Fabrication and characterization of cesium telluride photocathodes: A promising electron source for the Los Alamos Advanced FEL

The Advanced FEL at Los Alamos embodies a K2CsSb photocathode as an electron source. The photocathode consists of a K2CsSb film deposited on a molybdenum plug that can be inserted into the linac of the FEL. However, because K2CsSb is easily contaminated and has a half-life of less than a day when in use, switching to a more rugged high quantum efficiency (QE) material such as Cs2Te is considered as a means to lengthen the beam time. Cs2Te films were deposited on molybdenum plugs in an ultrahigh-vacuum research chamber. Several Cs2Te films were measured in situ for their spectral responses with a bias voltage of 90 V; the resulting QEs were 15–18% at a wavelength of 251 nm, 0.3–1% at 334 nm, 1–3 × 10−4 at 365 nm, and 10−7 – 10−6 at 546 nm. For this cathode to be useful, we need to frequency quadruple the 1053 nm line of the Nd:YLF laser to achieve a wavelength of 263 nm. Initial studies showed that the 251-nm QE of Cs2Te is much less sensitive to contamination than the 526-nm QE of K2CsSb. We exposed Cs2Te photocathodes to air at 10−4 Torr for five minutes. As a result, the QEs at 254 nm dropped from 16–18% to 1–2%. However, heating the cathode to 165°C revived the QE to about 10%. We conclude that Cs2Te is a very rugged photocathode material for use in an rf photoelectron source.