Transcript here
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 ~
T
4 Solid conduction • Operating conditions • • Free molecular flow > 10 -3 torr
Pressure gauges Ion Gauge
• Measurement of ion current • Gauge sensitivity • Bayard-Alpert gauge • • • • 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 m 3 s -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