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Plasma Properties and Problems

A plasma is called the fourth state of matter (after solid liquid and gas) and it is what makes the stars bright and fluorescent lamps work. An electric field is used tear electrons off a gas molecule and when they recombine the result is light. At low currents you get a relatively gentle light like you see in an aurora borealis or those novelty plasma globes. At higher currents and higher gas pressures you transition to an "arc" like those you see in arc welding or lightning.

The air you breathe is a relatively inert material but in an plasma it creates charged oxygen and nitrogen ions that are highly reactive. You can use air as a plasma medium but the ions will rapidly corrode and etch any container. Even after the plasma is turned off oxygen ions will combine with oxygen molecules to form ozone which is a toxic gas even in low concentrations. For my plasma experiments I needed a safe gas and that means one of the inert gases: Helium (He), Neon (Ne), Argon (Ar), or Krypton (Kr).

Noble Gas Supplies

There are easily obtainable industrial and scientific supplies of noble gases. Krypton is used in light bulbs to protect the filament and allow a brighter light. Argon is used extensively in welding to keep air away from the arc column especially for easily oxidized metals like aluminum. Neon is used in Neon signs and Helium is used in party balloons.

All noble gases can be purchased from a scientific supply house for about $200 a refill. The problem is you have to buy a gas bottle for them to fill. A small lecture bottle (about two feet long) or a big industrial argon bottle (about four feet tall) with cost you about $200. A little expensive for a hobbyist or poor starving inventor.

A Helium Bottle Comes with 30 Free Balloons

I decided to use helium and I found a much cheaper solution. You can buy "Balloon Time" disposable bottles of helium of about $35 and you get 30 free balloons with it. I originally wanted to use neon or the neon/helium mix used in lasers because I am familiar with those gases. It turns out helium has some properties that make it a better choice for my project.

Remove the balloon filling nozzle (plastic bendy thing) and you will find a standard automotive 1/4" flare connector.

Connect your helium source to the red side of the manifold. The color coding is accidental but having the blue pump on the blue line and the red gas bottle on the red line makes the system look like it was designed by a professional.

Using Paschen's to Measure Gas Pressure

I will still have to use air as a plasma gas to calibrate my Pirani gauge. Air and helium have different Paschen's Law curves and I will use this difference to verify my calibration.

A little background on the Paschen Formula that will be used to calibrate my Pirani gauge (see my Vacuum Gauges page). The breakdown voltage of a plasma depends on three factors; the type of gas, the pressure of the gas, and the distance between the electrodes. Measuring the breakdown voltage will give you the pressure if you know width of the electrode gap and the type of gas. See the "Measurements of the Breakdown Potentials For Different Cathode Materials in The Townsend Discharge" paper for a lot data and practical information on measuring the Paschen effect.

There are lots of circuits for neon tube relaxation oscillators available. It is just a neon bulb with a series resistor and a parallel capacity. When you power the circuit the neon bulb is off and capacitor has zero voltage. The resistor starts to charge the capacitor in a classic sawtooth until the capacitor reaches the breakdown voltage of the neon bulb. The capacitor then dumps its charge into the neon gas until the voltage is too low to sustain a plasma and the sawtooth starts again.

By constructing a relaxation oscillator I can measure the peak voltage of the circuit which is the breakdown voltage of the gas. By varying the gas pressure I can calibrate my Priani gauge. Since the Priani gauge curve is smooth I can get very good calibration with only a few data points. By using both air and helium I can verify my calibration so I have an accurate measure of the gas pressure in my vacuum chamber during later experiments.