Filter
Top Products
PRECAUTION - 6
Fig. 3 Status Transition of Adsorbed Gas Molecules on Cathode Tip
A : In the initial reduction period, or just after flashing, gas is completely discharged from
the cathode tip. (See (a) of Fig. 3.) As gas molecules are gradually adsorbed onto
the cathode, emission current decreases. In this stage, gas molecules are
desorbed, adsorbed and transferred. So noise (fluctuation of emission current)
occurs. (See (b) of Fig. 3.)
B : In the stable period approximately one layer of gas molecules is adsorbed onto the
cathode. In this status a stable emission current is usually available. (See (c) of
Fig. 3.) Although noise (fluctuation of emission current) may appear occasionally
due to desorption and adsorption of gas molecules a stable status returns in a
comparatively short time. So record an image after confirming the stable status.
In this period emission current slightly decreases with time.
C : In the unstable period a large quantity of gas molecules repeat desorption and
adsorption. In consequence emission current fluctuates considerably while
increasing. (See (d) of Fig. 3.)
Images are observed usually in the stable period.
The periods sectioned on the time axis in Fig. 2 vary depending on electron gun vacuum,
residual gas component, cathode/anode cleanliness and emission current value.
With the Model S-4800, they are roughly given as follows.
• Initial reduction period : 30 min to 3 hours
• Stable period : 2 to 12 hours
When the initial reduction period is half an hour, the stable period lasts 2 to 4 hours, and 4
to 10 hours when the initial reduction period is 1 to 2 hours (approximate values). Even in
the unstable period, flashing cleans the surface of the cathode as shown in Fig. 3 (a) and
emission current changes again as illustrated in Fig. 2, whereby the stable period returns.
(2) Flashing Practice
The flashing may be practiced in either of the following two fashions:
(a) Carry out flashing once at the start of use.
Then when the emission current becomes unstable after two to eight hours of use
(cumulative time of beam turn-on), perform flashing again.
(b) Carry out flashing once on completion of daily operation. On the next day start
operation without flashing, and perform flashing if the emission current becomes
unstable during use.
Cathode Tip
Gas Molecules
(d)
(c)
(b)
(a)