Vacuum Systems - Experimental Techniques - Lecture Slides, Slides of Electrical Engineering

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Vacuum

Systems

Why

much

of

physics

sucks

Why

Vacuum?

Anything cryogenic (or just very cold) needs to get rid of the air - eliminate thermal convection; avoid liquefying air - Atomic physics experiments must get rid of confounding air particles - eliminate collisions - Sensitive torsion balance experiments must not be subject to air - buffeting, viscous drag, etc. are problems - Surface/materials physics must operate in pure environment - e.g., control deposition of atomic species one layer at a time

Properties

of

a

vacuum

Vacuum Pressure (torr) Number Density (m

• ) M.F.P. (m) Surface Collision Freq. (m
• ·s
• ) MonolayerFormation Time (s) Atmosphere 760

 10 25 7  10  8 3  10 27

 10

Rough 10  3

 10 19

4  10 21

 10

High 10  6

 10 16 50 4  10 18

Very high 10  9

 10 13 50  10 3 4  10 15

 10 Ultrahigh 10  12

 10 10 50  10 6 4  10 12

 10

Kinetic

Theory

The particles of gas are moving randomly, each with a unique velocity, but following the Maxwell Boltzmann distribution:

The average speed is: - With the molecular weight of air around

g/mole (~75%

N

2

O

2

K:

m

29 

 10 ‐^27 kg

= 461 m/s

note same ballpark as speed of sound ( m/s)

Mean

Free

Path,

cont.

Now that we have the collision frequency, Z , we can get the average distance between collisions as:  = v / Z - So that - For air molecules, r  1.  10 ‐^10 m

So  

 10  8 m = 68 nm at atmospheric pressure

Note that mean free path is inversely proportional to the number density, which is itself proportional to pressure

So we can make a rule for  = ( cm)/(P in mtorr)

Relevance

of

Mean

Free

Path

Mean free path is related to thermal conduction of air

if the mean free path is shorter than distance from hot to cold surface, there is a collisional (conductive) heat path between the two - Once the mean free path is comparable to the size of the vessel, the paths are ballistic - collisions cease to be important - Though not related in a

way, one also cares about transition from bulk behavior to molecular behavior

above 100 mTorr (about

atm), air is still collisionally dominated (viscous) -  is about 0. mm at this point - below 100 mTorr, gas is molecular, and flow is statistical rather than viscous (bulk air no longer pushes on bulk air)

Evacuation

Rate

What you care about is evacuation rate of vessel

S

Q

P

1

but pump has

S

p

Q

P

2

Q

is constant (conservation of mass)

Q

P

1

P

2

C

from which you can get: 1/ S = 1/ S p

1/ C

So the net flow looks like the “parallel” combination of the pump and the tube: - the more restrictive will dominate - Usually, the tube is the restriction - example in book has 100 l/s pump connected to tube 2. cm in diameter, 10 cm long, resulting in flow of 16 l/s - pump capacity diminished by factor of 6! P 1 P 2 Q Q Q C pump: S p Docsity.com

Tube

Conductance

For air at

K:

In bulk behavior

mTorr): C = 180  P  D 4 / L (liters per second)

D , the diameter, and L , the length are in cm; P in Torr - note the strong dependence on diameter! - example: 1 m long tube 5 cm in diameter at 1 Torr: - allows 1125 liters per second - In molecular behavior

mTorr): C = 12  D 3 / L

now cube of D - same example, at 1 mTorr: - allows 0. liters per second (much reduced!)

Mechanical

Pumps

Form of “positive displacement pump” - For “roughing,” or getting the the bulk of the air out, one uses mechanical pumps - usually rotary oil ‐ sealed pumps - these give out at ~ 1– mTorr - A blade sweeps along the walls of a cylinder, pushing air from the inlet to the exhaust - Oil forms the seal between blade and wall

Lobe

Injection

Pumps

Can move air very rapidly - Often no oil seal - Compression ratio not as good

Cryopumping

A

cold

surface

condenses

volatiles

(water,

oil,

etc.)

and

even

air

particles

if

sufficient

nooks

and

crannies

exist

a

dessicant,

or

getter,

traps

particles

of

gas

in

cold

molecular

sized

“caves”

Put

the

getter

in

the

coldest

spot

helps

guarantee

this

is

where

particles

trap:

don’t

want

condensation

on

critical

parts

when

cryogen

added,

getter

gets

cold

first

Essentially

“pumps”

remaining

gas,

and

even

contin ed o tgassing

Ion

Pump

Ionize

gas

molecules,

deposit

ions

on

chemically

active

surface,

removed

by

chemisorption

Best

use

is

for

Ultra

High

Vacuum

applications

‐ 11

Torr)

Current

is

proportional

to

pressure

(pump

is

also

a

pressure

gauge)

No

moving

parts,

but

efficient

only

at

very

low

pressures

slide courtesy O. Shpyrko

Example

of

RGA

spectra,

He:Ne

mixture

slide courtesy O. Shpyrko

Typical

problems

in

achieving

UHV:

Actual

Leaks

(valves,

windows)

Slow

pump

down

times

“Virtual”

leaks

Outgassing

bulk

and

surfaces

Solutions: •

Leak

testing

Re

design

of

vacuum

chamber

Bake

out

slide courtesy O. Shpyrko