Sumitomo Marathon CP-8 - PRINCIPLES OF OPERATION; CRYOPUMP VACUUM GENERATION

PRINCIPLES OF OPERATION
7
A cryopump creates a vacuum by capturing gases from the vacuum chamber. The pump does
this by solidifying gases onto cold surfaces. See Figure 3.
Typical cryopumps have a cryogenic refrigerator (cold head) that produces refrigeration at two
temperature levels and cools two, extended-surface cryopanels onto which the gases can
freeze. The first stage of the cold head, operating in the range of 50 to 75 K, cools the outer
cryopanel that shields the colder, second-stage cryopanel against radiant heat and cools the
louver across the inlet of the pump. Water vapor freezes out when it hits the louver.
The second stage of the cold head, which operates between 10 and 20 K, cools the inner
cryopanel that freezes out nitrogen, oxygen and argon after they pass through the louver. On
the inside of the cold cryopanel, charcoal adsorbs gases that will not freeze at second-stage
temperatures.
A cryopump can reduce the pressure in a vacuum chamber to very low levels. Figure 2 shows
the relationship between the equilibrium vapor pressures and temperatures of frozen gases.
Because the equilibrium vapor pressures for neon, hydrogen and helium are too high at 20 K to
freeze these gases onto a bare surface, charcoal is used to adsorb them.
Figure 2 Equilibrium Vapor Pressure vs. Temperature
1E-13
1E-12
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
1E+1
1E+2
1E+3
1 10 100 1000
Temperature (K)
Vapor Pressure (torr)
He
H
2
N
2
Ne
CH
4
O
2
Ar
CO
2
H
2
O