Vacuum Technology

Rajeev Mehta

Part – 1: Basics

What is Vacuum ? What’s the need ?

Lets first see Pressure........

Molecules moves in straight path

Hit other molecule or wall of container

Imposes force on wall This force per unit area

is PRESSURE

P = F/A

Pressure due to 1 N on 1 m2 area is 1 Pa

Force = (mass*acceleration) or F=ma

The earth's gravity exerts an acceleration of 9.8 m/s2

A column of air 1 m2 in cross section, extending through the atmosphere,

has a mass of roughly 10,000 kg

=> F = 10000 (kg) 9.8 (m/s2) 1 105 kg . m/s2

=> F 1 105 N

Thus pressure exerted by this on area of 1 m2

=> P = F/A = 1 105 N / 1m2 = 1 105 N . m-2

P = 1 105 Pa

Fluid Pressure

Pressure is due to fluid weight above it

P = gh P = P

a±gh

Units of Pressure

Pa Bar Atm To rr

Pa (1) 1 1.0000E-5 9.8690E-6 7.5010E-3

Bar (1) 100000 1 9.8690E-1 7.5010E+2

Atm (1) 101325 1.01325 1 760

To rr (1) 133.22 1.3322E-3 1.3160E-3 1

Pressure Unit

1 atm= 760 torr = 1.0132 bar = 1.013x105 Pa = 14.7 psi

Pressure Equivalents

0

14.7

29.9

760

760

760,000

101,325

1.013

1013

gauge pressure (psig)

pounds per square inch (psia)

inches of mercury

millimeter of mercury

torr

millitorr or microns

pascal

bar

millibar

Atmospheric Pressure (Standard) =

THE ATMOSPHERE IS A MIXTURE OF GASES

PARTIAL PRESSURES OF GASES CORRESPOND TO THEIR RELATIVE VOLUMES

GAS SYMBOL PERCENT BY

VOLUME

PARTIAL PRESSURE

TORR PASCAL

Nitrogen

Oxygen

Argon

Carbon Dioxide

Neon

Helium

Krypton

Hydrogen

Xenon

Water

N2

O2

A

CO2

Ne

He

Kr

H2

X

H2O

78

21

0.93

0.03

0.0018

0.0005

0.0001

0.00005

0.0000087

Variable

593

158

7.1

0.25

1.4 x 10-2

4.0 x 10-3

8.7 x 10-4

4.0 x 10-4

6.6 x 10-5

5 to 50

79,000

21,000

940

33

1.8

5.3 x 10-1

1.1 x 10-1

5.1 x 10-2

8.7 x 10-3

665 to 6650

After Pressure............Vacuum

Latin word VACUUS => Empty Pressure above sea level....... @ Sea Level => P ~ 101325 Pa h ~ 10000 m => P ~ 26500 Pa h ~ 161 km => P ~ 0.0007 Pa

Why is a Vacuum Needed?

To move a particle in a (straight) line over a large distance

Why is a Vacuum Needed?

Contamination

(usually water)

Atmosphere

Clean surface

(High)Vacuum

To provide a clean surface

Few Basics.......................

Ideal Gas Law ... pV = n RT N

A = 6.022 X 1023 /mole

R = 83.14 mbar L/Kelvin mole k = R/N

A = 1.380650 X 10-23 J/K

p

n = 101325 Pa, T

n = 273 K, V

n = 22.414 L

1 mole of particles (6.022 X 1023) occupy

22.414 L @ pn & T

n In 1 L @ 1 mbar & 293 K there are 2.478 X 1019

particles

Some results from Kinetic Theory

Average kinetic energy 21 3

2 2m kT

8 8145

kT RT T

m M M

2

1

2 d n

Impingement Rate

Mean Velocity

Mean free path

2 2

ApNp

JmkT MRT

Mean Free Path

• Mean free path of a particle, is the average distance the particle travels between collisions with other particles.

• The magnitude of mean free path depends on the

characteristics of the system the particle is in:

diameter molecule d

eunit volumper particles of no.η

area sectional cross effective

Path FreeMean

constant22

12

where

pp

kT

d

Vacuum Range Pressure # of Molecules MFP

Ambient 1000 mbar 2.7 X 1019 68 nm

Low Vacuum 300 - 1 mbar 1019 - 1016 .1 – 100 m

Medium Vacuum 1 – 10-3 mbar 1016 - 1013 .1 – 100 mm

High Vacuum 10-3 -10-7 mbar 1013 - 109 10 cm – 1 km

UH Vacuum 10-7 -10-12 mbar 109 - 104 1 km – 105 km

Mean free path is important parameter for

vacuum system

If MFP < vacuum system dimensions then flow

is viscous flow, molecules are transported as bulk parcel

If MFP > vacuum system dimensions then flow is molecular flow, molecules are transported individually through diffusion

Viscous and Molecular Flow

Viscous Flow

(momentum transfer

between molecules)

Molecular Flow

(molecules move

independently)

d

λk

na) Nature of gas “Knudsen’s Number”

Gases flowing in a pipes are characterized by

Relative quantity of gas “Reynold’s Number”

dR

Where

= Mean Free Path

d = pipe diameter,

= gas viscosity

= mass density

v = flow velocity

FLOW REGIMES

FLOW

Viscous Kn < 0.01

High Pressure

<<d

Molecular Kn > 1

Low Pressure

> >d

Laminar R < 1200

Stream flow

Turbulent R > 2200

Chaotic flow

FLOW REGIMES

Viscous Flow:

Distance between molecules is small; collisions between

molecules dominate; flow through momentum transfer;

generally P greater than 0.1 mbar

Transition Flow:

Region between viscous and molecular flow

Molecular Flow:

Distance between molecules is large; collisions between

molecules and wall dominate; flow through random motion;

generally P smaller than 10-3 mbar

FLOW REGIMES Mean Free Path

Characteristic Dimension Viscous Flow: is less than 0.01

Mean Free Path

Characteristic Dimension Molecular Flow: is greater than 1

Mean Free Path

Characteristic Dimension Transition Flow: is between 0.01 and 1

Part – 3: Terminologies

Terminologies

Throughput or gas load – the quantity of gas

in pressure-volume units flowing in unit time

past some location in the system

Pumping speed – gives the volume of gas that

is removed from a system by a pump in unit

time

Conductance – gives the capacity of a tube to

allow a volume of gas pass from one end to

another in unit time

THROUGHPUT & GAS LOAD

Throughput and gas load can be defined as the

quantity of gas in pressure-volume units

flowing in unit time past some location in the

system.

Units: (torr x liters / sec) or (Pa x m3 / sec).

Gas in = gas out

or

Gas load = throughput

)( pVdt

dQ

PUMPING SPEED

Pumping speed is defined as the ratio of the

throughput of a given gas to the partial

pressure of that gas at a specific point near the

inlet port of the pump.

Less formally, it is the volume of gas (at any

pressure) that is removed from the system by

the pump in unit time.

Pumping speed is a measure of the pump's

capacity to remove gas from the chamber.

Units: liters per second (L/sec) or (m3/sec).

CONDUCTANCE Conductance is the ratio of throughput, under steady-

state conservation conditions, to the pressure

differential between two specified isobaric sections

inside the pumping system.

Informally, the conductance of a tube is its capacity to let

a volume of gas (at any pressure) pass from one end to

the other in a unit of time.

Conductance is a property of a passive (non-pumping)

component of a vacuum system, similarly as pumping

speed is a property of a pump.

Units: liters per second (L/sec) or (m3/sec).

Consider gas flowing

through the conductance, C.

The quantity of gas entering

in unit time must be the same

as that leaving.

Upstream, this mass occupies

a volume V1 and downstream

V2

So P1V1 = P2V2

Volumetric flow rate is

Throughput is

P1>P2

V

t

VQ P

t

Pumping speed is volumetric rate at which gas is

transported across a plane

inletP

QS

)( pVdt

dQ

dt

dVS

dt

dVP

inlet

SPinlet

Conductance is defined as

12PP

QC

Here P2 – P1 is drop in pressure

across the component

C1 C2 C3

Cp

Cp = C1 + C2 + C3

C1

C2

C3 Cs

Cs = 1

C1

1

C2

1

C3 + +

-1

For elements of a system connected in series,

we must add the conductance of these elements

as in an electrical circuit: 1/C = 1/C1 + 1/C2 +

1/C3 +…

For elements of a system connected in parallel,

we must add the conductance of these elements

as in an electrical circuit: C = C1 + C2 + C3 +…

Conductance depends on pressure in the low to

medium vacuum regions, and is independent of

pressure in high to ultrahigh vacuum regions.

Flow through cylindrical tube

In the viscous flow regime, the conductance is geometry,

pressure, and viscosity dependent.

here D & L are in meters, P is in Torr,

and η is in poise.

where D is tube diameter & L is length both

in meters, Pav is in Pa, and η is in Pa-sec.

Also if D & L are in cm, P is in Torr,

then η is in micropoise.

avP

l

4D32716Cs

l(

)

avP

l

4DC(

128)

3

sm

In the molecular flow regime, the conductance of a long

cylindrical pipe is given as

l

3DCM

TB

k

s

m

2

6

13

where D is tube diameter &

L is length both in meters,

M in Kg & T in K

Using M (air) = 28.97

Kg/KMole, T as 293K we

get

l

3DC

l

3DC

121120

12.1213

s

l

s

m

or

l

3DCM

Ts

l 81.3

here D & L both in cm, M

in AMU & T in K

And conductance of a short cylindrical pipe is given as

3l

4Dl

C l

D

M

TB

k

s

m

3

2

6

13 where D is tube diameter &

L is length both in meters,

M in Kg & T in K

Using M (air) = 28.97

Kg/KMole, T as 293K we get

3l

4Dl

C l

D

s

m

3

12.1213

As

l 12

C

Conductance of a apperture

of area A cm2 is

Pumping Speed

2P

QS

p

CPPQ )(21

1P

QS

eff

2P

QS

p

21

21)(

PC

QP

CPPQ

CSSPeff

111

Part – 2: How to Create it?

How to Create VACUUM...

Principle Used: Bernoulli’s

Principle (moving fluid reduces

pressure)

Operation is based on bulk flow

of fluids (viscous flow)

Ultimate vacuum is

approximately 24 Torr (vapor

pressure of water at 25°C)

P1, V1

P2, V2

ATM

V1 < V2, P1 >

P2

Types

1. Mechanical

2. Diffusion

3. Containment

1. Rotory Vane Pumps

2. Roots Blower

3. Scroll Pump

4. Diaphragm Pumps

5. Molecular Drag Pump 1 - 1x10-6 Torr

6. Turbo molecular Pump 1x10-3 - 1x10-8 Torr

1. Mechanical Pumps

Rough Pumps

Atm – 100mTorr

VACUUM PUMPING METHODS VACUUM PUMPS

(METHODS)

Gas Transfer

Vacuum Pump

Entrapment

Vacuum Pump

Reciprocating

Displacement Pump

Rotary

Pump

Diaphragm

Pump

Piston

Pump

Rotary

Piston Pump

Roots

Pump

Dry

Pump

Fluid Entrainment

Pump

Drag

Pump

Ion Transfer

Pump

Molecular

Drag Pump

Turbomolecular

Pump

Gaseous

Ring Pump

Turbine

Pump

Ejector

Pump

Diffusion

Pump

Adsorption

Pump

Cryopump

Getter

Pump

Getter Ion

Pump

Sputter Ion

Pump

Evaporation

Ion Pump

Bulk Getter

Pump

Cold Trap

Sublimation

Pump

Positive Displacement

Vacuum Pump

Kinetic

Vacuum Pump

Low Vacuum Pump

• Principle of operation :

(1) begin expansion cycle

(2) seal off expanded volume

(3) compress gas out exhaust

• Pump operation is based on bulk

flow of gas ; hence the pump works

in the viscous flow regime

• Used for obtaining "rough" vacuum

(10-3 Torr), which is the lower limit

of the viscous flow regime

•An oil seal between a phenolic vane and a steel

cylinder is used to remove gas from the vacuum

region and exhaust it to the atmosphere.

•This pump works from atmosphere to about 0.1

mTorr.

•Precautions must be taken at low pressures to avoid

oil back streaming into the vacuum vessel.

•It is also used as a backing pump for compression

pumps like a diffusion pump or turbo molecular

pump

The lobed (2 or 3)

rotors trap a

volume of air

against the stator

body and sweep it

around, exhausting

the air 180°from

the inlet.

Tight clearances

between the rotors

and the stator are

critical to trap and

moved through the

pump body.

Dry Pumps

Scroll

Gas transfer

10-2 torr

12-25 cfm

Screw

Gas transfer

10-3 torr

30-320 cfm

May handle aggressive gases

H V Pumps

Principle of operation: momentum

transfer by vapor jet stream

Individual molecules are "pushed"

toward exhaust by jet stream;

hence, the pump works in the

molecular flow regime

Used for obtaining "high" vacuum

(10-6 Torr)

2. Diffusion Pump

•Oil vapor forced through jets in

the stack transfer momentum to

gas molecules and force them

down through the pump and out

the exhaust

•Economical (no moving parts).

•If used with a cryogenic trap,

UHV can be routinely achieved.

•Problem: oil back-streaming

into the vacuum system

3. Turbo Molecular Pump

•Turbine blades rotating at high

speed transfer momentum to gas

molecules to force them out to the

exhaust (must be backed).

The pump is characterized by a

compression ratio and ultimate

pressure.

•UHV can be readily achieved

(better if used in combination with

a titanium sublimation pump).

•Sensitive to corrosive gases

•Mechanical wear

Gas Compression Ratio

• Since the pump works by

momentum transfer, the

compression ratio

depends on the atomic

mass.

• The thermal velocity of

light gas is much greater,

so the molecules are

pumped less efficiently.

OIL BACKSTREAMING

2

Pump Down Time

dt

dPVQS

eff

eff

V

tS

S

QePtP

eff

)(

0)(

)(

0)( V

tSeff

ePtP

eff

ultimate

S

QP

Early Stage Ultimate

P

P

S

Vt 0ln

Pump Down Time

GAS LOAD

Outgassing

Leaks

Virtual

Real

Backstreaming

Diffusion

Permeation

GAS LOAD (Q) IS EXPRESSED IN:

mbar liters per second

Typical Pump Down Curve P

ress

ure

(m

bar)

Time (sec)

10-11

10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 15 10 17

10+1

10-1

10-3

10-5

10-7

10-9

Volume

Surface Desorption

Diffusion

Permeation

Clean Surface

sec/0.5X10 (torr) P

2 RateImpacr Surface

221cm

TmK

P

B

Assuming a sticking coefficient of 1, time to cover a

clean surface with 1 monolayer of gas is

sec)(

1046

torrP

X

Part 4: Some Practical Aspects

Selection of Fore-pump

QI

QO

SM

SD

PI

PO

QI = Throughput of D.P.

= Pressure x PS DP

QO = Throughput of R.P.

= Fore Pressure x PS RP

QO > QI

PO x SM > PI x SD

Selection of Forepump Diffusion Pump: SD = 5,000 l/s; PI (max) = 1 mtorr; FP = 500 mtorr = PO (max)

•Speed of 10 l/s converts to: 36 m3h-1

•Use pump with at least 150 % safety margin: min 54 m3h-1 •CONDUCTANCES NOT INCLUDED

SM = PI x SD

PO

500

Size Forepump: ???

=

1*5000 10 (l/s) =

Case 1: A chamber is connected to a pump (300 l/s) through a opening having conductance of 12 l/s => Effective pumping speed is ~ 11.54 l/s

Case 2: Change pump (30000 l/s) => Effective pumping speed is ~ 12 l/s

Solution????????

Differential Pumping

A common requirement is to maintain

part of a system at a relatively low

pressure while another part is at a

relatively high pressure. We need to

calculate the pumping speed S2 required

to maintain the pressure P2

Assume C is small, so

P1 >> P2

then

1

2

2 2 1

LCP CQ

SP P S

VACUUM SYSTEM USE

1

2

4 5

1

2

3

4

5

6

7

8

9

Chamber

High Vac. Pump

Roughing Pump

Hi-Vac. Valve

Roughing Valve

Foreline Valve

Vent Valve

Roughing Gauge

High Vac. Gauge

9 8

8 7

3

6

7

VACUUM SYSTEM OPERATION

1

4

6

5

9

8

8 1

2

3

3a

4

5

6

7

8

9

Chamber

High Vac. Pump

Roughing Pump

Fore Pump

Hi-Vac. Valve

Roughing Valve

Foreline Valve

Vent Valve

Roughing Gauge

High Vac. Gauge

7

3

2

8

2

Start up

Rough Vacuum Gauge : 103 mbar

4

6

5

9

8 7

3

2 2

B : 103 mbar

Rotary Pump On

1.0 x 10-2 mbar

4

6

5

9

8 7

3

2 2

B : 103 mbar

Roughing Valve open

5.0 x 10-2 mbar

4

9

8 7

3

2 2

Roughing Valve

Close

Backing Valve Open

5.0 x 10-2 mbar

Rough Vacuum OK

Backing Vacuum OK

For DP • Start Water Cooling

• Start DP Heater

• Wait 20 minutes

For TP • Start Pump

1.0 x 10-3 mbar

8 7

3

2 2

5.0 x 10-2 mbar

Rough Vacuum OK

Open HV Valve

5.0 x 10-5 mbar

Experiment Chamber Ready

1.0 x 10-3 mbar

8 7

3

2 2

5.0 x 10-2 mbar

1. Close HV Valve

2. Switch off HV

Gauge

Shut down

3. Switch off heater

1000 mbar

8 7

3

2 2

5.0 x 10-2 mbar

2. Switch off HV

Gauge

Shut down

Open vent valve

For DP • Wait 30-40 min. for oil to cool • Close backing valve • Stop cooling water

For TP • Close backing valve • Stop the Pump • Vent if required

PRESSURE DISTRIBUTION IN A LONG TUBE WITH A STEADY STATE DISTRIBUTED GAS LOAD

q is the outgassing rate per unit area

Pressure at any distance X From the entrance to pipe

Calculate the pressure at the inlet and at 200 cm from inlet of a 10 cm diameter and 300 cm Long stainless steel pipe. Pumping speed at inlet = 100l/s , Outgassing of SS = 5 x 10-8 Torr. L cm-2s-1

VACUUM TECHNOLOGY

Measuring Devices

----Gauges----

For many purposes, it is necessary to measure the level of vacuum

achieved in a vacuum system. Given the very large range of pressures

produced in vacuum systems (up to 19 orders of magnitude in some

systems) there is no single gauge capable of measuring the full range of

pressures. Most vacuum systems must have at least two different types of

gauges, or even three.

Types Mechanical Gauges

Mechanical movement of a surface (diaphragm), Independent of gas

properties, P> 10-5 torr

Gas Property Gauges

Bulk property, e.g., thermal conductivity, viscosity, Dependent on gas

composition, 102 – 10-4 torr

Ionization Gauges

Charge collection, Dependent on gas composition, 10-4 – 10-10 torr

How to Measure Pressure

BAROMETER

WATER

Mercury: 13.58 times

heavier than water:

Column is 13.58 x shorter :

10321 mm/13.58=760 mm

(= 760 Torr)

10.321

mm

MERCURY

760

mm

29.9

in

Pressure is due to fluid weight above it

P = gh P = P

a+gh

Range: 1000 Torr to 1x10-4 Torr

Accuracy: 0.01%, Response: 10ms

Range : 10 to 1 x 10-3 mbar

Accuracy ± 10%

Pressure dependence of the ability of gas to conduct heat

Thermocouple Gauge

Heat Capacity is proportional to number of molecules in given volume

(in molecular flow region)

Thermal conductivity gauges

rs, l @ Ts

re, l @ Te

P

lrTTPQHsesC

2)(

This should dominate, at high T

radiative losses are dominant as

they depends as T4

How the gauge works

Typical filament temperatures

450OC at 10-3 mbar

40OC at 5 mbar

Above 5 mbar there is not much change in temp.

Not harmed by sudden exposure

Pirani Gauge

• Change in resistance is the measure of vacuum

• Heat loss to environment is proportional to no. of molecules

• At pressures above 10 mbar ?

Range : 1000 to 1 x 10-4 mbar

Constant – voltage Pirani Range : 10 to 1 x 10-3 mbar

Constant resistance [temperature] Pirani

• Range : 1000 to 1 x 10-3 mbar • Voltage is regulated to keep the resistance constant Irrespective Of the heat loss. • Voltage applied to balance the bridge is the measure of the pressure • Zero adjustment of the gauge head

Pirani Gauge

Ionization gauges

• Ionization of gas molecules by electrons • Collection of the Ions • Amplification of the Ion current by sensitive and stable electronics

PRINCIPLE

Mounting of Gauge Heads

WRONG

RIGHT

VACUUM TECHNOLOGY

Leak Detection

Introduction

The need for leak detection

• Everything leaks! It is a matter of how much of a leak is tolerable.

• Thus the question is:

What is the maximum acceptable leak rate consistent with reasonable performance life of the product?

Kinds of Leaks

• Real Leaks

– Air Leaks

– Porosities

• Permeation leaks

– O-rings

– Glass

•Virtual Leaks

– Outgassing

– Trapped Volumes

Sources of real leaks are:

• Imperfect joints or seals, including:

• Welds

• Brazed joints

• Soldered joints

• Glass to metal seals

• O rings and gaskets etc.

• Imperfections in materials

JOINTS

Problems that appear to be Leaks

Outgassing

Leaks

Virtual

Real

Backstreaming

Diffusion Permeation

Trapped Volumes

Leak Rates over Time

LEAK RATES

10 -1 STD CC/SEC --- 1 CC/10 SEC

10 -3 STD CC/SEC --- 3 CC/HOUR

10 -5 STD CC/SEC --- 1 CC/DAY

10 -6 STD CC/SEC --- 1 CC/2 WEEKS

10 -7 STD CC/SEC --- 3 CC/YEAR

10 -9 STD CC/SEC --- 1 CC/30 YEARS

Why helium is used ?

Helium is very light and small

Low concentration in air (0.0005%)

Permits dynamic testing

Permits non-destructive testing

Helium is safe

CONVENTIONAL LEAK DETECTOR

1

2

3

4

5

6

7

8

9

10

11

12

Test Piece

Test Port

High Vac. Pump

Roughing Pump

Fore Pump

RoughingValve

Test Valve

Pump Valve

Spectrometer Tube

Cold Trap

Roughing Gauge

Vent Valve

5

1

2

7 6

12 11

4

8

9 10

3

Ion Separation in Magnetic Field

Ion Source

To Pre-Amplifier

Collector

Magnetic Field

Deflects He Ions

90O, other ions

more or less than

90O.

He ions pass

through slit and

are collected

Lighter ions:

more

Heavier ions:

less

Ion Gauge

Tracer probe leak detection technique

Leak detectors are calibrated

with the “permeation leak”

A permanently evacuated and sealed device and working pressure

is below 10-5 torr. If the volume is 2cm3 estimate the life of the

device if a leak = 1 x 10-7 torr l/s is there in the device.

1 x 10-5 x 2

1 x 10-7 x 1000 =

= 0.2 s

Quartz and Pyrex : UHV – He Leak

Ceramics – Alumina : Good internal parts

Brass : Contains Zinc – Outgasses at 10-6

OHFC Copper : reduces outgassing for heating

Stainless Steel 300 series (304, 316) : Weldable

Aluminum 6000 series : Porous and Oxide surface – outgasses

more than stainless

Plastic : Outgas at 10-7

Polyimide, Delron, Kapton – Good, Be careful of wiring shield

Material

Practical Concerns

If 10-3 mol H2O in chamber surface and if the outgas pressure is

10-7 torr, how long it takes to pump down to 10-8 torr when the

pumping speed is 1000 ℓ/s?

Initial pumping throughput is

10-7 torr * 1000 ℓ/s = 10-4 torr ℓ/s

Total amount of water is

1 atm * 2.4x10-2 ℓ = 760 torr * 2.4x10-2 ℓ = 1.8 x 101 torr ℓ

Pump out time constant = t

hr50s101.8/storr10

torr101.8

rateinitial

amounttotalτ

5

4

1

Estimating the effect of Baking

When temperature rises by 100 oC, outgas rate rises by

roughly two orders of magnitude, i.e., 10-5 torr instead of

10-7 torr

Initial pumping throughput is 10-5 torr * 1000 ℓ/s = 10-2 torr ℓ/s

hr5.0s108.1s/torr10

torr108.1τ

3

2

1

to P = 10-10 torr, P0/P = 105

hr11.5P

PlnτΔt 0

Often for a chamber with big surface area

Big chamber ≠ big surface area

How to minimize surface area

Remove thick surface oxide:

electro polish chamber and parts

basic wash, acid wash

Oxide could be porous

Dirty surface is thicker

Strong detergent is much more efficient than solvent Cleaning

ultrasonic bath

bake uniformly is important to avoid distortion

Don’t bake oily surface. oil tar

Aluminum foil on chamber, heating tape on the

aluminum foil, another layer of aluminum foil to

reduce heat loss

Using plastic parts

plastic may absorb H2O to 1~2 % w/w

more troublesome is that most plastics cannot be baked

Use only

Inert material: Teflon, PE, PP, Kel-F, Viton,

Teflon insulated wire

High temperature material : polyimide (Vespel,

Kapton), Kalrez (O-ring)

Bakable to

200oC less inert than Teflon

Material outgas (volume outgas)

SS: H2 & CO.

SS316L can be vacuum firing at 1000 oC to remove

deeper contaminants

Al alloy: less H2 & CO. Bakable to 120 oC

Zn & Cd alloy have high vapor pressure

High temperature increase outgas @ bake out

Cooling can reduce outgas @ use

It lasts forever! More than your life!

Metal seal: copper gasket & ConFlat flange are preferred

Sealing Concern:

100% seal

low outgas

bakable

O-ring seal: Viton O-ring bakable to 100 oC

15 ~ 18 % compression to seal

volume compression is not allowed

sealing surface polish is important

small leak is possible (Hard to find small leaks)

convenient

non-consuming

Careful to use viton gasket on conflat flanges very easy to

leak for size

larger than 4.5” not cheap

Leak Check

Spread CH3OH or C2H5OH on a possible leak to see if pressure rises

Acetone is OK for metal, bad for O-ring, bad for health. Response

rise time ~ few seconds, Don’t move too fast. It takes very long to

dry out the solvent.

Helium leak check:

Spread He to see if PHe rises. RGA or He leak detector

fast ≲ 1 sec ∵ light mass ∴ fast speed

He low background inert

Good vacuum practices

No leak

Clean: traps for oil pumps: molecular sieve, LN2

Metal & non-porous ceramic is excellent

Plastic and grease: as less as possible

Confident sealing. Finding a leak is labor consuming.

Bakable for 10-10 torr or better

Good local conductance for pumping speed

Gas composition (partial pressure) is often more important than the

total pressure, as most vacuum parameters are species dependent.

e.g. surface laser burn, background masses

RGA is very nice to have

Thanks

“A vacuum is a hell of a lot better than

some of the stuff that human replaces it with.”

“Spaghetti can be eaten most successfully if you inhale

it like a vacuum cleaner.”

Multiple Flow Systems

Cold Cathode Gauge/ Penning Gauge

How the Gauge works

Cold Cathode Gauge

• Ionization current is the measure of vacuum

• Range 10-2 to 10-8 mbar

• Loses sensitivity below 10-7 mbar

• Accuracy + 100%, -50%

• 10-5 mbar ~ 5 x 10-6 mbar / 2 x 10-5 mbar

• Switch on < 10-2 mbar

• Gets contaminated

• Can be cleaned

Cold Cathode Gauge

6.4 Calibrated Leak

•Known flow of helium (ex., 5.0 x 10-7 std cc/sec at 20°C)

•Set leak detector scale zero with valve closed

•Leave valve open remainder of time

Valve Pyrex Finger

Port Adapter Helium Filled Reservoir

6. Non Evaporable Getter

• Gas molecules can be sorbed by a chemical reaction when

they impinge on the clean metal surface of the getter

material.

• To achieve a clean metal surface the oxide layer must be

removed in an activation process.

• For that the getter must be heated to a certain temperature.

• During activation the passivating layer diffuses into the

bulk material.

• The metal surface saturates with cumulative sorption of

gas molecules and a new oxide layer will be created.

• To achieve the full pumping speed the NEG must be

reactivated.

Non Evaporable Getter Pumps

NEG Strips

NEG Strips

• ST707 NEG strips

• 350-450°C activation temperature

• High pumping speed and pumping

capacity

water cooling electron channel NEG coating water cooling

NEG Coating

• NEG Coating, Ti-Zr-V, 2μm

• Activation temperature 200 °C

• High pumping speed and low

desorption rate.

Q1. A 22.4 L vessel contains 2 mole of H

2 and 1 mole of N

2 at

Tn. What are the mole fractions

and the partial pressures of the components in the vessel?

Molar Fractions:

Total no. of moles = 3, x(H

2) = 2/3 , x(N

2) = 1/3

Partial Pressures:

For H2,

p = nRT/V = 2026 mbar, For N

2,

p = nRT/V = 1013 mbar, p (Total) = 3039 mbar

Q2. A 0.5 L vessel contains H

2/N

2 at 500 mbar & T

n. If

partial pressures of H2 is 200

mbar, what are the mole fractions of H

2 & N

2?

Use pV = nRT as n = pV/RT

nH

2 = 4.41 X 10-3 mole

nN2 = 6.61 X 10-3 mole

n (Total) = 11.02 X 10-3 mole => x (H

2) = 0.4

x (N

2) = 0.6

Q1. A 4 L of N2 has a pressure of 500 mbar at 20°C. What is the mass of the

gas?

Q2. The composition of dry air in % volume is as follows : N

2=78.1, O

2=20.9, Ar=0.9, CO

2=0.03 and other 0.07

Find the partial pressure of each at Tn when total pressure is p

n.

Q3. The interior of a spherical vessel of radii 0.5 m is covered with monolayer of a gas particles each having cross sectional area of 10-19 m2. Find the increase in the pressure if all the particles are desorbed at 300°C.

Q4. Calculate the mean free path of N2 (molecular diameter = 3.7A ) at 1 torr

and 293 K.

Q1. What is the average velocity of gas molecules in terms of its molecular weight and temperature ? Fine this for nitrogen molecule at 20C

Q2. What is mean free path? What is the relationship between pressure and mean free path of a molecule. At 25C for nitrogen find number of molecules/cm3 and the mean free path at following pressures (in torr) a) 760 b) 1 c) 10-6

Q3. Find the relationship between following units Torr, mtorr, Pascal, atmosphere, mmHg and bar

Q4. Describe basic fluid flow regimes. How does it depends on characteristic dimension of the system.

Q1. For a system with gas load of 110-5 torr l/sec, required ultimate vacuum is 110-7 torr. System is connected with a pump by a cylindrical 4” long tube of diameter 2.5”. Find a suitable pump for this system. Can you improve on this system?

Q2. How can one determine the pumping speed of a pump experimentally?

Q1. A chamber is connected with a 3 m long tube of 20 mm radius to a pump of speed of 25 l/s. a) Find the conductance of the tube and effective pumping speed at the chamber. Assume molecular flow. b) What can be done to increase the effective pumping speed of the system? c) If pump is replaced with 1000 l/s pump OR connecting tube is replaced with 1m long tube having 40 mm diameter, which of the options suggested is more effective for increasing the effective pumping speed.

Q2. Explain the differential pumping.

Q3. At what temperature 4He atoms will have mean velocity of 150 ms-1?

Sorption Pump – Design and Physics

• Uses cryosorption to pump gases

• Liquid nitrogen cools the exterior of the pump to 75K (-196o C) and internal fins aid in cooling of sieve material

• Cooled porous trapping material (zeolite, charcoal) is used to cryosorb the gases entering the pump inlet

• Can be easily re-activated by warming to room temperature and removing stopper

Sorption Pump – Bank Design

• Banks of 2 or more

sorption pumps are often

staged together

• Faster pumping speeds are

achieved by connecting

the pumps in parallel

• Lower base pressures are

reached by operating a

bank of pumps in series

Rouging of 200 L volume using a bank of 3 pumps in series

Sorption Pump - Pros and Cons

• Pros

– Clean, oil free

roughing pump

– High pumping

speeds

– Inexpensive

– Easily regenerated

– Durable

• No moving parts

• Cons

– Limited base

pressure range

– Finite pumping

capacity

• Determined by

pump volume and

gas species

Roughing pump comparisons

Oil Sealed Pumps

Type Advantages Disadvantages

Rotary Vane

Low ultimate pressure.

Low cost

Long pump life.

Backstreams oil.

Produces hazardous

waste.

Rootes Lobe Very high pumping

speed.

Frequent maintenance.

Requires a purge gas.

Requires a backing

pump.

Must be absolutely

horizontal.

Roughing pump comparisons

Dry Roughing Pumps

Scroll

Clean.

Low "dry" ultimate

pressure.

Easily servicable

Quiet.

Evolved from air

conditioning

compressor so

technology is

well known.

Limited bearing life.

Limited scroll life.

Permeable to small

gases.

Not hermetically

sealed.

Clean applications

only.

Sorption Clean.

No moving parts.

Requires LN2.

Requires regeneration.

Limited capacity.

4. Ion Pump & Getter Pump

A high voltage combined with a

magnetic field causes electrons to

travel in a helical path with an energy

sufficient to ionize gas atoms.

The ions are accelerated so they strike

a Ti plate and become buried in the

plate.

• Can be started below 10-6 mbar.

• Pumping speed is gas dependent and drops off below 10-9 mbar.

• Buries the gas in the plate, so no backing pump is required.

• A little less expensive than turbo pumps.

Voltage ~ 3000 – 7000 V DC

B ~ 1000 – 2000 Gauss

Sublime Ti-Mo,

continuous/pulse

Chemical reaction

Gas selective: no inert gas

5. Cryo Pump

Cryopump – Physics • Uses cryocondensation, cryosorption as

well as cryotrapping to pump gases

• Cryocondensation happens when gas

molecules impinge on the cold surface

and adhere to the surface

• Cryosorption occurs when the gas is

adsorped by the surface

• Cryotrapping happens when the gases at

the surface are trapped in the growing

solid layer of a second gas.

• As gas enters pump heavier gas

molecules are captured by the first array.

• Lighter, harder to trap gases are captured

at the colder second array

Cryopumps - Design • Liquid He and liquid N used to cool the

system

• 1st stage array is held around 75K by

LN2

• 2nd stage array is held around 4K by

LHe

• 3rd stage array is held around 4K by

LHe

• Closed-loop, gaseous

helium refrigerator to cool

the system.

• 1st stage array is held

around 50K-80K

• 2nd stage array is held

around 10K-20K

Cryopump – Pros and Cons

• High speed pumping

• Clean, oil free pumping

• Easy regeneration

• Has ability to perform

several pumpdown cycles

before regeneration

• Durable

– No moving parts

• Can be rebuilt by various

specialists

• High pumping speed models

can be expensive

• Limited pumping capacity

before regeneration

Ultimate Vacuum

Steady decrease

interrupted by gauge

limitations 1920-1950

Bayard-Alpert gauge

introduced in 1950

Plateau ~1x10-14 Torr

for nearly 3 decades

again

10-14

Ultim

ate

Vacuum

(Torr

)

10-6

10-12

P.A. Redhead Vacuum 53 (1999) 137

10-10

10-8

Some Other Pumps

Turbo molecular pump: ultra-fast fan blades

knock molecules out of vacuum system

Cryo pump: molecules are frozen out

Sorption pump: molecules diffuse into

absorbing material

Sputter ion pump: molecules are ionized and

buried

Cold Traps

Purpose :

Prevents pump oil from

contaminating vacuum system

Prevents organic solvents from

reaching pump (and degrading oil and

seals)

Acts as a pump by condensing

molecules

How to Measure Pressure

Manometer

Pressure is due to fluid weight above it

P = gh P = P

agh

How to Measure Pressure

BAROMETER

WATER

Mercury: 13.58 times

heavier than water:

Column is 13.58 x shorter :

10321 mm/13.58=760 mm

(= 760 Torr)

10321

mm

MERCURY

760

mm

29.9

in

What if we take

bigger diameter

tube?