Experimental measurements of electron emission uniformity from cold cathodes*

We report measurements of electron emission, including spatial uniformity, from cold field emission cathodes. The measurements are taken on a system designed to examine the nanophysics of field emission from advanced Al, W and CsI cathodes that operate in the 1-1000 A/cm$^{2}$ regime. Operation is for UHV vacuum (10$^{-10}$ Torr) with bake-out up to temperatures of 450 C to eliminate residual water vapor or other contaminants. The current-voltage characteristics, field emission, work function, space charge effects and the Fowler-Nordheim coefficients are examined. Negative pulses of 0-20 kV and 1-5000 $\mu $s duration are applied between the cathode and anode to obtain the current-voltage characteristics. A linear translation stage is used to adjust the cathode-anode gap from 0-1.25 cm with a resolution of 0.025 mm. A small $\sim $4 mm$^{2}$ second ``local anode'' maps the spatial uniformity of the emission current density across the cathode surface. Measurements have been carried out on ALF (Ablation Line Focus) and knife edge Si-Ni cathodes. Additional tests will be accomplished on both single and multi-tip sharp tip/knife-edge cathodes to determine the interaction between local neighboring tips during electron emission.