05_vacuum

1k and all evaporate due to finite heat of

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Unformatted text preview: uum-tight clear window attached to outside • Can put another, nested, inner-inner shield hosting liquid helium stage • Note that LN2 in a bucket in a room doesn’t go “poof” in doesn’ 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 1 61 J/mL 2.6 J/mL • L4He is 0.0829 kJ/mol, 0.125 g/mL, 4 g/mol • H2 O 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 pressure vessel/outer shield Winter 2008 Lecture 5 19 Winter 2008 20 5 Vacuum Systems 01/24/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Nested Shields Assignments • 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 • 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 (OTurbomolecular, Cryo), ring joints), 3.6.3, 3.6.5 – (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 LN2 Winter 2008 Lecture 5 21 Winter 2008 22 6...
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This note was uploaded on 01/30/2014 for the course PHYS 121 taught by Professor Staff during the Winter '08 term at UCSD.

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