overview of vacuum systems tejas deshpande 24 july, 2014
TRANSCRIPT
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Overview of Vacuum SystemsTejas Deshpande24 July, 2014
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• Introduction
• Pumps
• Positive displacement pumps
• Momentum transfer pumps
• Entrapment pumps
• Pressure gauges
• Vacuum seals
• Manipulation and motion under vacuum
Outline
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Introduction
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Introduction: definitionsKinetic Theory of Gases
• Ideal gas equation
• Velocity of molecules hitting chamber walls
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Pumping
• Rate of mass flow
• Mass “conductance”
Introduction: definitions
http://fisica.ufpr.br/sharipov/tubefm.html
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Pumping
• Pump capacity & effective pumping speed
Introduction: definitions
• Rate of evacuation
• Example: Pfeiffer Hena-25 pump + cylindrical chamber
• Finite C with a hose
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Pumping
• Conductance of KF 40 hose (L = 10", D = 2.165")
Introduction: definitions
[1] http://www.vacuumlab.com/Articles/Knowing%20Eff%20Pump%20Speed.pdf
[1]
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Rotary vane pumps
• Principle of operation
Positive displacement pumps
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Scroll pumps
• Principle of operation
Positive displacement pumps
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Diaphragm pumps
Positive displacement pumps
• Principle of operation
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Roots “blower”
Positive displacement pumps
• Principle of operation
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Diffusion pumps
• Air diffuses to vapor stream
• Baffles condense vapor
• Oil backstreaming problem
Momentum transfer pumps
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Turbomolecular pumps
• Principle of a molecular “drag” pump
• Mechanically similar to rotary pump
• Good compression ratio
• Poor pumping speed
• Turbo pump
Momentum transfer pumps
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Turbomolecular pumps
Momentum transfer pumps
• Principle of operation
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Ion pumps
• Penning cell
• Magnetic field acts as electron trap cyclotron orbits
• Ions deposit on Cathode
Entrapment pumps
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Titanium sublimation pumps
• Titanium as evaporatable “getter” material
• Chemical sorption
• 40-50 Ampere current evaporates Ti
Entrapment pumps
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Liquid Manometers
• Simplest U-tube manometer
Pressure gauges
• Atmospheric pressure = 1 bar
The Diaphragm Manometer
• Capacitive sensor
• AC vs. DC
• DC capacitance bridge
• AC resonant circuit tuned to diaphragm vibration
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Thermal Conductivity Gauges
• Convection of heat through the gas
Pressure gauges
• Other heat losses
• Radiation ~ T4
• Solid conduction
• Operating conditions
• Free molecular flow
• > 10-3 torr
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Ion Gauge
• Measurement of ion current
Pressure gauges
• Gauge sensitivity
• Bayard-Alpert gauge
• Collector potential: -10 V• Grid potential: 180 V• Cathode filament: 30 V• Screen: 0 (ground)
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O-ring seal
• Elastomers, Viton-A, Polyimide
• Quick and reusable
• Limited baking range (< 250 °C)
Vacuum Seals
ConFlat flange seal
• “Leak-tight” (< 10-12 Pa m3s-1)
• Metal gaskets (typically copper)
• Bakable to 450 °C
• Not reusable
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Metal bellows
• Mechanical feedthroughs
• Bakable to 450 °C
• Limited linear/angular stroke
Manipulation and motion under vacuum
Continuous rotation
• Speeds up to 2000 RPM
• High torque bellows
• Low torque magnets
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Gate Valve
Manipulation and motion under vacuum
Right-angled valve
• Large bore
• Sample transfer
• Small bore
• Air evacuation
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Transfer arm
Manipulation and motion under vacuum
• Significantly greater stroke than bellow-sealed manipulator
• Magnetic power transmission
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• Introduction
• Pumps
• Positive displacement pumps
• Momentum transfer pumps
• Entrapment pumps
• Pressure gauges
• Vacuum seals
• Manipulation and motion under vacuum
Summary