We report electrical properties of radio frequency (RF)-driven hollow slot microplasmas operating in open air but with uniform luminous discharges at RF current densities of the order of A cm −2 . We employ interelectrode separations of 100-600 µm to achieve this open-air operation but because the linear slot dimension of our electrode designs are of extended length, we can achieve, for example, open-air slot shaped plasmas 30 cm in length. This creates a linear plasma source for wide area plasma driven surface treatment applications. RF voltages at frequencies of 4-60 MHz are applied to an interior electrode to both ignite and sustain the plasma between electrodes. The outer slotted electrode is grounded. Illustrative absolute emission of optical spectra from this source is presented in the region from 100 to 400 nm as well as total oxygen radical fluxes from the source. We present both RF breakdown and sustaining voltage measurements as well as impedance values measured for the microplasmas, which use flowing rare gas in the interelectrode region exiting into open air. The requirement for rare gas flow is necessary to get uniform plasmas of dimensions over 30 cm, but is a practical disadvantage. In one mode of operation we create an out-flowing afterglow plasma plume, which extends 1-3 mm from the grounded open slot allowing for treatment of work pieces placed millimetres away from the grounded electrode. This afterglow configuration also allows for lower gas temperatures impinging on substrates, than the use of active plasmas. Work pieces are not required to be part of any electrical circuit, bringing additional practical advantages. We present a crude lumped parameter equivalent circuit model to analyse the effects of changing RF sheaths with frequency of excitation and applied RF current to better understand the relative roles of sheath and bulk plasma behaviour observed in electrical characteristics. Estimates of the bulk plasma densities are also provided. Finally, we present results of afterglow plasma based bacteria inactivation studies (Escherichia coli, Bacillus atrophaeus and B. atrophaeus spores) in which we employ the flowing afterglow plume from a hollow slot microplasma device rather than the active plasma itself, which is fully contained between electrodes.

Escherichia coli, Bacillus atrophaeus, and Bacillus atrophaeus spores were exposed to a downstream plasma afterglow plume emitted from a slotted plasma device operating in open air at atmospheric pressure. The reactor electrodes were RF powered at 13.56 MHz to excite a mixture of argon and oxygen gases by a capacitive discharge as it flowed past the electrodes into open air. Bacterial inactivation experiments on surfaces exposed to the plasma afterglow were conducted with varying plasma exposure times. Experimental results demonstrated a colony forming unit (CFU) reduction of almost 5 log10 of E. coli with only 1 s of exposure per unit area. One log CFU reduction was observed in B. atrophaeus with the same treatment time of 1 s per unit area. B. atrophaeus spores showed a reduction of 3 log10 with exposure time of 10 min. Comparison on various growth media suggests that cells are killed rather than sublethally injured, while the mechanistic action of the plasma appears to affect both nucleic acids as well as the cell wall structure. These results present a promising means of inactivation of harmful microbes in a practical environment with an electrically grounded device that is handheld, much like a wand applicator. Results are applicable to the development of plasma sterilization tools for various environmental purposes.

Aim:  Nonthermal atmospheric‐pressure plasmas offer significant advantages as an emerging disinfection approach. However the mechanisms of inactivation, and thus the means of optimizing them, are still poorly understood. The objective of this study, therefore, was to explore differential gene expression on a genome‐wide scale in Escherichia coli following exposure to a nonthermal atmospheric‐pressure argon plasma plume using high‐density oligonucleotide microarrays. Methods and Results:  Plasma exposure was found to significantly induce the SOS mechanism, consisting of about 20 genes. Other genes involved in regulating response to oxidative stress were also observed to be up‐regulated. Conversely, the expression of several genes responsible for housekeeping functions, ion transport, and metabolism was observed to be down‐regulated. Conclusions:  Elevated yet incomplete induction of various DNA damage repair processes, including translesion synthesis, suggests substantial DNA damage in E. coli. Oxidative stress also appeared to play a role. Thus it is proposed that the efficacy of plasma is due to the synergistic impact of UV photons and oxygen radicals on the bacteria. Significance and Impact of the Study:  This study represents the first investigation of differential gene expression on a genome‐wide scale in an organism following plasma exposure. The results of this study will help enable the design of safe and effective plasma decontamination devices.

A coupled 2D computational model of nanosecond pulsed plasma induced flame ignition and combustion for a lean H 2 -air mixture (dry air) in a high pressure environment is discussed. The model provides a full fidelity description of plasma formation, combustion ignition, and flame development. We study the effects of three important plasma properties that influence combustion ignition and flame propagation, namely (a) plasma gas temperature, (b) plasma produced primary combustion radicals O, OH and H densities, and (c) plasma generated charged and electronically excited radical densities. Preliminary 0D studies indicate that plasma generated trace quantities of O, OH and H radicals drastically reduce the ignition delay of the H 2 -air mixture and become especially important for high pressure lean conditions. Multi-dimensional simulations are performed for a lean H 2 -air mixture (ϕ = 0.3) at 3.3 atm and an initial temperature of 1000 K. The plasma is accompanied by fast gas heating due to N 2 metastable quenching that results in uniform volumetric heating in the interelectrode gap. The spatial extent of the high temperature region generated by the plasma is a key parameter in influencing ignition; a larger high temperature region being more effective at initiating combustion ignition. Plasma generation of even trace quantities (~0.1%) of primary combustion radicals, along with plasma gas heating, results in a further fifteen-fold reduction in the ignition delay. The radical densities alone did not ignite the H 2 -air mixture. The generation of other plasma specific species results only in a slight ~10% improvement in the ignition delay characteristics over the effect of primary combustion radicals, with the slow decaying ions (H + 2 , O − 2 , O − ) and oxygen metastable species (O a1 2 , O b1 2 , O * 2 ) primarily contributing to combustion enhancement. These species influence the ignition delay, directly by power deposition due to quenching, attachment and recombination reactions, and indirectly by enhancing production of primary combustion radicals.

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