Effects of grid parameters on field emission characteristics in triode type CNT X-ray source

摘要

Field emission X-ray sources have been developed by using carbon nano-tubes (CNT) for diverse applications in fields of biological, medical and material diagnostic technologies. For such applications, it is required to improve the efficiency of field emission for obtaining higher X-ray flux. Previous studies have reported that several parameters, including the aspect ratio of CNT, the inter-tube distance and so on, can influence on the field emission properties of CNT emitter.1 One of the important factors in field emission of CNT is the field enhancement factor (FEF) which is defined as the ratio of the actual electric field at the apex of CNT and the macroscopic electric field.2 The FEF of CNT emitter in diode structure depends dominantly on the shape of individual CNT and the pattern of CNT array, whereas the FEF in the triode structure, especially in case of planar type emitter, is modulated by the grid structure, consisting of grid mesh size (i.e., grid mesh pitch, opening area rate, grid mesh line width) and gap distances between grid and CNT emitter.3 Here, we demonstrate that the grid parameter significantly affects the FEF of the planar type CNT emitter in triode structure. The experimental measurements as well as computer simulation works are performed by adjusting the gap distance and the grid mesh size to interpret characteristics of the field emission from CNT emitters. The results clearly show that the variation of grid structure — a decrease of the grid mesh size and an increase of the gap distance — induces modification of local electric field on individual CNT, resulting in an effective increase of the FEF.