Emerging and expanding streamer head in low-pressure air

The emergence of a streamer from an ionisation wave and its expansion are ultra-fast processes shaping the very first moments of the streamer development, and are usually accessible only by complex numerical models. In this Letter, we report experimental evidence of the emergence of a streamer from an ionisation wave in 1.3 kPa air, a laboratory analogue of early-stage streamers emerging in geophysical Blue Starters and Jets. The radially and temporally resolved electric field patterns of an expanding streamer are determined by sub-nanosecond optical emission spectroscopy. As the emerged streamer expands, the electric field decreases by a factor of 1.4 in 1 ns. We quantify the radial expansion of the streamer head and its axial acceleration, reaching the velocity of 107 m s-1. In combination with electrical measurements, the transferred charge, electron density, and mean electron energy are quantified, enabling detailed insight into this ultra-fast phenomenon at its characteristic time-scale. © 2020 IOP Publishing Ltd.This research was funded by the Academy of Sciences of the Czech Republic under project M100431201. TH was supported by the European Science Foundation Research Networking Programme TEA-IS (ex-change Grant No. 4219). ZB was supported by the Czech Science Foundation, project No. 15-04023S. FJGV was supported by the Spanish Ministry of Science and Innovation (MINECO) under project ESP2017-86263-C4-4-R. TH is grateful for the support of project LM2018097 funded by the Ministry of Education, Youth and Sports of the Czech Republic.Peer reviewe

Optical diagnostics of streamers: From laboratory micro-scale to upper-atmospheric large-scale discharges

Optical emission produced by streamers is determined by spatial distribution of electronically excited atomic and diatomic species within the streamer head and streamer channel. Peculiarities of emission and LIF diagnostics dedicated to investigating the basic structure of streamers with high spatio-temporal resolution are discussed. Possible strategies based on the 2D projections of cylindrically symmetric streamers to determine radial distributions of excited species within the streamer channel are illustrated for streamers produced in volume or on the dielectric surface at atmospheric and low pressures

Plasma modeling with Plasimo - design and applications

No abstract

Correlation between CF2-and C2F4-concentrations in pulsed capacitively coupled CF4/H2 rf plasma

The CF2 and C2F4 absolute concentrations were measured in pulsed low pressure CF4/H2 rf plasmas (13.56 MHz, CCP) by means of Infra-Red Tuneable Diode Laser Absorption Spectroscopy. For measurement of CF2 radicals the P2(21) line at 1096.3433 cm-1 was chosen with its calculated line strength of 4.09·10-20 cm/molecule. In case of C2F4 there is no detailed spectroscopic data available. Therefore, the C2F4 gas was produced by thermal decomposition of polytetrafluoroethylene for spectroscopic analysis. An absorption structure of several overlapping C2F4 lines was found around 1337.11 cm-1 and manually fitted. In pulsed plasma the time dependencies of the CF2 and C2F4 concentration correlated with each other. In plasma off-phase, the recombination of two CF2 radicals forming C2F4 was found to be dominant in CF2 kinetics, but of minor importance in C2F4 production

Structure formation in a DC-driven "barrier" discharge:

A DC-driven "barrier" discharge is a gas discharge layer and a high-Ohmic semiconductor layer sandwiched between planar electrodes to which a DC voltage is applied. The system resembles a dielectric barrier discharge, but is even simpler, as the external boundary conditions allow for a completely homogeneous and stationary state. Whole phase transition diagrams to purely oscillating states or to states with spatio-temporal structures were determined in experiments that actually explored the regime between Townsend and glow discharge. We discuss the experimental conditions and our present theoretical understanding, that is based on stability analysis and numerical solutions of a °uid model for the gas discharge with space charge e®ects and a linear model for the semiconductor