Instruction manual

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.
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.
Fig. 2 Change of Emission Current with Time
A
ccelerating 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