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Control unit connection
WARNING!
Voltage delivered by control units used with the
VacIon pumps are potentially lethal. To operate
the pump safely ensure that grounding connection
is correctly made.
Connect the control unit to the pump as follows:
1. Ground the pump to the power supply by plac-
ing the ground spring over the pump insulator
before attaching the high voltage lead. On ca-
bles with ground leads, attach the ground lead
to a ground lug on the system.
2. The power supply, in turn, is grounded through
its power lead. Therefore, to ensure a com-
pletely grounded circuit, the three-prong plug
on the power supply cord must be inserted in a
socket that has a ground connection. Do not
use socket convertors that prevent the comple-
tion of this ground circuit.
OPERATING NOTES
VacIon pumps operate as a multiple-cell Penning
discharge. A potential of about 3 kV is applied to
the anode cell; the cathodes are at ground poten-
tial. Electrons are constrained from going directly
to the anode by the presence of an axial magnetic
field of about 1200 gauss. A self-shielding insula-
tor isolates the anode from ground potential. When
the VacIon pump is operating properly (with mag-
net installed), the current drawn is proportional to
pressure (see the following figure).
Pressure versus current curve
After the system is rough-pumped to below 10
Torr, the pump can be started by simply switching
on the VacIon pump control unit. A current draw
on the control unit will indicate that the pump dis-
charge has started.
Always wait at least 30 seconds after turning off
the high voltage switch before disconnecting the
power supply leads. This should allow the power
supply output capacitor to discharge adequately.
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 and the system with a
bakeout oven unit or heating strips to tempera-
tures 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 com-
ponents must be compatible with the bakeout
temperature. The heating must be approxi-
mately even on all vacuum surfaces or water
vapour can recondence on the cooler surfaces
preventing achievement of UHV vacuum pres-
sures.
2. Leave the pump control unit on and monitor
the pressure. It must never increase above
-5
5x10
Torr (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 pump for at least eight hours.
Longer bakeout periods are recommended
when the pump has been used with heavy gas
load or when UHV pressure, 10
or less is desired.
4. As the pump and system cool down to room
temperature, a drop in pressure should be ob-
served.
5. Since VacIon pumps can be used in many dif-
ferent applications, some are being used suc-
cessfully in vacuum processing. The high
voltage feedthrough on the VacIon pump, can
stand a limited number of high temperature
bakeout cycles (to +400°C). Hence, Varian
recommends only a few high temperature
bakeouts to +400°C.
11
INSTRUCTIONS FOR USE
NOTE
-9
Torr (mbar)
87-400-085-01(D)
-2
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