PHYSICS 225, 2ND YEAR LAB

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PHYSICS 225, 2ND YEAR LAB

• VACUUM TECHNOLOGY

G.F. West
Thurs, Jan. 12
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INTRODUCTION

• Humans work in a gaseous environment.
• Although less dense than solids or liquids, the
  normal gas environment greatly influences much
  physics.
• Often, to do interesting and important physics,
  one must get rid of it.

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WHAT IS A VACUUM ?

• The absence of appreciable matter (i.e., atoms,
  molecules, ions, particles), usually as gas.
• How do we measure the amount of gas?
• As mechanical pressure on container walls or
  neighbouring gas.
• Pressure Force /unit area N/m2 Pa (SIU)
• bars (cgs)
• psi (USA/Imperial)
• Atm (Chem)
• mmHg Torr (traditional physics)

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KINETIC THEORY OF GASSES (The colliding
billiard ball model )

• Gas atoms have a range of velocities, increasing
  with temperature.
• Gas molecules therefore have appreciable energy
  and momentum.
• Pressure is the cumulative result of the momentum
  changes in collisions.
• Collision likelihood is usually expressed as
  mean free path (average distance molecules move
  between collisions).

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LEVELS OF VACUUM

• Air at 273 K, molecular Vrms 485 m/s
• Pressure (V HV UHV UHV )
• Atm, 1.0 1/760 - - - -
• kPa, 101.3 0.13, 0.13Pa - - -
• psi, 14.7 0.02 - - -
  -
• Torr, 760 1 1e-3 1e-6 1e-9
  1e-12
• Mean Free Path, at 273 K, mol radius 0.3 nm
• m 1e-7,7e-5,7e-2, 7e1,7e4, 7e7

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VACUUM PUMPS

• Mechanical with valves, vanes, diaphrams
• (Roughing pumps, forepumps).
• Entrainment principle
• Diffusion pumps,
• Turbomolecular pumps.
• Entrapment principle
• Cryopumps,
• ionpumps (gettering)
• TI sputtering
• molecular sieves (zeolites and other synthetic
  microporous compounds)

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PROBLEMS WITH PUMPING

• Need for a forepump.
• Contamination of vacuum by backflow.
• Gas selectivity.
• Need for regeneration.
• Virtual leaks.
• Speed, ease of cycling to lab conditions.

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VAPOUR PRESSURE
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MULTI-PUMP SYSTEM
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ROTARY VANE FORE PUMP
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DIFFUSION PUMPS
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TURBO-MOLECULAR PUMPS
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TURBO-MOLECULARPUMP
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ION PUMP
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ION VACUUM GAUGE
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THE VACUUM ENCLOSURE

• Materials -
• (Glass stainless steel predominate.)
• Requirements-
• Chemically inert
• Cleanable
• Bakeable
• Strong
• Workable, (e.g., machineable)

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TYPICAL VACUUM SYSTEM

• Forepump
• Main vacuum pump with cooling.
• Gate valve,(to allow pump turn off).
• Vacuum gauges, if not intrinsic to pumps.
• Cold trap(s) (Liquid air).
• Sample inlets, if required.
• View manipulation ports, experiment area.
• Bakeout system.

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VACUUM SYSTEM COMPONENTS

• Older systems mainly were hand fabricated from
  glass by artisanal glassblowing.
• New systems mainly are constructed from
  commercially manufactured stainless steel
  components using (e.g.,)-
• Bolted flange connections
• Thin metal seals
• Glass- to-metal sealed electrical connections
• Bellows connected or in-vac bakeable manipulators
• Special window glasses for radiation entry/exit

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VACUUM COMPONENTS
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USES OF VACUUM TECHNOLOGY
Some examples-

• Semiconductor lithography and surface coating.
• Analytical insts e.g., spectrometers,
  microscopes.
• Particle accelerators, HEP
• Space simulation
• Nanotechnoloy
• Surface physics.
• Gas lasers.
• Manufacturing of special materials.