Spatially and Temporally Resolved Electron Density Measurements of Air Breakdown Plasma Utilizing a 1.4 MW, 110 GHz Gyrotron

We present the latest results of the MIT microwave-frequency air breakdown experiment. The experiment utilizes a 1.4 MW, 110 GHz gyrotron producing 3 microsecond pulses. The linearly polarized beam is focused to a 3.2 mm diameter spot size. The resulting breakdown plasma spontaneously forms a two-dimensional array of filaments, oriented along electric field lines, that propagate toward the source.\footnote{A. M. Cook, J. S. Hummelt, M. A. Shapiro, and R. J. Temkin, ``Observation of plasma array dynamics in 110 GHz millimeter-wave air breakdown,'' Physics of Plasmas, Vol. 18, No. 10, 100704 (2011).} Two-wavelength laser interferometry is combined with a 2 nanosecond fast gating ICCD to make spatially and temporally resolved electron density measurements of the filament array. Electron density is measured as a function of incident microwave power in a range of pressures of atmospheric air from 25 to 700 Torr.