Spectroscopy Measurements of Air Breakdown Initiated with a Focused 110 GHz, MW-Level Quasioptical Gyrotron Beam*

We present spectroscopic measurements of air breakdown that is created with a focused 110 GHz gyrotron beam at fluxes exceeding 1 MW/cm$^{2}$. Excitation, rotational, and vibrational temperature measurements are made over the pressure range of 1-100 Torr. The gyrotron power is varied to measure the behavior of these plasma temperatures as the field is raised above the threshold field required for breakdown. Rotational temperature measurements of the plasma show minimal gas heating, with gas temperatures in the range of 300-500 K. The vibrational and excitation temperatures were measured to be on the same order and vary between 4200-6200 K and 0.4-0.65 eV, respectively. In order to calculate the excitation temperature it was necessary to include the deviation from Local Thermodynamic Equilibrium (LTE) in the analysis because the electron density of the discharge is not high enough to satisfy the conditions for LTE. The excitation temperature is observed to vary little with applied field, while the vibrational and rotational temperatures increase as the field is increased above threshold. The temperature measurements indicate that the plasma is in a state of thermal non-equilibrium, with heavy ions and neutrals heated by collisions with much hotter electrons.