Abstract We estimate the two-temperature generalized electron energy distribution function (GEEDF) of an atmospheric-pressure nonequilibrium argon plasma (APNEP) from optical emission line spectra alone—a fundamentally ill-posed inverse problem—and quantify which GEEDF parameters the line spectra determine, which they only bound, and which they leave undetermined. Two ingredients are combined. First, a 69-level Ar collisional-radiative model (CRM) is refined for atmospheric pressure with the heavy-particle kinetics that the classical low-pressure models omit: a re-evaluated Einstein- A database (NIST values on the Katsonis–Drawin base with electric-dipole selection rules enforced), a closed three-species ion chemistry ( Ar + , Ar 2 + , impurity ions) with n eff − 3 -weighted dissociative recombination of Ar 2 + , 4p excitation transfer, Hornbeck–Molnar ionization, and H 2 O /air quenching. Second, an ‘undetected-line penalty’ (ULP) turns every high-lying Rydberg line that falls below the noise floor into a one-sided upper bound on its upper-level population. A four-case GPU grid search shows that neither the penalty alone nor the refined CRM alone is self-consistent; only their combination is. Because the detected lines leave a cold and a hot bulk branch nearly degenerate, the reported cold-branch solution is selected by an explicit rule anchored to the continuum electron temperature of the same device, giving bulk temperatures T e 1 ≈ 0.2 – 0.8 eV. An identifiability analysis shows the ULP to be strictly one-sided: it bounds the Ar 2 + recombination feed—and thereby the absolute electron density—from above, and excludes only tails whose high-energy content exceeds the detection bound, leaving the individual tail parameters undetermined. The recovered densities ( n e ∼ 10 12 – 10 13 cm − 3 ) are effective values averaged over the observation chord, the drive cycle, and the discharge filaments; they agree with the continuum-based densities of the same device at low voltage but disagree by a factor growing from ≈ 4 to ≈ 50 across the voltage range, a systematic we attribute to the recombination model and to the line and continuum emission being weighted toward different discharge phases. The method thus converts the absence of lines into quantitative exclusion bounds and delineates what line-only OES can and cannot determine from time-averaged spectra of such discharges.
Abstract The spatiotemporal evolution of nanosecond laser-produced Sn plasma was investigated under N 2 pressures from 10 −3 to 10 4 Pa using ICCD imaging and time- and space-resolved optical emission spectroscopy. The plasma evolution exhibits an early “fast” process (<100 ns) and a later “slow” process (>100 ns)(10-10 3 Pa). The “fast” process is characterized by continuum radiation and spatially extended N 2 /N 2 + emissions reaching several centimeters, likely arising from VUV/EUV-induced photoionization of ambient N 2 and subsequent collisional excitation by photoelectrons and secondary electrons. The “slow” process is dominated by sequential emissions from Sn ions and neutral atoms. Spatially resolved spectra reveal clear charge-state-dependent separation(<~50 Pa), with higher-charge-state ions located closer to the plume front and lower-charge-state ions and neutral atoms distributed farther behind. Self-electrostatic acceleration and pressure-dependent collisional and recombination processes are discussed as possible contributors to this ordering. Electron density (n e ) and electron temperature (T e ) were determined by Stark broadening and a spectral matching algorithm(SMA), respectively. Within the measured time delay range, ne decreases from approximately 3.0×10 25 to 4.0×10 23 m −3 , while Te decreases from approximately 3.2 to 1.0 eV. Te shows an apparent transition around 100 ns, corresponding to the fast-to-slow transition, whereas the late-stage decay of ne is strongly regulated by pressure-induced collisional confinement. These results provide a pressure-resolved picture of early background-gas activation and charge-state-dependent transport in laser-produced Sn plasma.
Abstract Dielectric Barrier Discharge Plasma Actuator (DBD-PA) generates Electrohydrodynamic (EHD) force via the interaction between an electric field and charged particles. Linear Plasma Synthetic Jet Actuator (LPSJA) utilizes opposing DBD-PA to induce a vertical jet through the collision of induced flows; however, the effect of the electrode distance (λ) on its performance remains unclear. Clarifying this relationship requires investigating the electric field distribution because it directly drives the EHD force. In this study, we evaluated the electrical and fluid dynamic characteristics of the LPSJA using Pockels measurement and Particle Image Velocimetry (PIV). Our results demonstrate that reducing λ intensifies the local electric field due to the accumulation of surface charges in preceding discharge phase, which promotes initial EHD force generation. Nevertheless, this localized enhancement does not directly correlate with the global flow velocity. At small λ, the output is significantly restricted by complex fluid dynamic effects, particularly the increased pressure gradient from colliding ionic winds. We characterize the LPSJA performance as a result of the competition between electrical EHD force promotion and fluid dynamic losses. This study provides a practical basis for optimizing Plasma Synthetic Jet Actuator configurations, facilitating more effective design strategies that account for the complex interaction between electrical and fluid dynamic factors.
Abstract We report a tenfold increase in H atom ground state density from water vapour dissociation in the burst regime of an atmospheric He-based ns-discharge. The density increases gradually in the burst, reaching 10 16 cm − 3 for a water admixture of 0.07%, as measured in a 7-pulse burst by calibrated two-photon absorption laser-induced fluorescence. The energy cost per produced H atom drops from about 190 eV in the single-pulse regime to about 50 eV in the 7-pulse burst, as deduced from the conductive current extracted from the discharge waveforms. Both fast and slow growths in H atom density are observed, especially when the burst pulse separation time is short. The step-like fast increase in each plasma pulse is attributed to a direct electron impact dissociation of water molecules, as Stark spectroscopy reveals a similar growth in electron density. The increase of the electric field in each subsequent burst pulse, as deduced from the He 447 nm line profiles, suggests a facilitated electron-based chemistry due to enhanced pre-ionisation and streamer propagation in the discharge gap. At the same time, the slow H density increase observed in each afterglow likely results from the presence of vibrationally excited species, also facilitating water vapour dissociation. Neither mechanism may be attributed to He metastables, as follows from laser-collisions-induced fluorescence measurements showing that the He 3 S 1 metastable state gets quenched within 100 ns after each plasma pulse. Rotational spectroscopy performed on the OH (A 2 Σ + ) state additionally reveals that the gas heating along the burst remains moderate ( ≈ 80 K), the N 2 + (B) state being likely rotationally over-excited in this regime.
Abstract A 2D hybrid (particle-in-cell/fluid) model has been enhanced to analyze the time-dependent coupling of the RF emission with the 2D plasma discharge in a cylindrical radio frequency ion thruster. Electromagnetic fields are described with radial Maxwell's equations, assuming a purely axial magnetic field induced by the external coil and plasma currents. The full time-dependent electron momentum equation is integrated. The ion response is quasi-stationary while the electron current density is highly time-dependent and develops several harmonics of the exciting frequency. Phase shifts between the electromagnetic fields and the azimuthal electron current change much with the temporal derivatives of the electron current and the inclusion of the plasma-induced magnetic field. These two features lead to a significant time-averaged cross-field magnetic confinement even in the absence of a time-averaged magnetic field, which improves the thruster performance appreciably.
