Electric field determination in transient plasmas: in situ & non-invasive methods

One of the primary basic plasma parameters within transient nonequilibrium plasmas is the reduced electric field strength, roughly understood as the ratio of the electrical energy given to the charged species between two collisions. While physical probes have historically been used for electric field measurements, recent advances in high intensity lasers and sensitive detection methods have allowed for non-invasive optical electric field determination in nearly any discharge configuration with time-resolution up to the sub-nanosecond range and sub-millimeter spatial resolution. This topical review serves to highlight several non-invasive methods for in situ electric field strength determination in transient plasmas ranging from high vacuum environments to atmospheric pressure and above. We will discuss the advantages and proper implementation of (i) laser induced fluorescence dip spectroscopy for measurements in low pressure RF discharges, (ii) optical emission spectroscopy based methods for nitrogen, helium or hydrogen containing discharges, (iii) electric field induced coherent Raman scattering, and (iv) electric field induced second harmonic generation. The physical mechanism for each method will be described as well as basic implementation and highlighting recent results

Nanosecond pulsed barrier discharge in argon for various frequencies and theoretical analysis of 2p states density ratios for E/N determination

Nanosecond pulsed barrier discharges in atmospheric pressure argon are simulated using spatially one- and two-dimensional fluid-Poisson models using the reaction kinetics model presented by Stankov et al. [1], which considers all ten argon 2p states (Paschen notation) separately. The very first (single) discharge and repetitive discharges with frequencies from 5kHz to 100kHz are considered and a semi-automated procedure is utilized to find appropriate 2p states for electric field determination using an intensity ratio method. The proposed method is based on a time-dependent collisional-radiative model enabling a sub-nanosecond plasma diagnostics, it links the 2p state density ratios to the reduced electric field strength E/N by quantifying the excitation rate coefficients and by computing 2p states effective lifetimes from the fluid model simulation. The semi-automated procedure identifies several candidates for determination of E/N from given temporal profiles of the 2p state densities. Different approaches for effective lifetime determination are tested and applied also to measured data. The influence of radial and axial 2p state density integration on the intensity ratio method is discussed. The above mentioned models and procedures result in a flexible theory-based methodology applicable for development of new diagnostic techniques.Comment: 19 pages, 13 figure

High-resolution electric field and temperature distributions in positive streamers

In this work, we aim to take a detailed experimental picture of the positive streamer. We apply optical emission spectroscopy to the first negative system (FNS, (Formula presented.)) of (Formula presented.) and the second positive system (SPS, C3Πu → B3Πg) of N2. Large, centimeter wide, and highly reproducible streamers are created in pure nitrogen and synthetic air, at pressures ranging from 33 to 266 mbar. Direct time resolved spectral imaging of the space charge layer resulted in spatiotemporal maps of the calculated reduced electric field strength (E/N) and rovibrational temperature in sub-nanosecond and sub-millimetre resolution. The E/N peaks at approximately 540 and 480 Td, directly in front of the space charge layer, for synthetic air and pure nitrogen respectively, as determined by using the intensity ratio method of FNS and SPS. A global model for pure nitrogen in PLASIMO uses the experimentally determined E/N distribution to draw a picture of the gas kinetics around the space charge layer passage. In addition, the results of the global model serve as a reference to interpret the rotational and vibrational temperatures obtained from experimental FNS and SPS emissions.</p

Formation mechanisms of striations in a filamentary dielectric barrier discharge in atmospheric pressure argon

Formation mechanisms of striations along the discharge channel of a single-filament dielectric barrier discharge (DBD) in argon at atmospheric pressure are investigated by means of a time-dependent, spatially two-dimensional fluid-Poisson model. The model is applied to a one-sided DBD arrangement with a 1.5 mm gap using a sinusoidal high voltage at the powered metal electrode. The discharge conditions are chosen to mimic experimental conditions for which striations have been observed. It is found that the striations form in both half-periods during the transient glow phase, which follows the streamer breakdown phase. The modelling results show that the distinct striated structures feature local spatial maxima and minima in charged and excited particle densities, which are more pronounced during the positive polarity. Their formation is explained by a repetitive stepwise ionisation of metastable argon atoms and ionisation of excimers, causing a disturbance of the spatial distribution of charge carriers along the discharge channel. The results emphasise the importance of excited states and stepwise ionisation processes on the formation of repetitive ionisation waves, eventually leading to striations along the discharge channel.Comment: 18 pages, 15 figures, resubmitted to Plasma Sources Science and Technolog

Memory propagation in barrier discharge at water interface: Suspected Markov states and spatiotemporal memory effects

Barrier discharges are known for strong memory effects which shape their stochastic properties. We study memory propagation in a surface barrier discharge operated at the water interface in atmospheric pressure air using electrical measurements and synchronized intensified CCD imaging. Comprehensive electrical data sets of seemingly chaotic behaviour are evaluated from thousands of subsequent periods recorded with high temporal resolution and large dynamic range. We confirm known memory effects and identify new ones both in between the half-periods as well as between subsequent pulses within one half-period. We find two memoryless states for the first discharges in the positive polarity which are defined by the presence/absence of photoemission-induced collective behaviour. Given the determined probability for entering one of these two suspected Markov states, the system bifurcates and follows one of the two subsequent non-Markovian pathways. The identification of the collective behaviour in these pathways enables us to recognize the spatially resolved property of the system and to determine its probability of occurrence, both solely from the zero-dimensional electrical characterisation

Memory propagation in barrier discharge at water interface: suspected Markov states and spatiotemporal memory effects

Abstract Barrier discharges are known for strong memory effects which shape their stochastic properties. We study memory propagation in a surface barrier discharge operated at the water interface in atmospheric pressure air using electrical measurements and synchronized intensified CCD imaging. Comprehensive electrical data sets of seemingly chaotic behaviour are evaluated from thousands of subsequent periods recorded with high temporal resolution and large dynamic range. We confirm known memory effects and identify new ones both in between the half-periods as well as between subsequent pulses within one half-period. We find two memoryless states for the first discharges in the positive polarity which are defined by the presence/absence of photoemission-induced collective behaviour. Given the determined probability for entering one of these two suspected Markov states, the system bifurcates and follows one of the two subsequent non-Markovian pathways. The identification of the collective behaviour in these pathways enables us to recognize the spatially resolved property of the system and to determine its probability of occurrence, both solely from the zero-dimensional electrical characterisation.</jats:p