Vacuum Systems

1
Vacuum Systems

• Lecture 6
• G.J. Mankey
• gmankey_at_mint.ua.edu

2
Pumping Speed and Throughput

• The mass flow or throughput of a pump is given by
  the equation Q SP where S is the pumping speed
  and P is the pressure. For a conductive element Q
  C(P1-P2)
• For elements of a system connected in series, we
  must add the conductance of these elements as in
  an electrical circuit 1/C 1/C1 1/C2 1/C3
  
• Conductance depends on pressure in the low to
  medium vacuum regions, and is independent of
  pressure in high to ultrahigh vacuum regions.

Ref Inficon Instrumentation Catalog (2000-2001)
3
Calculations of Conductance

• In the molecular flow region, the conductance of
  a long straight circular tube is C 12 d3/z
  liters/sec where d is the diameter(cm) and z is
  the length(cm).
• For an orifice C 12 A liters/sec where A is the
  area in square centimeters.
• These equations should be used to estimate the
  effect of connecting pumps, hoses, etc. to a
  system to insure the pumps are properly utilized.
• The effective pumping speed of a system is then
  given by the equation 1/Seff 1/S 1/C where S
  is the pumping speed of the pump and C is the
  conductance of the associated flanges and
  fittings.

Ref Inficon Instrumentation Catalog (2000-2001)
4
Standard Flanges

• Conflat flanges use viton or copper gaskets with
  a knife edge for high to ultrahigh vacuum
  applications.
• Care must be taken not to damage the flange knife
  edge.
• Standard sizes are mini (¾" ID), 2 ¾" (1 ½" ID),
  4 ½" (2 ¾" ID), 6" (4" ID), 8" (6" ID) and 10"
  (8" ID).
• Medium vacuum applications use ISO and ASA
  flanges, low vacuum uses KF quik flanges.

5
Differential Pumping

• The amount of gas Q is equated SP2 Q C(P1
  P2)
• This trick can be used to maintain a constant
  pressure difference between two vessels.

6
Rotary Vane Pump

• An oil seal between a phenolic vane and a steel
  cylinder is used to scavenge gas from the vacuum
  region and exhaust it to the atmosphere.
• This pump works from atmosphere to about 0.1
  mTorr.
• Precautions must be taken at low pressures to
  avoid oil backstreaming into the vacuum vessel.
• It is also used as a backing pump for compression
  pumps like a diffusion pump or turbomolecular
  pump.

Ref Inficon Instrumentation Catalog (2000-2001)
7
Oil Diffusion Pump

• Oil vapor forced through jets in the stack
  transfer momentum to gas molecules and force them
  down through the pump and out the exhaust (must
  be backed).
• The pump is characterized by a compression ratio
  and an ultimate pressure.
• Economical (no moving parts).
• If used with a cryogenic trap, UHV can be
  routinely achieved.

Ref Inficon Instrumentation Catalog (2000-2001)
8
Turbomolecular Pump

• Turbine blades rotating at high speed transfer
  momentum to gas molecules to force them out the
  exhaust (must be backed).
• The pump is characterized by a compression ratio
  and ultimate pressure.
• Expensive (gt10k).
• UHV can be readily achieved (better if used in
  combination with a titanium sublimation pump).

Ref Inficon Instrumentation Catalog (2000-2001)
9
Gas Compression Ratio

• Since the pump works by momentum transfer, the
  compression ratio depends on the atomic mass.
• The thermal velocity of light gas is much
  greater, so the molecules are pumped less
  efficiently.

Ref Inficon Instrumentation Catalog (2000-2001)
10
Mass Spectrum of Turbo System

• The gas composition reflects the difference in
  compression ration of light gases and the
  composition dependent outgassing rates of
  stainless steel.
• Usually hydrogen is the main constituent of a
  well-baked system.

Ref Inficon Instrumentation Catalog (2000-2001)
11
Titanium Sublimation Pump

• High current (50 A) is passed through a titanium
  impregnated molybdenum filament to sublimate a
  fresh coating onto the cryoshroud walls.
• The film is highly reactive to H, CO and O and
  catalytically converts H2 and CO to CH4 which is
  more readily pumped by a turbo pump.
• Cooling the cryoshroud with liquid nitrogen goes
  the extra mile to get into the low 10-10 mbar
  range.
• Pumping speed depends on gas, activated area and
  wall temperature (can be quite high, i.e. limited
  by inlet flange size).

12
Ion Pump

• A high voltage combined with a magnetic field
  causes electrons to travel in a helical path with
  an energy sufficient to ionize gas atoms.
• The ions are accelerated so they strike a Ti
  plate and become buried in the plate.
• Can be started below 10-6 mbar.
• Pumping speed is gas dependent and drops off
  below 10-9 mbar.
• Buries the gas in the plate, so no backing pump
  is required.
• A little less expensive than turbo pumps.

Ref Inficon Instrumentation Catalog (2000-2001)
13
Ion Pump Types

• Diode pump center Ti electrode is biased
  positively to accelerate ions toward pump wall.
• Triode Pump Intermediate Ti electrode is biased
  negatively to accelerate ions toward pump wall.

Ref Inficon Instrumentation Catalog (2000-2001)
14
Cryopump

• A He refrigerator is used to cool a large-area
  surface where gas is condensed.
• The gas absorption depends on the bonding
  mechanism to the cryopanels.
• After prolonged use, the pump must be
  regenerated.

Ref Inficon Instrumentation Catalog (2000-2001)
15
Cryo Pump Speed for Various Gases

• Pumping speed depends on type of gas and area of
  selected cryopanel.

Ref Inficon Instrumentation Catalog (2000-2001)