The spherical double pole-piece electron lens i§ depend on some
fact0rs : the- minimum va}ue-of projector fo·cal length (Fp)i:J,Iill • the
maximum value of magnetic flux density (Bm) , the total hal[ width
,(W) o'f asyriunetrical clln'e of magnetic field distribution [because there are differe_nt values of bore diameter (D1) • i. the ratio (D)D2) with different values] and the air gap width (S) between tlw iron poles The rat'io (S!Om) is used to represent' this type of lenses and depend as
a primary factor to desi:gn the lenses . Therefore the effect of air gap width and; the ratid (D1 /Dx) on the varia_ble-above , especially (Fp)mili Wi;is studied to obtain the mos-t favorable design •. highest fje)d of view (r) and magnification (M) of doublet projeetor tens although, the
requirtXI projector lens is about (M=JOO x) .!ll All calculat.ic;ms done by
the finite element method sugges-ted by (Munro, l973) C2-l , which is a numerical method used to find the magnetic field in electron lenses .
