Reactive species in non-equilibrium atmospheric-pressure plasmas: Generation, transport, and biological effects

The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.

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Gas temperature determination from rotational lines in non-equilibrium plasmas: a review

Cold atmospheric pressure helium plasma jets are increasingly used in many processing applications, due to a distinct combination of their inherent plasma stability with excellent reaction chemistry often enhanced downstream. Despite their widespread usage, it remains largely unknown whether cold atmospheric plasma jets maintain similar characteristics from breakdown to arcing or whether they possess different operating modes. In addition to their known ability to produce a fast moving train of discrete luminous clusters along the jet length, commonly known as plasma bullets, this paper reports evidence of two additional modes of operation, namely a chaotic mode and a continuous mode in an atmospheric helium plasma jet. Through detailed electrical and optical characterization, it is shown that immediately following breakdown the plasma jet operates in a deterministic chaotic mode. With increasing input power, the discharge becomes periodic and the jet plasma is found to produce at least one strong plasma bullet every cycle of the applied voltage. Further increase in input power eventually leads to the continuous mode in which excited species are seen to remain within the inter-electrode space throughout the entire cycle of the applied voltage. Transition from the chaotic, through the bullet, to the continuous modes is abrupt and distinct, with each mode having a unique set of operating characteristics. For the bullet mode, direct evidence is presented to demonstrate that the evolution of the plasma jet involves a repeated sequence of generation, collapse and regeneration of the plasma head occurring at locations progressively towards the instantaneous cathode. These offer previously unavailable insight into plasma jet formation mechanisms and the potential of matching plasma jet modes to specific needs of a given processing application.

/pdf/three-distinct-modes-in-a-cold-atmospheric-pressure-plasma-k1n7loqqv0.pdf

Three distinct modes in a cold atmospheric pressure plasma jet

In atmospheric-pressure air preheated to 1000 K, nanosecond repetitively pulsed (NRP) discharges are shown to generate three plasma discharge regimes. In addition to the well-known corona and spark regimes, there exists a glow-like regime that develops through an initial cathode-directed streamer, followed by a return wave of potential redistribution. The applied electric field is then switched off before the formation of the cathode fall, resulting in an 'imminent' glow discharge. Previously, this regime had been observed only at 2000 K in air at atmospheric pressure. Measurements of the plasma dynamics, current–voltage characteristics, gas temperature and plasma chemistry of the excited species N2(B), N2(C), , NO(A) and O(3p 5P) in the pulsed glow regime are presented. Using 10 ns pulses applied repetitively at 30 kHz, we find that this glow regime generates an estimated electron number density of 1013 cm−3, while consuming only 1–10 µJ per pulse and heating the gas by less than 200 K.

https://iopscience.iop.org/article/10.1088/0963-0252/19/6/065015/pdf

Nanosecond repetitively pulsed discharges in air at atmospheric pressure—the glow regime

Chemical and bactericidal effects induced by plasma in water upon electro-spraying through DC-driven positive transient spark discharge in air were investigated. Inactivation of E. coli was determined in dependence on pH (controlled by buffers) and correlated with chemical changes induced in water. Productions of hydrogen peroxide, nitrites, nitrates, peroxynitrites, and pH changes were determined, and the extent of oxidative stress induced in bacteria was evaluated. The degree of inactivation and oxidative damage of bacteria increased with the increasing acidity of the solution. Acidified nitrites interacting with hydrogen peroxide were determined as the most important bactericidal ROS/RNS agents in plasma-treated water. A possible role of peroxynitrites, ozone, and metal nanoparticles is discussed.

Formation of ROS and RNS in Water Electro-Sprayed through Transient Spark Discharge in Air and their Bactericidal Effects

Emission spectroscopy of atmospheric pressure plasmas for bio-medical and environmental applications

Three types of DC electrical discharges in atmospheric air (streamer corona, transient spark and glow discharge) were tested for bio-decontamination of bacteria and yeasts in water solution, and spores on surfaces. Static vs. flowing treatment of contaminated water were compared, in the latter the flowing water either covered the grounded electrode or passed through the high voltage needle electrode. The bacteria were killed most efficiently in the flowing regime by transient spark. Streamer corona was efficient when the treated medium flew through the active corona region. The spores on plastic foil and paper surfaces were successfully inactivated by negative corona. The microbes were handled and their population evaluated by standard microbiology cultivation procedures. The emission spectroscopy of the discharges and TBARS (thiobarbituric acid reactive substances) absorption spectrometric detection of the products of lipid peroxidation of bacterial cell membranes indicated a major role of radicals and reactive oxygen species among the bio-decontamination mechanisms.

I. Jedlovský

Papers

Emission spectroscopy of atmospheric pressure plasmas for bio-medical and environmental applications

DC discharges in atmospheric air for bio-decontamination - spectroscopic methods for mechanism identification