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

Abstract

Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments using a pulsed 1.5 MW, 110 GHz gyrotron. The gyrotron Gaussian output beam is focused to an intensity of up to 4 MW/cm2. The plasma filaments develop in an array with a spacing of about one quarter wavelength, elongated in the electric field direction. The array was imaged using photodiodes, a slow camera, which captures the entire breakdown event, and a fast camera with a 6 ns window. These diagnostics demonstrate the sequential development of the array propagating back toward the source. Gases studied included air, nitrogen, SF6, and helium at various pressures. A discrete plasma array structure is observed at high pressure, while a diffuse plasma is observed at lower pressure. The propagation speed of the ionization front for air and nitrogen at atmospheric pressure for 3 MW/cm2 was found to be of the order of 10 km/s.

Computational study of microwave-induced ionization process using PIC-MCC method

“…We can find that the electron number density grows exponentially for all cases, and the growth rates are the same for all situations with identical microwave intensity. Even if the number density of the background electrons is sufficiently low at atmospheric environment and room temperature, the new ionization front is formed on a ns timescale while the experimental studies suggested the front should move to the new one in a ten ns timescale [3]. On the other hand, the influence of the incident microwave intensity is relatively strong on forming the ionization front.…”

“…Any relationship hence may exist between the formation of the filamentary structures and the thrust. Additionally, another experiment has shown clear formation of regular filamentary plasma arrays propagating toward the microwave source [3].…”

Asymmetric Shock Wave Generation in a Microwave Rocket Using a Magnetic Field

“…The maximum intensity of the incident field E y is fixed at 5 MV/m, and the microwave frequency is 110 GHz following Hidaka's experiments [6]. The 1D simulation domain has 2.25λ (the microwave wavelength is λ = 0.27 cm), and the number of grid points used is 40,000 in order to resolve the Debye length of the electrons.…”

“…Pressure dependencies of the breakdown structure and the thrust performance of a microwave rocket were assessed previously by placing a parabolic thruster in a vacuum chamber [2,3,6]. A discrete plasma was formed at atmospheric pressure because a standing wave was induced in front of the plasma due to waves reflected by the overcritical plasma.…”

“…The vehicle obtains an impulsive thrust from interactions between the thruster wall and the shock waves. Chemical fuel aboard the vehicle can be removed or reduced because ambient air acts as the fuel, which decreases the launch cost.Pressure dependencies of the breakdown structure and the thrust performance of a microwave rocket were assessed previously by placing a parabolic thruster in a vacuum chamber [2,3,6]. A discrete plasma was formed at atmospheric pressure because a standing wave was induced in front of the plasma due to waves reflected by the overcritical plasma.…”