New design for a field emission electron gun immersed in a magnetic lens field

This paper describes a new design for a field emission electron gun immersed in a magnetic- lens field. Its low aberration yields both a high brightness and a high emittance. A single-pole- like magnetic lens made a water-cooled pipe coil produces a magnetic flux density of 0.28 T at the emitter tip. The spherical aberration of the gun, as calculated by an electron ray tracing program, is only 1/3 that of a conventional gun with a magnetic lens. A prototype of the gun was constructed using a zirconium-oxide/tungsten thermal-field emitter. The measured crossover diameters agree with those calculated by the ray tracing program, confirming that the aberration is very low.