[DMCForum] OT - Cryogenics. was: PCV Routes Manifold Vacuum Through The Crankcase

["bkp944" <bk_pollard@xxxxxxxxxxx>]

--- In DMCForum@xxxxxxxxxxxxxxx, "checksix3" <jetjock11@xxxx> wrote:

> it will thru many other materials. It's why it's difficult to use 
> elastomers for sealing below 10E8 Torr or so.

The superconducting magnet is welded in a cylinder that contains the 
liquid helium. Some systems contain hundreds of liters of LHe. This 
chamber will have vent tubes to atmosphere for electrical service, 
LHe vent and fill.

All around the LHe chamber is the evacuated cryostat. Working our 
way out from the LHe chamber are several thermal radiation shields, 
a liquid nitrogen chamber, aluminized mylar sheets, and finally the 
room temperature outer. Although the LHe chamber's inside is at 
atmospheric pressure, there is ultra-high vacuum on the outside of 
the LHe chamber.

Think of it as a giant, fancy, Thermos bottle.

We pump the dewar down to 10E-6 torr or so then start adding the 
liquid cryogens. Ultimately, the vacuum level of dewar will get down 
to 10E-9 or so. The cold LHe chamber cryopumps the contaminents out 
of the evacuated space. This will cause the contaminents to "freeze 
out" on the LHe chamber's outer walls (the evacuated side). We also 
install charcoal getters on the LHe chamber's outer walls to absorb 
some of these contaminents.

We leak check the vessel using a mass spectrometer helium leak 
detector with minimum sensitivity of 4 X 10-10 std cc/sec.

The leak can appear as liquid helium surrounding the superconducting 
magnet is pulled or more correctly "pushed" through microscopic 
holes or cracks in the welds. 

Most of the time leaks don't happen, but you can imagine the 
headache of cutting apart a vessel to find a microscopic leak, that 
only appears when the vessel is at 4.2K and is under ultra high 
vaccum.

E-mail off list if you want to discuss this further. I think we are 
boring the DeLorean community.

-Brian
VIN# 4494

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