SPECS EBE-1, Manual

Page: 14 / 34

Operation
Set the beam and filament controls to zero. Turn the power supply on. The beam
voltage will be briefly seen rising and then fall back to around 180 V. This is a character-
istic of the power supply and not a sign of malfunction. Depending on component
availability this output voltage may not appear. In this case increase the HV setting until
it shows a few hundred volts.
Now increase the filament current. As the current passes ~4.5 A the 180 V will drop to
zero indicating rather usefully that the filament has reached electron emitting temper-
ature. The HV circuit now sees a load instead of open circuit and the driver can drop the
voltage to zero.
The beam voltage can now be gradually increased. As the beam voltage rises, the emis-
sion current will also rise. Emission current can be reduced by decreasing the filament
current and hence the filament temperature. To increase the emission current the fila-
ment current should be increased. However as the emission current rises, the beam
voltage will be seen to drop a little but it can be readjusted using the beam control.
Regulating the output voltage can mask certain physical phenomena of the evapora-
tion and lead to runaway effects, which are not seen until too late. Electronically stabil-
ised evaporation is best achieved by regulating the filament current from a deposition
rate signal. The filament temperature is effectively setting the impedance of the source.
By adjusting the filament current in conjunction with the Beam voltage it is possible to
get the desired combination of beam voltage and emission current. A voltage of 1-1.5
kV will be enough for most purposes. At least 1 kV is required to ensure adequate fo-
cusing of the electrons. 2 kV will only be required for the most high power and de-
manding applications.
4.2.1 Starting evaporation
When the rod or material to be evaporated is heated for the first time and also when
the instrument is being used for the first time, it is important to condition and outgas
both. Outgassing can cause brief local increases in pressure, which can cause a moment-
ary arc to occur. This will cause no damage. It can in some cases take some hours of op-
eration at low power to ensure the instrument is properly conditioned. A blown fuse in
the power supply is indicated by the fact that emission current cannot be obtained.
With the rod tip in the right position, the power can now be increased until the re-
quired evaporation rate is achieved.
10 EBE-1