Chapter 9 Vacuum Systems Chapter 9. Vacuum Systems Viscous and - - PowerPoint PPT Presentation

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Chapter 9 Vacuum Systems Chapter 9. Vacuum Systems Viscous and - - PowerPoint PPT Presentation

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Chapter 9 Vacuum Systems Chapter 9. Vacuum Systems Viscous and molecular flow Pumps Gages Chambers and components Chambers and components Techniques dolan swip 2009 1 EAST ASIPP Vacuum technology in China Carved leakage timer (

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Chapter 9 Vacuum Systems Chapter 9. Vacuum Systems

Viscous and molecular flow Pumps Gages Chambers and components Chambers and components Techniques

dolan swip 2009 1

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ASIPP

EAST Vacuum technology in China

Carved leakage timer (刻漏记时器）–invented in Western Zhou (1097 BC - 771BC） Blower(鼓风机)-this was invented in Spring and Autumn Period（770BC-221BC) Autumn Period（770BC 221BC) Cupping(拔火罐)-it was invented in the Warring pp g( ) g States Period (403BC-221BC)

杨庆喜

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17th Century Vacuum Technology

Galileo -- can raise water 10 m by suction 1638 Torricelli – mercury-filled tube  barometer 1640 Pascal -- air pressure vs altitude 1647 Pascal -- air pressure vs. altitude 1647 Perier – verified air pressure vs. altitude 1648 Von Guericke – vacuum pump 1650 Boyle – low pressures experiments, pV = constant 1670s

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ASIPP

EAST

“vacuum” discoverer~(1643) vacuum discoverer (1643)

vacuum

Evangelista Torricelli Experiment of “Torricelli” To commemorate Torricelli, we use “torr” as a vacuum unit 托里拆利“实验

  • unit. — 托里拆利 实验

杨庆喜

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ASIPP

EAST Von Guericke(葛利克)- invented air pump in 1654, Demonstrated effect of air pressure. Atmospheric pressure held evacuated spheres

“Magdeburg” hemisphere experiment

p together. （马德堡半球实验）

Magdeburg hemisphere experiment

杨庆喜

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Vacuum Technology Development

Charles – V  T at constant p 1787 Dalton – p = sum of partial pressures 1801 McLeod – vacuum gage 1874 McLeod – vacuum gage 1874 Crookes – cathode ray tubes 1879 Fleuss – piston-cylinder pump 1892 Kaufman & Gaede – rotary pump 10-3 Pa 1905

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ASIPP

EAST

1.3 Vacuum technology development

Thomas Alva Edison （爱迪生） Light-bulb This first industry product using vacuum technology This first industry product using vacuum technology

杨庆喜

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20th t id d l t 20th century -- rapid development

Light bulbs Light bulbs Vacuum tube electronics Vacuum tube electronics radio, radar, TV Accelerators Pl d i Plasma devices

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Units of Pressure Units of Pressure

1 Pa = 1 N/m2 = 1 J/m3 1 atm = 1 013x105 Pa 1 atm = 1.013x10 Pa 1 Torr = 133.3 Pa

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Viscous and Molecular Flow Viscous and Molecular Flow

Mean free path of gas molecules  = k1/p Mean free path of gas molecules  k1/p k1(N2) = 0.00813 Pa-m, At p=1 Pa, (air) = 7 mm Flow through tube diameter D: Flow through tube diameter D: Flow turbulent if Reynold’s Number

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Viscous and Molecular Flow Viscous and Molecular Flow

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From C. Day, FZK Summer School, 2008

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Throughput Q Throughput Q

Q=C(p2-p1) = (p2-p1)/Z (Pa-m3/s) C = conductance (m3/s), Z = impedance (s/m3) Electrical circuit I = V/R Z = impedance (s/m ) Electrical circuit I = V/R

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Average Velocities Average Velocities

Average flow velocity v = Q/pA A = tube area

1/2

Average molecular speed V = (8kT/m)1/2 Example: D = 0.1 m, p(N2) = 1 Pa, T = 293 K, p , p(

2)

, , Q = 0.1 Pa-m3/s Find v, V, dN/dt, and flow regime A=D2/4 = 0 00785 m2 A=D /4 = 0.00785 m , v = Q/pA = 12.7 m/s, m= 4.68x10-28 kg, V = (8kT/pm)1/2 = 469 m/s. dN/dt Q/kT 2 47 1019 / dN/dt = Q/kT = 2.47x1019 /s  = k1/p = 8.1 mm   /D = 0.08 transition region flow.

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Pumping Speed and Flow Rate p g p

Q = St(pp – pu)

St = pumping speed (m3/s)

t

p p g p ( ) pp = pressure at pump pu = ultimate pressure attainable Q = C(p-pp)

Eliminating pp

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Conductances

Viscous flow, circular duct with length L, air at 293 K  = viscosity if not air Molecular flow, circular duct with length L, air at 293 K Example: D = 0.1 m, L = 2 m, pav = 1 Pa Viscous flow C = 0.0715 m3/s Molecular flow C = 0.0572 m3/s /D= 0.0068/0.1 = 0.068  transition region, T iti i C C( i ) C( l l )

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Transition region: C ≈ C(viscous) + C(molecular)

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Conductances in Parallel Conductances in Parallel

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Conductances in Series Conductances in Series

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Conductances Relative to Air Conductances Relative to Air

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Pumpdown Pumpdown

V(dp/dt) = inflow – outflow V(dp/dt) = inflow – outflow = QL - Q where

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Example Example

L = 0.7 m, molecular flow. , = 0.118 m3/s Usually higher leak rates due to desorption of gases

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Usually higher leak rates due to desorption of gases

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Rotary Vane Mechanical Pump y p

pu ~ 0.1 Pa

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Roots Booster Pump Roots Booster Pump

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Turbomolecular Pump

1 kHz pu ~0.1 Pa

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Jet Pump Jet Pump

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Oil Diffusion Pump

Mechanical pump and cold trap needed needed.

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Cold Trap

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Chevron Baffle Chevron Baffle

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Ionization Pump Ionization Pump

Sputtered Ti buries gas atoms Less effective for He Ar pu ~ 0.1 Pa

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Less effective for He, Ar, …

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Sublimation Pumps (Getters)

Ti or Ba boiled off heated filament. Deposits thin film on walls Deposits thin film on walls. Traps gas molecules. Limited to a few hours.

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Cryosorption Pump Cryosorption Pump

R i li id N Require liquid N2 T = 77 K

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Cryogenic Pumps Cryogenic Pumps

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Saturation Pressures

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From C. Day, FZK Summer School, 2008

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Comparison of Pumps p p

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Bourdon Gage Bourdon Gage

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Thermocouple Gage Tube

0 01 – 100 Pa 0.01 100 Pa

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Pirani Gage Pirani Gage

Initially current G = 0 Change of p  resistance F changes  resistance F changes  Current in G

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Calibration of Pirani Gage Calibration of Pirani Gage

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Bayard-Alpert Ionization Gage Bayard Alpert Ionization Gage

Electrons spiral around ionizing Electrons spiral around, ionizing the gas. Ion collector current is Ion collector current is proportional to p. p < 0.1 Pa

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Magnetron Gage Tube Magnetron Gage Tube

B = 0 025 T Bz 0.025 T Inhibits radial electron motion Higher ion currents Higher ion currents p down to 10-11 Pa.

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Comparison of Gages p g

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Chamber stress

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Metal Joints Metal Joints

Solder poor Silver braze good for Cu, brass, SS Silver braze good for Cu, brass, SS Tungsten Inert Gas (TIG) welding is best

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Welds Welds

Weld on outside bad Weld on inside good. Virtual leak

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Welding Flanges Welding Flanges

Poor – warpage due to thermal Good – stress relieve groove Good – thin ridge can flex stress likely g can flex

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Glass-to-Metal Seals Glass to Metal Seals

Need to match thermal Need to match thermal expansion coefficients to prevent thermal stress.

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O-Ring Flange Joint O Ring Flange Joint

Temperature limited by elastomer O-ring T  200 C

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Bakeable Metal Gasket Flange J i Joint

High Temperature Capability More expensive

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Monolayers on Surfaces

Small amount of grease from fingerprint (1016 molecules) in 0.01 m3 chamber.  4 mPa. in 0.01 m chamber.  4 mPa. Usually several monolayers of adsorbed gas molecules. At 0.1 mPa a monolayer forms in 3 s. Each monolayer contains ~ 8x1018 molecules/m2 Extensive cleaning is needed. Extensive cleaning is needed.

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Cleaning Cleaning

Degreasing – detergent or solvent, hot water, cold water, d i i d t deionized water Oxide removal and smoothing – acid etching, electro-polishing (Anodized Al can adsorb 100 times as much as smooth Al) Rinsing – deionized water, then pure alcohol Bakeout after chamber pumped down. 400-700 K, many hours to remove water vapor, etc. Discharge cleaning – to remove more monolayers.

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Leak Detection Leak Detection

Stethoscope to listen for hissing sound Pressurize inside, apply soap solution, look for bubbles Glass systems – excite gas with Tesla coil. Spray acetone around metal sections, watch for glow in glass. Spray acetone or helium, watch pressre gage.

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He Mass Spectrometer He Mass Spectrometer

Spray helium around joints watch mass spectrometer signal Spray helium around joints, watch mass spectrometer signal. Most effective method for small leaks. Tighten flanges on leaky gasket. Reweld leaky welds, then reclean. If ultrahigh vacuum not required can use epoxy sealant If ultrahigh vacuum not required, can use epoxy sealant

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Constituents of Air Constituents of Air

H2 and He can diffuse through metals and glasses.

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ITER Vacuum Systems

From C Day FZK Summer School 2008

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From C. Day, FZK Summer School, 2008

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ITER Required Pumping Speeds

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From C. Day, FZK Summer School, 2008

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ITER Torus Cryopumps

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From C. Day, FZK Summer School, 2008

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Cryopump Regeneration Plan

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From C. Day, FZK Summer School, 2008

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Torus Cryopump Design

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From C. Day, FZK Summer School, 2008

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Neutral Beam Injector Cryopumps

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From C. Day, FZK Summer School, 2008

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NBI Cryopump Design

From C Day FZK Summer School 2008

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From C. Day, FZK Summer School, 2008

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Conclusions – ITER Vacuum System

Use experience from JET and elsewere C i t FZK d l h l t d l Cryopumping systems use FZK modular charcoal coated panels Torus and cryostat pumping systems testing prototype, starting manufacture. NBI system is in design phase, test bed for prototype begun Concepts for pellet vacuum systems developed. New mechanical pumps under development. p p p ITER vacuum systems = most complex in the world.

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From C. Day, FZK Summer School, 2008

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Summary – Vacuum Systems y y

Vacuum technology is well developed: theory pumps gages joints joints cleaning D i t id th l t d l k Design to avoid thermal stresses and leaks to facilitate cleaning and maintenance. ITER system is most complex in the world.

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