Recombination in deuterium plasma at cryogenic temperatures (down to 130 K)
Résumé
In the presented study variable temperature Flowing Afterglow with Langmuir Probe - VT-FALP was used to study plasma decay in D_2/He mixture at temperatures down to 130 K and total pressure up to 10 Torr. Large extent of partial number densities of D_2 (10^12-3x10^15cm^-3) were used in the experiments. Langmuir probes and mass spectrometers were applied to monitor decay of the plasma during the afterglow. The study is continuation of our previous measurements of recombination rate coefficients of D_3^+ and D_5^+ ions. In these studies we observed dependence of the recombination rate coefficient on partial pressure of deuterium indicating that third-body assisted recombination is efficient and significantly contributes to recombination in decaying deuterium-containing plasma. In He/D_2 mixture at low D_2 number densities ions D_3^+ dominate the ion composition in the afterglow, the plasma decay is controlled by recombination of D_3^+ ions (with rate coefficient \alpha_3). At high D_2 number densities and lower temperatures ions D_5^+ are effectively formed and plasma decay is controlled by recombination of D_5^+ ions (\alpha_5). In the intermediate region the decay of plasma is governed by recombination of both ions with electrons and by ion molecule reactions (forward three-body association and reverse collision induced dissociation), which are keeping equilibrium between D_3^+ and D_5^+: D_3^+ + D_2 + He <--> D_5^+ + He. In the equilibrium ratio [D_5^+]/[D_3^+]= K_C[D_2], where K_C is the equilibrium constant. If the mentioned four processes are dominant processes during the afterglow the decay of plasma can be described by an overall (apparent) effective recombination rate coefficient \alpha_eff, which can be expressed by equation: \alpha_eff= (\alpha_3 + \alpha_5K_C[D_2])/(1+ K_C[D_2]). The temperature dependence of observed recombination rate \alpha_eff is coupled with the temperature dependences of \alpha_3 and \alpha_5 and predominantly with strong temperature dependence of K_C given by van't Hoff equation. From the measurements at low partial number densities [D_2] (=10^12-3x10^13cm^-3) where influence of formation of D_5^+ can be neglected the dependence \alpha_3 = \alpha_3(T) was obtained. From the data at intermediate and higher [D_2] values K_C and \alpha_5 were estimated and are reported in the paper. Obtained results are in good agreement with thermodynamic data.