Some expressions of the variation of the beam divergence caused by apertures in each electrode versus the electric field intensity ratio for the extraction system with four and five electrodes are given. When the electric field intensity ratio is changed from 0.8 to 3.2 for a four‐electrode system, the divergence caused by an aperture in the third electrode is always divergent and slightly increases with the electric field intensity ratio. The divergence caused by an aperture in the second electrode increases linearly with the electric field intensity ratio and varies from divergence through the zero point to convergence. When the beam perveance is matched, the beam divergence formed by the ion emission boundary will cancel out the role of the second and third electrodes. Thus, the beam divergence caused by the ion emission boundary decreases with increases in the electric field intensity ratio and varies from convergence through the zero point to divergence. On one hand, the perveance ratio increases as the electric field intensity ratio increases, and on the other hand, the perveance of the plane diode, at a certain beam energy, will evidently decrease as the electric field intensity ratio increases. Therefore, the beam current only falls slightly with an increase in the electric field intensity ratio at a certain beam energy. When the beam divergence in each electrode aperture is smaller, the optimum electric field intensity ratio only decreases from 1.76 to 1.47. For an extraction system with five electrodes, the beam current is changed by changing the voltage across the extraction gap, and then the beam is accelerated to the given energy by the voltage across the third gap. Moreover, the beam current can be kept constant by adjusting the voltage across the extraction gap; in this case, the beam energy is changed by changing the voltage across the third gap.