diffraction. For these applications, the nonuniformityof the electric field near the cathode caused by surface roughness can be thedominant source of beam emittance. Therefore, improving the photocathode roughnesswhile maintaining quantum efficiency is essential to the improvement of beambrightness. In this paper, we report the demonstration of a bi-alkali antimonide photocathodewith an order of magnitude improved roughness by sputter depositionfrom a K2CsSb sputter target, using in situ and operando X-ray characterizations.We found that a surface roughness of 0.5 nm for a sputtered photocathode with afinal thickness of 42 nm can be achieved while still yielding a quantum efficiencyof 3.3% at 530 nm wavelength. ? 2017 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.
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