File:Fig218 Field Emitter Array.PNG

http://aoss-research.engin.umich.edu/sets/sets_old/Tether_Systems/FEAC/feac.htm

or

http://en.wikipedia.org/wiki/Field_emitter_array

SEM photograph of an SRI Ring Cathode developed for the ARPA/NRL/NASA Vacuum Microelectronics Initiative by Capp Spindt

Summary
Diagram of actual field emitter cathodes