Konstantinos Giotis

We developed a prototype reactor generating surface dielectric barrier discharges (SDBDs) in ambient air, designed for reliable operation while preventing constructive material degradation. It features stainless steel electrodes and quartz dielectric, both detachable to facilitate reproducible fabrication. The grounded electrode is fully immersed into transformer oil, drastically suppressing undesired parasitic discharges. The device efficiently sustains ns-pulsed and AC discharges at 10 kHz, enabling fundamental studies of their electrical characteristics and spatiotemporal dynamics. The mean electric power (P¯DBD) consumed exhibits a dissimilar non-linear increase with the rising peak voltage (Vp) in each case: P¯DBDpulsed≈0.8–2.5 W for ns-pulsed (Vp=7–9 kV) and P¯DBDAC≈0.9–5.3 W for AC (Vp=7–10 kV) operation. Using ICCD imaging, distinct ionization channels are recorded in the rising part of the pulsed voltage being detached from the driven electrode; during the voltage falling part, a glow-like discharge is formed, remaining anchored on the driven electrode. The rising part of the AC voltage is characterized by erratic, elongated ionization channels in a filamentary form, whereas the falling part leads to a glow-like behavior. During the rising and falling parts of the AC voltage, the discharge reaches maximum propagation lengths (Lmax) of ≈12 and ≈7 mm, respectively, while remaining attached to the driven electrode. The corresponding maximum discharge velocities (vmax) are about 5 × 102 and 3 × 102 m/s. For the ns-pulsed operation, Lmax ≈ 5 mm (vmax≈5 × 105 m/s) and Lmax ≈ 3.5 mm (vmax≈1.5 × 105 m/s) during the rising and falling parts of the voltage pulse, respectively. The SDBD dynamics generated with an ns-pulsed voltage are more reproducible than for the AC case, allowing for the use of a 500 times smaller ICCD gate width (2 ns) and a more accurate description of the discharge's spatiotemporal development. This reactor is suitable for performing fundamental studies and understanding key SDBD features for potential applications such as flow control, biomedicine, and agriculture. However, dedicated studies on the impact of the present results in such applications remain beyond the scope of the present work.

The occurrence of emerging micropollutants of pharmaceutically active compounds (PhACs) in the environment poses a public health concern. Due to PhAC persistence and toxicity even at low concentrations, advanced oxidation processes (AOPs) have gained interest as effective treatment methods. In this context, the present study focuses on the application of a dielectric barrier discharge (DBD)-based plasma jet to Diclofenac (DCF) and Sulfamethoxazole (SMX) degradation in aqueous media. Plasma is sustained by continuous-wave sinusoidal high-voltage of audio frequencies, and negligible total harmonic distortion, in a helium–air mixture. The target pharmaceuticals are chosen based on anticipation of their occurrence due to rehabilitation center (DCF) and hospital (SMX) effluents in sewage systems. The degradation rates are determined by Liquid Chromatography Triple-Quadrupole Mass Spectroscopy (LC-MS/MS). Removal efficiency close to 100%, after 20 min of plasma treatment in the case of DCF at an initial concentration of 50 ppb, is achieved. The post-treatment action of the plasma-induced reactants on PhAC degradation over a day-scale period is studied. The results provide an insight into the dynamic degradation (kinetics) of both DCF and SMX, and they overall highlight the potentiality of the process under consideration for sewage remediation.

The present study explores combining machine learning (ML) algorithms with standard optical diagnostics (such as time-integrated emission spectroscopy and imaging) to accurately predict operating conditions and assess the emission uniformity of a cylindrical surface dielectric barrier discharge (SDBD). It is demonstrated that these optical diagnostics can provide the input data for ML which identifies peculiarities associated with the discharge emission pattern at different high voltage waveforms (AC and pulsed) and amplitudes. By employing unsupervised (principal component analysis (PCA)) and supervised (multilayer perceptron (MLP) neural networks) algorithms, the applied voltage waveform and amplitude are predicted based on correlations/differences identified within large amounts of corresponding data. PCA allowed us to effectively visualise patterns related to the voltage waveforms and amplitudes applied to the SDBD through a transformation of the spectroscopic/imaging data into principal components (PCs) and their projection to a two-dimensional PCs vector space. Furthermore, an accurate prediction of the voltage amplitude is achieved using the MLP which is trained with PCs. A particularly interesting aspect of this concept involves examining the uniformity of the emission pattern of the discharge. This was achieved by analysing spectroscopic data recorded at four different regions around the SDBD surface using the two ML algorithms. These discoveries are instrumental in enhancing plasma-induced processes. They open avenues for real-time control, monitoring, and optimization of plasma-based applications across diverse fields such as flow control for the present SDBD.

This work deals with the experimental study of a surface dielectric-barrier discharge, as a part of the ongoing interest in the control of plasma induced electro-fluid dynamic effects (e.g. plasma actuators). The discharge is generated using a plasma reactor consisting of a fused silica plate which is sandwiched between two printed circuit boards where the electrodes are developed. The reactor is driven by narrow high voltage square pulses of asymmetric rising (25 ns) and falling (2.5 μs) parts, while the discharge evolution is considered in a temporarily and spatially resolved manner over these pulses. That is, conventional electrical and optical emission analyzes are combined with high resolution optical emission spectroscopy and ns-resolved imaging, unveiling main characteristics of the discharge with a special focus on its propagation along the dielectric-barrier surface. The voltage rising part leads to cathode-directed ionization waves, which propagate with a speed up to 10⁵ m s⁻¹. The voltage falling part leads to cathode sheath formation on the driven electrode. Τhe polarization of the dielectric barrier appears critical for the discharge dynamics.

The present report is devoted to the study of distinct ionization waves in terms of temporally-, spatially-, and wavelength-resolved analyses. The study is based on the technique of two-dimension fast imaging. However, appropriately selected ultraviolet to near infrared optical filters are employed to capture the propagation of specific species and digital image processing techniques are applied to explore the recorded snapshots. N₂(SPS), N 2 + (FNS), He I, OH(A–X), and O I, i.e., emissive neutral and ionic species, are investigated. On the other hand, the propagation of the NO _γ species is studied by means of laser-induced fluorescence spectroscopy, since the light intensity due to spontaneous emission of this species was not readily detectable. Digital image processing techniques are also applied for the NO _γ case. The crucial role of the above species in fields like plasma biomedicine and material processing is extensively recognized, and the present work: (i) provides gathered information on the propagation of these species within the atmospheric air; and (ii) introduces image processing algorithms to extract information which otherwise would remain hidden or uncorrelated with other plasma parameters. The present results unveil specific emission patterns due to the propagation of the N₂(SPS), N 2 + (FNS), He I, OH(A–X), and O I species. The intensity patterns consist of a first peak located in the vicinity of the reactor orifice, a second peak moving away from the reactor orifice, and a continuum which couples the two peaks. However, the detailed features of each pattern depend on the type of species considered. The fluorescence of the NO _γ species also suggests the division of the area downstream of the reactor orifice into two regions where distinct ionization-excitation effects take place. Propagation speeds of the species up to about 2 × 10⁵ m s⁻¹ were measured. Finally, qualitative correlation between the species propagation (as it is pronounced by the emission patterns) and the local electric field (as it was numerically calculated in a similar setup) is demonstrated.

This report gives prominence to the importance of the traditional Cockcroft-Walton voltage multiplier to the design of pulsed power supplies for dielectric barrier discharge (DBD) applications. Following an inventive concept, a simple Cockcroft-Walton generator is combined with a MOSFET-based switch (in either ``push'' or ``pull'' mode). Thus, the to-be-chopped stabilized dc high voltage is produced by a compact, transformer-less, modular unit, having high flexibility in terms of engineering. This approach leads to well-defined, square, high-voltage pulses of variable amplitude, frequency, and duty cycle. Design, implementation, and proof test of such a prototype are here demonstrated. The prototype yields square pulses as high as 10 kV (plateau value), as narrow as 350 ns (variable pulsewidth up to millisecond, depending on the frequency), rising or falling time close to 20 ns, pulse repetition rates up to 4 kHz, and output mean power up to 150 W. The functionality of the system is demonstrated by driving coaxial, ambient air DBDs of variable lengths, while principal electrical and optical parameters are recorded. Peak power values higher than 70 kW are measured on the DBD side, while the voltage multiplier power efficiency factor remains close to 95%.