BASIC CONCEPTS OF THE PLASMA

Partly ionized gas or vapour is called a plasma. It contains atoms, molecules, and ions from which some fraction may be in excited states, and free electrons. Several theoretical models have been presented to describe a plasma. One of these is the so called Thermal Equilibrium Theory, which is based on the micro reversible principle. According to this principle, each energy process is in equilibrium with a reverse process. For example, the number of transitions per time unit from the state f to the state (absorption) is exactly the same as the number of the reverse transitions (emission). According to Maxwell, the microscopic states of the plasma at thermal equilibrium may be calculated on the basis of the temperature, which is the only variable. The number of particles (dA) with the speed between v dv is  

The number of particles at a given state A can be obtained using equation 34  

Theoretically, there are an infinite number of states close to each other before the ionization energy. In practice, Z T) does not become infinite, because the electric field of close particles diminishes the ionization boundary and the number of energy levels becomes finite. 

The ionization degree can be obtained at any temperature from Saha s equation  

A complete thermal equilibrium is not achievable using laboratory plasmas. The temperature is very high inside the plasma, but a few millimetres away from the plasma is the lower room temperature. Hence, plasma could be considered to be formed from a number of layers, and each layer is at a local thermal equilibrium. Secondly, most of the radiation is emitted to the environment without back absorption, which results in an increase of atoms in the ground state. However, the thermal equilibrium theory is a good approximation for relatively large particle concentrations. 

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