Return to main page. An Introduction to Plasmas

What is the plasma in a plasma globe?
How does it work?

plasma








n 1: an electrically neutral ionized gas in an electric discharge; distinctly different from solids and liquids and normal gases 2: any watery animal fluid [syn: serum, plasm] 3: a green slightly translucent variety of chalcedony used as a gemstone 4: (physical chemistry) the gaseous state of hot ionized material consisting of ions and electrons and present in the stars and fusion reactors: sometimes regarded as a fourth state of matter distinct from normal gasses

Source: WordNet 1.6, 1997 Princeton University

A plasma ball in action. Plasmas exist in a wide range of settings and varieties. Most stars are made up of plasma. The Aurora Borealis is a plasma light show in our upper atmosphere caused by the bombardment from space of the solar wind - another kind of plasma. Lightning bolts are visible plasma trails left by the passage of the electric current that formed it.

As stated in the definition, plasma is a gaseous type of state where the matter making the plasma consists of electrically neutral and charged particles. Overall, plasma is electrically neutral having as many positive ions as free electrons distributed through it. Since negatively charged electrons and positively charged ions attract each other, the matter in plasma often binds itself together.

Accelerating electrons through ordinary gas can create plasma, in just the way they create lightning.

  • If you do it in a very low pressure gas - something like the interior of an incandescent light bulb - you usually just get a kind of diffuse glow. The kind of gas in the bulb determines its color. This technology gave us neon lights, and the variations of them that span the visible spectrum.

    • The color of the display depends on the element making up the gas, its pressure, and the way we put electricity through it.

    • At the atomic level, the process involves an area of physics called quantum mechanics.

  • Under standard atmospheric pressure, you have to use a lot of electricity to generate a plasma path. When that happens, you get something that looks like lightning.

    • The bright light we see comes from the superheated gases along the plasma path, just the way other very hot objects glow.

    • Thunder comes from the shock wave formed in the superheated gas - a natural sonic boom.

  • At in-between gas pressures, the plasma forms a lightning-like path but without the intense burst of energy. No "clap of thunder" from the shock wave formed in the superheated gas. This intermediate range* is where plasma globes work.

How does a Plasma Globe Work?

We apply an electric voltage to the metal electrode in the center of the plasma globe. This creates a steady electric field between the electrode and the outer globe. Under these conditions, electrons jump from the electrode into the gas within the globe and accelerate toward the surrounding glass sphere.

Simultaneously, we create a changing electric field inside the globe with another, oscillating electric voltage on the electrode. (Changing electric fields produce changing magnetic fields.) This alternating electromagnetic field helps to contain the plasma when it forms and to keep the free electrons from the cathode spiraling inside the globe.

Free electrons create the plasma from the regular gas atoms inside the globe. First, electrons in the electrode are given enough energy to break free of their bonds inside the metal. The newly freed electrons accelerate in the globe until they have many times the energy needed to ionize** a gas atom. As the free electrons move, they start to collide with neutral atoms along their way, ionizing them. The ion trail created makes it easier for more electrons to follow along in the same path, forming a plasma streamer or tendril.

Within the plasma tendril, all the collisions with energetic particles readily move atoms and ions from their ground atomic level to an excited level. Excited atoms relax back to their ground levels by emitting a burst of light energy known as a photon. The color of the emitted light depends on the atomic element and the particular excited state involved. Quantum physics tells us which states are likely to be involved for each kind of element.

Now, the intermediate pressure range of the plasma globe comes into play. Because the pressure is higher than what is really needed to create plasma, the density of atoms in the gas is higher - making them closer together than in regular plasma.

On the one hand, the frequency of collisions with energetic particles needed to ionize atoms into plasma happens more often this way. However, so do the collisions with low energy, neutral atoms that cause the ions to recombine with free electrons back into regular gas. And there are many more low energy neutral atoms than excited particles in the globe. This contains the ion trails in a narrow, well-defined tendril.

So, the path the electricity takes remains concentrated in the plasma tendril, heating the gas. As it warms, it expands and becomes less dense than the surrounding gas. This causes the plasma tendril to drift upwards in the globe, like rising hot air.

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* ionization -  Collision with a particle, like a free electron, can ionize an atom. This causes it to eject one of its outer electrons into free space, leaving it with a net positive charge.

** intermediate pressure -  At normal atmospheric pressure, there are around 440 billion billion molecules in a cubic inch of air. The pressures we are looking at now are about two thousand times less than normal atmosphere. In this range, a cubic inch of air will still hold about 200 million billion molecules.

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