One of the plural electrostatic quadrupole lenses nearest to the cathodes is configured so as to produce a lens action weaker on two side electron beams than on the center electron beam.

• 多个静电四极透镜中最接近阴极的静电四极透镜构形为能使透镜对两个侧边电子束的透镜作用比对中心电子束的透镜作用弱。