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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
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Introduction
A. VacuumB. Pressure Units
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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
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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
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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 !!
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Considerations on AcceleratorVacuum System
A. Accelerator Vacuum SystemB. Vacuum Related Beam Considerations
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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.
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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)
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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.
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Beam lifetime (electron rings)
-1= T-1+ RGS-1+ ion-1 : Beam lifetime (in general, T
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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.
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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
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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.
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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
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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)
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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
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Vacuum System Design Considerations
A. Basic Vacuum IssuesB. System Operation Issues
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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TMP
IPNEG
Distributed Ion Pump
TMP (commissioning) IP+NEG (normal operation)
TMP (commissioning)
IP+NEG (normal operation)
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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).
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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
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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
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NEG
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Turbomolecular Pump (TMP)
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Titanium Sublimation Pump (TSP)
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Non-Evaporable Getter(NEG)
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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)
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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
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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
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Surface Treatments
1. chemical cleaning
2. in-situbaking
3. glow discharge cleaning
4. surface coating5. high temperature degas
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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
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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
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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!!)
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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)
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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)
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Special Components
RF bridge
Be-window
Ceramic chambers
Glass- and ceramic-windows
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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)
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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
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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
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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
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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
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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)
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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
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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 )
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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 )
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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 )
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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 )
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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
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-- 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
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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
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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 )
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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
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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 /
H om e w o r k t o D e s i g n t h e TPS ( Le ss o n - 2 )
Q2: Effects o f the Movements o f Vacuum Chambers
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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
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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
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1) Pump ing s lots RF impedance
2) Gas molecules & ions
> 1,000,000/cm3
!! (@0.1nTo rr )
SGV
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SGV ( ) SGV ( )
RF fingers RF fingers
RF Fingers
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RF Fingers
RF Fingers
! RF Fingers
RF Fingers
Al Bellows (R6S6)
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( )
PT100
Thermal sensor
Heater
RF contact
Cu sheet
RF Fingers
H om e w o r k t o D e s i g n t h e TPS ( Le ss o n - 3 )
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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.
H om e w o r k t o D e s i g n t h e TPS ( Le ss o n - 3 )
4) In addition to the chambers and pumping ports, the bellows,
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(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
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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)
H om e w o r k t o D e s i g n t h e TPS ( Le ss o n - 4 )
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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
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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
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