A gated single carbon nanotube field emitter with magnetic focusing is proposed and simulated using a parallelized Poisson's equation solver, coupled with the ray tracing of electrons, on an unstructured tetrahedral adaptive mesh. The magnetic focusing for the electrons can be achieved by a vertically downward magnetic focusing field (-B-z) through the use of either external solenoids or permanent magnets around the field-emission array. The simulation results, assuming uniform magnetic field inside a field-emission unit, are compared with those conventional tetrode-type field emitters using an electrostatic focusing structure. The results reveal that the magnetic focusing design can promise much higher emission current, while a much smaller spot size results at the anode. In addition, the magnitude of the applied gate voltage in the range of 60-120 V shows little influence on the electron-beam diameter at the anode. The proposed magnetic focusing method can also possibly reduce the complexity of the fabrication without the electrostatic focusing structure. Noticeably, a distribution, similar to the Airy function, is obtained that shows the dependence of the spot size at the anode on the magnetic flux intensity. Thus, under suitable magnetic focusing conditions, it is possible to produce well-defined microelectron sources for many field-emission applications, such as novel parallel electron-beam lithography or field-emission displays. (c) 2007 American Vacuum Society.