Generation of excited species in a streamer discharge

Shirshak K Dhali

doi:10.1063/5.0033110

Abstract

At or near atmospheric pressure, most transient discharges, particularly in molecular gases or gas mixture containing molecular gases, result in a space charge dominated transport called a streamer discharge. The excited species generation in such discharges forms the basis for plasma chemistry in most technological applications. In this paper, we simulate the propagation of streamers in atmospheric pressure N2 to understand the energy partitioning in the formation of various excited species and compare the results to a uniform Townsend discharge. The model is fully two-dimensional with azimuthal symmetry. The results show a significantly larger fraction of the energy goes into vibrational excitation of the N2 ground state in a streamer-type discharge in comparison to a Townsend discharge. For lower applied voltages, the anode-directed (negative) steamer is slightly more efficient in channeling energy for excited species production in comparison to a cathode-directed (positive) streamer. Near 70% overvoltage, both types of streamers show very similar energy partitioning, but quite different from a Townsend discharge.

Highlights

In most overvolted transient discharges at or near atmospheric pressure, an electron avalanche quickly leads to a space charge dominated transport called streamers
We have studied in detail the formation and propagation of positive and negative steamers and compared the energy partitioning for the creation of various excited species to a uniform Townsend discharge at different applied voltages in a parallel plate gap
The vibrational energy partition in either streamer type is considerably higher than the uniform Townsend discharge

Summary

Introduction

In most overvolted transient discharges at or near atmospheric pressure, an electron avalanche quickly leads to a space charge dominated transport called streamers. Streamers are precursors to arc and lightening formation. Since the streamer mechanism was first proposed by Raether and Loeb and Meek, a considerable amount of evidence has been accumulated, showing the importance of streamers or fast ionizing waves to the transient electrical breakdown of gases. The formation and propagation of streamers have been studied extensively both numerically and experimentally.. Streamer properties depend on the density and composition of the gas, the temporal and spatial shape of the applied voltage, and its polarity. There are challenges to numerical studies of the streamer formation and propagation: The problem is inherently two-dimensional as a one-dimensional model has limited validity, and the steep density gradients encountered in the streamer front can lead to numerical instabilities.

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