DEPENDENCE OF ABERRATION COEFFICIENTS OF TRIODE FIELD-EMISSION GUN ONELECTRODE GEOMETRIES FOR ACCELERATING AND DECELERATING MODES

Numerical analysis of spherical and chromatic aberration coefficients have been conducted for a field emission gun consisting of a field emi ssion tip, a 1st anode and a 2nd anode for both accelerating and decel erating modes, A comparison of the aberration coefficients has been ma de for different electrode geometries such as electrode thickness and electrode aperture angle under the condition of the infinite magnifica tion and a constant voltage ratio of K = V-0/V-1, where V-1 is the tip -to-1st anode voltage and V-0 is the tip-to-2nd anode voltage, A prefe rable electrode geometry under a constant voltage-ratio condition is s omewhat different from that under a constant tip-position condition, U nder a constant voltage-ratio condition, a combination of a thicker 1s t electrode with a thinner 2nd electrode is preferable for the acceler ating mode, while the opposite combination of electrode thicknesses is desirable for the decelerating mode. The electrode thickness and elec trode aperture angle have been found to have very little effect on the chromatic aberration coefficient for both accelerating and decelerati ng cases, For the decelerating mode, a smaller distance between the 1s t and 2nd anodes is especially suitable for smaller spherical and chro matic aberration coefficients, since the distance can safely be reduce d without any serious problem of electric discharge between electrodes .