Schottky-type electron gun
Schottky effect means a phenomenon where the potential barrier of a substance decreases in a strong electric field, resulting in ease of thermoelectron emission. In the Schottky-type electron gun, the tungsten (W) tip emitter is heated at a lower temperature (～1800 K) than the temperature that can effectively emit thermoelectrons, and a strong electric field is applied to the tip, thus decreasing the potential barrier to emit electrons from the emitter. In the actual Schottky-type electron gun, the surface of the tip is covered with a thin layer of zirconium oxide (ZrO) to make electron emission easy by a decrease of the work function of the tip (～2.7 eV). The energy spread of the emitted electrons is ～0.7 eV. Its brightness is as high as 4×108 A/cm2.sr at 200 kV. The size of the virtual source produced is >10nm. The Schottky type gun is broadly used because of its high stability of the emission current. This type of gun is not the field emission type because the tunnel effect is not used.