06_vacuum - Vacuum Systems Why much of physics sucks UCSD:...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Vacuum Systems Why much of physics sucks UCSD: Physics 121; 2011 Why Vacuum? Anything cryogenic (or just very cold) needs to get rid of the air eliminate thermal convection; avoid liquefying air Atomic physics experiments must get rid of confounding air particles eliminate collisions Sensitive torsion balance experiments must not be subject to air buffeting, viscous drag, etc. are problems Surface/materials physics must operate in pure environment e.g., control deposition of atomic species one layer at a time Winter 2011 2 UCSD: Physics 121; 2011 Measures of pressure The "proper" unit of measure for pressure is Pascals (Pa), or Nm-2 Most vacuum systems use Torr instead based on mm of Hg Atmospheric pressure is: 760 Torr 101325 Pa 1013 mbar 14.7 psi So 1 Torr is 133 Pa, 1.33 mbar; roughly one milliatmosphere Winter 2011 3 UCSD: Physics 121; 2011 Properties of a vacuum Vacuum Pressure (torr) 760 Number Density (m-3) 2.7 1025 3.5 1019 3.5 1016 3.5 1013 3.5 1010 M.F.P. (m) 7 10-8 0.05 Surface Collision Freq. (m-2s-1) 3 1027 4 1021 4 1018 4 1015 4 1012 Monolayer Formation Time (s) 3.3 10-9 2.5 10-3 2.5 2.5 103 2.5 106 Atmosphere Rough 10-3 High 10-6 50 50 103 50 106 Very high 10-9 Ultrahigh 10-12 Winter 2011 4 UCSD: Physics 121; 2011 Kinetic Theory The particles of gas are moving randomly, each with a unique velocity, but following the Maxwell Boltzmann distribution: The average speed is: With the molecular weight of air around 29 g/mole (~75% N2 @ 28; ~25% O2 @ 32), 293 K: m = 29 1.67 10-27 kg <v> = 461 m/s note same ballpark as speed of sound (345 m/s) Winter 2011 5 UCSD: Physics 121; 2011 Mean Free Path The mean free path is the typical distance traveled before colliding with another air molecule Treat molecules as spheres with radius, r If (the center of) another molecule comes within 2r of the path of a select molecule: Each molecule sweeps out cylinder of volume: V = 4r2vt in time t at velocity v If the volume density of air molecules is n (e.g., m-3): the number of collisions in time t is notZ = 4nr2vt Correcting for relative molecular speeds, and expressing as collisions per unit time, we have: Winter 2011 6 UCSD: Physics 121; 2011 Mean Free Path, cont. Now that we have the collision frequency, Z, we can get the average distance between collisions as: = v/Z So that For air molecules, r 1.75 10-10 m So 6.8 10-8 m = 68 nm at atmospheric pressure Note that mean free path is inversely proportional to the number density, which is itself proportional to pressure So we can make a rule for = (5 cm)/(P in mtorr) Winter 2011 7 UCSD: Physics 121; 2011 Relevance of Mean Free Path Mean free path is related to thermal conduction of air if the mean free path is shorter than distance from hot to cold surface, there is a collisional (conductive) heat path between the two Once the mean free path is comparable to the size of the vessel, the paths are ballistic collisions cease to be important Though not related in a 1:1 way, one also cares about transition from bulk behavior to molecular behavior above 100 mTorr (about 0.00013 atm), air is still collisionally dominated (viscous) is about 0.5 mm at this point below 100 mTorr, gas is molecular, and flow is statistical rather than viscous (bulk air no longer pushes on bulk air) Winter 2011 8 UCSD: Physics 121; 2011 Gas Flow Rates At some aperture (say pump port on vessel), the flow rate is S = dV/dt (liters per second) A pump is rated at a flow rate: Sp = dV/dt at pump inlet The mass rate through the aperture is just: Q = PS (Torr liter per second) And finally, the ability of a tube or network to conduct gas is C (in liters per second) such that Q = (P1 -P2) C Winter 2011 9 UCSD: Physics 121; 2011 Evacuation Rate What you care about is evacuation rate of vessel S = Q / P1 but pump has Sp = Q/P2 Q is constant (conservation of mass) Q = (P1 - P2)C, from which you can get: 1/S = 1/Sp + 1/C Q P1 C P2 Q Q pump: Sp So the net flow looks like the "parallel" combination of the pump and the tube: the more restrictive will dominate Usually, the tube is the restriction example in book has 100 l/s pump connected to tube 2.5 cm in diameter, 10 cm long, resulting in flow of 16 l/s pump capacity diminished by factor of 6! Winter 2011 10 UCSD: Physics 121; 2011 Tube Conductance For air at 293 K: In bulk behavior (> 100 mTorr): C = 180 P D4/L (liters per second) D, the diameter, and L, the length are in cm; P in Torr note the strong dependence on diameter! example: 1 m long tube 5 cm in diameter at 1 Torr: allows 1125 liters per second In molecular behavior (< 100 mTorr): C = 12 D3/L now cube of D same example, at 1 mTorr: allows 0.1 liters per second (much reduced!) Winter 2011 11 UCSD: Physics 121; 2011 Pump-down time Longer than you wish Viscous air removed quickly, then long slow process to remove rest to go from pressure P0 to P, takes t = (V/S) ln(P0/P) note logarithmic performance Winter 2011 12 UCSD: Physics 121; 2011 Mechanical Pumps Form of "positive displacement pump" For "roughing," or getting the the bulk of the air out, one uses mechanical pumps usually rotary oil-sealed pumps these give out at ~ 110 mTorr A blade sweeps along the walls of a cylinder, pushing air from the inlet to the exhaust Oil forms the seal between blade and wall 13 Winter 2011 UCSD: Physics 121; 2011 Lobe Injection Pumps Can move air very rapidly Often no oil seal Compression ratio not as good Winter 2011 14 UCSD: Physics 121; 2011 Turbomolecular pumps After roughing, one often goes to a turbo-pump a fast (24,000 RPM) blade achieves a speed comparable to the molecular speed molecules are mechanically deflected downward Work only in molecular regime use after roughing pump is spent (< 100 mTorr) Usually keep roughing pump on exhaust Winter 2011 15 UCSD: Physics 121; 2011 Cryopumping A cold surface condenses volatiles (water, oil, etc.) and even air particles if sufficient nooks and crannies exist a dessicant, or getter, traps particles of gas in cold molecular-sized "caves" Put the getter in the coldest spot helps guarantee this is where particles trap: don't want condensation on critical parts when cryogen added, getter gets cold first Essentially "pumps" remaining gas, and even continued outgassing Called cryo-pumping Winter 2011 16 UCSD: Physics 121; 2011 Ion Pump Ionize gas molecules, deposit ions on chemically active surface, removed by chemisorption Best use is for Ultra-High Vacuum applications (10-11 Torr) Current is proportional to pressure (pump is also a pressure gauge) No moving parts, but efficient only at very low pressures Winter 2011 slide courtesy O. Shpyrko 17 Residual Gas Analyzer (mass spectrometer) Electronic "nose", sniffing inside the chamber Can detect partial pressure down to 10-14 Torr Useful as a He leak-detector Measures mass-to-charge ratio by ionizing a molecule and accelerating it in EM field UCSD: Physics 121; 2011 Winter 2011 slide courtesy O. Shpyrko 18 UCSD: Physics 121; 2011 Example of RGA spectra, He:Ne mixture 10:1 Winter 2011 slide courtesy O. Shpyrko 19 UCSD: Physics 121; 2011 Typical problems in achieving UHV: Actual Leaks (valves, windows) Slow pump-down times "Virtual" leaks Outgassing bulk and surfaces Solutions: Leak-testing Re-design of vacuum chamber Bake-out Cryopumping Winter 2011 slide courtesy O. Shpyrko 20 UCSD: Physics 121; 2011 Dewars Evacuating the region between the cold/hot wall and the ambient wall eliminates convection and direct air conduction Some conduction over the lip, through material minimized by making thin and out of thermally nonconductive material Radiation is left, but suppressed by making all surfaces low emissivity (shiny) Heat paths cut holds temperature of fluid 21 Winter 2011 UCSD: Physics 121; 2011 Liquid Nitrogen Dewar Many Dewars are passively cooled via liquid nitrogen, at 77 K A bath of LN2 is in good thermal contact with the "inner shield" of the dewar The connection to the outer shield, or pressure vessel, is thermally weak (though mechanically strong) G-10 fiberglass is good for this purpose Ordinary radiative coupling of (Th4 - Tc4) = 415 W/m2 is cut to a few W/m2 Gold plating or aluminized mylar are often good choices bare aluminum has 0.04 gold is maybe 0.01 aluminized mylar wrapped in many layered sheets is common (MLI: multi-layer insulation) MLI wants to be punctured so-as not to make gas traps: makes for slooooow pumping Winter 2011 22 Dewar Construction cryogen port perforated G-10 cylinder UCSD: Physics 121; 2011 cryogen (LN2) tank vacuum port Cryogen is isolated from warm metal via G-10 but in good thermal contact with inner shield science apparatus inner shield Metal joints welded Inner shield gold-coated or wrapped in MLI to cut radiation Windows have holes cut into shields, with vacuum-tight clear window attached to outside Can put another, nested, inner-inner shield hosting liquid helium stage 23 pressure vessel/outer shield Winter 2011 UCSD: Physics 121; 2011 Cryogen Lifetime Note that LN2 in a bucket in a room doesn't go "poof" into gas holds itself at 77 K: does not creep to 77.1K and all evaporate due to finite "heat of vaporization" LN2 is 5.57 kJ/mole, 0.81 g/mL, 28 g/mol 161 J/mL L4He is 0.0829 kJ/mol, 0.125 g/mL, 4 g/mol 2.6 J/mL H2O is 40.65 kJ/mol, 1.0 g/mL, 18 g/mol 2260 J/mL If you can cut the thermal load on the inner shield to 10 W, one liter of cryogen would last 16,000 s 4.5 hours for LN2 260 s 4 minutes for LHe Winter 2011 24 UCSD: Physics 121; 2011 Nested Shields LHe is expensive, thus the need for nested shielding Radiative load onto He stage much reduced if surrounded by 77 K instead of 293 K (2934 - 44) = 418 W/m2 (774 - 44) = 2.0 W/m2 so over 200 times less load for same emissivity instead of a liter lasting 4 minutes, now it's 15 hours! based on 10 W load for same configuration at LN 2 Winter 2011 25 Coolest place on earth: UCSD: Physics 121; 2011 Antarctica -89 C, or 183K San Diego: Dilution fridges Mayer Hall (Maple, Goodkind), NSB (Butov) ~300 mK Cambridge, MA: Sub-500 picoKelvin achieved in Ketterle group at MIT See "Cooling Bose-Einstein Condensates Below 500 Picokelvin" Science 301, 5639 pp. 1513 - 1515 (2003) Winter 2011 slide courtesy O. Shpyrko 26 UCSD: Physics 121; 2011 Adiabatic Magnetization Cooling Winter 2011 slide courtesy O. Shpyrko 27 UCSD: Physics 121; 2011 Photos: Displex Cryostat insert Winter 2011 slide courtesy O. Shpyrko 28 UCSD: Physics 121; 2011 Photos: Ultra High Vacuum chamber Winter 2011 slide courtesy O. Shpyrko 29 UCSD: Physics 121; 2011 Photos: Turbomolecular "Turbo" Pump Winter 2011 slide courtesy O. Shpyrko 30 Photos: Dilution Refrigerator UCSD: Physics 121; 2011 Winter 2011 slide courtesy O. Shpyrko 31 Photos: Dilution Refrigerator UCSD: Physics 121; 2011 Winter 2011 slide courtesy O. Shpyrko 32 UCSD: Physics 121; 2011 Helium Flow Cryostat Winter 2011 slide courtesy O. Shpyrko 33 UCSD: Physics 121; 2011 Assignments Read 3.1, 3.2, 3.3.2, 3.3.4, 3.4: 3.4.1 (Oil-sealed and Turbomolecular, 3.4.3 (Getter and Cryo), 3.5.2 (Oring joints), 3.6.3, 3.6.5 applies to both 3rd and 4th editions Winter 2011 34 ...
View Full Document

Ask a homework question - tutors are online