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.
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
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