Hitachi Microscope & Magnifier S-4800 Manual Do Utilizador

PRECAUTION - 5  

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. 

 
 

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. 
 
 
 
 
 
 
 
 
 
 
 

Accelerating voltage

（Vacc） 

First anode

Cathode 

Second anode

Extracting voltage 
(Vext) 

Flashing power supply 

Unstable Period 

Stable Period

Initial  
Reduction Period 

C 

B 

A 

(d) 

(c)

(b) 

(a)

Time 

Emission current