PHYSICS 225, 2ND YEAR LAB

VACUUM TECHNOLOGY

G.F. West

Thurs, Jan. 12

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.

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);

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).

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, ~7e+1,~7e+4, ~7e+7

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)

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.

VAPOUR PRESSURE

MULTI-PUMP SYSTEM

ROTARY VANE FORE PUMP

DIFFUSION PUMPS

TURBO-MOLECULAR PUMPS

TURBO-MOLECULARPUMP

Specifications

MDP 5011

Pumping speed (L/s N2) 7.5

Ultimate pressure (Torr) 7.5 x 10-7

Compression ratio:

......N2 1 x 109

......He 2 x 104

......H2 1 x 103

Speed (rpm) 27,000

Max. ambient temp (°C) 50

Exhaust flange QF16

Pump weight (lb) 5.5

Power supply (VAC) 115

Other voltages available on request

ION PUMP

ION VACUUM GAUGE

THE VACUUM ENCLOSURE

Materials: -

(Glass & stainless steel predominate.) Requirements:-

Chemically inert Cleanable Bakeable Strong Workable, (e.g., machineable)

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.

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

VACUUM COMPONENTS

USES OF VACUUM TECHNOLOGY

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

Some examples:-