Download 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.