Optimization of Operating Conditions for the FE Gun System of HVEM

Numerical calculations have been conducted on the electron optical characteristics of the field-emission (FE) gun system of the high-voltage electron microscope, which consists of an FE gun, an accelerating tube (AT) and a transfer lens placed between them. Emitted electrons are firstly accelerated to V0 by the FE gun and, finally, to Va by the AT, which includes 34 stages of accelerating electrodes and has an overall length of 1,423 mm. The AT is equipped with a retarding field between the first and second electrodes, and operates as a thick electrostatic lens. The spherical aberration coefficient of the AT lens was calculated as a function of the object position, i.e. the crossover position of the electron beam entering the AT. The operating conditions of the FE gun and the transfer lens have been established so that the aberration effect of the AT lens may be as small as possible for any accelerating voltage (Va) between 400 and 1,000 kV: FE gun voltage (V0) should preferably be larger than 30 kV and the transfer lens should be operated so as to bring the beam crossover to a position near the entrance plane of the AT. These operating conditions can also satisfy the requirement that the electron beam never collides with the AT electrodes.