Simulation of Field-Emission Triode Using Carbon Nanotube Emitters

An X-ray source based on a field-emission electron source is currently being developed using carbon nanotubes (CNTs) for applications to biological, medical and material technologies. In the design of triode-type X-ray sources, it is important to ensure that fine beam focus and efficient electron emission can be simultaneously obtained. Therefore, the geometrical parameters, such as electrode shape and grid mesh design, including mesh wire thickness and mesh opening area, and the gaps between parts (for example, CNT cathode and grid mesh), must be optimized. In this study, for four different types of triode structures, the beam focusing performances and emission currents, with respect to grid mesh design, were simulated. The results show that the concave-shape cathode structure allows the optimum spot size and structural simplicity among the structures tested in this study. From the simulation results, it was estimated that a beam spot size of 1.5 mm can be reduced to less than 150 µm on the anode plate with the aid of the concave cathode.