Plasma balls are pretty demonstrations of plasma. The research group I'm part of uses a flat one to attract new students. Like a lava lamp or a fire, the constantly changing pattern is something that can capture your attention for a long time. If you look at it, you can see a small central ball, from which luminous threads emanate to an outer glass ball.

So, how does it work? A detailed description of this device would be a good subject for a PhD thesis rather than a node, so this will be a rough description only.

The plasma is generated by a high-voltage (several kV) high-frequency (several kHz) power supply, that uses the small central ball as an antenna. Because the voltage is so high, free electrons in the gas surrounding the central ball get very high energies, so high that they can excite and ionize the gas. This ionization produces more electrons, which help to sustain the plasma.

The excited species are basically gas atoms from which one of the outer electrons is kicked into an higher orbit, which is at a higher energy level. It will stay in this higher orbit for a very short time, say a few nanoseconds to a few microseconds. It will then fall back to a lower orbit, emitting the excess energy as photonslight. Because atoms have discrete energy levels, the photons emitted also have discrete energies, causing the light emitted by the plasma ball to have discrete colours.

Various gases can be used to create the plasma in. Because every gas has different energy levels, each gas will give a different colour of light. Some popular gases are:

Oxygen or water are bad news for the plasma ball, as the electronegative oxygen will gobble up electrons, making discharges very weak. It is possible to create threads, or streamers (They are not arcs, arcs are something entirely different!) as plasma physicists call them, with various colours using a mixture of gases. Because the electric field gets weaker near the glass, the electrons collisions are likely to have less energy there than in the center. If you put in a gas with a smaller energy distance between atomic levels and one with larger distance between energy levels, you can get the plasma to excite the former near the wall and the latter near the center, giving you a nice multicoloured streamer.

The gas in the plasma ball is at subatmospheric pressure, usually around one kPa. The reason why you don't want too high a gas pressure is that in that case, electrons collide so often that they haven't gained enough energy between collisions to excite or ionize the gas, which is bad for the discharge. If the pressure is too low on the other hand, the electrons will collide less often, also diminishing the strength of the discharge. This leads to an optimum of the ratio of electric field and pressure. This effect is expressed in a Pashen curve, which is different for each gas or mixture of gases. Home builders often use atmospheric pressure and a much higher voltage to compensate.

The glass sphere surrounding the gas acts as a return circuit. While glass is obviously an isolator, the high-frequency electric field can travel over the surface of the globe, closing the current loop. However, if you touch the globe, your body, which is a very good conductor of high-frequency electric current, will act as a return path, causing most of the pretty plasma streamers to collect near your hand.

Why the plasma organizes in streamers, rather than in a homogeneous glow is a difficult matter. A key mechanism is however the tendency of current to seek the path of the least resistance. If a piece of plasma is more ionized than the pieces of plasma around it, it has a lower restance. This will draw more current in, further increasing the ionization. This is a self-amplifying mechanism, causing the plasma to organize in discrete streamers.

Sources: I got the list of plasma colours off at July 10th 2004.

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