Non-equilibrium plasmas

A brief description of a low-density non-equilibrium plasma is given followed by a review of its characteristic features and of tire relevant collisionprocesses in tire plasma. Principles for tire generation of plasmas in teclmical devices are discussed and examples of important plasma chemical processes and tlieir technical applications are presented. 

A non-tliennal, non-equilibrium plasma is characterized by an electron temperature much larger tlian tire ion temperature and tire neutral gas temperature (T T. Typical non-tliennal, non-equilibrium plasmas... 

Low Temperature (Non Equilibrium) Plasmas The rigorous classification of plasma treatments is difficult, however, from the viewpoint of the treated surface there are three broad categories ... 

Nozaki, T., Kimura, Y., and Okazaki, K., Carbon nanotubes and hydrogen co-production from methane using atmospheric pressure non-equilibrium plasma, Proc. 16th ESCAMPIG and 5th... 

Since pyrolysis converts waste into CO, CH4, and H2, the product gases can be processed in an atmospheric pressure non-equilibrium plasma reformer to improve the energy-recovery potential of the product gas. Energy-recovery options include heat and chemical energy recovery. 

In non-thermal plasmas, also known as non-equilibrium plasmas, there is a significant difference in temperature between electrons and ions/neutrals. Non-thermal plasmas can initiate a chemical reaction even at relatively low temperatures by generating free radicals (i.e., H, O, OH, CH3, etc), which propagate the reaction. 

We will now develop the transport equations in L-space from the above Green functions. Following the Keldysh approach in //-space, the transport equations for non-equilibrium plasmas and radiation have been given by DuBois [29]. A similar transport equation for a system of ions may be found in Kwok [30], which is based on the Green function associated with ion positions. In a separate paper [31], we will derive the appropriate transport equations for the coupled system of electrons, ions, and electromagnetic fields. 

Dissociation of molecular oxygen in non-equilibrium plasmas has been subject of numerous experimental investigations, mostly at pressures below about lOmbar26-31 ... 

Numerous plasmas exist very far from the thermodynamic equilibrium and are characterized by multiple different temperatures related to different plasma particles and different degrees of freedom. It is the electron temperature that often significantly exceeds that of heavy particles (7 > To). Ionization and chemical processes in such non-equilibrium plasmas are directly determined by electron temperature and, therefore, are not so sensitive to thermal processes and temperature of the gas. The non-equilibrium plasma of this kind is usually called non-thermal plasma. An example of non-thermal plasmas in nature is the aurora borealis (Fig. 1-2). 

The non-equilibrium EEDF for different discharge systems and different plasma conditions will be discussed in Chapter 3. Sometimes, however (even in non-equilibrium plasmas), the EEDF is determined mostly by the electron temperature and, therefore, can be described by the quasi-equilibrium Maxwell-Boltzmarm distribution function ... 

The Einbinder formula can be generalized to non-equilibrium plasma conditions when the electron temperature exceeds that of aerosol particles (Tj, > T ) (Fridman, 1976) ... 

A detailed description of non-equilibrium plasma-chemical systems and processes generally requires an application of kinetic models. The application of statistical models leads sometimes to significant errors (Slovetsky, 1980). In some specific systems, however, statistical approaches can be not only simple but also quite successful in describing non-equilibrium plasma, which is discussed next. 

A second example is related to strongly non-equilibrium plasma gasification processes,... 

The steady-state solution of the Fokker-Planck equation (3-45) forEEDF in non-equilibrium plasma, corresponding to /(e oo) = 0, ( oo) = 0, canbe presented as... 

In quasi-equilibrium plasma, Da = 2Dj in non-equilibrium plasma (Te T ), the ambipolar diffusion D = corresponds to the temperature of the fast electrons and mobility of the slow ions. To determine conditions of the ambipolar diffusion with respect to free diffusion, we should estimate the polarization field from equations (3-88) ... 

This relation is usually referred to as the Unsold-Kramers formula, where ga and g are statistical weights of an atom and an ion, x = fioj/T,x = 1 jT and I is the ionization potential. It is interesting to note that the Unsold-Kramers formula can be used only in quasi-equilibrium conditions, while the relation (3-107) can be apphed for non-equilibrium plasma as well. 

Vibrational distributions in non-equilibrium plasma are mostly controlled by W-exchange and VT-relaxation processes, while excitation by electron impact, chemical reactions, radiation, and so on determine averaged energy balance and temperatures. At steady state, the Fokker-Planck kinetic equation (3-116) gives J(E) = const. At E oo = 0,... 

In non-equilibrium plasma, the hot atoms can be generated in endothermic chemical reactions as well. For example, vibrationally excited molecules participate in endothermic reactions with some excess of energy, which goes into the translational energy of products (Fridman Kennedy, 2004) ... 

Vibrationally excited molecules are very effective in the stimulation of endothermic chemical reactions. But the exothermic reactions with activation barriers are not stimulated by molecular vibrations (see Section 2.7), which slows down the whole process. In this case the hot atoms can make a difference by accelerating exothermic processes. This effect will be illustrated in Section 6.3.7 in the discussion of NO synthesis in non-equilibrium plasma. 

Figure 3-13. Coefficients of selectivity (x) characterizing isotopic effect in non-equilibrium plasma-chemical reactions of different molecules as a function of relative molecular mass difference AA//A/ of the molecular isotopes. Numbers on curves represent activation energies of specific plasma-chemical reactions (in electron volts) circles represent positions of the specific plasma-chemical reactions of vibrationally excited molecules.

Figure3-15. The non-equilibrium plasma (Ty To) isotopic effect dependence on translational gas temperature To temperature Tq corresponds to the maximum value of the selectivity coefficient of the inverse isotopic effect.

Figure 3-16. Influence of a chemical reactionon the vibrational distribution function in the weak excitation regime of non-equilibrium plasma Ty To).

Energy Efficiency of Quasi-Equilibrium and Non-Equilibrium Plasma-Chemical Processes... 

The total energy efficiency of ary non-equilibrium plasma-chemical process can be subdivided into three main components an excitation factor (jjex), a relaxation factor (jjrei), and a chemical factor ( chem) ... 

Low-pressure are diseharges. The positive column of arc discharges at low pressures (10 -1 Torr) consists of non-equilibrium plasma. The ionization degree in the non-thermal arcs is higher than that in glow discharges because arc currents are much larger (see Table 4-7). 

Figure 4-54. Consequent phases of evolution of a gliding arc discharge (A) region of gas breakdown (B) quasiequilibrium plasma phase (C) non-equilibrium plasma phase.

Non-Equilibrium Plasma-Chemical Microwave Discharges of Moderate Pressure... 

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