Observation of a comb-shaped filamentary plasma array under subcritical condition in 303-GHz millimetre-wave air discharge

Abstract

Gas breakdown in the millimetre-wave frequency band is an interesting phenomenon in nonlinear dynamics such as self-organized structure formation. We observed the transition between two types of filamentary plasma arrays in air discharge driven by a 303-GHz millimetre wave. Plasma is ignited at a parabolic mirror’s focal point in the overcritical condition. One array parallel to the electric field vector appears with a spacing of λ/4 at the focal point. Filaments then separate into plasma lumps ~10 μs after ignition. At 20 μs, a new comb-shaped array grows in the subcritical condition. Filaments are parallel to the incident beam with spacing of 0.96 λ and elongate towards the incident beam. This comb-shaped array appears only in the electric field plane; bulk plasma with a sharp vertex forms in the magnetic field plane. This array is created by a standing wave structure generated by waves diffracted from the plasma surface. Filamentary plasma array formations can influence the energy absorption by the plasma, which is important for engineering applications such as beamed energy propulsion.

Computational Analysis of Plasma Evolution during High Power Millimeter Wave Breakdown using Mesh Refinement based Fluid Simulations

“…The process of standing wave formation and enhancement of the front E-field follows and results in continuous ionization and diffusion-assisted front propagation. Further, it continues till the end of simulation (around 100 ns) and we observe a self-organized fish-bone like plasma filamentary structure exactly similar to experimental observations [3], [4].…”

“…However, only recently, detailed experimental investigations of the plasma dynamics during breakdown have been possible with the use of sophisticated high-speed cameras. Particularly, in the past few years, several experiments and numerical simulations using millimeter and sub-millimeter wave irradiation (100s of GHz) at high pressures (ten to hundreds of Torr) have been carried out [3], [4]. Several types of gas discharges have been reported experimentally such as streamer, overcritical, subcritical, volumetric and attached to an initiator [5].…”

“…The modeling and simulation of the interaction of high power high frequency electromagnetic (EM) wave with air/gas is a computationally challenging problem [6]. The interaction results in the breakdown of air/gas leading to plasma formation, a highly complex and nonlinear phenomenon [1], [4], [6]- [10]. Different computational techniques have been used to study this problem [11]- [15].…”

“…II. PHYSICAL AND COMPUTATIONAL MODEL The mmWave air breakdown at high pressure is highly collisional and nonlinear process [1], [4], [6], [8], [9], [11]- [13], [16], [23], [27], [28]. It can be captured by a plasma fluid model that follows a Maxwell-Boltzmann Probability Distribution Function (PDF) to define the plasma species, and various macroscopic quantities such as particle number density, mean velocity and mean energy are obtained by integrating the PDF over the momentum space of the Boltzmann equation [11].…”

“…It can be captured by a plasma fluid model that follows a Maxwell-Boltzmann Probability Distribution Function (PDF) to define the plasma species, and various macroscopic quantities such as particle number density, mean velocity and mean energy are obtained by integrating the PDF over the momentum space of the Boltzmann equation [11]. A well established coupled EM plasma fluid model [1], [27]- [29] exists which has been used by several researchers [4], [7], [8], [10], [11], [13] to reproduce the experimental observations [3], [20], [21].…”