Vacuum Fundamentals

High-Vacuum Technology Course

Week 6

Paul Nash

HE Subject Leader (Engineering)

Vacuum Fundamentals

• Recap on last session

• Progress on assignments to date

Vacuum Technology

Vacuum Fundamentals

General requirements:

Very low rates of outgassing

Tolerant to bakeout temperatures (can be up to 450oC)

Requirements are more stringent as we head to higher vacuum levels

Materials for use in a vacuum

Vacuum Fundamentals

Specific risks:

Materials may sublimate in vacuum(transition from solid to gas)

Gases can be released from porous materials or cracks/crevices

Traces of lubricants and residues from machining may be present on surfaces

Materials for use in a vacuum

Vacuum Fundamentals

• Guest Speaker– Dr Nick Singh – VG Scienta

• Vacuum fittings and accessories

Learning Objectives

Vacuum Fundamentals

Vacuum Fittings and Accessories

Vacuum Fundamentals

Joints and Seals

• ‘O’ Rings– Made of elastomer materials trapped and

compressed between two surfaces– May be fitted into a machined groove in the

flange if there is enough room– May be fitted in a special carrier for thin

flanges where machining is not possible

Vacuum Fundamentals

Joints and Seals

Vacuum Fundamentals

Joints and Seals

‘O’ Ring in machined groove‘O’ Ring in carrier

Vacuum Fundamentals

Joints and Seals

• Metal Seals– For very high or low temperatures– Long service life– Require high contact pressures to seal but

must have lower hardness than the mating face

– Plastic deformation of the material occurs on assembly and they can generally only be used once

Vacuum Fundamentals

Joints and Seals

• Metal Seals - CONFLAT– Uses a copper gasket – knife

edges cut into the copper– Suitable for UHV systems– Must use oxygen-free copper

to avoid reaction with hydrogen during bakeout

– Clamping must be uniform around the seal

Vacuum Fundamentals

Joints and Seals

• Metal Seals – Contact Forces and operating temperatures

Vacuum Fundamentals

Joints and Seals

• Areas to watch for ………– Elastomers can absorb large quantities of

cleaning solvents – avoid this method of cleaning. Remove ‘O’ rings from flanges or carriers before cleaning metalwork in this way

– ‘O’ Rings should be flexible and have no surface damage or nicks

– Store in clean non-static generating bags (ideally) to avoid dust contamination

Vacuum Fundamentals

Joints and Seals

• Areas to watch for ………– Avoid touching metal gaskets without gloves– Store in a dust and contamination free

environment– Do not use if any damage is evident– Clamping must be uniform– Tightening must be done in a controlled

sequence to balance forces

Vacuum Fundamentals

Joints and Seals

• Areas to watch for ………– Avoid heating the system too rapidly to avoid

excess stress on gasket or bolts – 150oC per hour maximum rise

– Do not touch knife-edges in order to avoid burrs

– Use as few detachable joints as possible– If in doubt, don’t use it!

Vacuum Fundamentals

Feedthroughs• Electrical

– In addition to vacuum considerations the curent and voltage also affect material choice

– Ceramic feedthroughs (eg. Aluminium Oxide) give good insulation resistance and permit high voltages

Vacuum Fundamentals

Feedthroughs• Issues

– High temperatures reduce insulation properties of ceramics and current carrying capacity of wire

– Too small clearances can lead to flashovers in the pressure range down to 10-6Pa

– Feedthroughs are sometimes potted with resins to reduce the risk (select vacuum compatible ones though and ensure they are fully cured)

Vacuum Fundamentals

Valves• A necessary part of most vacuum systems• To control flow and provide a ‘shut-off’

capability• Three main operating methods

– Manual– Electromagnetic– Pneumatic

Vacuum Fundamentals

Typical Valves• Rough to medium vacuum

– Diaphragm valves• Flexible elastomer diaphragm deformed onto a polished

‘seating’ surface by screw action• Mechanism seperated from gas path so no contamination to

operating parts• Choice of diaphragm material dictated by content of process

gases• Manual operation

Vacuum Fundamentals

Typical Valves• Diaphragm Valve

Vacuum Fundamentals

Typical Valves• Medium to High Vacuum

– Bellows • Can be manual, electromagnetic or pneumatic

actuation• Mechanism seperated from gas path by flexible

stainless steel bellows• Seal achieved by elastomer ‘O’ ring on ‘seat’• Longer gas path – poorer ‘conductance’

Vacuum Fundamentals

Typical Valves• Bellows Valve

Vacuum Fundamentals

Typical Valves• Medium to High Vacuum

– Gate Valves • Can be manual, electrical or pneumatic actuation• Seal achieved by balls forcing the two gates apart• Large diameter possible• No disruption to gas path – good ‘conductance’

Vacuum Fundamentals

Typical Valves• Ultra High Vacuum

– Must have high conductance, low outgassing and maximum cleanliness

– Stainless steel construction– Similar construction to bellows valves but use knife-

edge seals on copper pads – CONFLAT system– Precision guidance is necessary to ensure

repeatability of seating

Vacuum Fundamentals

Typical Valves• Ultra High Vacuum

– Bakeable to high temperatures (450oC)– Can be linked to control systems

Vacuum Fundamentals

Typical Valves• Actuation Systems

– Manual• Requires accesibility to system – not integrated into

control system

– Electromagnetic• Solenoid operated• Fast acting• Can be linked to control system • Can be designed to fail ‘safe’

Vacuum Fundamentals

Typical Valves• Actuation Systems

– Pneumatic• Operated by compressed air• Single-acting – spring return• Double-acting – air return• Fast acting• Can be linked to control system • Can be designed to fail ‘safe’ (single-acting)