1.11 Vacuum Chenjr

7/28/2019 1.11 Vacuum Chenjr

1/83


2/83

Vacuum System

I. Introduction (Vacuum and Pressure Units)

II. Considerations on Accelerator Vacuum

System

III. Vacuum System Design ConsiderationsIV. Outgas, Pumping and Pressure Distribution

V. Vacuum Components and Reliability

VI. Case Study


3/83

Introduction

A. VacuumB. Pressure Units


4/83

Vacuum

Vacuum : an environment w ith a pressu re < 1 atm

Low Vacuum : 760 25 torr

Medium Vacuum: 25 10-3 torr

High Vacuum (HV): 10-3 10-6torr

Very High Vacuum: 10-6 10-9 torr

Ultra High Vacuum (UHV): 10-9 10-12 torr

Extreme High Vacuum (XHV): < 10-12 torr


5/83

Pressure units

Pressure: force per unit of area

Pa: Newton/m2 (SI unit), 1 Newton = 1 kg-m-sec-2

bar: (kg/cm2), 106 dyne/cm2, 1 dyne =1 g-cm-sec-2

mbar: milli-bar, 10-3bar, 103 dyne/cm2

Torr: mm-Hg (at 0)

1 torr = 1.333 mbar = 133.3 Pa 1.31610-3 atm1 Pa = 10-2 mbar7.5 10-3 torr 9.869 10-6atm

1 atm 760 torr1013 mbar 1.013 105Pa


6/83

Pressure

PV= nRT

Pressure is equivalent to number density.

Number density (at room temperature):

at 1 Torr, N ~ 3.2 x 1016 molec./cm3

at10-10Torr, N ~ 3,200,000 molec./cm3 !!


7/83

Considerations on AcceleratorVacuum System

A. Accelerator Vacuum SystemB. Vacuum Related Beam Considerations


8/83

Accelerator Vacuum System

--- to provide a comfortable path for the particlebeam (to increase the beam lifetime and alsothe beam quality)

--- to provide a clean environment for the criticalcomponents (to keep their high performance)

--- a vacuum system contains vacuum chamber,

pumps, gauges, valves, mechanical andelectrical feedthroughs, the related controlunits, and many other subsidiary components.


9/83

Vacuum Related Beam ConsiderationsA. Beam Lifetime Issues

Pressure: scatteringIon/Dust Trapping: scattering

B. Beam Stability IssuesMechanical Stability: Beam OrbitBeam Duct Cross section: Impedance

Chamber Material: Frequency Response

Ion Effects: Beam Lifetime, Beam Size andEmittance

(Electron Clouds: Beam Size and Emittance)


10/83

Beam Lifetime and Beam Size Issues

The less the gas molecules density

the less the interactions between theparticle beams and the gas molecules

the less the blow up of the beam bunch

and also the less the beam loss.

The less the gas molecules densitythe less the interactions between theparticle beams and the gas molecules

the less the blow up of the beam bunch

and also the less the beam loss.


11/83

Beam lifetime (electron rings)

-1= T-1+ RGS-1+ ion-1 : Beam lifetime (in general, T


12/83

Bremsstrahlung-scattering lifetime

BS-1= c BSN = c(/X0)Wwhere

X0: radiation length of the residual gas (g - cm-2)

: density of the residual gas (g - cm-3),c : velocity of l ight (3x1010cm-sec-1)

W = 4/3 ln( / )(5/6), probabi l i ty to loss energy > , = Ee/mec2 = MP/24500760 at room temperatureM : mass of the residual gas (a.m.u.)

P: pressure (torr)

Ref: J. Kouptsidis and A. G. Mathewson, DESY report, DESY 76/49, 1976.


13/83

Bremsstrahlung-scattering lifetime

Assume / =1%

BS-1= 8539 MP/ X0sec-1= 3.1107MP/ X0 hr1M/X

0

=i

(M/ X0

)i

36.119.437.335.945.534.242.558X0444028181616121M

CO2ArCOH2OCH4OCH


14/83

Nuclear-scattering lifetime

NS-1=[c1(E2A02/P 0)(1/)]x-1+[c1(E2A02/P 0)(1/)]y-1

where

C1: 1.010-7hr- GeV-2- nTorr-1E : electron energy

P : pressure (nTorr)

A0: l imiting aperture (min.[vacuum chamber, dynamic aperture])

0: Betatron function at the limiting aperture = ring ds/L , average betatron function

Ref: H. Wiedemann, Coulumb scattering and vacuum chamber aperture, SSRL-

ACD-NOTE, Dec.13,1983.


15/83

Assume: d= 5 cm, < > = 10m EN-1= c ENN= 3x 10104 [(2.8 x 10-13)2Z2/ 2 max2](61023/24500)(P/760)= 1.4105(Z2/E2)P hr-1

EN-1= c ENN EN= 4 r2Z2/ 2 max2 max= (d/2)/where,

r : classical electron radius= 2.8 x 10-13 cm

Z: atomic number

= Ee/meC2

d: diameter of vacuum chamber

< >: average betatron function


16/83

Electron-electron-scattering lifetime

ee-1=c eeNwhere

ee : electron-electron scattering cross section= 5.0 10-25(Z/)(/ ) (cm2)

Z: atomic number of the residual gas

N = 3.21016P (# of molecules/cm3), at RTP : pressure (Torr)


17/83

Beam Stability IssuesMechanical stability: as stable as possible

vibration or thermal expansion of vacuum

chambers movement of Magnets or BPMs Beam Orbit Change

Beam duct cross section: as smooth as possible

abrupt change of cross section wake field

Induce Beam Instability (and the lost energy could alsoheat up vacuum components)

Chamber material and thickness: Frequency Response

AC or pulse magnetic field

Eddy currentShielding or Changing the Original Magnetic Field and

Heating the vacuum Chamber


18/83

Vacuum System Design Considerations

A. Basic Vacuum IssuesB. System Operation Issues


19/83

Vacuum System Design Considerations

A. Basic Vacuum Issues

1. How to reduce pressure

2. How to overcome thermal problems

B. System Operation Issues1. How to keep a precise mechanical structure even after

baking2. How to reduce the impact from the stringent environment

(radiation, humidity, dust, etc.)3. How to protect the vacuum system in case of an

accident


20/83

Basic Vacuum Issues

--- How to reduce pressure

reduce outgassing rate (material, sealing, treatment)

effective pumping configuration

--- How to reduce thermal problems

increase thermal conductivity (material, direct cooling)

absorbers, grazing incident, differential heat removal (low Z

material), cooling system


21/83

System Operation Issues

--- How to protect the vacuum system in case of an accident

device self protection (IP, IG), electrical or pneumatic actuated valves,reliable vacuum interlock system (e.g. PLC), redundant sensors, reliableutility systems (e.g. compressed air and cooling water systems)

--- How to reduce the impact from the stringent environmenthigh radiation resistance material, installation under clean roomconditions, to avoid the condense of water vapor, and to prevent thecontact with humid air (e.g. with isolation coatings, to avoid corrosion)

--- How to keep a precise mechanical structure even after bakingcareful dimension control during machining and welding, rigid fixed

points (at BPMs, heavy components, critical positions), bellows andflexible supports, pre-displacement so as to have an optimized-forcecondition for some critical components during baking, to use springs toreduce the load of heavy components


22/83

Outgas, Pumping and PressureDistribution

A. Outgas1. Thermal outgas2. Photon-induced desorption

B. Pumping and Pressure Distribution1. Throughput, Conductance and Effective Pumping Speed

2. Pumping Configurations

3. Pressure Distribution4. Pumps


23/83

In order to get a lower pressure in the UHV range,

it is much more effective to reduce outgassing ratethan to increase pumping speeds.

P = Q / Swhere

P: pressure

Q: outgassing rate

S: pumping speeds


24/83

Thermal desorption1. Qth ~ exp(-Ed/kT)

Ed--- surface binding energy of the desorbed gas

k --- Boltzmann constant (8.6x10-5eVK-1)T --- temperature (K)

2. Qth :

a) mechanism: surface desorption and diffusion

b) can be effectively reduced by the treatments of chemical cleaning and

in-situ baking

c) water vapor is the major outgas before baking, hydrogen is the major

outgas after baking

d) Elastomers and the materials with high vapor pressure are not

recommended for an UHV system.


25/83

Photon-stimulated desorption, PSDebeamSynchrotron RadiationPhoto-electron Gas molecules

I d/dt (d2N()/dI d) Y(hv)F() 2Qpsd= I d/dt(d2N()/dI d )Y()F () d 2

Y , F( ) , Qpsd (normal incident, =90F( ) minimum)where, I: beam current (mA)

Y( ): photoelectron yield (# of electrons/ # of photons)for aluminum, Y() 2.61 -0.94 10eV


26/83


27/83

Qpsd= I d/dt(d2N()/dI d )Y()F () d 2 8.61017I E c-1/3Y( c) F() 2

whered/dt (d2N()/dI d) 1.51 1014/E2(/ c)-2/3, for c 0 for > cI: beam current (mA)

E: electron beam energy (GeV) c : critical photon energy = 2.21103I E3/F() sin-1/2 : bending radius (m)for aluminum, Y( c) = (0.41 - 1.66 c-0.6) hv 1560eV

= (1 - 216.2 c-0.6) hv > 1560eV


28/83


29/83

Throughputis the volume of gas at a known pressure and temperature that

pass a plane in a known time.Throughput = Outgassing rate(if no absorption in the path)

Q = P (ch)S (ch) = P(pump) S(pump)

= C (P (ch) P(pump))C : conductance of the tube (uni t: l/s)

= function of geometry, independent of

pressure for the molecular f low regime

1/S (ch) = 1/S(pump) + 1/C

S (ch) : effective pumping speed at the chamber

C : conductance of the tube

I t is useless to use a large pump with a narrow tube!

Throughput, Conductance and

Effective Pumping Speed


30/83

Pumping

Pumping Configurations

The conductance of the beam duct in an accelerator is

always very small so that special pumping conf igurations

are necessary to meet the str ingent low pressure

requirements.

a) Distributed Pumping

b) Localized Pumping


31/83

Insertion Device Chamber (extremely conductance limited)

(Distributed pumping) (NEG Strip / NEG coating)Insertion Device Chamber (extremely conductance limited)

(Distributed pumping) (NEG Strip / NEG coating)

Heavy Gas Load

Ante-chamber + Localized PumpingHeavy Gas Load

Ante-chamber + Localized Pumping

Conductance Limited Area

Discrete Absorber + Localized PumpingConductance Limited Area

Discrete Absorber + Localized Pumping

IP NEG

IP

IP

IP

IP

NEG

NEG

NEG

DIP

DIP

DIP


32/83

TMP

IPNEG

Distributed Ion Pump

TMP (commissioning) IP+NEG (normal operation)

TMP (commissioning)

IP+NEG (normal operation)


33/83


34/83


35/83

Pressure Distribution

SiPi= Qi+ Ci(Pi -1Pi) + Ci+1(Pi+1Pi)

Ref: D.C. Chen et al., J. of Vac. Soc. of ROC 1(1), 24(1987).


36/83


37/83


38/83

Pump considerations

a) pumping speedsb) preferred gases

c) ultimate pressure

d) oil freee) vibration free

f) micro-dust free

g) failure safe (or interlocked)h) long lifetime and maintenance free


39/83

Pumps

a) Mechanical Pumpsb) Sputter ion pumps

c) Getters (NEG, TSP)

(NEG: Non-evaporable getter, TSP : Ti-sublimation pump)

d) Adsorption pump

e) Cryo-pump


40/83

NEG


41/83

Turbomolecular Pump (TMP)


42/83

Titanium Sublimation Pump (TSP)


43/83

Non-Evaporable Getter(NEG)


44/83

Sputter Ion Pumpgas molecule

electron

N S

magnet

magnet

ion Ti cathode

Sputtered-Ti Sputtered-Ti gas molecule

(trapped)

Magnet

field

N S

Ti cathode anode (cell)


45/83

Vacuum Components and Reliability

A. Vacuum Chamber Material and Treatment

B. Sealing Technique

C. Valves

D. Bellows

E. Mechanical feedthrough

F. Electrical feedthrough

G. Special components


46/83

Vacuum Chamber Material(& thermal absorber)

UHV Considerations

--- low defect (to avoid virtual or real leak)

--- low outgassing rate, low vapor pressure

--- easy machining, easy welding (increase reliability)

--- bakable

High Thermal Load Considerations

--- high thermal conductivity--- grazing incident (to reduce thermal density)

--- differential heat removal, the first layer with low Z material


47/83

Surface Treatments

1. chemical cleaning

2. in-situbaking

3. glow discharge cleaning

4. surface coating5. high temperature degas


48/83

Sealing Technique

Welding, Tungsten Inert Gas (TIG), metal-to-metal

Brazing, between two different materials, metal-to-

ceramics, different metals,

E-beam welding

Flange sealing, Con-Flat Flange, O-ring, Helicoflex,

metal wires (e.g. indium wire, aluminum wire, etc.)

leak check, He-gas mass spectrometer

Leak rate unit: Torr-L-sec-1, Pa-m3-sec-1, atm-cc-sec-1


49/83

Valves

Gate Valves, Angle Valves, Variable Leak Valves,

Fast Closing Valves

All metal valves and O-ring valves

Considerations:

leak tight, tunability, response time, baking

temperature, type of actuation, mechanicalreliability and lifetime


50/83

Bellows

Flexibility, expansion/suppression dimension

Rf sliding fingers (touch force, flexibility)

Thermal conductivity

Mechanical reliability (strength and lifetime)

How to fix ? or free suspended (vacuum force!!)


51/83

Mechanical Feedthrough

Applications:scrapers, screen monitors, rf tunners, front-end and beamline components, etc.

Considerations:

Stroke, Precision, Heat removal (thermal contact and

cooling), Mechanical Reliability (wearing and lifetime)


52/83

Electrical Feedthrough

Applications:beam position monitors, stripline monitors, excitationelectrodes, gauges, pumps, etc.

Considerations:

Frequency response, HV range, Current range

Radiation induced damage (corrosion, degrade of contactor insulation)


53/83

Special Components

RF bridge

Be-window

Ceramic chambers

Glass- and ceramic-windows


54/83

Case Study

A. TLS Vacuum System1. Vacuum Chamber Fabrication and

Treatments

2. System Installation and OperationB. TPS Vacuum System Design

(Lessons learned from the TLS vacuum system)


55/83

TLS Vacuum System

Vacuum Chamber Fabrication and Treatments

1. Aluminum vacuum chambers

2. Oil-less Fabrication Process

3. Low Impedance Structure

System Installation, Operation, and Commissioning

4. Oil-less and Effective Pumping System

5. Low-Dust Treatments6. Vacuum Safety Interlock System

Th e TLS V a cu u m Sy s t em


56/83

A. Vacuum Chamber Fabrication

1. Aluminum vacuum chambers

2. Oil-less Fabrication Process3. Low Impedance Structure

B.System Installation and Operation4. Oil-less and Effective Pumping System

5. Low-Dust Treatments

6. Vacuum Safety Interlock System

Al i h b


57/83

Aluminum vacuum chambers

Aluminum Components

(B-chamber, S- chamber, flanges, gaskets, bellows,BPMs, etc.)

Aluminum TIG Welding

Al-Al and Al-S.S. Seals with Al Gaskets

(between two chambers or components)

(no transition material was used)Co-extruded or Co-machined Cooling Channels


58/83

Oil-less Fabrication Process

A. Bending ChambersOil-less numerical control machining in an ethyl-alcoholenvironment

Degreased cleaning

B. Straight Chambers

Extrusion

Detergent + Acid + DI water ultrasonic cleaning


59/83

Low Impedance Structure

1. Smooth Cross Section

(main chamber: 38mm-H x 80mm-W)

2. Gate Valves, Bellows, Flange Gaps shielded

with rf bridges3. Smooth Transitions in Cross Sections

4. Port with small holes or slots

5. Long Slots with a Large Width to Height Ratio(in B-chamber for extraction SR to beamlines)


60/83

Oil-less and Effective Pumping System

1. Oil-less pumps were adopted

sorption pump, dry pump (membrane pump + moleculardrag pump), magnetic bearing turbo-molecule pump,sputter ion pump, and non-evaporable getters

2. The pump locations and pumping speeds determined bycomputer simulations

3. Localized pumping + distributed ion pump in thebending chamber

4. Heavy dynamic gas loads mainly evacuated out of thevacuum system (by the TMPs) in the beginning ofcommissioning


61/83

Low-Dust Treatments

1. Welding and pre-assembly in clean rooms.

2. Clean booths were used during installation

3. Ion pumps turned on after baking (at ~10-8 torr)

4. Slow venting (if necessary)5. Low IP voltage (HV ~ 3kV)

TLS V a cu u m Sy s t e m ( Fa b r i c a t i o n )


62/83

1) NC Machining

with Ethyl Alcohol

2) Dimension Check

After Machining

3) Surface Cleaning4) DIP Installation

5) Welding

in Clean Room

6) Deformation Check

After Welding7) Leak Test

8) Pre-assembly

In Lab

9) Installation

in the Tunnel

88

TLS V a cu u m Sy s t e m ( Fa b r i c a t i o n )


63/83

88

80

38 44

80

60

17

17

21.5

13

171

174

Standard S-Chamber

ID-Chamber for EPU5.6,

U5, U9 Undulators

B-Chamber

ID-Chamber for Wiggler

(W20)

4.16 m

10 mm

TLS V a cu u m Sy s t em ( Co l d Ch am b e r )


64/83

S.S. Taper

(1) Al Beam duct (Extruded)

Al/SS Bimetal adaptor

(4) Flatness Check

(5) TIG Welding on the other side (with Al beam duct installed in SW6)

(2) TIG welding on one side (3) Leakage Check

SW6

11 mm inner height

Temperature of beam duct ~ 100 K

ID-Chambers for Superconducting WigglerSWLS (2002), SW6 (2003), IASW x3 (2005-6)

TLS O p e r a t i o n Re su l t s ( B e am D o s e )


65/83

8622 Ah

1). Accumulated Beam Dose : ~ 8622 Ah

1993.07 ~ 2005.11 (12 years)

Yearly operation hour: ~5000-5500 hours

TL S O p e r a t i o n Re s u l t s ( Re l ia b i l i t y )

2) High Reliability: Vacuum Failure < 2 hr/ year


66/83

-- About 100 hour (~2%) of the users time was lost in a year.

-- Less than 2% of the failures (< 2 hours in a year) wasattributed to the vacuum failure.

-- The most popular items of the vacuum failures are

utility related components.

0

200

400

600

1996 1997 1998 1999 2000 2001

Year

Hou

r

PS

Booster

RF

Control

Magnet

Vacuum

Utility

Safety

OtherTotal

Machi ne Fai l ure Hours

2). High Reliability: Vacuum Failure < 2 hr/ year


67/83

Le ss o n L e a r n e d f r o m TLS - 1

1) Beam Cleaning Interrup ted by New ID Ins tal lat ion s


68/83

Busy with Installation Work

EPU5.6

U5U9 SWLS SW6

SRF Cavity

Replace new

Kicker Chambers

Top-up

300 mA

W20

P/I vs. time

The data of P/I and I scattered dueto frequent installation of new devices.

1) Beam Cleaning Interrup ted by New ID Ins tal lat ion s

H om e w o r k t o D e s i g n t h e TPS ( Le ss o n - 1 )


69/83

Q1: Beam Cleaning Interrupted by New ID Ins tal lat ions,How to Avo id?

A1:1) -- Most of the ID-chambers are to be fabricated and installed before

the TPS is commissioned, to prevent the vacuum from beingfrequently broken and to allow the beam dose on the ID-chamber

to be accumulated effectively.

-- Some ID-chambers will be unavailable at the commissioning of the

TPS, they will be cleaned in a photon beam line before installation.

2) Effective pumping system is necessary for the ID-Chamber.

-- NEG strip is to be installed in a side-channel of the beam duct as a

distributed pumping. The arrangement is effective to reduce the

potential effects caused by the drop off of the NEG powders in thebeam channel.

-- Some other pumps (e.g. Ion Pump) are required to remove the

inert gases and methane, which the NEG cannot do.

Le ss o n Le a r n e d f r o m TLS - 2

2) Effect of the Movements of Vacuum Chambers


70/83

0 200 400 600 800 1000 1200 1400-0.08-0.06-0.04-0.02

0 200 400 600 800 1000 1200 1400

0.51.01.52.0

0 200 400 600 800 1000 1200 1400

2425262728

0 200 400 600 800 1000 1200 1400

0

10 0

20 0

Beam Position

mm

m in

BPM Displacement

um

Vac-chamber Temp

Temp(C)

Beam Current

mA

The expanded vacuum chamber moves the components touched orconnected to it. The force transferred to the girder, to the magnetsand then to the beam orbit.

2) Effect of the Movements of Vacuum Chambers

Movement of the vacuum chamber, sensitivity to water temp.: ~10 m / Movement of the girder (~0.3 m/) and BPM (~1 m/)Induced beam orbit drift: ~10-30 m /


Q2: Effects o f the Movements o f Vacuum Chambers


71/83

A2:

For vacuum chambers:

1) Independent supports fixed directly to the ground.

2) A 3mm gap between the magnet and vacuum chamber.

3) The vibration caused by water flow must be suppressed. A

heavy chambers is helpful to reduce the vibration amplitude.

Q2: Effects o f the Movements o f Vacuum Chambers,

How to Reduce?

Le s s o n L e a r n e d f r om TLS - 3

3) Vacuum Pressu re and RF Impedance Need be Better


72/83

3) Vacuum Pressu re and RF Impedance Need be Better

V a cu um Re l a t e d B eam I n s t a b i l i t i e s


73/83

1) Pump ing s lots RF impedance

2) Gas molecules & ions

> 1,000,000/cm3

!! (@0.1nTo rr )

SGV


74/83

SGV ( ) SGV ( )

RF fingers RF fingers

RF Fingers


75/83

RF Fingers

RF Fingers

! RF Fingers

RF Fingers

Al Bellows (R6S6)


76/83

( )

PT100

Thermal sensor

Heater

RF contact

Cu sheet

RF Fingers



77/83

Q3: Vacuum Pressure and RF Impedance Need be Better,How to Improve?

1) A large B-chamber can confine more PSDs locally.

2) It is easier to design with more pumps and also with a

differential pumping structure in a large B-chamber to

benefit the ante-chamber design, which is good in

reducing the number of gas molecules (and ions) in the

beam channel.

5mA3:A large B-chamber with

ante-chamber structure.


4) In addition to the chambers and pumping ports, the bellows,


78/83

(BPM-chamber: 70mm*13mm Left side: SGV, Right side: ID )

Fixed end of RF fingers

Movable end

of RF fingers

Movable end

of RF fingers

flange gap, gate valve, tapers, BPMs, and other monitorswill be carefully designed to reduce the impedance.

TPS V a c u u m


79/83

1/4~ 0.3nTorr~1.3nTorrPressure increase (design value)

at = 1x10-5 molec./ e

1x~1x10-6~1x10-6Q (for one cell)

1/4 (1/2)

less

same

more

4x

Remark

7.520Bending Angle of Dipole Magnet (deg.)

92.8%77%Percentage of the synchrotron lightinside the B- chamber

|Z/n| (Chamber/Total)

Pump ports per cell

Nominal Pumping Speed (per cell)

Beam Duct Material

QTot at = 1x10-5 molec./e (Torr*l/s)Beam current (mA)

Beam energy (GeV)

Parameter

10 (off axis)13 (on axis)

~2.4x10

-5

~5.9x10

-6

AluminumAluminum

~ 4000 L/s~ 4000 L/s

0.003/0.00850.012/0.0163

400200

3.01.5

TPSTLS

3 GeV, 400 mA, ~ 22W/mm2at L = 3.3 m(from BM source)



80/83

1) The thermal problem is reduced by designing a larger

B-chamber, so that the crotch absorber in the B-

chamber is farer away from the source point. The

criteria are met by a B-chamber with ~ 5 m long.2) By using stepped surfaces (to keep a smaller photo

electric yield) and fins in the cooling channel enables

the maximum temperature of the aluminum chamber

surface to be reduced from ~196C to ~109

C.

Stepped

surfaces

f ins in the

coo l ing channel

~196C~109CSaw too th (0.4 mm / 2 mm -step)

Crotch-1

Crotch-2


81/83

Vacuum Safety Interlock System

device self protection or alarm (IP, IG, TMP)

electrical or pneumatic actuated valves

reliable vacuum interlock system (e.g. PLC)

redundant sensors

reliable utility systems (e.g. compressed air and coolingwater systems)

Thermal problem protections


82/83


83/83

Documents

1.11 Vacuum Chenjr