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Re: glass vacume gaps

Subject: 
        Re: glass vacume gaps
  Date: 
        Mon, 07 Apr 1997 12:27:00 -0500 (EST)
  From: 
        Benson_Barry%PAX5-at-mr.nawcad.navy.mil
    To: 
        tesla-at-pupman-dot-com


Hi Stephen, All,
I tried some experiments with sparkgaps in sealed glass
tubes about 25 years ago for a high school research project.
I tried copper, tantalum, tungsten, and platinum at atmospheric
pressure.
They all eventually melted down (vaporized) below the distance
where the spark could jump (12kv,30ma neon). The platinum
lasted much longer than the rest.  My Tesla coil worked best
with this gap.  Eventually the inside of the tube became
coated with a white (oxide ?) layer but still worked.  I still have
that platinum spark gap tube and it still works!  It gets very hot
though.

Extrapolation:

I've examined cutaways of commercial spark gap tubes.
The failure mode is for the metal to sputter the inside
surface of the tube until a metallic bridge forms and shorts
the tube out.  Getting the wall away from the electrodes
is a good thing but the atmospheric force upon the vacuum
chamber makes that difficult.  For glass the best seal is
Kovar.  Then the Kovar has to be brased  to the metal.
I'd buy a commercial unit before making one unless
you have access to a machine shop, oxyacetalene
torch, high vacuum system (turbo or/and cryo pump),
Kovar alloy foil or sheet, and elkonite or stainless
steel disks 3" in diameter and 1/4" thick for the
electrodes.

For a demountable design use cajun fittings brased into
a brass block at each end of the pyrex or silica tube.
The elkonite electrodes can be braised to the brass blocks
in such a way that the glass tube can be slid into the
cajun fitting (vacuum seal) and over the electrode at
the same time.  Do this on both sides and pump from
one side.  Make the electrodes long say 4" to get the
heat away from the o ring seals on the cajun fittings.
To adjust the electrode separation put some
fiberglass threaded rods between the brass blocks
with nuts.  Adjust the nuts for the best separation.

I built a carbon dioxide laser with this method.  The
glass tube can be changed out quickly and the
thermal expansion is allowed for by the sliding
o ring seals.  It also makes it possible to change
the tube when it becomes sputtered although
the very long creepage surface from this
method will not necessitate this very oftern.

Barry


<<  but how is the considerable heat
 buildup of the gap electrodes going to be removed if they are sealed
 in a glass enclosure?  I'd worry about that unless the coil is just a
 5 or 10 watt job.

 rwstephens >>
I didnt think of that, they used to use this type of " quenched" gap
with no
problems but then the power systems and caps were not what they are
today.
The first book I read on the subject said the gap will always be the
most
aggravation and it is so. Tesla work can run into some money at home
depot
quickly. I will probably try it anyway, maybe I could heat sink it
somehow or
use a fan by putting the glass tube in another tube and force air over
it.
   Stephen