Hitachi S-4800 manual Flashing

7.FLASHING

(1)Principle of Field Emission Electron Gun

Figure 1 shows the principle of the field emission electron gun. The gun consists of cathode, first anode and second anode.

An extracting voltage (Vext) is applied between the cathode and first anode, and electrons are emitted from the cathode. The electrons are accelerated by an accelerating voltage (Vacc). The cathode need be cleaned occasionally by turning on the flashing power supply in order to remove adsorbed gas on the surface of the cathode.

Flashing power supply

Extracting voltage

(Vext) Cathode

First anode

Second anode

Accelerating voltage（Vacc）

Fig. 1 Principle of Field Emission Gun

Figure 2 plots a usual change of emission current with time after flashing. The abscissa and ordinate represent time and emission current, respectively.

Figure 3 sketches a status transition of gas molecules adsorbed on the cathode tip. The status (a) through (b) correspond to (a) through (d) on the curve in Fig. 2.

A, B and C in Fig. 2 are generally called an “Initial Reduction Period”, “Stable Period” and “Unstable Period”, respectively.

	(a)
current	(b)	(c)	(d)
Emission	A	B	C
	Initial
		Stable Period	Unstable Period
	Reduction Period
		Time

Fig. 2 Change of Emission Current with Time

PRECAUTION - 5