Overview of Vacuum Systems Tejas Deshpande 24 July, 2014

Introduction Pumps Positive displacement pumps Momentum transfer pumps Entrapment pumps Pressure gauges Vacuum seals Manipulation and motion under vacuum Outline

Introduction

Introduction: definitions Kinetic Theory of Gases Ideal gas equation Velocity of molecules hitting chamber walls

Pumping Rate of mass flow Mass “conductance” Introduction: definitions

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] [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 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 ~ T 4 Solid conduction Operating conditions Free molecular flow > 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” (< Pa m 3 s -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