introduction to vacuum systems for particle … to vacuum systems for particle accelerators ......

Introduction to Vacuum Systems

for Particle Accelerators

Dr. Oleg B. Malyshev,

ASTeC Vacuum Science Group,

STFC Daresbury Laboratory, UK

Particle Accelerator School - 2013

Oleg Malyshev CI PAS, October 2013 slide 2

Outline

• Introduction • Definition of vacuum

• Classification of vacuum ranges

• A few simple formulas for vacuum

• Vacuum for particle accelerators • What is Vacuum Science and Technology?

• Gas composition in the atmosphere and in vacuum

• Pumps

• Gauges and RGAs

• The interaction between charged particles and residual gas molecules

• Sources of gas in a vacuum system

• BIEM and ion induced pressure instability

• NEG coated vacuum chamber

• Vacuum chamber at low temperatures

• Rough Calculation and Design of Vacuum System


• Vacuum (from Latin “vacua”) means empty

• this is an unreachable aim and dream

• this means that everybody who needs VACUUM is a dreamer!

• In practice, Vacuum is the gas state when P < Patm

• As soon as gas from a closed volume is pumped out all that

remains is called the vacuum

• this is the realistic approach

• or real science

• Vacuum is a problem for many applications and researchers and it is

a subject of

• Rarefied Gas Dynamics for studying gas flows, heat transfer, etc.

• Vacuum Science and Technology for applications and implimentation.

Introduction


There’s nothing in it!

Vacuum

Particles/m3

Atmosphere 2.5 x 1025

Vacuum Cleaner 2 x 1025

Freeze dryer 1022

Light bulb 1020

Thermos flask 1019

TV Tube 1014

Low earth orbit (300km) 1014

SRS/Diamond 1013

Surface of Moon 1011

Interstellar space 105


Classification of Vacuum Ranges

Vacuum Ranges

Criteria

Pressure (mbar) Gas density at RT (particles/cm3)

min max min max

Low (LV) ‹› << d 1 1000 ~1016 2.51019

Medium (MV) ‹› ~ d 10-3 1 ~1013 ~1016

High (HV) ‹› > d 10-6 10-3 ~1010 ~1013

Very High (VHV)

‹› >> d 10-9 10-6 ~107 ~1010

Ultra-High (UHV)

‹› >> d 10-12 10-9 ~104 ~107

Extreme Ultra-High (XHV)

‹› >> d < 110-12 < 104

‹› is the mean free path,

d is the effective size of a vacuum chamber (in many cases 0.5 cm d 50 cm)


• SI pressure Unit – Pascal (1 N/m2)

• Pa is used by all metrology labs, in Asia and ex-USSR

countries

• In Europe – mbar (100 Pa)

• In USA/Asia – Torr (133.322 Pa)

• Atmosphere = 1 bar = 105 Pa = 103 mbar 750 Torr

• Gas density unit - particles/m3

P – pressure, n – gas density (number gas density),

kB – Boltzmann coefficient, T – temperature.

Vacuum Units

BP nk T


Vacuum required in the particle accelerators

High energy particles collide with residual gas molecules that results in:

• loss of particles,

• loss of the beam quality.

Examples of vacuum specification for a high energy particle accelerator:

• 100 h vacuum life time at I = 560 mA after 100 Ah conditioning (for DLS);

• P(N2 eqv) = 10-8 mbar after bakeout and a week of pumping (for a buster);

• n(H2 eqv) = 1015 m-3 after 2 years conditioning (for the LHC); etc...

H2

H2

H2O

CO2

CH4

CO Beam

CO

CH4

CO2


• The main reason is beam-gas interaction (e.g.

scattering)

• Single pass machines

• Increases beam size (emittance)

• Increases radiation hazard

• Encourages recombination

• Stored beam machines

• Increases beam size

• Reduces beam lifetime

• Increases radiation hazard

Vacuum in Accelerators


Interaction between the Beam and Residual Gas Molecules

Interaction

Inelastic

Elastic

),( pX Z

The beam current I decays with time t as:

where is the total beam lifetime given by

the beam lifetime beam due to different

Quantum, Touschek, particle lifetime, etc,

and gas lifetime defined as:

)/exp(0 tII

nvgas /1

gasbeam

111

Critera for ‘good vacuum’ for the accelerator is

Beam quality shouldn’t be affected by residual gas beamgas

Bremsstrahlung (e+, e-)

Ionisation energy loss (all particles)

Electron capture (low energy X+, XZ+)

Electron loss (X+, X-, XZ+)

Nuclear reactions

Single Coulomb scattering (all particles)

Multiple Coulomb scattering


• All need Vacuum to a greater or lesser extent e.g.:

• 10-5 – 10-6 mbar in small linacs, Van de Graafs

• 10-7 – 10-8 mbar in proton synchrotrons

• 10-9 – 10-10 mbar in synchrotron light sources

• 10-11 – 10-12 mbar in antiproton accumulation rings

Examples of required vacuum in accelerators


• “So if vacuum science is the science about nothing,

what does vacuum scientist know?”

• Where does gas come from?

• Leaks and leak detection, outgassing, induced desorption...

• How to suppress the gas sources?

• Choice of materials, cleaning, baking, coatings, mechanical design, etc...

• How to remove the gas out of vacuum system?

• Different types of pumps based on very different physics principles.

• How to measure vacuum?

• Different types of gauges for different pressure ranges, RGAs, indirect

(non-gauge) measurements…

• Vacuum system design

• Gas dynamics, surface physics and chemistry, material properties, a lot of

measurements, a lot of engineering.

Vacuum Science and Technology


Gas Composition in the Atmosphere and in Vacuum

Atmosphere

(at sea level)

Unbaked

vacuum

chamber

Baked

vacuum

chamber

NEG coated

vacuum

chamber

At cryogenic

temperatures

(1 to 80 K)

N2 (78.1%) H2 H2 H2 H2

O2 (20.9%) H2O CO CH4 CO

H2O (0.1-1.0%) CO CO2 CxHy CH4

Ar (0.93%) CO2 CH4 CO CO2

CO2 (0.033%) CH4 CxHy

Atmospheric air is a mixture of gases with over 99% of nitrogen and

oxygen, while the rest of gas in UHV consist mainly of hydrogen.

The gas composition is varied depending on many factors: choice of material,

cleaning, baking, pumping system design, type of pumps, temperature,

photon, electron or ion bombardment of the surface and many others.


What are usual considerations for vacuum

Required pressure P is defined by gas

desorption Q in the vessel and effective

pumping speed Seff.

In a simple case it is

Q

Pump,

S (l/s)

P

U (l/s)

1 1

eff

QP Q

S S U

e e ion ionQ qA

Thermal, photon, electron and ion

stimulated desorption


A Few Simple Formulas for Vacuum

Pressure in complicated vacuum system is always higher

because the vacuum conductance, U, of vacuum chamber

should be taken into account. For example, for a long

vacuum chamber:

u Q S

0 L x

1( ) ;

L x UP x Q u

S u L


A Few Simple Formulas for Vacuum

21

2

2CxC

u

xqP

u S

0 x

Gas load q can be distributed along the vacuum

chamber, in this case:

In a complicated vacuum system it is very difficult or

sometimes impossible to describe pressure by a simple

function but it is always necessary to know the gas loads,

pumping speed and geometry of vacuum system.


• Long tube with length L >> a, where a - transversal dimension

• Average pressure depends on vacuum conductance u(L,a) of

the beam vacuum chamber

Usual accelerator vacuum chamber

1

12 2B

eff

LP qL k T

u S

S S S S

P

z

L


The pumps can be divided:

• into three groups depending upon the vacuum range

• Roughing

• HV and VHV

• UHV and XHV

• and into three groups depending upon the pumping mechanism

of the pump

• Positive Displacement Vacuum pumps

• Kinetic vacuum pumps (Molecular pumps)

• Capture Vacuum pumps

Pumps


• Positive Displacement Vacuum pumps

• Oil Sealed pumps

• Liquid Ring pumps

• Dry Vacuum pumps*

• Kinetic Vacuum pumps (Molecular pumps)

• Diffusion pumps

• Drag and Turbomolecular* pumps

• Capture Vacuum pumps (no exhaust !)

• Chemisorption pumps and getters (TSP, NEG*)

• Sputter Ion pumps*

• Cryopumps

* 1st choice pumps for accelerators

Pumps


Specification of vacuum pumps includes:

• Working pressure range

• Pumping speed

• Maximum start up pressure

• Minimum pressure

• Capacity and regeneration conditions (for getters and cryopumps)

• Pump orifice and flange type

• Overall dimensions

• Mass

• Operation conditions

• Cable length

• Power supply parameters

• Cost

• …..

Pumps

Pressure Ranges of Vacuum Pumps Pressure (mbar)

10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102

10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102

Rotary Piston Mechanical pump

Oil-Sealed Mechanical pump

Scroll pump

Roughing pumps

HV pumps

UHV pumps

Roots Blower, Booster

Liquid Nitrogen Trap

Cryopump

Diffusion pump

Turbomolecular pump

Titanium Sublimation pump

Ion Sputter pump

Non evaporable getter


Sorption pump

Diaphragm pump

Cryogenic pump


Commonly used gauges can be separated by the method of

measurement:

• Deformation transducer:

• Bourdon tube

• Membrane gauges

• Capacitance manometer

• Hydrostatic transducer (U-tube)

• Thermal transducer

• Viscosity transducer

• Ionisation gauges

• Hot cathode gauge ionisation gauge, Bayard-Alpert

• Extractor ionisation gauge

• Cold Cathode gauges

• Penning gauge

• Inverted Magnetron gauge (SRS main gauge in recent years)

Vacuum Gauges


Seldom used techniques to measure vacuum:

• Radio isotope transducer

• Flash filament, adsorption/desorption method

• Scattering of neutral atoms from a molecular beam

• Beam lifetime (in accelerators)

• Recharge of beam particles (trapped or lost electron)

• Photocathode lifetime (in GaAs photocathode gun)

• Residual gas luminescence

• Photo-ionisation

Vacuum Gauges


Pressure Ranges of Vacuum Gauges Pressure (mbar)

10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102

10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102

Bourdon gauge

U-tube

Diaphragm gauge

Pirani gauge, Thermocouple

McLeod gauge

Penning gauge

Spinning rotor gauge

Capacitance

Cold cathode Discharge gauge

Hot Cathode Ionisation gauge, Bayard-Alpert


Extractor Ionisation gauge, Modified Bayard-Alpert

Thermistor


Residual Gas Analysers

Total pressure measurements are

not sufficient to characterise

vacuum, for example:

If vacuum beam lifetime of N hrs

corresponds to 1·10-9 mbar in

nitrogen equivalent this will be

either 8·10-9 mbar for H2

or 1.4·10-9 mbar for CH4

or 1·10-9 mbar for CO

or 6.4·10-10 mbar for CO2

Generally, the heavier the gas

molecules are, the better the

vacuum is required.

There are a few methods to

analyse the gas composition

based on different behaviour of

ionised molecules in different

configurations of electric and

magnetic fields:

Magnetic Sector Spectrometer

Quadrupole Mass Analyser

Cycloidal Mass Spectrometer

Omegatron

or in kinetic properties:

Time-of-Flight Mass Analyser


• Compatibility with the required vacuum range have to be always

checked for:

• Vacuum chamber and components (elbows, T-pieces, crosses, etc.)

• Flanges and gaskets (ISO, CF, KF...)

• Bellows

• Electrical feedthroughs

• Liquid feedthroughs for cooling water or LN2, or LHe

• Motion feedthroughs and manipulators

• Valves

• (All metal) right angle valves

• (All metal) gate valves

• Needle valves and UHV fine leak valves

• Other vacuum components to be considered

• Power supplies,

• Controllers for vacuum components

• Software for vacuum equipment

Other Vacuum Components


Mechanisms Contributing to Outgassing

Atmosphere

Vacuum

Thermal

Desorption

Recombination

So to reduce outgassing,

we must inhibit or reduce

these processes

Vaporisation

Adsorption

Permeation

Surface Diffusion Bulk

Diffusion

Real Leaks

Virtual

Leaks

Back-

streaming


• Leaks and leak detection

• Residual gas

• Thermal outgassing

• Choice of material for UHV system

• Vacuum components cleaning and pre-treatment

• Equilibrium vapour pressure

• Beam induced gas desorption

• Photon simulated desorption

• Electron simulated desorption and BIEM

• Ion simulated desorption and ion induced pressure instability

• Dust particles


• Cryogenic vacuum chamber: recycling and cracking

Sources of Gas in a Vacuum System


The aim is building a vacuum-tight vessel, i.e. no gas

from atmosphere should be able to find its way into the vessel,

there must be no leaks to atmosphere.

Sources of Gas in a Vacuum System: Leaks

To minimise the possible leaks it is necessary:

• to engineer an appropriate design (both mechanical and vacuum)

• to use vacuum-tight components (checked by vacuum support)

• to perform quality welding and careful assembly of components

• to use tested types of valves and flanges, only new gaskets

After the production of the vacuum vessel,

assembly of components and installation, a

leak detection is needed to locate possible

leaks and to guarantee the required pressure

will be reached.


Thermal desorption

(or thermal outgassing) means:

• Molecules diffusing through the bulk

material of the vacuum chamber,

entering the surface and desorbing

from it

• Molecules adsorbed on the surface

(initially or after the air venting) and

desorbing when vacuum chamber is

pumped

Outgassing rate depends on many

factors: choice of material,

cleaning procedure, pumping

time, etc...

Sources of Gas in a Vacuum System: Thermal Desorption

Air Vacuum


The cleaning procedure may include the following operations:

• Degreasing

• Washing

• Mechanical or electrical polishing

• Chemical cleaning/etching/passivation

• Pre-baking or Vacuum oven firing

• Argon discharge cleaning

• Surface coating

• In-situ baking

The exact procedure depends upon the material, its history,

required vacuum, contaminant(s), cost, availability of certain

facilities and cleaning agents.

Cleaning and Pre-Treatment of Vacuum Components


Choice of Material for UHV System

Material t [mbar l/(scm2)]

Aluminium (fresh ) 910-9

Aluminium (20h at 150C) 510-13

Cupper (24h at 150C) 610-12

Stainless steel (304) 210-8

Stainless steel (304, electropolished) 610-9

Stainless steel (304, mechanically polished) 210-9

Stainless steel (304, electropolished, 30h at 250C ) 410-12

Perbunan 510-6

Pyrex 110-8

Teflon 810-8

Viton A (fresh) 210-6

The outgassing rates may vary in order of magnitudes depending on factors:

choice of material, cleaning procedure, history of material, pumping time, etc...

Not all materials are compatible with UHV and XHV system!

The example of the outgassing rates after one hour pumping:


Could an Accelerator Be Built in Space?

A 1-m and 10-m long circular beam

‘vacuum’ chamber with a diameter d

somewhere in space where the pressure is

10-12 mbar (i.e. 2.5107 molecules/cm3).

For thermal desorption only:

t = 10-11 mbarl/(scm2).

To fulfil the vacuum

requirements for circular

accelerators would be not an

easy task even in Space!!!


Equilibrium Vapour Pressure.

When liquid, condensed gas or a very porous material is present in a

vacuum chamber the pressure is limited by the equilibrium vapour pressure:

Sources of Gas in a Vacuum System:

Material Temperature, K Equilibrium pressure, mbar

Mercury 293 210-3

Pump oil 293 10-6 to 10-8

Water 293 20

H2 4.2 810-7

CO2 77.8 210-8


• Leaks and leak detection

• Residual gas

• Thermal outgassing

• Choice of material for UHV system

• Vacuum components cleaning and pre-treatment

• Equilibrium vapour pressure

• Beam induced gas desorption

• Photon simulated desorption

• Electron simulated desorption and BIEM

• Ion simulated desorption and ion induced pressure

instability

• Dust particles


• Cryogenic vacuum chamber: recycling and cracking

Sources of gas in a vacuum system


Sources of Gas in a Vacuum System: PSD

e- H2

H2

H2O

CO2

CH4

CO

Photon stimulated desorption (PSD) is

one of the most important sources of gas

in the presence of SR.

Gas molecules may desorb from a

surface when and where photoelectrons

leave and arrive at a surface

e-

The same as thermal

desorption, PSD

depends on:

• Choice of material

• Cleaning procedure

• History of material

• Pumping time

Additionally it depends

on

• Energy of photons

• Photon flux

• Integral photon dose

• Temperature



Photodesorption yields, (molecules/photon), as a function of

accumulated photon dose, D, for different materials measured up to

certain doses, these results are extrapolated for use in the design

of new machines

PSD yield for CO for prebaked and in-situ baked stainless steel vacuum chambers.

Yields for doses higher then 1023 photons/m (1 to 10 Amphrs for diamond) are extrapolations.

165.0,00

D

D

Photodesorption yield as

function of accumulated photon

dose can be described as:



Photodesorption yields, (molecules/photon), as a function

of accumulated photon dose for different materials for

vacuum chamber (data from A. Mathewson, CERN):


PSD as a function of critical photon energy

O.B. Malyshev et al. J. Vac. Sci. Technol. A 25 (2007) 791


Sources of Gas in a Vacuum System: ESD

Electron stimulated desorption (ESD) can be a

significant gas source in a vacuum system in a number

of cases when the electrons bombard the surface.

The same as thermal

desorption and PSD,

ESD depends on:

• Choice of material

• Cleaning procedure

• History of material

• Pumping time

Additionally it depends on:

• Energy of electrons

impacting the surface

• Electron flux to the

surface

• Integral electron dose

• Temperature O.B. Malyshev, C Naran. Vacuum 86 (2012) 1363


ESD as a function of electron energy

O.B. Malyshev et al. J. Vac. Sci. Technol. A 28 (2010) 1215

10 100 1 103

1 104

1 106

1 105

1 104

1 103

0.01

0.1

H2

CH4

CO

CO2

H2 fit

CH4 fit-1

CH4 fit-2

CO fit

CO2 fit

Energy [eV]

Yie

ld [

Mo

lecu

les/

elec

tro

n]


Sources of Gas in a Vacuum System: ESD and BIEM

H2

H2O

CO2

CH4

CO

Beam induced electron multipacting (BIEM) and build up of electron cloud (e-

cloud) are significant problems in a vacuum chamber with a positive charged

beam: • a free electron is accelerated towards the first positively charged bunch;

• when the bunch passes the accelerated electron moves with accumulated energy up to

hundreds of eV towards the opposite wall and strikes it, this causes: • ESD, which results in a pressure rise

• Secondary electrons are then accelerated by the next bunch

• An electron cloud space charge can increase the beam emmitance

+

e-

e-

+ +

e- e-

e-

e- e-

e- e-

e- e- e- e-

e-

e-


Ion stimulated desorption (ISD) can be a significant gas source in a

vacuum system where the ion beam bombards the surface. There is

very little data, most work has been done at CERN.

The same as thermal desorption, PSD and ESD, the ISD depends on:

choice of material, cleaning procedure, history of material and

pumping time.

It is also depends on:

• Mass, charge and energy of ions impacting the surface

• Ion flux to the surface

• Integral ion dose

• Temperature

Sources of Gas in a Vacuum System: ISD


Ion Induced Pressure Instability

H2+

COH2

CH4

CO2

+

where

Q = gas desorption,

Seff = effective pumping speed,

= ion induced desorption yield

= ionisation cross section,

I = beam current.

IIe

Q

e

IS

Qn

ceff

e

ISeff

eSI

eff

c

When I Ic (or )

then gas density (pressure) increases

dramatically!

When the positive charged beam particles

colliding with residual gas molecules ionise

them, these ions are accelerated towards the

vacuum chamber wall. This causes ion

induced gas desorption, the pressure rises

and more molecules will be ionised,

accelerated and bombard the wall…


Heavy Ion Induced Pressure Instability

The heavy ion beam particles colliding with residual gas molecules may

lose or trap an electron and be lost in the bending magnet. These very high

energy ions or neutrals bombard the vacuum chamber wall which results in

a very high desorption yield (up to a few thousands molecules per ion). This

causes further gas desorption, resulting in a pressure rise and more lost

beam particles bombarding the wall…

H2

H2O

CO

A+ Ao

A+

A+

< --- Dipole --- > | < --------------------- Straight ----------------------------- >


Dust Particles in a Vacuum Chamber

The dust micro-particle in the beam vacuum chamber

might be ionised by photons or photoelectrons and then

be trapped by the beam electric field. This may cause the

significant loss of the beam.

Potential sources of the dust micro-particles:

• Dust from the atmosphere during storage,

installation or venting

• Dust from moving parts: manipulators,

bellows, valves, etc

• Micro-particles from getters, cryosorbers

• Micro-particles from working Ion Pumps.

How to avoid:

• Proper cleaning and storing of components

• Positioning of potential dust sources in regard to the

beam

• Clean environment when vacuum chamber is open

• Clean gas for venting (for example, boil-off nitrogen)


Two concepts of the ideal vacuum chamber

Traditional:

• surface which outgasses as little as

possible (‘nil’ ideally)

• surface which does not pump

otherwise that surface is

contaminated over time

Results in

• Surface cleaning, conditioning,

coatings

• Vacuum firing, ex-situ baling

• Baking in-situ to up to 300C

• Separate pumps

‘New’ (C. Benvenuti, CERN, ~1998):

surface which outgasses as little as

possible (‘nil’ ideally)

a surface which does pump,

however, will not be contaminated

due to a very low outgassing rate

Results in

NEG coated surface

There should be no un-coated parts

Activating (baking) in-situ at 150-

180C

Small pumps for CxHy and noble

gases


NEG coated vacuum chamber under SR

Dynamic pressure rise for the Stainless Steel (baked at 300C for 24 hrs)

and TiZrV coated vacuum chambers (activated at 190C for 24 hrs)


• Reduces gas desorption:

• A pure metal film ~1-m thick

without contaminants.

• A barrier for molecules from

the bulk of vacuum chamber.

• Increases distributed pumping

speed, S:

• A sorbing surface on whole

vacuum chamber surface

S = Av/4; where – sticking probability,

A – surface area,

v – mean molecular velocity

What the NEG coating does

Vacuum NEG Subsurface Bulk

Coating Layers


• A and B are

vacuum

chamber

without a liner

Vacuum Chamber at Low Temperature: PSD and Recycling

C and D are

experiments

with a liner with

pumping holes

E is the beam

lifetime limit

SR

SR

Low temperature does not

necessary provides good

vacuum in a vacuum chamber!


Vacuum Chamber at Low Temperature

Average removal coefficient

as a function of surface

coating

10 -6

10 -5

10 -4

10 -3

10 -2

10 18 10 19 10 20 10 21 10 22

Yie

ld (

mo

lec

ule

s/p

ho

ton

)

Dose (p hotons/m)

H2

CO

CO 2

CH 4

Photodesorption yield at 77

K as a function of photon

dose

Primary photodesorption Secondary photodesorption:

recycling and cracking


Vacuum Chamber at Low Temperature:

Molecular Cracking There are four main photodesorbed gases in a cryogenic

vacuum chamber: H2, CH4, CO and CO2, and two of them

(CH4 and CO2) can be cracked by photons, : ~

24 2~ HCCH 22 2~2 OCOCO

The additional amount of H2, CO and O2 appears in a

vacuum chamber due to photo-cracking of CH4 and CO2.

The efficiency of photo-cracking of CH4 and CO2 is about

10 times higher then CH4 and CO2 desorption from their

cryosorbant!

and


Vacuum Chamber at Low Temperature: P and n!

• Pressure and gas density :

• Two vessels at temperatures T1 and T2: T1 > T2

TnkP B

and

21

2

121 PP

T

TPP

21

1

221 nn

T

Tnn

T1

P1

n1

T2

P2

n2

Molecular regime:

21

1

22121 nn

T

TnnPP Viscous regime:

2211 vnvn


Tools:

• Kinetic Theory of Gases

• Gas Flow Through Tubes and Orifices

• Results of Dedicated Experiments

• Specially performed experiments

• Analytical Approximations

• Numerical Methods

• PC

• Our Brains

• … and Our Experience

Calculation and Design of Vacuum System


“Vacuum is not exact science”

A.G. Mathewson

• Desorption yields may differ (factor 2 or even more) for

vacuum components made of the same material after exactly

the same cleaning procedure and treatments

• Mechanical tolerances may result in a difference between

estimated and real vacuum conductance

• Results of Experiments: 10-20% accuracy for all gauges at

UHV

• Approximations: extending of experimental results on a few

order of magnitude, - it is just a reasonable guess!

• Pumping speed is also approximation: up to +60% of nominal

pumping speed after baking but -30% to -50% at UHV

Uncertainties in Calculation of Vacuum System


Diamond LS:

• Thermal and Photon

Stimulated Desorption

• Conductance limited

pumping

• High photon intensity on

crotch absorbers

• No in-situ baking: use of

photons scrubbing,

conditioning for 100 A·hrs.

Calculation and Design of Vacuum System:

Examples


LHC (CERN):

Vacuum chamber at 1.9K, 4.5K,

78K, 300K, transitions at the ends,

beam screen at 5K to 20K.

Vacuum problems:

1st year: PSD (to reduce SEM)

2nd year: PSD + ESD (due to BIEM)

3rd year: normal operation

Vacuum system designed with

consideration to avoid Ion Induced

Pressure Instability

Calculation and Design of Vacuum System:

Examples The H2 equivalent average gas density in the vacuum chamber of the IR1&5.

<neff> (mol/m3),

1st year 2nd year 3rd yearElement L, (m) beginning

I = 0.2 In,

We=0

after 70 days

I = 0.2 In,

We=0

beginning

I ~ 0.3 In,

We=0.2W/m

+10 days

I ~ 0.3 In,

We=0.2W/m

+90 days

I = In,

We=40mW/m

Intercon 0.83 1.51014

21013

21015

21014

11014

Q1 7.70 21013

31012

51013

81012

61012

Intercon. 1.40 21013

31012

31015

31014

1.51014

Q2 12.58 21013

31012

31013

51012

31012

Intercon. 1.90 21013

31012

31015

31014

1.51014

Q3 8.40 21013

31012

51013

81012

61012

DFBX 3.23 21013

31012

31015

31014

1.51014

D1 (RT) ~25 11015

21013

61016

61014

51012

Conus'(RT)

~57 ~1012

~1012

~1012

~1012

~1012

TAN (RT) 4.9 11016

11014

91016

81014

71012

Ring 1. The beam from MB to IP.

VC (RT) 7.50 31014

11013

71016

61014

51012

D2 11.67 <1012

<1012

61015

31014

1.51014

Q4 8.65 31012

91011

61014

41013

21013

VC (RT) 19.38 11015 ~10

12610

16610

14510

12

Q5 8.25 61012

1.21012

51014

41013

21013

VC (RT) 24.76 21015

21013

61016

61014

51012

Q6 8.25 1.51013

21012

41014

41013

21013

VC (RT) 17.73 41015

2.51013

61016

61014

51012

DFBA 8.58 71013

71013

61014

31014

1.51014

Q7 9.00 1.51014

1.21013

1.81013

1.61013

21013

DS&Arcs 21014

11013

1.51014

31013

41012

Ring 2. The beam from IP to MB.

VC (RT) 7.50 <1012

<1012

71016

61014

51012

D2 11.67 <1012

<1012

61015

31014

1.51014

Q4 8.65 <1012

<1012

61014

41013

21013

VC (RT) 19.38 11015 ~10

12610

16610

14510

12

Q5 8.25 31012

1.21012

51014

41013

21013

VC (RT) 24.76 81014

11013

61016

61014

51012

Q6 8.25 21012

11012

51014

41013

21013

VC (RT) 17.73 51014

11013

61016

61014

51012

DFBA 8.58 21012

11012

61014

31014

1.51014

Q7 9.00 21012

11012

1.81013

1.61013

21013


• It is a very hard job: to do nothing, i.e. vacuum

• It requires :

• Comprehensive knowledge of Gas Dynamics, Surface

Physics and Chemistry

• Skilled experimental scientists

• Experience to deal with real problems in a working machine

• Knowledge of operational principles of all vacuum equipment

• Expertise of new equipment

• A lot of engineering and drawings

• Lease closely with numerous groups

• Strong muscles to lift pumps and to bolt flanges!

Conclusions


Select Bibliography

• CERN 99-05: CAS - CERN Accelerator School: Vacuum Technology, Snekersten, Denmark, 1999, (http://cas.web.cern.ch/cas/CAS_Proceedings-DB.html)

• CAS - CERN Accelerator School : Vacuum in Accelerators, Platja d'Aro, Spain, 2006, to be published (http://cas.web.cern.ch/cas/Spain-2006/Spain-lectures.htm)

• Handbook of Vacuum Technology, Edited by K Jousten, Wiley-VCH, 2008, ISBN 978-3-527-40723-1

• Basic Vacuum Technology (2nd Edn), A Chambers, R K Fitch, B S Halliday, IoP Publishing, 1998, ISBN 0-7503-0495-2

• A User’s Guide to Vacuum Technology (3rd Edn), J F O’Hanlon, Wiley- Interscience, 2003. ISBN 0-471-27052-0

• Modern Vacuum Physics, A Chambers, Chapman & Hall/CRC, 2004, ISBN 0-8493-2438-6

• The Physical Basis of Ultrahigh Vacuum, P A Redhead, J P Hobson, E V Kornelsen, AIP, 1993, ISBN 1-56396-122-9

• Vacuum Science and Technology, Pioneers of the 20th Century, AIP, 1994, ISBN 1-56396-248-9

http://cas.web.cern.ch/cas/CAS_Proceedings-DB.html




http://cas.web.cern.ch/cas/Spain-2006/Spain-lectures.htm






First considerations for vacuum design

• Lattice design

• Location and required space for key components:

• Magnets, collimators,

• Detectors,

• SR absorbers

• Beamlines

• Choice of materials and coatings

• Electric and magnetic properties

• Vacuum chamber wall thickness

• Beam size (close orbit) => Apertures (lower limit)

• Magnet design => Apertures (upper limit)

• Preliminary mechanical layout

• Specific components

• E-gun; gas target, etc.

• Specific problems

• Electron cloud; ion instability, ion induced pressure instability

• High power loss, high radiation damage, etc.

Possible locations of pumps,

distance between pumps

Vacuum chamber

cross section


First considerations for vacuum design

• For the storage ring Lattice design

• Location and required space for key components:

• Magnets, collimators,

• Detectors,

• SR absorbers

• Beamlines

• Choice of materials and coatings

• Electric and magnetic properties

• Beam size (close orbit) => Apertures (lower limit)

• Magnet design => Apertures (upper limit)

• Preliminary mechanical layout

• Specific components

• E-gun; gas target, etc.

• Specific problems

• Electron cloud; ion instability, ion induced pressure instability

• High power loss, high radiation damage, etc.


Vacuum chamber cross sections

Beam pipe

Circular or elliptical

4 mm d, a, b 200 mm

Vacuum chamber with an antechamber

for larger vacuum conductance, U,

and for SR and the SR absorber

Distributed pumping In dipole magnetic field With NEG strips

(LEP in CERN)

d a

b


Initial rough calculations

• Roughly estimate an internal surface area

• A (m2) =

• Assume an achievable outgassing rate

• qth (mbarl/(sm2)) • for 316LN qth = 10-15 mbarl/(sm2)

• Determine total required pumping speed, S (l/s) to

reach the base pressure, PB

• typical specification for a storage ring is 10-9 mbar

• Since conductance was not considered here, this a

lower limit estimate of total required pumping speed.

th th

B B

Aq QS

P P


Initial rough calculations

• Work out the significance of any stimulated desorption

• Location

• Direct

• Scattered

• Intensity

• Desorption coefficients

• This will result in a dynamic gas load, Qd (integrated

along the machine)

• If Qd << Qth, it may be ignored and Pd~PB.

• Otherwise, the minimum total required pumping speed,

Sd, calculated from d th

d

d

Q QS

P


Initial Rough Design

• Determine type of pumps to

use

• Sputter Ion pump (SIP)

• Lumped

• Distributed

• TSP

• NEG

• Lumped

• Distributed

• Coatings

• Turbo-molecular pump (TMP)

• Cryo-pumping

• Lumped

• Distributed

• From a knowledge of what

is available, work out how

many pumps of each type

will be required overall.

• Then, using the preliminary

mechanical layout, draw up

a rough vacuum design

layout.


• Long tube with length L >> a, where a - transversal dimension

• Average pressure depends on specific vacuum conductance

u(L,a) of the beam vacuum chamber

Average pressure calculations

23; 121

12 eff

qL qLP u d

u S

P

z

L


Towards the final design

• If stimulated desorption is important, then it is

necessary to calculate the conditioning behaviour of

the machine.

• Calculate the SR critical energy

• Calculate photon flux


SR from a dipole magnet

=> This is for an ideal orbit and is very sensitive to the real beam position


Example: photon stimulated desorption (PSD)

PSD yields, (molecules/photon), as a function of accumulated photon

dose, D, for different materials measured up to certain doses, these results

are extrapolated for use in the design of new machines

PSD yield at room temperature as

function of accumulated photon

dose can be described as:

165.0,00

D

D

PSD yield for CO for prebaked and in-situ baked stainless steel vacuum chambers.

Yields for doses higher then 1023 photons/m (1 to 10 Amphrs for Diamond LS) are extrapolations.

=> Input data for a gas dynamic model are very approximate


PSD yield and flux as a function of distance from a dipole magnet

=> These data for each gas can be used in the gas dynamics model.

=> Uncertainty in desorption flux is less than in photon flux and desorption yield


• These results, formulae and data are sufficient for the first

rough estimations of average pressure for your design.

Average pressure calculations

introduction to vacuum systems for particle … to vacuum systems for particle accelerators ......

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