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BAKEOUT OPERATION
When a VacIon pump does not reach the desired
base pressure, and there are no leaks, it is neces-
sary to bake the system to remove water vapour.
This is done by heating the pump and all the com-
ponents in the system.
1. Heat the pump body with a bakeout oven or
heating strips to temperatures between 150 °C
and 250 °C (250 °C is the maximum allowable
for most bakeable high voltage cables). This
temperature is high enough to degas the pump
surfaces of water vapour without damaging the
magnet and high voltage connector. Note that
the system components must be compatible
with the bakeout temperature. The heating
must be approximately even on all vacuum sur-
faces or water vapour can recondence on the
cooler surfaces preventing achievement of
UHV vacuum pressures.
2. Leave the pump control unit on and monitor the
pressure. It must never increase above 5x10
-5
Torr (6.6x10
mbar); if this value is exceeded,
turn the bakeout off and then on again when
low pressure is restored.
To control the heaters and to monitor to high
pressure limit during bakeaout in automatic
mode, a pressure-sensitive relay may be used.
3. Bake the VacIon Plus pump for at least eight
hours. Longer bakeout periods are recom-
mended when the pump has been used with
heavy gas load applications or when UHV
-9
pressure, 10
Torr (mbar) range or less is de-
sired.
4. As the pump and system cool down to room
temperature, a drop in pressure should be ob-
served.
PUMP SPEED
Pumping speed varies with pressure and with dif-
ferent gases.
Virtually all gases and vapours can be pumped
without stopping the pumping action in VacIon
pumps. The sputtering action of the pump re-
moves contaminants and continues to provide
fresh titanium for pumping. An inert gas load in a
vacuum system is typically only "volume gas" - in-
ert gas absorption to surfaces of the system is
negligible. Thus, the need for long-term inert gas
pumping only occurs when argon or air is intro-
duced continuously into the system or if a large
leak exists (argon is 1% of atmospheric air). Cycli-
cal pumping occurs above 10
gon. These pressure variations typically cease
when the leak rate of argon is reduced or elimi-
nated.
-5
-5
Torr (mbar) on ar-
18
TECHNICAL INFORMATION
MAINTENANCE
VacIon Plus pumps are maintenance free. In case
of life time expiry or accidental premature failure of
the pump, please contact your nearest Varian
sales/service office for repair.
Leakage current check
The current value displayed on the controller is the
total of the operating current, and leakage current
from controller, cables, feedthrough, and pump in-
sulators.
If the pump current reading is to be used as pres-
sure measurement, check the pump leakage cur-
rents as follows:
1. Turn off the pump control unit.
2. Remove the pump magnet.
3. Turn on the pump control unit and wait for cur-
rent stabilization. The current reading should
not be higher than 1 μA. Make sure that the
control unit and the high voltage cable leakage
current is negligible.
4. If leakage current comes from the pump, per-
form "high-potting" per the procedure below;
then recheck the pump and install the magnet.
5. If it is not possible to "high-pot" the pump, the
pressure reading is biased by the leakage cur-
rent value.
Hi-potting
A constant pump current when no vacuum leak
exists is often caused by field emission currents at
the pump cathodes which prevent the use of the
pump current as a pressure indicator. To reduce
this field emission current, "hi potting" should be
performed. "High potting" is the term used to de-
scribe the application of higher than normal oper-
ating voltage (10-12 KV, 20-50 mA) for the
purpose of burning off "whiskers" (sharp edges) on
the pump cathode.
The output of an appropriately sized AC trans-
former (i.e. neon sign type) may be applied to the
pump for a period of about 30 seconds. This
should effectively remove any "wiskers" or sharp
edges on the pump cathode.
WARNING!
Voltages developed in the High Potter power sup-
ply are potentially lethal. Use caution during opera-
tion and ensure correct grounding connection.
87-900-063-01(C)
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