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