Re: [TCML] Large Plasma Tube

[Bill Lemieux <gomezaddams@xxxxxxxxx>]

> David N. Van Doren wrote:
> > Thanks Scott - Those globes on the link, what pressure are they being
> > run at.
> > Getting gas is not a problem. Designing and building the fixture  
> > isn't
> > a problem.
> > My only concerns are, can I do this with out vacuum and what kind of
> > power will be needed.

The voltage you need will depend upon the gas pressure, and the gas  
pressure
you choose will depend upon A) the gases you choose (different gases  
ionize
more easily than others, as other people have already pointed out) and  
the
appearance of the streamers you desire.  In my own experiments, I  
found that
higher pressures tended to result in narrower, thread-like discharges,  
while
higher pressures tend to result in fatter, more diffuse discharges.   
Frequency
also affects the appearance, any can also change the

You will probably find that you don't need very much current.  That  
too will
depend on pressure and voltage.  Generally speaking, the higher the  
pressure
and voltage, the more brightness you will get, up to the point where  
current
starts to go down from too much gas.  And as others have pointed out,  
some
gases just seem to be brighter than others.  There are many I have not  
experimented
with, ever, because of the cost.  Price a small lab bottle of Krypton  
- you'll
be shocked.

You have many factors to experiment with!

One caveat: if you intend to seal your tube after you have the "look"  
you want,
you had best make sure the interior and all materials, electrodes, and  
so forth,
are unbelievably, ATOMICALLY clean.  ( I use that word just because it  
sounds
impressive, not because it has any technical meaning).  What I am  
getting at is
that over time, even tiny contaminants (like a fingerprint you can't  
even see)
can poison your gas mixture and screw up your electrodes.  You WILL  
need a vacuum
system capable of very high vacuums, and you will want to not only  
clean the
interior with some very pure but pump-able solvent (like optical grade  
acetone
or reagent grade isopropanol) but you will also want to bake your  
assembled system
at the highest temperature it and your sealing materials can stand,  
while it is
at a very high vacuum, for many hours, to remove contaminants that can  
be baked
out.  Only then should you back-fill with the gas mix at whatever  
partial pressures
you have previously determined.  This will ensure the longest life of  
your finished
plasma tube.

You will find that some gases are hideously expensive.  You can save a  
lot of money
(although it will dramatically limit the appearance of your discharge)  
by filling
your tube with very clean glass beads.  This is what is done with  
"crackle" tubes,
which don't actually crackle audible, but look like that ought to.

> > I suppose that if I need a power source other
> > than what I have I'll have to decide if
> > I want to buy/build such a thing. And vacuum scares me with such a
> > large GLASS tube. I don't particularly want to put an implosion gage
> > around it.

I have done inverse hoop stress calculations on tubes for vacuum before,
and I assure you that a .25 inch wall tube is more than sufficient to
hold a vacuum.  Remember, a vacuum is only 15 PSI, and a cylinder is  
about
an order of magnitude stronger under compression from outside than it is
from pressure on its inside.

regards,
 - Bill "Gomez" Lemieux

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