Vacuum Systems Lecture 6 G.J. Mankey gmankey@mint.ua.edu
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)
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)
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.
P1 C P2 S 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.
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)
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)
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 (>$10k). • UHV can be readily achieved (better if used in combination with a titanium sublimation pump). Ref: Inficon Instrumentation Catalog (2000-2001)
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)
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)
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).
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)
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)
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)
Cryo Pump Speed for Various Gases • Pumping speed depends on type of gas and area of selected cryopanel. Ref: Inficon Instrumentation Catalog (2000-2001)
