Alexandros Gerakis

Dual-colour single-shot coherent Rayleigh scattering (CRS) is utilised to simultaneously measure the temperature and relative number density in a weakly ionised DC glow discharge. The discharge is produced using xenon gas at a pressure of 15 Torr. By analysing the lineshapes, both the translational temperature and relative number density of neutral xenon atoms are obtained. Measurements at the centre of the discharge are obtained as a function of discharge current. Additionally, spatially resolved measurements are performed across the discharge at an operating discharge current of 15 mA. The spatial results are compared with a one-dimensional radial fluid model of neutral gas flow informed by a 0D plasma global model of the glow discharge. The comparison shows good agreement, demonstrating the capability of CRS toward diagnostics for weakly ionised, low temperature plasmas.

Direct measurement of the particle velocity distribution function (VDF) of neutral gases and plasmas allows for their thermodynamic characterization (temperature, density, and velocity vector) and is of crucial importance for fields such as aeronautics, aerospace, microelectronics, etc. In recent years, the mechanical probes that have formed the state of the art — Pitot tubes, hot wires, Langmuir probes, etc. — are steadily being replaced by laser-based diagnostic techniques. Among the latter, single-shot coherent Rayleigh-Brillouin scattering (CRBS), a four-wave mixing technique, has been demonstrated to perform VDF characterization of atomic and molecular species within a single laser shot (∼ 100 ns). CRBS requires the use of highly specialized laser systems capable of generating laser beams with controllable chirps, variable pulse duration, controllable pulse shaping, and high pulse energies. However, the characterization of environments exhibiting fast flows, such as gas boundary layers, or low particle number densities, such as low-temperature glow discharges, have proven extremely challenging with current state-of-the-art laser systems mainly due to the need for pulse energies beyond 1 J. In this work, we report on the improvement of a custom nanosecond-pulsed laser system tailored for optical particle diagnostics and manipulation, and which is capable of pulse energies exceeding 2 Joule/pulse. In addition to the notable output energy, the laser system is capable of generating laser pulses with customizable temporal profiles and variable durations (10 ns to 1 μs), with a chirping range of several GHz over the pulse duration. To the best of the authors’ knowledge, this is the highest reported energy per pulse generated by a lab-scale frequency-chirped laser source at nanosecond timescales. The expanded output energy range is anticipated to greatly broaden the laser system’s application potential via CRBS for thermodynamic diagnostics of both supersonic/hypersonic gas flows in wind tunnel and shock-tube facilities, as well as plasma species in space-like conditions, e.g. ion thrusters.

A novel, to the best of our knowledge, laser diagnostic technique enabling simultaneous velocity measurements at different spatial locations is presented. Using single-shot coherent Rayleigh–Brillouin scattering (CRBS) and spatially splitting the signal beam, we show that distinct regions within the probe volume can be measured independently and simultaneously, thus enabling localized flow diagnostics at the micrometer scale. To demonstrate the new technique, measurements are performed in the flow of an under-expanded jet featuring sharp radial velocity gradients and microscale flow structures, which are probed through the spectroscopic signatures in two different signal beams. Velocity component extracted from the acquired spectra reveals flow-dependent signatures. The method presented here is inherently scalable through optical design modifications, offering a powerful, flexible diagnostic tool for detailed thermodynamic characterization of neutral gas and plasma flows in complex environments.

We demonstrate neutral gas-flow velocimetry by exploiting the optical dipole forces exerted by precisely tailored high-intensity laser fields on polarizable particles, in the form of single-shot coherent Rayleigh-Brillouin scattering (CRBS). This enables mapping of the velocity distribution function of the particles in a single laser shot of approximately 200 ns duration, allowing us to perform spectroscopic measurements in the time, rather than the frequency, domain. Importantly, CRBS results in a coherent signal beam, which renders the technique ideal for measurements in optically noisy environments. By correlating the time shift of the resulting spectrum to the Doppler shift caused by the bulk motion of particles, we perform neutral gas-flow velocimetry without the need to seed the flow, nondestructively, for a variety of gases, atomic or molecular. We demonstrate gas-flow velocimetry at flow velocities as low as 1 m s − 1 , with similar velocity resolution, while temperature and gas density can also be estimated from the same single-shot spectra. To prove the effectiveness of the measurement technique, we benchmark it against Pitot tube measurements, which show excellent agreement.

A numerical study on electron, ion, and neutral density perturbation is presented in low-temperature plasma (LTP) sources driven by the frequency detuning of two laser beams. Our study is performed in the hydrodynamic regime, which is valid when the plasma lattice period is larger than the Debye and mean free path lengths, and longitudinal plasma modes can be excited. Our attention is focused on a long-timescale in which ion acoustic waves could be excited when the frequency difference between the two laser beams approaches the value predicted by the ion acoustic wave dispersion relation. The density modulation induced by the laser beams leads to the formation of a periodic quasi-neutral plasma lattice, which causes periodic refractive index perturbations. The potential of the ion-acoustic wave resonance is discussed as a diagnostic tool for the precise measurement of plasma parameters in LTP sources through coherent scattering from the periodic index refraction perturbations induced in the plasma by the optical lattice.

Single-shot coherent Rayleigh-Brillouin scattering (CRBS) is modeled using direct simulation Monte Carlo (DSMC). In CRBS, an optical lattice generated by the interference of two pump lasers traps the neutral particles via the dipole force. The gradient of refractive index due to the trapped particles of the gaseous media leads to coherent scattering of a probe beam, resulting in the CRBS signal. Additionally, using a chirped laser beam, CRBS signals can be obtained in a single laser shot, shortening the measurement time of the gas flow (on the order of 100 ns). The DSMC results of single-shot CRBS assuming Maxwellian velocity distribution functions (VDFs) show good agreement with experimental data, including argon, carbon dioxide, and xenon, for different gas pressures. One of the key observations obtained from the present simulations is that the CRBS signals become asymmetric when using a fast chirp rate due to finite time for collisionless trapping and collisional thermalization. Published by the American Physical Society 2024

We present a numerical study on the electron and ion density perturbation in low-temperature plasmas driven by the frequency detuning of two intense laser beams. Our study is performed in the hydrodynamic regime, which becomes applicable when the plasma grating period induced by the beating of the laser beams is greater than the Debye length and collective processes such as plasma oscillations can be excited. Our findings show a resonance in electron density perturbation as the frequency detuning approaches a value consistent with the Bohm–Gross dispersion relation in low- and high-pressure plasmas. We discuss the potential of this resonance as a diagnostic tool for precisely measuring electron temperature and density in low-temperature plasmas through coherent scattering.

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.