[TCML] Large Plsma Tube

[Bill Lemieux]

PS: I forgot to mention the end-seals.  Because the ends will probably  
have to be flat, you should consider using 1/2" thick plate to span a  
6" distance.  This is a bit on the overkill side for 15PSI of  
pressure, but better safe than sorry!  Make sure you don't introduce  
any small cracks when you drill the plates for your electrodes, and  
polish all edges to eliminate sharp edges or pits from drilling or  
cutting.  Use fine emery paper for this.

Forget about using plastics for your end plates, it won't last.  Among  
other things, atmospheric helium and hydrogen will leak in (through  
the material) over the passing years... possibly changing the  
appearance of your discharge significantly. You're going to have  
problems with that using epoxy seals anyway, but if the exposed area  
of the epoxy seal is tiny and the thickness great (ie; .25") they  
ought to be good for many years.
The higher pressure you can operate at, (approaching atmospheric) the  
slower this leak problem will be.

This is why old epoxy-sealed HeNe lasers died after about ten years -  
the helium would diffuse out through the seals.  Amazingly, because  
there was still a low pressure inside the tube, you could rejuvenate  
those old tubes by sticking them in a bag of cheap balloon helium, and  
operating the tube - you'd check it every few days and eventually it  
would be working as good as new.  Take it out of the bag, and it would  
once again be good for years.  Of course, with modern diode lasers and  
even cheap hard-seall HeN lasers being so plentiful, that isn't worth  
the effort.  Epoxy-sealed HeNe lasers are worthless now.

  - "Gomez"