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Physics 121 2008 turbomolecular pumps cryopumping

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Unformatted text preview: der, pushing air from the inlet to the exhaust • Oil forms the seal between blade and wall Winter 2008 13 Winter 2008 14 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Turbomolecular pumps Cryopumping • After roughing, one often goes to a turbo-pump • A cold surface condenses volatiles (water, oil, etc.) and even air particles if sufficient nooks and crannies exist – a fast (24,000 RPM) blade achieves a speed comparable to the molecular speed – molecules are mechanically deflected downward – a dessicant, or getter, traps particles of gas in cold molecular-sized “caves” • Put the getter in the coldest spot • Work only in molecular regime – helps guarantee this is where particles trap: don’t want condensation on critical parts – when cryogen added, getter gets cold first – use after roughing pump is spent (< 100 mTorr) • Usually keep roughing pump on exhaust Winter 2008 Lecture 5 • Essentially “pumps” remaining gas, and even pumps” continued outgassing • Called cryo-pumping 15 Winter 2008 16 4 Vacuum Systems 01/24/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Dewars Liquid Nitrogen Dewar • 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 • 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 is shield” the dewar ...
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