Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Amateur Small Array Radio Telescope
Date: Mon, 3 Jul 2000 22:59:11 GMT

In article <20000703155800.11058.00000196@ng-da1.aol.com>,
CLVANCIL <clvancil@aol.com.de-spam> wrote:
>Is a liquid helium system that much more expensive than a liquid nitrogen
>system?

In a word, yes -- it's a whole different order of magnitude of problems.
There's a reason why people wanting to do things with superconductors were
very happy when liquid-nitrogen superconductors were discovered, and why
they have since worked very hard at taming some very uncooperative
materials.  It makes a huge difference.

Helium itself is scarce and in somewhat limited supply.

Making liquid helium is vastly harder (i.e., more expensive) than making
liquid nitrogen.

Insulation becomes extremely critical, because the heat capacity of liquid
helium is two orders of magnitude lower.  A given system won't leak heat
much faster into liquid helium than it will into liquid nitrogen (with
some caveats), but a given amount of heat leaked in will boil about 120
times as much helium as nitrogen.

Internal heat generation, e.g. by electronics, raises similar issues.

Liquid-air condensation in the insulation is almost a non-issue with
nitrogen but very serious with helium.

There are one or two elastomers, notably Teflon, which are still somewhat
flexible at LN2 temperatures and can be used for gaskets etc.  Even they
are brittle at LHe temperatures.

Helium will leak *through* a number of solid materials, e.g. glass.

Various types of superfluid and superconductive phenomena in LHe lead to
unexpected behavior which complicate design.  For example, a thin film of
liquid helium will creep up over the edge of an open container.

Etc.

--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)