Steady-state plasma electron kinetics

The significant differences between the cross sections for the electron collisions in the Ne and N2 plasmas in terms of their intensity and the energy region of their occurrence can be clearly seen from Fig. 1. These differences are the main reason for the very different kinetic properties of the electrons in the two plasmas, which will be illustrated in the following discussion for both steady-state plasmas. 

So far, the kinetics of the electrons in steady state has been considered for plasmas in pure gases. However, in many applications, mixtures of some gases occur, and the kinetic treatment of the electrons has to include all important electron collision processes with each mixture component. Because of the above-mentioned short range of the eleetron-heavy particle interaction, all these... 

According to the relevant power and momentum balance, Eqs. (38) and (39), the electron kinetics in steady-state plasmas is characterized by tbe conditions that at any instant the power and the momentum input from the electric field are dissipated by elastic and inelastic electron collisions into the translational and internal energy of the gas particles. This instantaneous complete compensation of the respective gain from the field and the loss in collisions usually does not occur in time-dependent plasmas, and often the collisional dissipation follows with a more or less large delay—for example, the temporally varying action of a time-dependent field. Thus, the temporal response of the electrons to certain disturbances in the initial value of their velocity distribution or to rapid changes of the electric field becomes more complicated, and the study of kinetic problems related to time-dependent plasmas naturally becomes more complex and sophisticated. Despite this extended interplay between the action of the binary electron collisions and the action of the electric field, the electron kinetics in time-... 

Furthermore, the detailed procedure ultimately used to determine the velocity distribution sensitively depends on the type of plasma and is quite different when studying the electron kinetics in steady-state, time-dependent, or space-dependent plasmas. 

Equations (12), simplified by the assumption of isotropic scattering in exciting and dissociating collisions, represent the basic equations for studying many quite different problems in electron kinetics. In particular, the additional simplification to steady-state, purely time-dependent, or purely space-dependent plasma conditions allows a detailed microphysical analysis of various electron kinetic problems related to each of these plasma conditions. 

It should be mentioned that the additional inclusion of nonconservative electron collision processes in the kinetic study of a plasma in steady state does not really make sense from a strict point of view. In such a case, fulfillment of the consistent electron particle balance would require the production and the loss of electrons to completely compensate for each other in any small volume of the plasma and at any time. But, for a given gas and its specific atomic data, such a requirement can not naturally be satisfied for any reduced field strength. Thus, E/N would no longer be a parameter of Eq. (36) if it was extended to nonconservative collision processes. 

To illustrate the behavior of electron kinetic quantities in steady-state conditions, weakly ionized plasmas in neon and molecular nitrogen are considered as typical representatives of atomic and molecular gas plasmas. 

In these time-dependent kinetic studies, a variety of electron collision processes similar to those treated in the steady-state kinetics has been treated. In addition to these processes, nonconservative electron collision processes, such as ionization and attachment, and even the nonlinear electron-electron interaction have been taken into accoimt. Besides the various types of electron collisions, other electron generation and destruction processes, such as the chemo-ionization in collisions between excited heavy particles in decaying plasmas or the injection of beamlike electrons into plasma, have been included as particle sources or sinks... 

By the preceding representations, an attempt has been made to give, on the basis of the electron Boltzmann equation, an introduction to the kinetic treatment of the electron component in steady-state, time-dependent, and space-dependent plasmas and to illustrate by selected examples the large variety of electron kinetics in anisothermal weakly ionized plasmas. 

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,... 

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