Observation of Large Arrays of Plasma Filaments in Air Breakdown by 1.5-MW 110-GHz Gyrotron Pulses

We report the observation of two-dimensional plasma filamentary arrays with more than 100 elements generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW, 110-GHz gyrotron operating in 3-� s pulses. Each element is a plasma filament elongated in the electric field direction and regularly spaced about one-quarter wavelength apart in the plane perpendicular to the electric field. The development of the array is explained as a result of diffraction of the beam around the filaments, leading to the sequential generation of high intensity spots, at which new filaments are created, about a quarter wavelength upstream from each existing filament. Electromagnetic wave simulations corroborate this explanation and show very good correlation to the observed pattern of filaments. We report the observation of large, two-dimensional regular arrays of plasma filaments generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW 110-GHz gyrotron with 3-� s pulse length. These filamentary arrays consist of a large number of regularly spaced elements, more than 100 in some cases. They are observed both involume breakdown and in breakdown at surfaces. The filamentary arrays are notably different from the well-known filamentation observed in laserinduced plasmas. Laser generated filaments are aligned along the direction of propagation, while the filaments observed here are perpendicular to the direction of propagation. Each filament is aligned parallel to the direction of the electric field. There is an extensive literature on gas breakdown at microwave frequencies [1‐3] and at optical frequencies [4,5]. Recent research has also addressed a number of important issues in breakdown at surfaces [6]. However, due to the scarcity of powerful sources in the millimeter wavelength range, there have been relatively few studies on gas breakdown at these wavelengths. Studies have been carried out with gyrotrons at frequencies of 35 to 86 GHz [7‐10] at power levels up to 250 kW and pulse lengths of 0.05 to 20 ms. The available power in these studies was insufficient to achieve breakdown in air at atmospheric pressure, but careful studies were conducted either by using lower pressure (where the breakdown threshold is reduced), using alternate gases such as helium, or using initiators such as a collection of fine wires placed near the beam focus. These studies yielded some interesting evidence of filamentary structures in the breakdown volume. However, none of these studies produced the large, twodimensional, filamentary regular arrays seen in the present experiments. Also, the present study differs substantially from these studies in terms of time scales (3 � s vs 0.05‐ 20 ms) as well as intensities (on the order of 10 6 vs 10 4 W=cm 2 ) used for initiating and sustaining the breakdown plasma. A number of theoretical studies have also been reported to explain the structures such as ‘‘snakes’’ and ‘‘chains’’ that have been observed in high-pressure microwave breakdown research [11‐13]. These explanations emphasize the role of plasma kinetics and instabilities. In this study, we explain the array as developing from a sequential process of electromagnetic wave diffraction. Incident microwave power is diffracted from plasma filaments formed early in the breakdown process. As the technology of high power millimeter wave sources matures and their applications broaden, breakdown will inevitably be an issue in transporting through a gas, or on the surface of a window through which such radiation is transmitted. This was our motivation for conducting volume and surface breakdown experiments in this relatively unexplored frequency band. Figure 1 shows the experimental setup for the breakdown studies. The 110 GHz gyrotron, operating at 96-kV beam voltage and 40-A beam current, can produce 1.5 MW of output power in 3-� s pulses [14]. The TE22;6 mode