PLASMA SPHERESPlasma spheres are those glass globes with lightning bolts (or sometimes fuzzier streamers) streaming from a central electrode towards the surface. If you touch the surface or sometimes even have your hand very close to the surface, the sparks usually largely stream towards your hand.
One example of these is Radio Shack's "Illuma-Storm".
What is in these and how they workThe globe typically contains an inert gas or an inert gas mixture at a reduced pressure. Pressures somewhat below atmospheric pressure favor longer sparks, or a less severe high voltage requirement, with little loss of spark brightness or appearance. If the pressure is too low, the sparks will be dim and fuzzy. I would guess that a typical gas pressure may be around 1/10 or 1/20 of an atmosphere.
The central electrode is sometimes surrounded by glass. In this case, the globe usually consists of two concentric bulbs that join at the base. The inner bulb is filled or largely filled with metal, and the space between the bulbs contains the inert gases.
High voltage is applied to an electrode in the center of the globe. This high voltage must be high frequency AC or high frequency pulsating DC in order for any current to get through the glass of the globe and surrounding air by capacitive coupling. Typical voltages are around a few thousand volts for most commercial plasma globes, sometimes around 10,000 volts for some homebrew ones. Typical frequencies are from a few to a few tens of kilohertz.
A continuous oscillation of near or over 100 kilohertz is not recommended unless the current is limited to around a milliamp. Otherwise, if you touch the globe, excessive current may flow and overheat your finger or that spot of the globe.
Most plasma spheres seem to contain xenon, krypton, or a mixture of at least one of these with neon. Xenon and krypton favor more lightning-like sparks rather than fuzzy streamers. Xenon is especially good for this. Xenon and krypton (especially xenon) conduct heat the least and confine heat toward the sparks, which favor any continuously maintained sparks rising upward like the arc in a Jacobs ladder.
However, xenon is particularly expensive. Plasma spheres containing xenon probably have the lowest pressure that is favorable to lightning-like sparks.
Colors and Effects of Various GasesHelium - In spectrum tubes it glows a brilliant whitish yellow-orange color, somewhat like that of a high pressure sodium lamp. I have heard that this sometimes varies with pressure, current, and container dimensions.
Neon - Usually produces dim red blurry streamers with brighter orange "pads" at the ends. If neon is mixed with another gas (other than helium), the streamer color and character is often dominated by the other gas, but the ends of the streamer are orange or pink "pads".
Carbon Dioxide - Glows a whitish or blue-white color. It is probably good to have no direct contact with metal electrodes for long life with gases that are not completely inert. Carbon dioxide probably requires more voltage than the noble gases. Generally, gases and vapors with monoatomic molecules work with less voltage than others.
Nitrogen - Streamers are usually a whitish or grayish pink or light orange. The color may be more gray or lavendar at very low currents. The apparant color varies with what kind of lighting it is in contrast with. Requires somewhat higher voltage than noble gases.
Air, Oxygen, water Vapor - These require more voltage than the noble gases and do not glow brightly. I do not recommend these. If you must use any of these, you may also want no direct contact of gas or vapor to metal in order to avoid corrosion problems.
Argon - Streamers are violet-lavendar. The ends are blue-violet-lavendar.
Argon and neon have the lowest voltage requirements. A mixture of around 99.5 percent neon, .5 percent argon has the lowest voltage requirement, but may not look as good as other gases.
Argon-Nitrogen mixture (as found in many light bulbs) - Streamers are whitish or grayish pink or orange, but more lavendar at low currents. The ends are blue-violet-lavendar. Requires a bit more voltage than pure argon.
Krypton - Generally lightning-like and close to white or light gray, sometimes purplish or pinkish, depending on background lighting. Sometimes fuzzier and/or gray-greenish, especially if the pressure and/or peak current are low.
Xenon - Usually lightning-like and bluish white or bluish gray. May get fuzzier and more gray or lavendarish gray at lower pressure and lower peak current. Peak currents over a few milliamps favor a more lightning-like appearance even if the RMS current is less than a milliamp.
How to Make Your Own Plasma SphereFirst, please beware that doing this could be very tricky and hazardous. Even if you cheat and use a light bulb as the globe (recommended), things may go wrong. I recommend just buying an Illuma-Storm from Radio Shack. They only cost about US$ 45.
One book that has been suggested:
"Build Your Own Fiberoptic, Infrared, and Laser Space Ace Projects"
by Robert I. Iannini.
Tab Books Inc.
Blue Ridge Summit, PA 17214 (USA)
You may also want to see some plasma sphere info at Bill Beatty's site.
If you must make one of your own, I seriously recommend getting a large, globe-shaped clear light bulb. Get one of a higher wattage, preferably at least 60 watts. Most 120 volt bulbs under 40 watts have a vacuum. Most 120 volt 40 watt bulbs have a gas fill unless the filament is very long, as in tubular refrigerator bulbs. However, I recommend at least 60 watts to maximize your chances of having a gas fill, which is usually an argon nitrogen mix.
Next, you need a high frequency AC or pulsating DC power supply. AC is better, since you have twice as much peak-to-peak voltage as peak voltage. Excessive peak voltage (more than [I guess] about 10-12 KV) has a higher risk of sparking through the glass. Peak-to-peak voltages, however, should be at least 10 KV.
Please note that if some types of glass are exposed to excessive high voltage DC for prolonged periods of time, ions may migrate through the glass and cause weak spots.
If you need to use a slightly lower voltage and you don't mind a smaller bulb, then use a 40 watt oven bulb. These have a lower fill gas pressure and will work at lower voltages.
For some high voltage supply ideas, check out my high voltage generator page. (Still somewhat in the works.)
A rough guide to peak voltage: Measure the distance between needle points that the voltage can break through. It is typically 1.1 kilovolts per millimeter. This is the distance that can be broken through, not the distance through which an arc can be maintained (which is largely more dependent on wattage than voltage).
After you get this working, you need to put it in some sort of case that will protect people from the high voltage. The base of the bulb and the lower portion of the bulb must be well insulated or contained so that nobody can get shocked by touching the bulb. It may even be a good idea to put a high value, high wattage resistor between the high voltage supply and the bulb to limit current to a few milliamps if something should go wrong.
Please note that the above is a high voltage project and has risks, including shock and fire hazards of high voltage as well as the risk of breaking/imploding light bulbs and/or plasma spheres. If you still want to do this rather than just buy one, please use all due caution. For some electrical safety hints, look at my safety hints, which is 15K long, slightly long-winded, but still may not cover everything. This was originally written for safely working on xenon strobes, but parts of it apply here.
Written by Don Klipstein.