Effects of pulse repetition rate on active species production for nanosecond spark discharges in air

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Hifi, Iliane and Timoshkin, Igor and Burt, Graeme (2026) Effects of pulse repetition rate on active species production for nanosecond spark discharges in air. IEEE Transactions on Plasma Science. pp. 1-11. ISSN 1939-9375 (https://doi.org/10.1109/tps.2025.3650404)

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Abstract

Nanosecond pulsed discharges are capable of producing highly transient nonequilibrium plasma in gases which are characterized by high electron temperatures and relatively low rotational and vibrational temperatures. These conditions enable the formation of unstable molecules, namely radicals, which are beneficial to a wide range of biomedical and environmental applications including air and water purification, bio-decontamination, and advanced oxidation processes. This research investigates the effects of repetition rate of ns discharges in static air and the average power delivered into the plasma on the production of the hydroxyl radical (OH ⋅ ) and the nitric oxide radical (NO ⋅ ) through optical emission spectroscopy (OES). Plasma pulses were generated with voltage pulses with a constant voltage rate of rise (dV/dt) of 9 kV/ μ s resulting in the formation of a spark discharge with current impulse duration of 16 ns. These pulses were applied at repetition rates from 5 thousand pulses per second (kpps) to 38 kpps. The individual discharge energy was observed to decrease as a result of increasing pulse repetition rate, showing a drop from 1.17 mJ per pulse to 0.28 mJ per pulse at 5 and 38 kpps, respectively. Despite this decrease, the average power delivered into the plasma increased as a result of pulse repetition rate from 5.80 to 11.56 W. The optical emission spectra from both the OH ⋅ and NO ⋅ radicals, and atomic oxygen showed a strong dependence on pulse repetition rate for low repetition rates, followed by a clear correlation with power delivered into the plasma for repetition rates over 15 kpps. The plasma rotational and vibrational temperatures were obtained using 3 methods, with a high resolution N2 second positive system (SPS) measurement used to evaluate the baseline temperatures. This was compared with a low resolution N2 SPS measurement using a local thermal equilibrium (LTE) assumption, which showed an error of over 1000 K between the LTE Tvib and the actual Tvib , proving the limitations of the LTE assumption with relation to transient spark discharges. The rotational temperature of OH ⋅ was evaluated and found to follow a non-Boltzmann behavior, resulting in an error in Trot of up to 189% as compared to the N2 SPS Trot . This study provides clear insights into the optimization of nanosecond pulsed plasma systems for efficient active species production, with wide ranging implications for biomedical, environmental, and industrial applications. The findings provide further insights into the characterization of transient plasmas, advancing the understanding of nonequilibrium plasmas.

ORCID iDs

Hifi, Iliane ORCID logoORCID: https://orcid.org/0009-0001-7995-6325, Timoshkin, Igor ORCID logoORCID: https://orcid.org/0000-0002-0380-9003 and Burt, Graeme ORCID logoORCID: https://orcid.org/0000-0002-0315-5919;
CORE (COnnecting REpositories)