What is a Plasma?

Plasma makes up 99% of all visible matter in our universe. Although naturally occurring plasma is rare on Earth, there are many man-made examples. Inventors have used plasma to conduct electricity in neon signs and fluorescent bulbs. Scientists have constructed special chambers to experiment with plasma in laboratories.

The Tokamak fusion reactor, courtesy of the Princeton Plasma Physics Laboratory.

Plasma is relatively rare on Earth, occurring only in lightning discharges and in artificial devices like fluorescent lights.
Plasma is everywhere in our space environment, however. Examples include:

• The aurora, or northern lights, flickering in the uppermost reaches of Earth's atmosphere. (Aurora photo courtesy of David Fritts © 1995.)
• The solar wind generates an immense sheet of electrical current that spirals like a ballerina's skirt as the Sun rotates. (Artist's impression courtesy of Tom Potemra.)
• The Sun itself, all other stars in our galaxy, and colossal exploding jets from distant galaxies. (The X-ray image of the Sun shown here is from the Japanese Yohkoh satellite, courtesy of Lockheed Palo Alto Research Lab.)

Hannes Alfvén (1908-1995), a Swedish physicist, is the father of modern day plasma science. He believed that the effects of electricity and magnetism played important roles in many astrophysical domains. He received the Nobel Prize in 1970 for his contributions to basic plasma physics and for his studies of space plasmas.

How are Plasmas Created?

Atoms are so tiny that more than a million can fit across the head of a pin. They are composed of one or more negatively charged electrons that orbit a positively charged nucleus (made up of neutral particles, called neutrons, and positively charged particles, called protons). Atoms are electrically neutral; they have the same number of positive and negative electrical charges.

When gases are exposed to enough heat or other radiation, their electrically neutral atoms split into positively charged fragments called ions and negatively charged free electrons. Another term for plasma is "ionized gas". Because plasma consists of electrically charged particles, it acts very differently from ordinary forms of gas.

What are electric and magnetic fields?

Electric charges and changing magnetic fields produce electric fields. Here on Earth, we have learned how to produce electricity by using motors, called generators, that cause magnetic fields to change. These changing magnetic fields create the electric current we use in our homes.

Electric charges in motion (an electric current) and changing electric fields produce magnetic fields. For instance, flows of molten metals deep inside Earth's core cause currents which sustain a huge magnetic field that extends far into space.

As seen in the diagram at left, magnetic fields are represented by lines of force. When the lines are close together, the force of the field is great; when they are far apart, the force is weak. Earth's magnetic field lines spread out from the south polar region and come together in the north polar region. (Diagram courtesy of Marshall Space Flight Center/NASA.)

Understanding how plasmas interact with electric and magnetic fields gives us a better idea of what is happening between the Sun and Earth and elsewhere in the universe, but many mysteries remain.

Can Plasmas Be Controlled?

The plasma discharge inside Princeton's Tokamak Fusion Reactor.
(Courtesy of the Princeton Plasma Physics Laboratory.)

A fission reaction occurs when heavy radioactive nuclei like uranium split apart or "decay" into lighter nuclei. The excess energy can be used to operate a nuclear power plant or in a more sinister way it can produce the incredible energy in a nuclear explosion.

A fusion reaction, on the other hand, occurs when hydrogen nuclei collide and form a heavier nucleus like helium. More energy is required to "ignite"a fusion reaction, but when ignited the excess energy is far greater than in a fission reaction. The only fuel required in a fusion reactor is the heavy hydrogen found in seawater, a virtually limitless source of energy. If controlled fusion were to become reality it would be one of the great breakthroughs for civilization. The scientific and engineering problems associated with a fusion reactor are great. They can be divided into three categories:

1. Confinement (how long does the plasma stay around?)
2. Heating (does the plasma get hot enough?)
3. Technology (reactor design -- many problems remain for future research)