Prevention of dark currents from photocathodes (Vol. 46 No. 4)

Prevention of dark currents from photocathodes
Field emission maps of an annealed oxide-free (left) and thermally oxidized Mo surface (right). Obviously, the number of emitters providing 1 nA current at fields up to 100 MV/m is reduced by the oxide.

Alkali-based photocathodes deposited in the centre of molybdenum substrates are used as pulsed electron sources in linear particle accelerators. Operation at high electric dc or rf fields is required to obtain a low beam emittance, thus increasing the probability of unwanted dark currents from the cathode surface. Therefore, a field emission scanning microscope was used to localize parasitic electron emitters on single crystal and polycrystalline Mo plugs. In contrast to well-polished and dry-ice cleaned Mo surfaces with native oxide, strong field emission occurred after heat treatments above 400 °C (see figure), which are usually applied before the coating process. Thermal oxidation, however, partially weakened the emitters. X-ray photoelectron spectroscopy confirmed the corresponding changes of the surface oxide layer. These results suggest a selective removal of the native Mo oxide prior to the photocathode deposition to prevent the dark currents in accelerators.

S. Lagotzky, R. Barday, A. Jankowiak, T. Kamps, C. Klimm, J. Knobloch, G. Müller, B. Senkovskiy and F. Siewert, Prevention of electron field emission from molybdenum substrates for photocathodes by the native oxide layer, Eur. Phys. J. Appl. Phys. 70, 21301 (2015)