Day 11: Introduction to Plasma
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
Web Design: Theresa Valentine
Last Updated: 8/11/2000