الفهرس 
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Abstract
This work is based on the modification of a previously developed model by (Mahmoud and Gamal, 1995). This model was initially developed to study theoretically the time evolution of the energy distribution of free electrons created by collisional ionization of sodium vapor resonantly excited with CW laser due to different physical mechanisms basing on Laser Ionization Based on Resonance Saturation (LIBORS) technique. The modification is devoted to include into the model an ionization process which acts to increase the seed electrons density namely tri-atomic associative ionization process. The cross section of this process was determined experimentally by Tapalian and Smith (Tapalian and Smith, 1993) in their measurements of the trimer ion yield in sodium vapor. In this experiment two continuous dye lasers are tuned to the transitions Na (3s)-Na (3p) (589 nm) and Na (3p)-Na (4d) (569 nm) at intensities of up to 100 W/cm2. Under these conditions associative ionization (AI) processes involving high energy states (Na(4d), Na(5s)) are studied, for example AI collision of Na(4d) atoms with Na2 ground state molecule (present at ≈ 0.5% percent in the atomic beam) resulting in Na3+ trimer ions. Accordingly an additional term refers to this process is included into the numerical model. This in turn implied the modification of the rate equations which describes the variation of the excited states population, ion currents as well as the Electron Energy Distribution Function (EEDF). Following to this modification the model is applied first to calculate the tri-atomic ions current against the density of the sodium vapor for different exposure time intervals taken into account the experimentally determined cross section given by Tapalian and Smith. The calculated values showed reasonable agreement with the measured ones for exposure time of 500 ns. Moreover, the comparison between the calculated and measured molecular ions as a function of atomic sodium vapor density (2x1011- 2x1012 cm-3) resulted from photo ionization of ground state molecules as well as AI by (3p-3p) collision showed reasonable agreement. This result reveals that these processes could contribute by about 104 electrons as seed electrons to plasma generation. To confirm this result study is performed for the electron energy distribution function and its parameters corresponding to the experimental measurements that carried out by Farooq et al (Farooq et al., 2001) taken into account the effect of trimer ions generation on the plasma formed in atomic sodium vapor. In doing so calculations are carried out to determine the EEDF in the presence and absence of this process at different values of atomic vapor density (2 x 1015 cm-3 and 4 x 1015 cm-3) and exposure time. The result of computations showed that the EEDF tends to be non-Maxwellian with its structure revealed a number of peaks over the whole energy range (0.0 - 5.25 eV). The presence of tri-atomic associative ionization process results in its noticeable increase over the low energy region. This increase becomes more efficient with the increase of both atomic density and exposure time. Similar result is also shown for the temporal variation of the parameters; electrons density, atomic ions and excited states population. Different behavior is shown for the molecular ions where their values did not show any change in the presence and absence of tri-atomic associative ionization process. This was attributed to the fact that in the absence of this process associative ionization of the 3p-3p interaction leads to the increase of molecular ions. While in its presence Horn-beck Molnar (HM) process is the main process responsible for the generation of molecular ions. The effect of these two processes compensates each other leading to this observed result. Finally, the separate contribution of each of the seed electrons production processes is performed to test their important individual role played in plasma generation.