Modeling of a millisecond pulsed glow discharge: Investigation of the afterpeak
We have developed a comprehensive modeling network for a millisecond pulsed glow discharge in argon (Ar) with copper (Cu) cathode, to describe the behavior of the various plasma species. Typical results of the model are shown, such as the potential distribution during and after the pulse, the densities of Ar+ ions, Ar atoms in the metastable and various other excited levels, Cu atoms and Cu+ ions, in ground state and excited levels, as well as optical emission intensities as a function of time during and after the pulse. Special attention is paid to the mechanisms giving rise to the so-called “afterpeak”, i.e., the peak in Ar and Cu excited level populations and optical emission intensities, which is experimentally observed in the afterglow of pulsed discharges. This afterpeak is attributed to electron-ion recombination to the highest excited levels, followed by radiative decay to lower levels. It is expected that the electron temperature decreases drastically upon pulse termination, resulting in a significant rise in electron density, making electron-ion recombination more plausible. Because we were not yet able to model these mechanisms, we worked in reversed order, to find out which recombination mechanisms account for the experimentally observed afterpeaks. Collisional-radiative recombination (i.e., three-body recombination with an electron as the third body) is the most plausible candidate, both for Ar and Cu, but it requires a rise in electron density in the afterglow, estimated to be about two orders of magnitude relative to the steady state, or voltage-on period. Therefore, as an alternative, we think that dissociative recombination of Ar2+ ions in high vibrational levels cannot yet completely be ruled out.