Negative electron mobility in attachment-dominated plasmas

The temporal evolution of the electron velocity-distribution function (EVDF ), the concentration, mean energy and the drift velocity of the electrons i s studied on a kinetic basis in a weakly ionized Ar/F-2 mixture plasma unde r conditions when the electron concentration temporally decreases as a resu lt of the electron attachment to fluoride molecules. Using an appropriate r elaxation model, the time-dependent electron Boltzmann equation,vas solved in multiterm and two-term approximations of the velocity distribution funct ion. The multiterm results confirmed predictions on the occurrence of negat ive electron mobilities in such a decaying Ar/F-2 plasma, which were made i n a former study using the conventional two-term approximation. The investi gations particularly showed that this approximation gives almost accurate r esults for the EVDF and related electron swarm parameters except for in the very beginning of the relaxation process. It has been further shown that f or a certain range of the reduced electric field strength, the drift veloci ty becomes negative in the process of temporal evolution and remains negati ve even when approaching the hydrodynamic stage of the electron swarm. In a ddition, the role played by the back heating from the gas by elastic collis ions on the EVDF formation is studied and various comparisons with correspo nding Monte Carlo results are performed.