Zhili Zhang

In situ diagnostics for defect detection and compositional assessment remain critical for addressing persistent challenges in Additive Manufacturing (AM). Among the available diagnostic techniques, Optical Emission Spectroscopy (OES) stands out for its strong capability to provide in situ and non-intrusive monitoring of the AM process. As part of our ongoing efforts to develop a low-cost, real-time AM monitoring tool, this study represents an initial investigation focused on the concentration of key species in the Tungsten Inert Gas (TIG) wire-arc plasma generated during welding of an aluminum (Al-6061-T6) sheet using OES. Emission spectra from the arc plasma were analysed to identify and quantify key spectral lines associated with aluminum and potential alloying elements. The plasma temperature and electron number density were estimated to support quantitative analysis through Boltzmann and Saha methods to enable accurate concentration determination by calibration-free approach. The results demonstrated the potential of OES as an effective tool for monitoring and optimizing the aluminum welding operations through in situ non-intrusive plasma composition analysis.

The detection of naturally occurring xenon (129Xe) in ambient air was demonstrated using the magnetically induced depolarization of Radar REMPI (coherent microwave Rayleigh scattering from Resonance-Enhanced Multi-Photon Ionization). The study employs a 2 + 1 REMPI scheme, where two photons (222–226 nm) from a tunable narrow-linewidth laser excite a xenon atom to resonant intermediate states (6p′[1/2], 6p′[3/2], 7p[1/2], 7p[3/2], and 7p[5/2]), followed by the absorption of a third photon to ionize the excited atom. Magnetically induced depolarization of microwave scattering was applied to enhance the sensitivity by a factor of ∼3. The time-dependent quantum perturbation theory was used to calculate the multiphoton absorption cross section of xenon near 222–226 nm region, which qualitatively agrees with the experimental results. This study enables the identification and quantification of naturally occurring xenon in air, demonstrating detection sensitivity in the tens of parts per billion (ppb) range.

A one-dimensional (1D) thermometry using oxygen-tagging resonantly ionized photoelectron thermometry (O₂ RIPT) was employed to investigate thermal gradients within a Mach 4 Ludwieg tube. The Ludwieg tube is pulsed with a test duration of approximately 100 ms, providing a cold supersonic flow at Mach 4 ideal for studying aerothermal effects. This study focused on measuring freestream temperatures, capturing shock-induced heating behind a detached bow shock from a blunt cylinder, and resolving sharp temperature variations across a bow shock generated by a cylinder. The O₂ RIPT technique produced strong emission signals extending approximately 4 cm long, demonstrating its capability for precise temperature measurements in high-speed wind tunnel environments. The results confirm that O₂ RIPT is well-suited for applications in large-scale aerodynamic testing facilities, particularly in regions with strong compression effects, enabling the resolution of sharp thermal gradients. This method presents a promising solution for thermometry in dynamic flow conditions relevant to various experimental ground-test facilities.

This paper presents an extensive parameter study of a non-intrusive and non-seeded laser diagnostic method for measuring one dimensional (1D) rotational temperature of molecular nitrogen (N₂) at 165 - 450 K. Compared to previous efforts using molecular oxygen, here resonantly ionized and photoelectron induced fluorescence of molecular nitrogen for thermometry (N₂ RIPT) was demonstrated. The RIPT signal is generated by directly probing various rotational levels within the rovibrational absorption band of N₂, corresponding to the 3-photon transition of N₂ (X¹Σ _g ⁺,v=0→b¹Π _u ,v^′=6) near 285 nm, without involving collisional effects of molecular oxygen and nitrogen. The photoionized N₂ produces strong first negative band of N₂⁺ (B²Σ _u ⁺−X²Σ _g ⁺) near 390 nm, 420 nm, and 425 nm. Boltzmann analyses of various discrete fluorescence emission lines yield rotational temperatures of molecular nitrogen. By empirically choosing multiple rotational levels within the absorption band, non-scanning thermometry can be accurately achieved for molecular nitrogen. It is demonstrated that the N₂ RIPT technique can measure 1D temperature profile up to ∼5 cm in length within a pure N₂ environment. Multiple wavelengths are thoroughly analyzed and listed that are accurate for RIPT for various temperature ranges.

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.

To fulfill the increasing needs of diagnostic support for researchers in plasma technology, a portable diagnostic package (PDP) equipped for both laser Thomson scattering (TS) and optical emission spectroscopy has been designed and constructed at Oak Ridge National Laboratory (ORNL), aiming to measure the temperature and number density of electrons and temperatures of ions in plasma devices. The PDP has been initially implemented on a high density and low temperature electrothermal arc source (ET-arc) at ORNL to test its TS capability. TS from the plasmas in the ET-arc has been obtained using the PDP. The electron temperature and number density were determined from TS spectra. These results were then compared to measurements from previous studies on the ET-arc. The TS diagnostic measured 0.8 ± 0.1, 1.3 ± 0.2, and 0.7 ± 0.1 eV and (4.4 ± 0.5) × 1021, (5.9 ± 0.7) × 1021, and (4.3 ± 0.5) x 1021 m-3, respectively, from three lines of sight that transect the plasma column.