The energy cost of N₂ dissociation in a microwave discharge: combining modeling and experiments

The production of N radicals in microwave (MW) plasma discharges is investigated through experiments and modeling. Particular attention is given to vibrational excitation as a means to reduce the energy cost of dissociation of N2 molecules as a first step for optimization of nitrogen fixation in plasmas.

A self-consistent 0D global model of plasma core is developed for this purpose, including the novel Fokker-Planck approach to describe N2 vibrational kinetics. Simulated values of gas, vibrational and electron temperature, and electron density are compared with experimental results, obtained through in-situ laser scattering diagnostics in the pressure range between 50 and 400 mbar, yielding good agreement.

Dissociation is shown by the simulations to be mostly reliant on the reaction N2(X,v) + N2(A) → N2(v=0) + N + N, and hence is strongly dependent on the population of vibrational levels. Investigation at p < 100 mbar reveals that low power density (< 120 W/cm3) limits vibrational excitation , resulting in an energy cost per atom of about 200 eV/atom. At p ≥ 100 mbar, partial contraction of the discharge allows power density in the core to increase up to 1000 W/cm3 at 400 mbar, allowing Tv to increase up to 8000 K and reducing the energy cost per atom to 100 eV/atom.