OCI BDL800IR, Instruction Manual

Page: 27 / 61

Chapter 5 Operating Instructions 27
constant at the minimum beam energy. The unit will stop keeping the beam current constant
if any of the following occurs:
 The beam energy falls below 80% of the value at the time of initialization.

The focus voltage is decreased below 90% of the value at the time of initialization.
 The Wehnelt voltage is increases 0.5 V above the value at the time of initialization.
5.8.3 Filament Forming
It is necessary to form a newly installed filament to relieve mechanical stress. Before pro-
ceeding, change the Controller Mode to Programming and select Program Selection  Pa-
rameter Limitation . Set about 2.7 A (thoriated tungsten) as the Max Filament Current or
0.05 A (LaB
6
) above the working “Current” as specified on the accompanying sheet. Press
Save to commit changes.
Change the Controller Mode to Operating and select Program Selection

Fil. Form-
ing program. Make sure the filament current is set to 1.2 A. If it is not, change it to 1.2 A in
the Programming mode. Once set, initiate the filament forming by pressing the Filament
Current On/Off button. The filament current will be steadily and slowly increased from
1.2 A to the maximum pre-set value at a rate of 0.01 A per minute.
It is possible to have the unit turn off after filament forming. To use this feature, change
the Controller Mode to Programming and select Program Selection  Fil. Forming Mode
 Shut-down and save.
5.8.4 Filament Activation
This sub-section applies to thoriated tungsten filaments only. It is necessary to activate this
type of filament every time it has been exposed to ambient conditions for any period of time.
Failing to form a filament may lead to unreliable data professing itself by delivering high
beam current for a short period of time and then falling down to its usual range.
This is performed by setting the filament current close to its maximum allowable value
2.85–2.90 A, turning it on, and letting it run for 1–2 hours. Optionally, using the Outgassing
procedure described below, one may set a timer, which will automatically turn the filament
current off at the end and switch to a default program.
5.8.5 Outgassing
Filament outgassing should be performed infrequently and understood only as a temporary
remedy to reduce the amount of adsorbed material on the filament. Whenever possible, a full
system bake-out should be carried out for a minimum of 8 hours followed by a 4-hour-long
cooldown.