Surface-Emission Studies in a High-Field RF Gun based on Measurements of Field Emission and Schottky-Enabled Photoemission.

We report on investigations into the fundamental surface emission parameters, the geometric field enhancement factor (β) and the work function (ϕ), by making both field emission and Schottky-enabled photoemission measurements. The measurements were performed on a copper surface in the Tsinghua University S-band RF gun in two separate experiments. Fitting our data to the models for each experiment indicate that the traditionally assumed high value of β(≈ 50-500) does not provide a plausible explanation of the data, but incorporating a low value of ϕ at some sites does. In addition, direct measurements of the surface conducted after the experiment show that β is on the order of a few, consistent with our understanding of the electron emission measurements. Thus we conclude that the dominant source of electron emission in high gradient RF cavities is due to low ϕ sites, as opposed to the conventionally assumed high β sites. The origin of low ϕ at these sites is unclear and should be the subject of further investigation. [ABSTRACT FROM AUTHOR]