lasma, also referred to as “ionized gas” is mixed state of atoms, molecules, ions, electrons and radicals. Plasma has two general states: equilibrium and non-equilibrium. The equilibrium state indicates the temperatures of electrons, ions and neutrals become almost equal, and the background gas is heated from a few thousands to more than ten thousands Kelvin degrees. Because of this, the plasma getting equilibrium state is called as “thermal plasma”. On the other hand, the non-equilibrium state means that the temperatures of electrons, ions and neutrals are quite different, and in general the electron temperature is substantially higher than other particles. Therefore, the rise of background gas temperature is quite low in non-equilibrium state and the plasma being non-equilibrium state is called as “non-thermal plasma”. Figure 1 shows a typical example of non-thermal plasma [8]. This figure shows the background gas temperature of non-thermal plasma is enough low to touch by a finger. In the non-thermal plasma, the majority of the discharge energy goes into the production of energetic electrons, rather than ion and neutron heating. The energy in the plasma is thus consumed preferentially to the electron impact dissociation and ionization of the background gas for production of radicals that, in turn, decompose the toxic molecules. In short, non-thermal plasma can remove toxic molecules near room temperature without consuming a lot of energy in background gas heating.
For low pressure plasma process such as semiconductor production, the non-equilibrium plasma which is often named “cold plasma” is typically used. Prof. Oda [9] defined that non-thermal plasma is high pressure (typically 1 atmospheric pressure) non-equilibrium plasma. Compared with that cold plasma, the electron temperature and ionization rate are quite lower in non-thermal plasma. Typically, the electron temperature of cold plasma is tens of eV. Meanwhile, in atmospheric pressure, the electron temperature is generally 1 to 10eV and ionization rate is around 0.1%. However, it is important for gas processing in atmospheric pressure because electron and molecular density is overwhelmingly high in comparison to low pressure condition. If the gas processing is done in low pressure condition, the absolute molecule quantity is low. That is, large amount of energetic electron having more than dissociation energy of objective molecules are need in order to generate more radicals and decompose more toxic molecules. Later on, the required value of electron energy for air pollution control is approximately 10eV.