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Abstract
The laser photodetachment experiment in a diffuse helium–oxygen barrier discharge is evaluated by a 1D fluid simulation. As in the experiment, the simulated discharge operates in helium with
400
ppm
oxygen admixture at
500
mbar
inside a discharge gap of
3
mm
. The laser photodetachment is included by the interaction of negative ions with a temporally and spatially dependent photon flux. The simulation with the usually applied set of reactions and rate coefficients provides a much lower negative ion density than needed to explain the impact on the discharge characteristics in the experiment. Further processes for an enhanced negative ion formation and their capabilities of reproducing the experimental results are discussed. These further processes are additional attachment processes in the volume and the negative ion formation at the negatively charged dielectric. Both approaches are able to reproduce the measured laser photodetachment effect partially, but the best agreement with the experimental results is achieved with the formation of negative ions at the negatively charged dielectric.
This thesis highlights the impact of surface charges and negative ions on the pre-ionization, breakdown mechanism, and lateral structure of dielectric barrier discharges operated in binary mixtures of helium with nitrogen or electronegative oxygen. Sophisticated diagnostic methods, e.g., non-invasive optical emission spectroscopy and the electro-optic Pockels effect as well as invasive laser photodetachment and laser photodesorption, were applied at one plane-parallel discharge configuration to investigate both relevant volume and surface processes. Moreover, the experimental findings were supported by numerical fluid simulations of the discharge. For the first time, the memory effect of the measured surface charge distribution was quantified and its impact on the local self-stabilization of discharge filaments was pointed out. As well, it turned out that a few additional seed electrons, either desorbed from the charged dielectric surface or detached from negative ions in the volume, significantly contribute to the pre-ionization resulting in a reduced voltage necessary for discharge breakdown. Finally, effective secondary electron emission coefficients of different dielectrics were estimated from the measured breakdown voltage using an analytical model.