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Description
Background
The Electric Arc Shock Tube (EAST) at NASA Ames Research Center produces incident shock waves for validation and improvement of non-equilibrium chemical kinetics and radiation models relevant to atmospheric entry flows. The main experimental technique employs four spectrometers, capable of measuring spatially resolved emission spectra of the radiating gas from the vacuum ultraviolet to the mid-infrared wavelength range [1]. However, emission measurements provide information about excited states, and the achievable spatio-temporal resolution is constrained by the camera gating time and repetition rate. In contrast, Scanned Laser Absorption Spectroscopy (LAS) has been concurrently employed in recent test series, allowing detection of low-lying states of atoms and molecules with high spectral resolution and scanning rates of up to 1 MHz [2]. Yet, its application is limited to a narrow wavelength range, typically spanning only a few absorption lines. The present study investigates different approaches for broadband absorption measurements in EAST, targeting molecular and atomic transitions spanning the ultraviolet to near-infrared wavelength ranges, using different light sources and detector configurations. The objective is to extend diagnostic access to ground and low-lying states over a broad spectral domain.
Methodology
In the ultraviolet wavelength range, a laser-driven light source (LDSL-EQ1500, Energetiq Technology, USA) provides broadband continuous emission, peaking between 180 and 400 nm. The light is directed into the shock tube through a collimating lens and a turning mirror. In the visible to near-infrared range, a supercontinuum laser source (SuperK Fianium FIU-6) delivers pulses of approximately 0.85 ns duration at a repetition rate of 78 MHz, with a tunable output spanning 400–900 nm over a selectable bandwidth of up to 100 nm, thereby providing high-intensity illumination across the visible and NIR spectral regions. Owing to fluctuations in the laser output, the beam is split prior to entering the tube in order to simultaneously record a reference signal.
For both light sources, dedicated collection optics are used to capture the transmitted signal and couple it into a spectrometer (SP2300 or SP2500, Acton) through optical fibers. In the ultraviolet range, an intensified detector (PIMAX4, Teledyne Technologies) is operated in kinetic acquisition mode to record absorption spectra with a temporal resolution of 25 μs. In the visible range, a high-speed camera simultaneously records the absorbed and reference laser signals at acquisition rates up to 100 kHz. Due to the lower spectral power density of the UV source, acquisition of a complementary emission signal is obtained by a similar set of optics when required, and subtracted from the transmitted signal.
Additionally, a fiber-coupled MEMS-VCSEL source (Thorlabs SLZ1002) is employed to generate a swept-wavelength laser centered at 1060 nm, with a tuning range of 33 nm and sweep rates reaching up to 1 MHz.
The transmitted signal is measured using a balanced photodetector with a 1 GHz bandwidth and acquired with a 20 GHz oscilloscope.
Preliminary results
The diagnostic techniques are applied in two EAST experimental campaigns, namely Test 65B and Test 68. Test 65B focused on the characterization of CN radiation under incident shock conditions representative of Titan entry backshell radiative heating, employing a Titan-like mixture consisting of 2.2% $CH_4$ in $N2$ in the Low Density Shock Tube (LDST). Test 68 primarily aims at reproducing previously investigated air conditions in the LDST in order to achieve improved signal-to-noise ratios and extended test times.
The UV broadband absorption spectroscopy technique, using measurements acquired during Test 65B, is demonstrated here through observations of the $\mathrm{CN}(X^{2}\Sigma^{+} \rightarrow B^{2}\Sigma^{+})\ \Delta v = 0 \text{ and } \Delta v = +1\ $ band sequences. Due to the significant emission contribution under these conditions, single-frame acquisitions were employed to simultaneously capture both emission and absorption signals, while the measurement location was selected to maximize absorbance and minimize emission contamination. Although the relative contribution of emission with respect to absorption increased with both fill pressure and shock velocity, absorbance signals were still obtained after emission subtraction, even under less favorable conditions. At higher shock velocities, however, the measurements exhibited reduced signal-to-noise ratios due to the combination of weak absorbance and elevated emission to absorption ratios. The inferred CN(X) number densities and rotational temperatures obtained from broadband absorption spectroscopy were found to be consistent with tunable diode laser absorption spectroscopy (TDLAS) measurements
Conclusions and Future Work
A suite of broadband absorption spectroscopy diagnostics is currently being implemented at the EAST facility to provide access to ground and low-lying atomic and molecular states over a broad spectral range. These diagnostics employ multiple light sources and detector configurations to spectrally resolve broadband absorption features spanning from the ultraviolet to the near-infrared. The first implementation of this approach, targeting ultraviolet CN absorption bands, successfully measured CN(X) number densities and internal temperatures during Test 65B, yielding results in agreement with concurrent TDLAS measurements. The experiments also demonstrated the feasibility of broadband absorption measurements in EAST under radiating shock-tube conditions, while highlighting key challenges associated with light coupling efficiency, detector sensitivity, and emission contamination at higher shock velocities.
Further developments are planned during Test 68, scheduled to begin in July 2026, where measurements will target NO ultraviolet transitions as well as atomic O, N, and H transitions in the visible and near-infrared spectral ranges. These efforts will also support the evaluation of updated spectroscopic databases and non-Boltzmann population models. Overall, the ongoing development of broadband absorption diagnostics at EAST provides a promising pathway toward improved characterization of non-equilibrium shock-layer chemistry and enhanced validation of kinetic and radiation models relevant to atmospheric entry applications.
References
[1] B.A. Cruden et al. AIAA paper 2009-4240.
[2] D. Drescher et al. AIAA paper 2026-1678.
Summary
This work presents the development and initial implementation of broadband absorption spectroscopy diagnostics at the NASA Ames Electric Arc Shock Tube (EAST) to probe ground and low-lying atomic and molecular states over a wide spectral range, from the ultraviolet (UV) to the near-infrared (NIR). The approach combines multiple broadband light sources, including a laser-driven UV source, a supercontinuum laser, and a MEMS-VCSEL swept-wavelength laser, with high-speed detection systems to enable time-resolved absorption measurements under non-equilibrium shock-tube conditions. Initial experiments conducted during EAST Test 65B successfully measured CN ultraviolet absorption bands in Titan-relevant N₂–CH₄ mixtures, yielding CN ground-state number densities and rotational temperatures consistent with concurrent tunable diode laser absorption spectroscopy (TDLAS) measurements. The results demonstrate the feasibility of broadband absorption diagnostics in radiating shock layers and provide a promising pathway for improved characterization of non-equilibrium chemistry and validation of kinetic and radiation models relevant to atmospheric entry flows.