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How of He cryo

by Rajini
Tags: cryo
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Rajini
#1
Nov26-08, 10:54 AM
P: 602
Hi All,
I need some basic info of He closed cycle cryos..(like working principle, etc)..
I have a compressor, which compress He gas and passed in to a cryo's coldhead. So He gas expands in side the coldhead and cools the coldhead..then it again passes in to compressor..so the cycle repeats..
I just want to know why and how vibrations are produced in such cryo (i.e., in coldhead) ?
[The compressor and coldhead are connected via tube, so i think vibrations are not from compressor!]
Is there any clever way to damp these vibrations ??
If i am correct there are no moving parts in coldhead !!!!?
Does any have some schematic sketch of coldhead or cross section of a coldhead..i searched the internet..but dint get any idea!!! or what is inside coldhead..
thanks for your help
Rajini.
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mgb_phys
#2
Nov26-08, 11:37 AM
Sci Advisor
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P: 8,953
Stirling cycle and griffen-mcmahon coolers have a piston in the head and vibrate. Newer pulse tube cryo-coolers don't vibrate but don't get as cold or have as much cooling power (or at least didn't 10years ago - don't know about now).

To avoid vibration all you can do is some clever mechanical design with lots of balanced springs - I was trying to find a picture of some of the cameras on UKIRT (uk ir telescope) they are works of suspension art.
f95toli
#3
Nov26-08, 01:14 PM
Sci Advisor
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P: 2,250
The main problem has always been vibrations from the compressor. The simplest way to avoid this is to have the compressor located as far as possible from the cryostat and to connect the two using tubes that do not transmit vibrations well, Where I work we generally try to keep the compressors in another room if possible (in one case it is actually in a room on the other side of the corridor).

The second problem has been electrical noise from the pumps. Grounding etc is very tricky but some of the manufacturers claim to have solved that problem in the latest generations of dry (dry=cryogen free) fridges; at a conference a few months ago I even saw a dry dilution fridge with a turbo mounted on top of the fridge (I think it was the VeriCold design now sold by Oxford Instruments); the manufacturer claimed that there shouldn't be any problems with either mechanical nor electrical noise (although I am not sure I believe them).

Note that single stage coolers are a mature technology now, if you only want to reach temperatures of about 20K or so vibrations should not be an issue. It gets a bit trickier with two-stage coolers that can reach 4K and sub-1K fridges (He3 and dilution fridges).

Rajini
#4
Nov28-08, 09:04 AM
P: 602
How of He cryo

Hi thanks for all the replies..
Compressor is no problem..it makes very less vibrations..but it produces a bit of sound...in any case the vibration produced by compressor will not be transferred to coldhead (coldhead and compressor are connected by flexible tubes, which are very poor in transmitting the vibrations produced from compressor)...
But when each time He gas is compressed and injected in to coldhead..it makes vibrations inside coldhead..I just want to damp it...is it possible,,,another problem is..my sample is fixed to coldstage, which is fixed tightly to coldhead...so the vibration from coldhead can easily transfer to cold state and then to sample..
I searched a lot of scientific paper...but all are different..for e.g., Sample is not directly fixed to cold stage but it is hanging inside..heat transfer is only be exchange gas (He gas)..
Rajini


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