TECHNICAL INFORMATION
Leakage current check
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 10 nA at 3 kV. 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 following procedure;
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.
High-potting
A constant pump current when no vacuum leak
exists is often caused by field emission currents
which prevent the use of the pump current as a
UHV pressure indicator. To reduce this field emis-
sion current, "high potting" should be performed.
"High potting" is the term used to describe the ap-
plication of higher than normal operating voltage
(10-12 KV, 20-50 mA) for the purpose of burning
off "whiskers" (sharp edges) on the pump cathode.
High potting should be done under vacuum and
preferably without pump magnets installed (to re-
duce the drawn current).
The output of an appropriately sized AC trans-
former (i.e. neon sign type) may be applied to the
pump. High-potting should be done carefully and
in voltage steps since uncontrolled arcing inside
the pump can cause permanent damage. Slowly
increase the applied voltage and watch the current
meter for indication of arcing inside the pump as
whiskers are burned away. If arcing occurs wait at
this voltage until the current is stable again. Then
slowly increase voltage again in steps up to a
maximum of 12 kV. The current should never ex-
ceed 50 mA.
,
WARNING!
Voltages developed in the High Potter power sup-
ply are potentially lethal. Use caution during opera-
tion and ensure correct grounding connection.
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87-900-106-01(B)
