Transcript W. Johnson
Plumbing and Gas, for better optical cryostats Warren Johnson LSU Motivation for cryogenic interferometers Higher mechanical Q (especially for metals) Lower Temperature (noise) Creaking and creeping frozen out? Dimensional stability. Gas and dust frozen out. Every material (?) becomes vacuum qualified. Superconductivity (ultra stable magnetic fields) Practical Price? • Extra layers in vacuum system – 77-K shell – Thermal radiation shields – X-K shell, where 4K < X < 300K • One added vacuum feedthrough layer for electrical wiring. • Three gas-liquid handling systems (LN, LHe, X ) Fundamental Price • New mechanical design issues – Thermal contraction is large, and material dependent. – Thermal conductivity of can have huge changes. • New electrical design issues – Electrical conductivity can have huge changes • New optical design issues – Optical absorption in bulk and ? Test Mass Material • Fused silica is disqualified (mechanical Q goes bad fast at lower temperature) • Single crystal sapphire is limited by relatively high optical absorption • Single crystal Silicon appears superior is almost everyway. [Industry wins again !] – except for need to change lambda to >1.5 micron. Lessons learned with cryogenic bar detectors • Handle the practicalities with good engineering (we physicists are unnaturally good at fooling themselves). Reliable and low maintenance cryogenics can be bought. • Need many trials of each new feature. That’s why they call it research. Run cycle must be short, changes and measurements frequent. Lessons … • Low temperature can allow some tremendous technical improvements. Normal cryostats are a problem • Experimental chamber (X shell) must be raised and lowered during each run • need a different configuration Want cryogenic bell jar configuration • A a fixed cold platform or table for construction of optical layouts, suspensions etc. Platform in ‘rigid’ contact with floor. • 2 pi of access gas heat exchanger on underside of platform 77K heat exchange below that • That space enclosed (by rings) and a jar. – Optical, electrical and gas feedthroughs through rings Cont… • All isolation and suspension in the cold X space. • X space gas pressure easily variable between 1e-6 Torr and 1 Torr Standard thermal control of platform. (heaters, Thermometers. Jena cryostat has most (or all?) of these features. • Cryogenics should be as easy as vacuum. – Specify the size, buy from vendors, work the controls.