ABSTRACT
It is understood from investigations that 99% of the matter in the observable universe is in the plasma state. Sun and stars, interplanetary medium, the magnetospheres and ionospheres of the earth and other planets, as well as the ionospheres of comets lightning, etc., are examples of natural plasmas. Even in space around the earth, the plasma has densities dramatically lower than those achieved in laboratory plasma sources. In the laboratories, the plasma density is about 10 billion particles per cubic centimeter, whereas the density of the densest magneto-spheric plasma region is about 1000 particles per cubic centimeter. The temperatures of plasmas are very high, ranging from several tens of thousands Kelvin. As the temperature is very high, it is convenient to express it in terms of the average kinetic energies of their constituent particles measured in “electron volts.” An electron volt (eV) is the energy that an electron acquires as it is accelerated through a potential difference of 1 volt and is equivalent to 11,600 Kelvin. As plasma comprises of charged particles (ions and electrons), which are caused by ionization of the background, hence to make plasma the energy given to the gas must be higher than the ionization
potential. For hydrogen, the ionization potential is 13.6 eV, i.e. the energy needed to eject one electron from the hydrogen atom. It is a big number if it has to be expressed in degree Kelvin. The ions, electrons and neutrals may have different temperatures. Normally in plasmas, if the electron temperature is much higher than the ion and neutral temperature, the plasma is called as “non-thermal”, as the plasma cannot be defined with one temperature. Usually these plasmas have low heat content/volume and do not heat the material surface with which it is interacting, to temperatures where it can modify the bulk properties of the material. This type of plasma is usually used for surface modification. Here the electrons can be at temperatures of few electron volts with ions being at temperature of 0.1 eV. There are plasmas where the electron, ion and neutral are at similar temperature. This plasma is called as “thermal” and has high heat content/volume. This type of plasma is used as heat source and can be used for bulk material disintegration. The temperature of the plasma species can be of the order of 1.0 eV. Figure 1.2 shows the entire plasma parameter space. Out of these the glow discharges and thermal processing plasmas are used for industrial plasma applications.
