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

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 ( 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

17 th Century Vacuum Technology Galileo -- can raise water 10 m by suction 1638 Torricelli – mercury-filled tube  barometer 1640 Pascal -- air pressure vs altitude Pascal -- air pressure vs. altitude 1647 1647 Perier – verified air pressure vs. altitude 1648 Von Guericke – vacuum pump 1650 Boyle – low pressures experiments, pV = constant 1670s dolan swip 2009 3

EAST ASIPP “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 托里拆利“实验 unit 杨庆喜

EAST ASIPP Von Guericke( 葛利克 )- invented air pump in 1654, Demonstrated effect of air pressure. Atmospheric pressure held evacuated spheres p together. （马德堡半球实验） “Magdeburg” hemisphere experiment Magdeburg hemisphere experiment 杨庆喜

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 dolan swip 2009 6

EAST ASIPP 1.3 Vacuum technology development Thomas Alva Edison Light-bulb （爱迪生） This first industry product using vacuum technology This first industry product using vacuum technology 杨庆喜

20 th century -- rapid development 20 th t id d l t Light bulbs Light bulbs Vacuum tube electronics Vacuum tube electronics radio, radar, TV Accelerators Pl Plasma devices d i dolan swip 2009 8

Units of Pressure Units of Pressure 1 Pa = 1 N/m 2 = 1 J/m 3 1 atm = 1 013x10 5 Pa 1 atm = 1.013x10 Pa 1 Torr = 133.3 Pa dolan swip 2009 9

Viscous and Molecular Flow Viscous and Molecular Flow Mean free path of gas molecules   = k 1 /p Mean free path of gas molecules k 1 /p k 1 (N 2 ) = 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 dolan swip 2009 10

Viscous and Molecular Flow Viscous and Molecular Flow dolan swip 2009 11 From C. Day, FZK Summer School, 2008

Throughput Q Throughput Q Q=C(p 2 -p 1 ) = (p 2 -p 1 )/Z (Pa-m 3 /s) C = conductance (m 3 /s), Z = impedance (s/m 3 ) Z = impedance (s/m ) Electrical circuit Electrical circuit I = V/R I = V/R dolan swip 2009 12

Average Velocities Average Velocities Average flow velocity v = Q/pA A = tube area Average molecular speed V = (8kT/  m) 1/2 1/2 Example: D = 0.1 m, p(N 2 ) = 1 Pa, T = 293 K, p , p( 2 ) , , Q = 0.1 Pa-m3/s Find v, V, dN/dt, and flow regime A=  D 2 /4 = 0 00785 m 2 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 = 2.47x10 19 /s 2 47 10 19 / dN/dt Q/kT  = k 1 /p = 8.1 mm   /D = 0.08 transition region flow. dolan swip 2009 13

Pumping Speed and Flow Rate p g p Q = S t (p p – p u ) S t = pumping speed (m 3 /s) p p g p ( ) t p p = pressure at pump p u = ultimate pressure attainable Q = C(p-p p ) Eliminating p p dolan swip 2009 14

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, p av = 1 Pa Viscous flow C = 0.0715 m 3 /s Molecular flow C = 0.0572 m 3 /s  /D = 0.0068/0.1 = 0.068  transition region, Transition region: C ≈ C(viscous) + C(molecular) T iti i C C( i ) C( l l ) dolan swip 2009 15

Conductances in Parallel Conductances in Parallel dolan swip 2009 16

Conductances in Series Conductances in Series dolan swip 2009 17

Conductances Relative to Air Conductances Relative to Air dolan swip 2009 18

Pumpdown Pumpdown V(dp/dt) = inflow – outflow V(dp/dt) = inflow – outflow = Q L - Q where dolan swip 2009 19

Example Example L = 0.7 m, molecular flow. , = 0.118 m 3 /s Usually higher leak rates due to desorption of gases Usually higher leak rates due to desorption of gases dolan swip 2009 20

Rotary Vane Mechanical Pump y p p u ~ 0.1 Pa dolan swip 2009 21

Roots Booster Pump Roots Booster Pump dolan swip 2009 22

Turbomolecular Pump 1 kHz p u ~0.1  Pa dolan swip 2009 23

Jet Pump Jet Pump dolan swip 2009 24

Oil Diffusion Pump Mechanical pump and cold trap needed needed. dolan swip 2009 25

Cold Trap dolan swip 2009 26

Chevron Baffle Chevron Baffle dolan swip 2009 27

Ionization Pump Ionization Pump p u ~ 0.1  Pa Sputtered Ti buries gas atoms Less effective for He Ar Less effective for He, Ar, … dolan swip 2009 28

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. dolan swip 2009 29

Cryosorption Pump Cryosorption Pump R Require liquid N 2 i li id N T = 77 K dolan swip 2009 30

Cryogenic Pumps Cryogenic Pumps dolan swip 2009 31

Saturation Pressures dolan swip 2009 32 From C. Day, FZK Summer School, 2008

Comparison of Pumps p p dolan swip 2009 33

Bourdon Gage Bourdon Gage dolan swip 2009 34

Thermocouple Gage Tube 0 01 – 100 Pa 0.01 100 Pa dolan swip 2009 35

Pirani Gage Pirani Gage Initially current G = 0 Change of p  resistance F changes  resistance F changes  Current in G dolan swip 2009 36

Calibration of Pirani Gage Calibration of Pirani Gage dolan swip 2009 37

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 dolan swip 2009 38

Magnetron Gage Tube Magnetron Gage Tube B = 0 025 T B z 0.025 T Inhibits radial electron motion Higher ion currents Higher ion currents p down to 10 -11 Pa. dolan swip 2009 39

Comparison of Gages p g dolan swip 2009 40

Chamber stress dolan swip 2009 41

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 dolan swip 2009 42

Welds Welds Weld on inside good. Weld on outside bad Virtual leak dolan swip 2009 43

Welding Flanges Welding Flanges Poor – warpage Good – stress Good – thin ridge due to thermal relieve groove g can flex can flex stress likely dolan swip 2009 44

Glass-to-Metal Seals Glass to Metal Seals Need to match thermal Need to match thermal expansion coefficients to prevent thermal stress. dolan swip 2009 45

O-Ring Flange Joint O Ring Flange Joint Temperature limited by elastomer O-ring T  200 C dolan swip 2009 46

Bakeable Metal Gasket Flange J i Joint High Temperature Capability More expensive dolan swip 2009 47

Monolayers on Surfaces Small amount of grease from fingerprint (10 16 molecules) in 0.01 m 3 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 ~ 8x10 18 molecules/m 2 Extensive cleaning is needed. Extensive cleaning is needed. dolan swip 2009 48

Cleaning Cleaning Degreasing – detergent or solvent, hot water, cold water, d i deionized water i d t 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. dolan swip 2009 49

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. dolan swip 2009 50

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 dolan swip 2009 51

Constituents of Air Constituents of Air H 2 and He can diffuse through metals and glasses. dolan swip 2009 52

ITER Vacuum Systems From C Day FZK Summer School 2008 From C. Day, FZK Summer School, 2008 dolan swip 2009 53