Xenon plasma tube: Construction and operation
Channel: Carl Willis
Date: 2013-01-08

I demonstrate and discuss my kitchen-table construction of a xenon-filled "plasma tube" (a glass tube that is filled with xenon gas at 50 torr and excited with a high-voltage, high-frequency power supply).  I explain the glassblowing operations, the vacuum system, the gas, and the RF power supply.

The discussion is rather rambling (sorry, no script or rehearsals here!), but it hits on some of what I consider to be important equipment and practices in the construction of such toys, based on my very limited experience so far.  Below are some more pointers.  Advice, suggestions, and questions are much appreciated.

The piece of glass shown is an old 3" OD Chinese borosilicate ("chinex") pipe that I obtained free as a scrap.  It is full of inclusions and one end of it has a lot of devitrification in evidence.  A quality piece of Pyrex glass could probably be obtained new for $50 and not have these problems, but I am a beginner and right now I use what I can get cheaply.

My glassblowing advice: The best torches for this kind of thing are the National #3 blowpipe hand torches!  One often benefits from the versatility of a hand torch in this work.  I use one with a big 40-hole premix tip and another with a small, 7-hole premix tip, frequently employing both at once.  They operate adequately from a single medical oxygen concentrator machine.  I also use a National 8M surface-mix bench burner fed from tanked O2 in order to deal with very large pieces of glass like the 3" pipe.  This is as big a glass tube as the 8M will handle, in my opinion.  You can easily spend ten times what the 8M costs to get a ridiculous gas cannon that will blow through 200 cu. ft. of oxygen in half an hour for dealing with even bigger tubes, but I'll leave that to the pros.  (In all honesty, stuff much larger should probably be done on a lathe.)  Lastly, I use a Meker burner to anneal large pieces of glass.  The procedures to make this tube are as follows: (1) Neck down one end of the tube, closing the end and blowing / rotating to produce a uniform rounded end. (2) Make a tubulation to attach the tube to the vacuum pump.  I use only heavy-wall 1/4" inch pyrex for this purpose. (2) heat the center of the closed end of the large tube and blow it open, then fuse down, spin out, and true up the end to form a butt seal with the tip-off tubulation; (3) neck down and close the opposite end of the tube; (4) wash the inside of the tube with a mixture of sulfuric acid and hydrogen peroxide, followed by DI water rinse and thorough drying (5) blow out the closed end of the tube and form a flange for a ring seal; (6) take a standard borosilicate neon electrode (commercially made, cost is about $10), form a maria in its center, and then ring seal that maria into the receptacle in the big tube, carefully annealing afterward.

Vacuum system: as I mention, you do not need a turbo pump (or diff pump, or drag pump) to fool around with plasma.  However, the turbo pump keeps the system clean and gives a big speed boost at the high-vacuum exhaustion stage of the work.  The vacuum gauge can be simple like my little Bourdon gauge, but whatever is used should give an absolute measurement.  You could use a Baratron (classy!) or a McLeod gauge (old-school) also.

Gas: The noble gases range from dirt cheap (argon can be purchased in huge cylinders at a welding store for about $50 for the gas), to moderately expensive (neon runs about $160 for 250 liters at 99.995% purity, exclusive of the cylinder that you must rent or buy, and the fee to transport this uncommon gas to your local distributor), to more expensive than gold by weight (xenon runs about $10 per standard liter in 250 liter volumes, more expensive in smaller volumes, once again exclusive of the costs of the cylinder and hazmat charges for delivery).  None of these gases other than Ar and He will be off the shelf at your local gas supplier, so plan on many weeks lead time and a hazmat charge for delivery.  Most distributors will not refill or credit returned LB ("lecture bottle") cylinders, but will still charge you $200 for the sucker.  A small, returnable, refillable steel cylinder like Matheson's #3 will cost $300-400 to own and about $0.30 per day to rent.  So for hardcore plasma tinkerers, that's the cost-effective way to go.  Buy through a local gas distributor: if you buy gas from an online company, you will not be able to return the cylinder for a refill unless you are dangerous-goods certified. 

Power supply: A high-frequency (~50 kHz), high-voltage (~25 kV) source of power is used for plasma toys.  Solid-state Tesla coils, including flyback-style transformers operated near resonance like the one I display, are common approaches.

That's all, folks!