virtual source

The virtual source is the real electron source of the field-emission electron gun (FEG) and the Schottky-emission electron gun.
The radius of the curvature of the tip of the tungsten (W) emitter for FEG is 100 to 200 nm. A strong electric field is applied to the tip by an extraction voltage, and then the electrons are emitted from the surface of tip. As shown in Fig.1, the electron beam behaves as if it emanates from a source of 5 to 10 nm diameter. This source is called the virtual source. As shown in Fig.2, the emitted electrons are accelerated up to a designated voltage by the voltage applied to the anode.
The high brightness of FEG results from the extremely small virtual source. The virtual source of the Schottky-emission electron gun is a little larger than that of FEG, 15 to 20 nm in diameter.

Fig. 1 Conceptual diagram of the virtual source

Fig. 2 Structure of FEG

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