Plasma Physics

Russell M Kulsrud (1960)

Plasma is the fourth state of matter. When a gas is heated and ionised (an electron is liberated from the atom) it forms a neutral collection of electrons and positive ions. Plasma is extremely abundant and makes up almost 99 percent of the universe. Some examples of plasma are lightning, the tails of comets and stars. Plasma has two unique properties, collective behaviour and quasi neutrality. Collective behaviour is where rather than each particle in the plasma acting individually, the sum of the forces experienced by the plasma is experienced by the entire collection of particles. Quasi neutrality is where the plasma does work to expel any excess charges to ensure it remains neutral. Plasma does not form in abundance naturally on earth. So scientists have to make plasma themselves. This is done under extremely high temperatures. Despite not being present in abundance on earth, plasma is present on the outer level of atmosphere called the ionosphere, where ultraviolet light from the sun ionises the outer region of the atmosphere.

Perhaps the most impactful application of plasma in today’s world is controlled nuclear fusion. Hot deuterium or tritium (isotopes of hydrogen) plasma are confined by a magnetic filed in a fusion reactor, ensuring that none of the plasma comes into contact with the reactor walls. This is because, reactor is operating at extremely high temperatures (around a 100 million kelvin) which can cause the reactor walls themselves to become ionised and react with the plasma. Current research looks to maintain stability in these reactors at such high temperatures.

Another application of plasma physics is in astrophysics. Since most of the universe is in the plasma state, understanding the general behaviour of plasma can aid in improving our understanding of the universe. As mentioned earlier, stars are made of plasma. When matter is thrown out from a star it is in the form of plasma. The plasma is able to escape from the surface
of the star and travels along the lines of force of the magnetic field of the star. For example, our own sun. Plasma that is thrown out and makes its way to earth are responsible for sunspots. Furthermore, plasma is also responsible for a process called solar wind. When plasma escapes from the sun, it can form a stream of charged particles. These particles are extremely radioactive and could do serious damage if exposed to those on Earth. However, Earth’s magnetic field is able to protect it from these solar winds.

Summary written by Saimur Rahman

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