Arthur Dogariu

Using only a pulsed laser and an imaging system, the molecular-based optical diagnostic of femtosecond laser electronic excitation tagging (FLEET) provides a simple and yet powerful tool for monitoring and quantifying high-speed flow dynamics, without requiring the introduction of particles or foreign species into the flow. FLEET is typically operated for measurements at continuous repetition rates up to 1 kHz, limited by the trade-off between pulse energy and repetition rate of commercially available Ti:sapphire femtosecond lasers delivering millijoule pulse energies at kHz rate. Higher temporal sampling, up to the MHz range, has only been achieved using burst-lasers or high-repetition rate imaging systems, albeit with an inherently discrete temporal coverage at the much lower burst repetition rate. Herein, using a Ludwieg tube facility as a benchmark case, we demonstrate 100 kHz continuous repetition rate FLEET velocimetry in supersonic nitrogen flow across the entire operating envelope. This capability provides a two order of magnitude increase in temporal sampling of flow dynamics over previous reports. The microjoule-range pulse energy delivered by the 100 kHz optical source was efficiently harvested for generating and tracking tracer molecules in the flow and high precision velocimetry, by using higher-energy photons, thereby avoiding plasma filamentation and preserving the non-intrusive nature of measurements. The comparison with 500 Hz measurements performed under the same tunnel operating conditions highlights the benefit of the higher temporal sampling in capturing the steady and unsteady flow dynamics throughout the wind tunnel operating time of ≈ 700 ms , producing unique experimental data for Ludwieg tube characterization. The substantial enhancement of FLEET flow velocimetry capability paves the way for high-resolution dynamic measurements of turbulence, shock-waves, and boundary-layer transitions, providing insight into the fundamental physics and computational models validation, crucial for many aerospace applications.

In this work, spatially and temporally resolved atomic hydrogen profiles were quantified using femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF) at the plasma-biodiesel interface, at low pressure and room temperature. A pronounced near-surface hydrogen depletion was observed, corresponding to an incident H-atom flux of 1.9 × 10 ¹⁸ cm ^-2 s ^-1 , approximately four orders of magnitude higher than molecular hydrogen fluxes typical of thermal catalytic hydrogenation. Within 2 h of plasma processing, the H-atom density gradually recovered to the no-fuel baseline, indicating progressive elimination of the surface C=C bonds. Ex situ gas chromatography mass spectroscopy (GC-MS) showed that the saturated component such as octadecanoic acid (C18:0) increased from 39% to 86%. With Ni catalyst addition, near-surface H density was further decreased by 50% and remained depleted throughout the plasma processing. Kinetic modeling reveals that non-equilibrium hydrogenation proceeds via two distinct regimes: a surface-limited, self-terminating process in the plasma-only case, and a persistent first-order, H-flux-controlled process in the plasma-catalysis case. These findings establish a quantitative mechanistic link between plasma-generated H flux and hydrogenation kinetics, offering fundamental understanding of upgrading biofuels with renewable electricity and electricity-derived hydrogen.

Solute transfer from the liquid to the gas phase in plasmas has been widely used in analytical chemistry often focused on liquid cathode discharges. In this study, we report on time-resolved absolute density measurements of sodium in a glow discharge plasma with a NaCl solution anode using two-photon absorption laser induced fluorescence (TaLIF). The averaged maximum sodium density is found to be similar to the electron density in the discharge consistent with plasmainduced vapor generation. Random sodium flashes were also observed extending into the plasma afterglow and are attributed to sodium particles falling back on the liquid surface. Cataphoresis can explain the location of the maximum sodium density near the cathode side of the electrode gap rather than near the liquid anode region.

This article presents the design and detailed characterization of a new supersonic wind tunnel at the Aerospace Laboratory for Lasers, ElectroMagnetics, and Optics of Texas A&M University, tailored for optical diagnostic development and sub-scale fundamental compressible fluid dynamics research. A Ludwieg tube tunnel architecture was selected due to its robustness, versatility, and low operational costs. The tunnel consists of a 50-foot-long driver tube constructed from modular Tri-Clamp spools, a Mach 4 nozzle with 3 in. exit diameter configured as a free jet, and a fast-acting valve with 14 ms opening time for high-duty-cycle operation. Such construction proved to be a robust, compact, and affordable solution for academic applications. Characterization methods consisted of simultaneous high-speed dot-schlieren, total and static pressure measurements, and femtosecond laser electronic excitation tagging. Average flow velocity for the first steady-state test time was measured via FLEET at (668.0 ± 5.7) m/s. The Mach number was calculated based on the angles of the attached oblique shocks formed near the 30° cone model. Calculated Mach number was repeatable from run to run and had small oscillations near the average value of 3.96 ± 0.03. Based on the simultaneously measured velocity and Mach number, the static temperature was calculated to be between (68.6 ± 0.3) K and (66.3 ± 0.3) K throughout the 400 ms test time, completely defining the thermodynamic state of the generated freestream flow.

Single-pulse pin-to-pin nanosecond pulsed discharges in ambient air (gap distances of 3–7 mm) were investigated at high pulse energies (∼20–30 mJ deposited energy per 11 ns full width at half maximum pulse). A Michelson interferometry setup (532 nm continuous-wave laser) was employed to record time-resolved interferograms of the discharge, enabling spatially resolved calculation of the electron number density. An intensified charge-coupled device camera was used to capture the spatiotemporal evolution of the discharge (streamer formation and spark channel development), and Coherent Anti-Stokes Raman Spectroscopy (CARS) was used to measure post-discharge N2 vibrational temperatures. Discharge current and voltage were monitored with a back-current shunt. The discharge was initiated with simultaneous cathode-directed and anode-directed streamers that bridge the gap within ∼1 ns and form a luminous, filamentary plasma channel immediately after breakdown. Laser interferometry measurements showed peak electron number densities of the order of 1017–1018 cm−3, occurring about 15–20 ns after pulse arrival at the discharge gap. Shorter gap discharges yielded higher peak electron densities, consistent with the higher energy density in the smaller gaps. Spatially, the electron density was highest near the electrodes and decreased toward the midgap region, with 5 and 7 mm gaps exhibiting a pronounced drop in the central channel. CARS indicated initial vibrational temperatures of approximately 4000–6000 K in the spark core of ∼50 ns after the discharge onset, decaying on a ∼500 ns timescale as the plasma cooled and recombined.

Initial plasma densification by odd-parity rotating magnetic fields (RMFo) applied to the linear magnetized Princeton field-reversed configuration (PFRC-2) device with fill gases at pressures near 1 mTorr proceeds through two phases: a slow one, characterized by a rise time τslow∼100μs, followed by a fast one, characterized by τfast∼10μs. The transition from slow to fast occurs at a line-integral-averaged electron density, tne, near 2×1011 cm−3, independent of magnetic field. Over most of the range of experimental parameters investigated, as the PFRC-2 axial magnetic field strength was increased, RMFo power decreased, gas fill pressure lowered, or lower atomic mass unit (AMU) fill gas used, the duration of the slow phase lengthened from 50 μs to longer than 10 ms after the RMFo power began. The post-fast-phase maximum ne increases with the fill-gas AMU, exceeding 5 × 1013 cm−3 for Ar. The slow phase is consistent with atomic physics processes and field-parallel sound-speed losses. The fast phase may be explained by improved axial confinement, possibly augmented by radial or axial contraction of the plasma. Another possible explanation, a large increase in electron temperature, is inconsistent with x-ray emission. The ne behavior is discussed in relation to the E to H transition.

The frame rate of conventional high-speed imaging devices is limited by exposure time and signal read-out time. Structured imaging is able to push the frame rate beyond the limit of the imaging device. This work demonstrates the feasibility of potential TeraHz rate structured imaging with a multiplexed structured imaging setup. In this work, a multiplexed structured image capture system was employed to image UV laser induced krypton-II emission lines at 10 GHz. The measured emission intensity of the 764 nm line over time suggests a temporal resolution of 97 ps. The temporal resolution is adjustable, and the frame rate can be further increased to the TeraHz level.