High-Energy Molecular Lasers by V. V. Apollonov

Author:V. V. Apollonov
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

(26.3)

Here, n +(0), and are the initial concentrations of positive and negative ions.

Because and for , the second term in (26.3) can be neglected starting from a certain instant of time. Taking into account that , this equation is transformed into the recombination equation only for negative ions with the smaller recombination coefficient . As applied to the SF6 plasma, this means that within a certain time interval after the recombination onset, negative ions and , which have close values of the mobility and recombination coefficient (see above), start dominating in the plasma along with the positive SF5 + ions. This justifies the use of (26.1) and (26.2). This is clearly shown in Fig. 26.2b. One can see that for t > 8 μs, the time decay n i (t) of the ion concentration calculated from (26.1) strictly follows the recombination relation (26.2). The values of ion mobilities used in calculations were borrowed from [13].

The complex (SF6) ion was considered as the main negative ion in [5, 6, 8]; however, the gas pressures were 5–10 times higher in these works than in our experiments. Taking into account that the rate of clusterization of the ions quadratically depends on p, we can expect that the fraction of the complex (SF6) ions in the investigated plasma is insignificant. Moreover, according to [13], the mobilities of ions and (SF6) differ by just a few percent in the range E/N ~ 100–250 Td, so that the formation of complex ions under the conditions of our experiments is insignificant at all.

In addition to the ions, the SF6:C2H6 = 10:1 mixture also contains positive ions formed upon the electron impact ionization of C2H6. According to [17, 18], the mechanism of dissociative ionization with the formation of C2H4 + ions and H2 molecules dominates in this case. Using the Langevin approximation and the Blank law [19], we find that the mobility of and C2H4 + ions in the SF6:C2H6 = 10:1 mixture is determined by their interaction with the SF6 molecules. Despite the fact that the mobility of C2H4 + ions (estimated in the polarisation limit ) is approximately 1.8 times higher than the mobility of ions, the C2H4 + ions do not make a significant contribution to the total current I(t) for the above-mentioned ratio of the concentrations of SF6 and C2H6. As a result, the set of negative ions remains unchanged. Taking into account that positive ions dominate in the plasma under study, expression (26.2) describing the recombination kinetics remains applicable for the mixture as well.

While the formation of positive ions in the SF6–C2H6 mixture is also possible upon the charge exchange of the ions with the C2H6 molecules (although, in our opinion, the situation is not quite clear), estimates show that the charge exchange times are ~10−8–10−7 s even for the lowest C2H6, concentrations of ~1017 cm−3. Therefore, only one positive ion will dominate on the time scale exceeding 1 μs of interest to us.

The polarisation approximation was used several times while estimating the

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