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In order to study the tip field of the carbon nanotube (CNT) or CNT array standing upright on a cathode plane, we proposed a model in which the long CNT is replaced by a floating top segment at the cathode potential. Using the fictitious charge and image method, a simple approximate formula for the field enhancement factor of a single CNT linear with the height-to-radius ratio has been deduced. This result is evidently improved compared to that from the floating sphere model. The normalized field strength distribution on the top hemisphere is also calculated and plotted, with the result depending slightly on the height-to-radius ratio. An estimation shows that the field emission intensity on the cylindrical surface of CNT is negligible compared to that on the top hemisphere.

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