Bombarding is one way to bake-out impurities in the glass, electrodes, phosphor, etc in a neon tube. Generally it's done using a current-limited high-voltage source, running at a much higher current (hundreds of milliamps, which is deadly) than normal operation (20-50mA, depending upon tube diameter). From what I recall, bombarding is started once the vacuum is around a few torr and the current is adjusted until the glass reaches a high temperature (around 500F I think), during which the electrodes will be almost red-hot. As the gas heats up, the pressure will increase and you continue pumping to remove the impurities as well as maintain bombarding current.
I recommend reading-up on Paschen's Law, which describes how the ionization voltage decreases with pressure until a minimum is reached, and then increases as pressure is lowered further. As the pressure is lowered below 1 torr at high temperature, the impurities are released and drawn out thru the vacuum port. This is where things can get really scary, because I've heard of cases where flashover occurs and an arc will go thru the neon work, into your manifold (multiport section for managing your pump, fill-gas, vacuum gauge, etc), and thru the pump. Yikes. Most setups I've read about use a 2-stage mechanical pump for the primary pump-down, and a diffusion pump to get below 1 torr. You also need good vacuum gauges. You can easily spend over $2000 US on the equipment. I also recommend reading about diffusion pumps; I never heard about them before researching neon work. After some amount of time, the bombarding is stopped and the glasswork cools, the vacuum is continued and I think I've seen people going down to 50 microns. Then the fill gas is introduced and pressurized to get optimum glow. Some mixtures require mercury; I'm not sure how they introduce it into the tube. Finally, the tubulation (electrode with a glass connector) is tipped-off and the tube is run for several hours to make sure it's properly filled, etc. It's really amazing how much work goes into a neon-sign tube. There are several good books on neon; See "Neon Techniques" by Strattman. "Neon Engineer's Notebook" (Fishman & Crook) is very good; I was able to get a personally signed copy -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/87b239d3-016f-405d-b34a-80fc9dd68b99%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
