Dimitrios Stefas

Dual-frequency tailored voltage waveforms at MHz range are applied to an atmospheric-pressure plasma jet source operating in He:O2 mixture to enhance Atomic Oxygen (AO) generation. In previous work, this approach was implemented to increase AO number density by a factor of 1.5 – 3 in the discharge region, with limited gas heating. In this further study, AO number density in the effluent region is quantified using picosecond two-photon absorption laser-induced fluorescence (ps-TALIF). Effects of oxygen concentration, operational power and harmonic phase shift are systematically investigated and coupled with a 1-D plasma model and a 2-D axisymmetric computational fluid dynamics simulation. Both experimental and numerical results reveal a pronounced Geometric Asymmetry Effect (GAE), which promotes AO population in the jet center. By superimposing a harmonic component onto the driving waveform with a relative phase of θ = 270°, an Electric Asymmetry Effect (EAE) is induced via modification of the sheath dynamics. This leads to an overall enhancement of AO density in the effluent, particularly in the far field. In contrast, shifting the harmonic phase to θ = 90° reverses the EAE, thereby limiting AO generation. Compared with the phase-modulation effects on plasma source, the changes in the effluent region are more limited due to the interaction between the plasma jet and the entrained ambient air. This study demonstrates effective waveform-based control of AO production and transport in atmospheric-pressure plasma jets, providing new insights into processing of heat-sensitive materials with plasma jets.

Accurate gas temperature (T Gas ) determination in microplasmas is critical for optimizing their applications, yet isolated diagnostic approaches may yield misleading results, especially under strong non-equilibrium conditions. Here, high resolution rotational spectra of N 2 (C), OH(A), NH(A) and NO(A), generated in the plasma jet of a DC Ar/N 2 microhollow cathode discharge (MHCD), are recorded and their associated rotational temperatures (T rot ) are cross compared. A detailed experimental analysis and robust fitting of the rotational spectra are performed, achieving a reliable estimation of T Gas . The dominant formation mechanisms of these species and their corresponding impact on rotational population distributions are also interrogated. Particularly, our findings indicate that the T rot of N 2 (C) is significantly influenced by energy transfers from argon metastables (Ar m ) and spectral interference from NH(A). This makes it unreliable as a thermometric probe in Ar-rich MHCD, unless complex analyses are employed. In contrast, OH(A) rotational population distribution appears to be less sensitive to Ar-induced perturbations across various discharge currents and pressures, providing more straightforward results. For all molecules considered, this study reveals the MHCD conditions under which all the measured T rot can be reliably considered to be in equilibrium with T Gas . This highlights the importance of cross-validating multiple thermometric probes and investigating relevant excitation kinetics when measuring T rot in reactive microplasmas.

Picosecond two-photon absorption laser induced fluorescence (ps–TALIF) is combined with a streak camera (∼1 ps highest temporal resolution) to probe the effective lifetime ( τ eff H ( n = 3 ) ) and absolute density ( N _H ) of atomic hydrogen in the effluent of a helium atmospheric-pressure microplasma jet ( μ APPJ). This approach allows for improved temporal resolution compared to conventional nanosecond diagnostics, enabling measurements of τ eff H ( n = 3 ) as small as 50 ps, the corresponding N _H reaching down to 10 ¹⁴ cm ⁻³ . Atomic densities are calibrated by performing identical ps–TALIF measurements in krypton gas contained in a custom-built low-pressure cuvette. Physical mechanisms that may be involved in the TALIF scheme such as photoionisation (PIN) and/or stimulated emission (SE) are also assessed, ensuring reliable studies. The determined τ eff H ( n = 3 ) and N _H increase with the helium flow rate ( Q _He ) in the range Q _He = 0.3–1 slm. Both quantities are maximised near the exit nozzle of the μ APPJ, obtaining values below 400 ps and 6 × 10 ¹⁴ cm ⁻³ , respectively. As the axial distance increases, τ eff H ( n = 3 ) declines with a rate of ≈ 65 ps mm ⁻¹ for Q _He = 1 slm which is about 3 times smaller than for Q _He = 0.3 slm. These findings reveal a strong correlation between the experimentally-measured τ eff H ( n = 3 ) and the local air entrainment into the jet as indicated by their comparison with calculated effective lifetimes based on published quenching rates. Furthermore, operating the μ APPJ in the burst mode allows for the estimation of the residence time ( t _res ) of ground state H–atoms in the helium gas channel, which is larger for Q _He = 0.3 slm ( t _res ≈ 1.2 ms) compared to Q _He = 1 slm ( t _res ≈ 0.5 ms). H-atoms consumption in the gas channel can be affected by diffusion and mechanisms involving neutral ground-state and metastable species among others. Furthermore, based on an error propagation analysis, density uncertainty as high as 64% (depending on the operating condition) is revealed. This mainly originates in the ratio of the two-photon absorption cross sections of Kr and H atoms ( σ ( 2 ) Kr / σ ( 2 ) H ) which has not yet been measured in the picosecond regime. Nevertheless, the combined utilisation of ps–TALIF and streak camera diagnostics demonstrates high sensitivity and temporal resolution for directly probing key reactive species in atmospheric pressure microplasmas.

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.

Boron nitride (BN) is deposited on a Si 〈100〉 wafer (≈20 cm2) via plasma enhanced chemical vapor deposition (PECVD) using a ns-pulsed N2/Ar micro hollow cathode discharge (MHCD) as a microplasma source. Aluminum nitride (AIN) is employed as the dielectric material in the MHCD instead of more conventional Al2O3 to exclude a potential source of film contamination by atomic oxygen. For the first time, in situ diagnostics such as optical emission spectroscopy and intensified CCD imaging are used to monitor plasma properties during BN synthesis, including species emission profiles, gas temperature, and discharge morphology. Furthermore, a comprehensive ex situ multi-diagnostics approach is followed to characterize the films, including Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray photoelectron microscopy (XPS), and transmission electron microscopy coupled to energy dispersive x-ray spectroscopy (EDX). Raman spectra reveal the E2g phonon mode of BN around 1366 cm−1 (representative of hexagonal, rhombohedral, or turbostratic BN polytypes), confirming successful synthesis. Further analysis using selected area electron diffraction reveals diffraction rings, characteristic of BN films with a turbostratic structure. SEM imaging reveals an almost complete surface coverage by the film, with localized delamination though. This is probably due to an uneven resistive heating of the Si wafer, rapid post-deposition cooling (∼13 K/min), and ambient exposure. AFM analysis indicates an average thickness of about 33 nm after 90 min of deposition (∼22 nm/h deposition rate). XPS measurements reveal an average B/N atomic ratio of ∼1.5 along the wafer diameter. EDX analyses confirm the dominant presence of B- and N-atoms in the film, in fair agreement with XPS. Deviations from ideal film properties (e.g., stoichiometric unity and uniform morphology) are attributed to plasma-induced inhomogeneities (such as non-uniform species flux and temperature gradients) among other factors (e.g., oxygen impurities in the chamber and ambient exposure post-deposition), which affect nitrogen and boron incorporation and localized film properties. Despite these challenges, the MHCD-driven PECVD process demonstrates strong potential toward h-BN synthesis, with further optimization of the reactor design, plasma conditions, and gas chemistry required to grow ideal films.

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.

In this work, we present a straightforward approach to retrieve gas temperatures directly from the effective lifetime τH of the ps-laser excited species measured with picosecond Two Photon Absorption Laser Induced Fluorescence (ps-TALIF). This approach allows for simultaneous measurements of absolute atom densities and gas temperatures performed in moderate-pressure reactive plasmas using an advanced ps-TALIF diagnostic and has been demonstrated for highly collisional hydrogen microplasmas. Furthermore, this methodology was applied to obtain a 2D distribution of gas temperature and H-atom densities in the downstream region of the microplasma discharge.