Discharge electron energy distribution

Plasmas typical of C02 laser discharges operate over a pressure range from 1 Torr to several atmospheres with degrees of ionization, that is, nJN (the ratio of electron density to neutral density) in the range from 10-8 to 10-8. Under these conditions the electron energy distribution function is highly non-Maxwellian. As a consequence it is necessary to solve the Boltzmann transport equation based on a detailed knowledge of the electron collisional channels in order to establish the electron distribution function as a function of the ratio of the electric field to the neutral gas density, E/N, and species concentration. Development of the fundamental techniques for solution of the Boltzmann equation are presented in detail by Shkarofsky, Johnston, and Bachynski [44] and Holstein [45]. 

The effectiveness of a given plasma-assisted surface treatment depends primarily on the nature of the feed gas, and on a number of externally controllable parameters pressure, power, gas flow rate, frequency of the electrical energy used to excite the discharge, reactor geometry, etc. These "external variables, in turn, affect the "internal" plasma parameters which control the overall processes, namely the electron density ne, the average electron energy , the electron energy distribution function f(E), and the plasma potential... 

Previous work in this laboratory (4,5) had established the usefulness of a mass spectrometer to obtain relative ion concentrations in the various regions of glow discharges. The work reported here is an extension of these earlier studies and represents an attempt to (a) measure the absolute concentrations of ions and electrons in conjunction with the mass spectrometric studies and, (b) to determine the electron energy distributions, particularly in the negative glow to assist in the interpretation of processes that result in the production of ions. 

Figure 2. Schematic electron energy distribution function for a discharge tube (based on interpretation of theoretical calculations for H2 gas (1)). The dashed line indicates the further possibility that the distribution function may be skewed in the direction of lower energy as the result of vibrational excitation processes...

Electron energy distribution functions (EEDFs) in non-thermal discharges can be very sophisticated and quite different from the quasi-equilibrium statistical Boltzmann distribution discussed earlier, and are more relevant for thermal plasma conditions. EEDFs are usually strongly exponential and significantly influence plasma-chemical reaction rates. 

Figure 5-38. Electron energy distribution function (EEDF) in glow discharge at different currents (1) 70 mA, (2) 30 niA.

A typical gas mixture for a self-sustained discharge-pumped XeCl laser is 3-atm mixture of Xe/HCl/Ne = 1/0.1 — 0.2/balance(%). The electron energy distribution in the discharge mixture can be calculated using a Boltzmann equation code, an example of which is shown in Fig. 4. The case is treated of the above mixture pumped by a 100-nsec discharge pulse at an excitation rate of 3 MW/cm. ... 

If the mixing ratio of Xe/HCl/Ne is varied, the electron energy distribution in the discharge plasma changes. As a result, formation of precursors Xe+, Xe, Ne+, andNe is greatly affected. If helium is used as a diluent gas in place of Ne, the electron temperature also changes, resulting in a different pathway for the XeCl(B) formation and a less effective formation than that of the Ne diluent. 

Mizuochi, J. Sakamoto, T. Matsuura, H. Akatsuka, H. (2010). Evaluation of Electron Energy Distribution Function in Microwave Discharge Plasmas by Sp>ectroscopic Diagnostics with Collisional Radiative Model. Jpn. J. Appl Phys., Vol. 49, No. 3, (March 2010), pp. 036001-1-036001-14, ISSN 1347-4065... 

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