Standing striations due to ionization instability in atmospheric pressure He/H2O radio frequency capacitive discharges
One dimensional particle in cell (PIC) simulations of a narrow gap atmospheric pressure He/2% radio frequency capacitive discharge showed standing striations in the bulk plasma region while previously conducted PIC simulations of a narrow gap atmospheric pressure He/0.1% discharges [1] showed no such instabilities. We successively modified the base He/ chemistry to make it more similar to the He/ chemistry in order to determine the cause of the striations. Setting the e scattering, attachment, vibrational and rotational excitation rates to zero did not suppress the striations. However, a systematic reduction of the e ion recombination cross section resulted in a transition to a stable state with no striations. The results are interpreted in terms of a model in which the balance between bulk direct ionization and bulk recombination loss determines the bulk plasma equilibrium. Perturbing the equilibrium, we find that the striations are consistent with an ionization instability induced by non local electron kinetics that form a spatially varying high energy tail of the electron energy distribution, causing the ionization rate coefficient to decrease with increasing electron temperature T e and root mean square electric field E in the instability regime.