Junhwi Bak

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.

While Langmuir probes (LPs) are relatively simple and inexpensive plasma diagnostics for the electron density, temperature, and the electron energy distribution function (EEDF), the interpretation of the measured current–voltage (I–V) characteristic is complicated considerably by the presence of a magnetic field. In regimes where the electron gyroradius is comparable to the probe radius, the electron flux to the probe surface is retarded by reduced mobility across field lines and can no longer be described by a thermal model. Predicting the current collected by the probe in these regimes requires accurate estimates of the plasma diffusion coefficients, which are usually difficult to obtain. In this work, we measure electron energy distribution functions in E×B plasmas with magnetized electrons and non-magnetized ions in argon and krypton gases, using both a LP and laser Thomson scattering (LTS) at various magnetic fields. Using the LTS measurements to provide a robust benchmark for comparison, we compare existing theories describing the flux to the probe under magnetized conditions. We find that even when the electron gyroradius associated with the effective electron temperature is small compared to the probe radius, the EEDF computed using classical probe theory is still robust at energies higher than the energy at which the gyroradius becomes larger than the probe size. For plasmas that are not strongly non-Maxwellian, we formulate a method to extract robust density and temperature measurements using physics-informed fitting techniques to analyze EEDFs computed using classical theory.

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.

In this work, the decay of femtosecond laser-induced plasmas has been explored through both experimental and numerical studies. Understanding the plasma decay in different gas compositions at varying background pressures is crucial in order to establish the capability of localized control of laser energy addition that ultimately can enable various applications. This study investigates the decay dynamics of femtosecond laser-induced plasmas in a varying N2–O2 mixture at varying pressures, with a focus on the role of oxygen in the plasma life and optimization of dual-pulse energy deposition of a second, nanosecond laser pulse. Experimental and simulation analyses show how the presence of oxygen affects the dual-pulse laser plasma decay. Using the self-consistent three-temperature plasma kinetic solver, the key reactions that dominate the temporal behavior of plasma are identified, such as recombination with N4+, electron attachment with O2, associative ionization producing N2+ or NO+, and photo-detachment. The findings are expected to contribute to a deeper understanding of plasma kinetics in both N2-only and N2–O2 environments at various pressures and be of value for advancing the understanding of laser-induced plasmas, optimizing localized energy addition, and advancing low-temperature plasma applications.

Laser Thomson scattering, which provides thermal properties of electrons, has been extensively utilized in plasma diagnostics. However, being a non-resonant linear light scattering technique, it often encounters several challenges in weakly-ionized plasma diagnostics due to spectral overlap with different scatterings induced from other existing species, including Mie, Rayleigh, and rotational Raman scatterings. To address this challenge, we propose an imaging spectroscopy technique, polarization-separated double-imaging spectroscopy (PoDIS), which selectively separates specific scatterings based on their polarization characteristics. Using an atmospheric plasma jet as a plasma source for demonstration, we show that PoDIS can effectively separate rotational Raman and Thomson scatterings from a superimposed spectrum without prior knowledge or assumptions about the thermal properties of neutrals or electrons. This separation enables independent fitting of the rotational Raman and Thomson scattering spectra, allowing for precise determination of the thermal properties of neutral particles and electrons separately.

Laser Thomson scattering, which provide thermal properties of electrons, has been extensively utilized in plasma diagnostics. However, being a non-resonant linear light scattering technique, it often encounters several challenges in weakly-ionzed plasma diagnostics due to spectral overlap with different scatterings induced from other existing species, including Mie, Rayleigh, and rotational Raman scatterings. To address this challenge, we propose a novel imaging spectroscopy technique, Polarization-separated Double-Imaging Spectroscopy (PoDIS), which selectively separates specific scatterings based on their polarization characteristics. Using an atmospheric plasma jet as a plasma source for demonstration, we show that PoDIS can effectively separate rotational Raman and Thomson scatterings from a superimposed spectrum without prior knowledge or assumptions about the thermal properties of neutrals or electrons. This separation enables independent fitting of the rotational Raman and Thomson scattering spectra, allowing for precise determination of the thermal properties of neutral particles and electrons separately.

Anomalous electron transport in Hall thrusters is traditionally viewed as detrimental to thruster performance due to the power consumption associated with electron back-flow. The electron back-flow can significantly increase when the magnetic barrier near the acceleration region is deteriorated. However, enhanced cross-field transport in the near-anode or plume region, which contributes less to the total resistivity across the space, can potentially control thruster operation while minimizing known detrimental effects. In this work, we explore how enhanced cross-field transport in such regions affects various aspects of Hall thruster operation and explore the potential applications of artificially enhanced cross-field transport using a radial-axial 2D hybrid particle-in-cell simulation. We report that, in certain cases, even though electron back-flow increases, induced changes in plasma formation and ion acceleration due to manipulated anomalous transport can occur favorably, overall improving thruster performance. This finding motivates further comprehensive studies focused on the control of cross-field electron transport.

Spontaneous Raman scattering, also known as common old ordinary Raman scattering (COORS), is revisited to evaluate its applicability for hypersonic flow characterization. Due to its very low cross section, Raman scattering is often considered unsuitable for measuring low-pressure gas properties that are found in ground test simulations of high-altitude hypersonic flights. Utilizing a recently developed one-dimensional (1D) light scattering technique with a volume Bragg grating filter and Stokes sideband windowing, we demonstrate 1D rotational Raman measurements of temperature and neutral gas density across a bow shock in front of a blunt wedge model under Mach 6 hypersonic flow. The experiment was conducted in the Hypervelocity Expansion Tunnel at Texas A&M University. The measurements were successfully obtained during a single run of the tunnel operation, capturing the temperature and density distributions with dynamic ranges of 200–2000 K and 5×10²³−4×10²⁴/m³, respectively, over both the free-stream and post-shock regions, covering approximately 10 mm in length with a spatial resolution of <0.5mm. Time-resolved high-speed measurement capability at 100 kHz was also demonstrated, showcasing the robustness of 1D COORS for gas diagnostics.