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The Filament control panel displays settings and allows control over the filament (W/LaB6).
The Filament button switches the heating of the filament on or off. The filament button has three possible settings:
When the filament heating is switched on, the filament is heated
to the value indicated in the Heat to: spin-enter-edit
control at a rate defined by the delay steps.
The Heat to spin
buttons change the filament heating step to which the
filament will be heated. If the filament is off (Filament button is gray), any
changes have no direct effect. If the filament is on (button yellow), the
changes (defined by the spin buttons) are implemented once the Enter
button has been pressed.
Changes in heating current are executed with the delay steps
defined.
The Heat to value cannot exceed the filament limit set.
A progress bar displays the current value of the heating as a fraction of the total range. The total range is determined by the filament limit setting.
The Emission Step spin
buttons change the emission settings of the gun. Press the Enter
button to set the new value. Possible values are in the range 1
to 6. In general, the emission should be set so as not to exceed
normal filament emission values (typically 10 uA for LaB6,
50 uA for W).
At lower high tension values, the emission value must be
increased to keep the emission current similar to the values
obtained at higher high tension settings.
A progress bar displays the current value of the emission current. The total range displayed is one of a series of values (10, 25, 100) that are dynamically set, depending on the value of the emission current itself.
The status displays (when relevant) waiting time before the filament has reached the status requested by the operator.
Pressing the arrow button displays the flap-out containing the Filament Settings control panel.
Setting the filament saturation
Thermionic filaments have a specific saturation setting. If a
filament is run at a lower heating current, the emission
typically will be somewhat unstable (as well as lower). If a
filament is run oversaturated it will not emit more electrons but
the lifetime of the filament will be decreased because filament
tip evaporation increases with filament heating temperature. It
is important therefore to set the filament to its proper
saturation setting.
There are several ways to judge whether a filament is saturated
or not :
1. Observe the emission of the
filament as the heating is increased
As the heating current of a filament is increased, the
emission will increase until a maximum is reached. At higher
heating currents the current will stay roughly the same or even
drop a little. Turn down the heating current until the emission
starts falling appreciably (>10% per step). Set the filament
limit to the value just above the step where the current started
falling strongly.
2. Observe the emission pattern of
the filament
As the heating current is increased, the image of the filament
(make sure the beam is focussed and visible on the viewing
screen) changes. Typically at low values some emission is seen.
Then, as the heating is increased, the emission appears to come
from a ring with a dark center. With further increase of the
filament heating current, the dark center becomes smaller and, at
filament saturation, disappears.
With tungsten filaments, the structure in the focussed beam
normally disappears totally.
With LaB6 filaments, very often the structure doesn't
disappear totally. LaB6 filaments get damaged by
evaporation of the lanthanum-hexaboride into the vacuum, leaving
pits and grooves that remain visible. One shouldn't overheat the
filament to try and remove this structure (the structure will
disappear when the beam is defocussed so it doesn't interfere
with microscopy).
Note : an overheated filament may result in an instability
in the high tension (looking like a ripple of ~5 volts) because
the bias feedback cannot work properly anymore.
Emission: a technical explanation
The emission parameter influences the bias voltage on the
Wehnelt of the thermionic gun. If the bias voltage is high (which
corresponds to a low emission setting), the electrons can only be
extracted from the tip of the filament. When the bias voltage is
lowered (at higher emission settings), the electrons are
extracted from larger and larger areas (higher up) of the
filament.
Decreasing the bias voltage increases the total emission current,
but at the same time the area of the filament that is emitting
(and therefore usually also the effective source size) is
increasing. The higher emission typically also results in a
stronger Boersch effect (Coulomb interactions between electrons)
in the first cross-over below the filament (the 'source') which
increases the energy spread of the electron beam.
Page last modified on 11/26/01
