Space and time evolution of light emitted from microwave nitrogen breakdown

A two-dimensional model coupling Maxwell's equations with plasma fluid equations is used to simulate nitrogen breakdown in a microwave field radiated from a circular waveguide. The balance equations of an excited molecule (ion) density, taking into account the production of excited states as well as quenching in radiation and collisional processes, are included in this model to predict the brightness of light produced in the breakdown. The electron energy distribution function (EEDF) obtained from the analytic expression is adopted to compute the rate coefficients in the model, and its effects on the light brightness are considered. The light brightness in the high electric field initially increases sharply over time. After the occurrence of the breakdown, the electron density saturates at a high level, but the local electric field collapses, leading to the decrease in the mean electron energy. In this case, the light brightness decreases in time since the production of excited molecules (ion) is less than its quenching. The plasma produced in nitrogen breakdown modifies the electric field distribution in space, and the light brightness in the enhanced electric field increases because of the enhancement of the electron density and mean electron energy. The difference in the evolutions of light brightness of different wavelengths is discussed. When the gas pressure increases, the critical electron density above which the incident wave is disturbed increases, and the corresponding light brightness for several wavelengths in the range of visible light first increases and then decreases, showing the similar trend as the experiment. The breakdown formation times predicted by the model also show the similar trend as the experiment. (paper)