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Overview of Vacuum SystemsTejas Deshpande24 July, 2014
• Introduction
• Pumps
• Positive displacement pumps
• Momentum transfer pumps
• Entrapment pumps
• Pressure gauges
• Vacuum seals
• Manipulation and motion under vacuum
Outline
Introduction
Introduction: definitionsKinetic Theory of Gases
• Ideal gas equation
• Velocity of molecules hitting chamber walls
Pumping
• Rate of mass flow
• Mass “conductance”
Introduction: definitions
http://fisica.ufpr.br/sharipov/tubefm.html
Pumping
• Pump capacity & effective pumping speed
Introduction: definitions
• Rate of evacuation
• Example: Pfeiffer Hena-25 pump + cylindrical chamber
• Finite C with a hose
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]
Rotary vane pumps
• Principle of operation
Positive displacement pumps
Scroll pumps
• Principle of operation
Positive displacement pumps
Diaphragm pumps
Positive displacement pumps
• Principle of operation
Roots “blower”
Positive displacement pumps
• Principle of operation
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
Momentum transfer pumps
• Principle of operation
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
Entrapment pumps
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
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
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)
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
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
Gate Valve
Manipulation and motion under vacuum
Right-angled valve
• Large bore
• Sample transfer
• Small bore
• Air evacuation
Transfer arm
Manipulation and motion under vacuum
• Significantly greater stroke than bellow-sealed manipulator
• Magnetic power transmission
• Introduction
• Pumps
• Positive displacement pumps
• Momentum transfer pumps
• Entrapment pumps
• Pressure gauges
• Vacuum seals
• Manipulation and motion under vacuum
Summary