In the physics of plasmas, it is often described as a state of matter, like a liquid or a gas. Unlike biology where a plasma has a different meaning. A plasma in physics may however form within a gas. For most telestial plasmas the gas and plasma state coexist. Plasma can also coexist with the solid state as demonstrated by the recent experiments in plasma crystals. In a partially ionised plasma some of the atoms or molecules are ionized, meaning that the electrons have enough energy to escape their nuclei. To the right we see what a gas plasma on earth might look like. Not all the gas is ionized; the black dots are neutral gas atoms or molecules. The blue dots represent atoms that have lost an electron and become positive charged ions. The lost electron is seen as the free electrons, the red dots. In general, it is thought that plasma is the fourth state of matter in order of solid, liquid, gas and finally plasma. The order is set by the energy of the particles. The more energy the particles in the system have then the higher the velocity of particles and the temperature of the system. This picture is accurate for equilibrium systems. It would take a temperature of many 10’s of thousands of degrees Celsius to start to ionise a gas to get to the plasma state. This concept is somewhat misleading as plasmas are not defined by the temperature of the particles. It is possible to ionise the gas by using a non-equilibrium system such as a beam of electrons not in thermal equilibrium with the gas. This is referred to as a cold, low temperature plasma and is in fact the most common plasma seen on earth. It is the cold plasma that has the most practical uses. I will explain that later.
Definition of Plasmas in Physics
The best definition of a plasma is then not as a hot gas but in terms of its collective behaviour. Plasma are a collection of charged particles, both positive and negative that behave in a collective way because of attractive and repelling electric forces. So this is the important feature, plasmas behave collectively. The interaction that makes them behave together is the electric fields created by the free charges.
In the centre of a plasma it is considered to be electrically neutral, that is the same number of negative electrons as positive ions in any given volume. The net charge is zero and the average electrical field is zero. If you put a metal probe into a plasma and put a voltage of +100 Volts on the metal probe the plasma would react to shield the probe so that the electrical field created by the probe would go to zero very close to the probe. This shielding is called Debye shielding.
The three criteria to define a plasma in physics
There needs to be enough charged particles in a given volume so that the plasma can act to shield an external potential. This means there needs to be many thousands of charged particles of both charges (positive and negative) inside a radius which is equal to the Debye length. The volume of the plasma needs to be big enough so that is covers many Debye lengths in each direction. If it takes a Debye length to shield a potential and the plasma is just one Debye length long then the shielding will not work. The number of collisions that the shielding particle has with neutral gas particles has to be small enough as not to prevent the shielding being effective. It is normally the electrons that do the shielding so that the time between electron-neutral collisions has to be longer that the response time of an electron to an electromagnetic field.
What is a Plasma in Physics part II