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Day 11: Introduction to Plasma
Background Information

Plasma is the term used for the 4th state of matter, after solid, liquid, and gas. A plasma is a gas which is ionized, and thus has particles with positive and negative charge within it. The ionization can be initially created by any process, but the key here is that the ionization is at a level of greater than 0.01% of the atoms, and that the recombination rate is less than the ionization rate. In practice, this means the plasma is hot and rarified. The vacuum conditions required are generally less than 1 Torr and often approaching micro Torr levels. Recallthat 760 Torr is the same as 1 atmosphere of pressure.

Plasmas are really quite common in the Universe. Some estimate that 99% of the visible material in the Universe is in the form of plasma. In our lives, neon light bulbs are examples of confined plasmas, while lightning, aurora, and flames are unconfined. Plasmas are involved in over a trillion dollars worth of industrial applications annually, from glass coating processes to computer chip etching applications. One of the most prized potential uses of plasma is to facilitate fusion energy process.

Confinement of a plasma is achieved with strong magnetic fields. This is possible due to the charged nature of the particles making up a plasma. The high temperatures of plasmas makes confinement quite difficult for any extended time period except in cases of very thin, relatively cool plasmas such as found in neon light bulbs.

The plasma of interest here is the Sun. While people often think of the sun as "burning" and as a "chunk of matter", it is in reality a very complex plasma. The fusion process at the core is powered by gravitational pressures creating a phenomenally hot, dense environment. The temperature (10 million degrees or more) tells us that the core particles are moving very fast and the density tells us the particles hit each other very often. Each collision spreads the general energy of the particles out among all the particles present. On occasion, collisions are violent enough that nuclei are fused together. This typically releases a great deal of energy. For instance, the fusion energy released by 10e(-4) grams of deuterium and tritium (fused to form helium and an extra neutron) is equivalent to the energy released by burning an entire gallon of gasoline. The energy percolates up through the sun, crossing layers where radiative transport dominates and finally reaching layers where convective processes dominate. It is the top of the convective layer that we see and associate with the solar surface. We say it has a temperature of about 6000o C. Higher above the "surface", we find layers with much higher temperatures, especially in the corona, where temperatures are thought to be in the millions of degrees. The necessary time to complete the process of transporting energy from the core to the outer surface of the sun is thought to be on the order of 1,000,000 years.

The movement of charged particles produces magnetic fields. It turns out that the sun is threaded with strong and complex magnetic fields. These fields are largely responsible for the observed cyclical behavior of the sun and releases of plasma and magnetic energy storms, some of which strike the earth. To be sure, gravitational processes play a significant role also.

Lesson Development/Writing: Ed Eckel
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Last Updated: 8/11/2000

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