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A numerical investigation of the topographic field-enhancement factor for structures including individual vertically aligned carbon nanotubes (VACNTs) and arrays of VACNT is presented. Some previously reported results for simple structures are reviewed first. Then, the extent of the zones of field enhancement and significant screening effects surrounding a given structure is discussed. The investigation with combined VACNT confirms the criterion that the spacing between identical CNT should be about twice their height to minimize screening effects. This statement is generalized to structures having different height ratios. The possibility of combining patterns of different height VACNT to minimize screening effects while allowing a larger surface density of such emitters is then investigated. The results show that height anisotropies in VACNT arrays can significantly reduce the field-emission current for a given applied field. A subsequent study that takes into account Joule heating and radiation losses during field emission demonstrates that, for height anisotropies larger than 5%, the VACNT tips reach temperatures above the onset temperature for selective field-assisted evaporation. This phenomenon occurs before the field-emission current from the nonideal films matches the targeted current value deduced from ideal VACNT arrays.