Operation
Figure 12 Evacuate the Cryopump Figure 13 Evacuate the Vacuum
Vacuum Enclosure Chamber and Cool Down
the Cryopump
1. Before starting the cryopump, rough pump the vacuum enclosure to 5 x 10
-2
torr
(50 microns of mercury) to establish an insulating vacuum. See Figure 12.
During rough pumping, use a suitable trap such as molecular sieve or liquid nitrogen to
prevent oil from backstreaming into the vacuum enclosure.
2. Turn on the cryopump by starting the compressor. The compressor's power switch will light.
Close valve V3 between the vacuum enclosure and the roughing pump to isolate the
roughing pump from the vacuum enclosure. See Figure 13.
Rough pump the vacuum chamber down to crossover pressure or the lowest possible
pressure while the cryopump cools down. Discussed later, crossover pressure is the
pressure at which changing from rough pumping to cryopumping occurs.
As the vacuum chamber is being rough pumped, residual gases in the vacuum enclosure
are cryopumped. Pressure around the cryopump drops to low levels before the cryopump
reaches minimum temperature. Cooldown time to 20 K for the Marathon
®
CP-8 Cryopump
is approximately 75 minutes.
3. After the cryopump has cooled down to minimum temperature and the main vacuum
chamber has been rough pumped to crossover pressure, open the gate valve. The
cryopump is now operating in steady state. See Figure 14.
During steady-state cryopumping, monitor the system pressure and second-stage
temperature. When frozen gases and water vapor form deposits on the cryopanels, the
cryopanel temperature may rise, causing the partial pressure of the gases to increase. To
maintain an adequate vacuum in the system, the second-stage temperature must remain
below 20 K.
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