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

Abstract

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.

Plasma structures observed in gas breakdown using a 1.5 MW, 110 GHz pulsed gyrotron

“…The temporal behavior of the breakdown process is also reported. The present results are an extension of our previous study, 6 in which we reported the first observation of a regular two-dimensional array of filaments in atmospheric air breakdown at 110 GHz. These filaments are qualitatively different from, and should not be confused with, the well-known filaments observed in laser breakdown.…”

“…Third, and most importantly, the series of fast camera images have experimentally confirmed that the sequential development of the plasma filamentary array is indeed occurring as we hypothesized in our purely electromagnetic wave model described in Ref. 6. That model predicts that after the formation of a first filament, subsequent filaments will be created upstream of that filament, with a spacing of about a quarter wavelength.…”

“…In Fig. 9, breakdown images of SF 6 at 115 Torr are shown. Each filament appears to have even less separation in the upstream region than in Fig.…”

“…Thus, we see explicitly that the breakdown plasma propagates toward the microwave source, as we reported in our previous measurements using two axially displaced photodiodes. 6 Second, the filaments existing at an earlier stage are extinguished at a later stage. The upstream filaments, which are being created closer to the microwave source with increasing time, absorb and reflect most of the incident power, resulting in the downstream filaments being extinguished.…”

“…4,5 However, gas breakdown research results in the frequency range between these two regimes, specifically between 100 and 800 GHz, have only recently been reported. 6,7 This is mainly because of the dearth of millimeter wave sources that can generate megawatts of power to achieve gas breakdown. Currently, a gyrotron is the only device that can deliver such high power in this frequency range.…”