OPTIMIZATION OF OPERATING-CONDITIONS FOR THE FE GUN SYSTEM OF HVEM

Numerical calculations have been conducted on the electron optical cha racteristics of the field-emission (FE) gun system of the high-voltage electron microscope, which consists of an FE gun, an accelerating tub e (AT) and a transfer lens placed between them. Emitted electrons are firstly accelerated to V-0 by the FE gun and, finally, to V-a by the A T, which includes 34 stages of accelerating electrodes and has an over all length of 1,423 mm. The AT is equipped with a retarding field betw een the first and second electrodes, and operates as a thick electrost atic lens. The spherical aberration coefficient of the AT lens was cal culated as a function of the object position, i.e. the crossover posit ion of the electron beam entering the AT. The operating conditions of the FE gun and the transfer lens have been established so that the abe rration effect of the AT lens may be as small as possible for any acce lerating voltage (V-a) between 400 and 1,000 kV: FE gun voltage (V-0) 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 ent rance plane of the AT. These operating conditions can also satisfy the requirement that the electron beam never collides with the AT electro des.