Ronald K. Hanson

The Stanford High-enthalpy Optical Tube/Tunnel (SHOTT) is a newly-commissioned, dual-use impulse facility which can operate as either a reflected shock tube or reflected shock tunnel and is intended for the study of hypersonic flight-relevant phenomena. SHOTT is a pressure-driven facility rated to 15 MPa and is built around optical access to enable the development and deployment of advanced spectroscopic and imaging sensors. The facility is up to 15 meters long and has a 10.6-centimeter inner diameter. When operating as a reflected shock tube, SHOTT briefly generates stagnant test gases at temperatures which can exceed 10,000 K. When operating as a reflected shock tunnel with its current nozzle, SHOTT impulsively produces high-enthalpy, supersonic test gases at Mach 3, though additional nozzles may be deployed in the future to achieve different test-gas conditions. This work describes some of SHOTT’s design features and details initial characterization experiments of the facility in both operational configurations. In reflected shock tube mode, incident shock velocity is measured by a series of high-speed pressure transducers at a range of conditions, test times are measured, and experimental repeatability is characterized. Similar measurements are conducted in reflected shock tunnel mode, with the addition of a stagnation pressure probe mounted at the nozzle exit. Moving forward, SHOTT will be an integral part of hypersonics research at Stanford University, providing a testbed to investigate non-equilibrium thermochemistry and high-speed flow physics and acting as a proving ground for advanced optical diagnostic development.

Two laser absorption spectroscopy diagnostics were developed and deployed to measure temperatures and species concentrations at the inlet and exhaust of a solid fuel ramjet (SFRJ) test facility. Custom optically accessible hardware enabled non-intrusive, in-situ measurements under representative facility conditions and was designed for seamless integration into the settling chamber and exhaust throat for the inlet and exhaust diagnostics, respectively. The inlet diagnostic used two distributed-feedback lasers near 1.4 μm, targeting H2O transitions, using a time-division multiplexed, scanned direct absorption scheme at 5 kS/s. The exhaust sensor used a quantum cascade laser near 4.9 μm to probe two CO transitions with a scanned wavelength-modulated spectroscopy technique at 4 kS/s. Time-series measurements at a baseline condition (505 K, 2.72 atm, and 0.34 kg/s at the inlet) demonstrate the sensors’ ability to resolve thermodynamic behavior, as well as initial low-frequency facility-driven fluctuations consistent across both measurement locations. Furthermore, estimates of soot volume fraction in the exhaust are provided. These initial results provide the foundation for a complete sensor suite to support future full-facility characterization and numerical model validation.

Owing to its flammability and high vapor pressure for a long-chain hydrocarbon, n-hexane is a valuable component in surrogate mixtures for commodity fuels and proxy fuel for fire safety testing in aircraft certification. Despite n-hexane’s utility, experimental measurements of key combustion properties at high temperatures and pressures relevant to engine environments are lacking. In this work, we experimentally characterized n-hexane flame propagation using the Imaging Shock Tube (IST) facility at Stanford University. Laminar flame speeds of n-hexane mixtures with lean, stoichiometric, and rich equivalence ratios (�� = 0.6, 1.0, and 1.5) were measured in the 600–1000 K temperature range at pressures between 1–5 atm. For the first time in this temperature and pressure regime, experimental data was directly compared against flame speed predictions from simulations using the NUIGMech 1.1 and Lawrence Livermore National Laboratory chemical kinetics models. This research provides novel data at previously inaccessible conditions for developing and improving chemical kinetics models for n-hexane and larger alkane oxidation. By extension, this work also supports advances in computational reacting flow modeling for propulsion system design.

Vibrational-state-resolved relaxation and chemistry of carbon monoxide and nitrogen mixtures from 2000-10 250 K probing carbon monoxide in the ground to twelfth excited vibrational levels Physics of Fluids (September 2025) On the vibrational excitation of shock-heated air.I. CO/O 2 /Ar mixtures

Laminar flame speed, temperature, and pressure measurements were conducted in high-temperature, spherically expanding ethane-air flames. The experiments were conducted in a shock tube, which allows access to a high-temperature regime previously under-explored for premixed ethane-air flames. The stoichiometric ethane-air mixtures were initially shock-heated to unburned gas conditions of 461-537 K, 1 atm. An Nd:YAG laser was used to spark-ignite the heated gas mixtures and initiate laminar flame propagation. High-speed, OH* endwall imaging was used to record the propagation of the spherically expanding flames in time, and the images were analyzed to determine the unburned, unstretched laminar flame speed. The measurements show close agreement with available literature results and kinetic model simulations (AramcoMech 3.0, NUIGMech1.3, and FFCM-2). A comprehensive survey of available ethane-air flame speed data was conducted to enable a high-fidelity power-law fit to describe the temperature dependence of ethane-air flame speeds. A single line-of-sight laser absorption diagnostic was additionally used to measure burned-gas temperature and pressure. The temperature and pressure measurements confirmed that flames generated using the shock-tube laminar flame method are adiabatic and constant-pressure. • Ethane-air laminar flame speed measurements were conducted at elevated temperatures (up to 537 K). • A new power-law fit describing the temperature dependence of stoichiometric ethane-air flame speeds (taking into account 79 data points generated in 17 different studies) is presented. • The first experimental study of temperature and pressure time-histories in the burned-gas region of laminar flames generated in a shock tube is presented.

Vibrational relaxation of the ground to twelfth excited vibrational levels (v″ = 0–12) of carbon monoxide (CO) has been probed with multi-pass absorption spectroscopy (MPAS) on a ring amplified shock tube (RAST) targeting absorption transitions near 5055 nm (1978 cm−1). Streicher et al., “High-temperature measurements of nitrogen vibrational relaxation through pathlengthamplified probing of carbon monoxide vibrational states in shock-tube experiments,” AIAA Paper No. 2025–1991, 2025. Experiments probed mixtures of 0.02%–5% CO diluted in nitrogen (N2) from 2000 to 7500 K and 0.07 to 0.84 atm, with conditions selected for sensitive inference of N2–N2 vibration-translation (VT) relaxation times (τVTN2−N2) using CO as an optically accessible tracer of N2. Two additional types of experiments leveraged argon (Ar) dilution of either 50% or 91%; 50% Ar was used to extend the test time, while 91% Ar was used to access a condition at 10 250 K where chemical reactions occur. High-temperature experiments probed 24 absorption features covering all vibrational states v″ = 0–12 and many rotational levels from J″ = 2–116, as well as weak features from the C13O16 isotopologue. Low-lying vibrational states follow an expected Boltzmann distribution, although states above v″ = 5 are observed to populate faster than expected from their Boltzmann populations—both for full N2 dilution and for 50% Ar dilution. Modal temperatures, inferred from v″ = 0–5 states, remain in good agreement with simulations of the vibrational relaxation processes based on literature values of vibrational relaxation times. Non-Boltzmann behavior for v″ > 5 was investigated with a state-to-state model, with model predictions improving when multiquantum transitions were included in the model. Overall, the inferred values of τVTN2−N2 remain consistent with literature values, although the low scatter and uncertainty of the RAST measurements suggest a rate approximately 10% faster than the Millikan and White correlation.

Laser absorption sensors have been developed for measuring carbon monoxide (CO), carbon dioxide (CO 2 ), and unburnt hydrocarbons, immediately downstream of the exhaust valves in an internal combustion engine. A spectroscopic database containing absorption linestrengths and collisional broadening parameters for CO and CO 2 , as well as absorption cross-sections for E85 fuel, was developed at engine-relevant temperatures. The sensors were designed with a 31 kHz bandwidth to measure high-speed exhaust dynamics with crank-angle resolution. Their utility was demonstrated in the exhaust manifold of a high-performance, single-cylinder development engine. At engine-relevant conditions, the measurement uncertainties for both the CO and CO 2 sensors are 10%. These sensors, when combined with simultaneous temperature, pressure, and water mole fraction measurements, form an exhaust sensor suite, which can provide insights into rapid processes in the exhaust of an internal combustion engine.

High-speed sensors are necessary to provide crank-angle-resolved data for the validation of computational models of high-performance internal combustion engines. To this end, a near-infrared laser-absorption sensor was developed for measurements of temperature and water (H 2 O) mole fraction near the exhaust valves of such an engine. A spectroscopic database was assembled for the target transitions, and the database was expanded to include broadening and narrowing by carbon dioxide (CO 2 ). Sensor accuracy was validated in benchtop experiments at representative conditions. The sensor was deployed on a single-cylinder development engine during dynamometer testing. Engine exhaust dynamics were captured at a rate of 31 kHz, sufficient for crank-angle-resolved measurements up to 5200 RPM, with 2 σ uncertainties of 5.1% and 5.8% in temperature and H 2 O mole fraction, respectively. Together with simultaneously deployed carbon monoxide, carbon dioxide, and unburnt hydrocarbon sensors and a high-speed pressure transducer, the sensor developed in this work forms a comprehensive suite for characterizing the exhaust conditions of an internal combustion engine.

We report on measurements of temperature and enthalpy in the 60-MW Interaction Heating Facility (IHF) Arcjet at the NASA Ames Research Center using tunable diode laser absorption spectroscopy. Measurements were made in the add-air plenum of the IHF, positioned between the downstream electrodes and the nozzle, which serves as the reservoir condition for the ensuing supersonic expansion. Spectroscopic measurements of atomic oxygen (O), atomic nitrogen (N), and molecular nitrogen ([Formula: see text]) serve as the basis for the reservoir characterization. Data was collected at four nominal IHF conditions spanning 11 different locations within the plenum. Measurements suggest greater radial flow uniformity at the end of the plenum than at the beginning, but at the price of flow enthalpy in the core. Agreement between separately inferred oxygen and nitrogen temperatures was variable between different conditions in the range of 1–5%. The IHF routinely demonstrates a high degree of stability in measured parameters over many minutes, but large fluctuations on the order of 500 K on millisecond timescales are noted and discussed. Inferred enthalpy profiles are compared with CFD predictions for two IHF conditions. Proposed bulk enthalpies from measurements are found to be lower by [Formula: see text], while normalized enthalpy profiles agree within 3%.

A novel method was used to directly measure the reaction rate, k 1 , of NH 3 +OH<=>NH 2 +H 2 O in shock tube experiments behind incident and reflected shock waves from 910–2474 K and 0.23–3.59 atm. NH 3 concentration of test gases was measured prior to each shock with a scanned laser absorption NH 3 diagnostic near 10.36 µm. OH was produced via thermal decomposition of tert ‑butyl hydroperoxide behind incident and reflected shock waves, and post-shock OH time-histories were measured via laser absorption at 308.6 nm. Measured OH profiles were fit with a detailed chemical kinetic model to find best-fit values for k 1 at each experimental condition, and results are compared to previous data, calculations, and recommendations for the NH 3 +OH reaction rate. To the authors’ knowledge, this is the first direct measurement of the NH 3 +OH reaction rate above 1425 K and significantly reduces the uncertainty of k 1 compared to previous indirect determinations at high temperatures. A recommendation is made for continued use of the NH 3 +OH rate expression k 1 = 10 6.31 T[K] 2.04 exp(-285/T[K]) cm 3 / mol/s suggested by Salimian et al. from 230 < T < 2474 K, which agrees well with the current data and prior low-temperature measurements. The technique used in this work also provides a new strategy for direct measurement of +OH reaction rates at reflected-shock temperatures above ∼1450 K, which has previously been a practical high-temperature limit when using tert ‑butyl hydroperoxide as a source of OH radicals.

The negatively charged tin-vacancy (SnV-) center in diamond has emerged as a promising platform for quantum computing and quantum networks. To connect SnV- qubits in large networks, in-situ tuning and stabilization of their optical transitions are essential to overcome static and dynamic frequency offsets induced by the local environment. Here we report on the large-range optical frequency tuning of diamond SnV- centers using micro-electro-mechanically mediated strain control in photonic integrated waveguide devices. We realize a tuning range of &gt;40 GHz, covering a major part of the inhomogeneous distribution. In addition, we employ real-time feedback on the strain environment to stabilize the resonant frequency and mitigate spectral wandering. These results provide a path for on-chip scaling of diamond SnV-based quantum networks.

This paper investigates the kinetics of nitric oxide and molecular oxygen under nonequilibrium conditions observed in the relaxation zone behind an air shock. We focus on thermochemical processes in air mixtures at temperatures exceeding 6,000 K. The kinetics of molecular species is studied by means of a master equation model, resolving each vibrational state of oxygen, nitrogen, and nitric oxide. Absolute concentrations of O2(v=4–6) and NO(v=0, 2) as well as total concentrations of O2 and NO are measured in the relaxation zone behind a shock wave reflected from an end-wall in the shock tube facility. These measurements are compared with the results of the master equation model employing rate coefficients obtained by the quasi-classical trajectory method on the most accurate potential energy surfaces available to date. Sensitivity analysis of the incident-reflected shock system is performed using Sobol indices. We observe generally good agreement with the experimental results and conclude that further investigation of the primary Zeldovich reaction and vibrational energy transfer in O2–N2 collisions is needed to improve agreement between computational and experimental results. This observation underscores the importance of shock tube experiments in air.

A new, pathlength-amplified tunable diode laser absorption diagnostic has been developed to probe carbon monoxide (CO) absorption features ranging from the ground to twelfth excited vibrational level (v" = 0-12) near 5055 nm. The application of this new diagnostic method in shock-heated mixtures of CO highly diluted in N2 yields a new method for inferring N2-N2 vibration-translation (VT) relaxation times, where the probing of CO vibrational states serves as a surrogate for N2. For each combination of temperature and pressure studied, at least three vibrational states of CO were resolved, with the full set of v" = 0-12 measured for experiments above 5000 K. The integrated absorbance for each probed absorbance feature was fit to infer CO number density and modal temperature (Trot and Tvib) time-histories, and justification is provided to approximately equate TvibCO and TvibN2. The use of 0.02%- 5% CO mixtures ensures vibrational relaxation measurements remained sensitive to the N2-N2 VT relaxation, while maintaining sufficiently strong absorbance signals across the measurement range. A high-temperature case with initial post-reflected shock temperature of 7490 K and pressure of 0.029 atm was used to demonstrate the probing of integrated absorbance areas for 23 absorption features, and the resulting inferences of vibrational-state-specific number density and modal temperatures show good agreement with a vibrational relaxation simulation. This demonstration shows general consistency with expected vibrational relaxation rates from literature, although some high-lying vibrational states show earlier population than the vibrational relaxation model suggests, resulting in some deviation from the expected Boltzmann distribution. Finally, a CO/N2 chemistry model has been developed to justify a chemically frozen assumption for the experiments, although future work can employ similar pathlength-amplified NO and CN diagnostics to probe high-temperature CO/N2 chemistry.

Excited air species, such as the excited electronic states of atomic oxygen and nitrogen, remain important targets for understanding nonequilibrium in flow fields around hypersonic vehicles. Recent work has shown that these species can exist out of Boltzmann equilibrium with their corresponding ground states at high temperatures due to radiative processes, slow heavy- particle excitation pathways, and preferential ionization, necessitating collisional-radiative models to accurately predict the degree of excitation. The preferential ionization of these states also make them important targets for investigating the onset of ionization behind shock waves. A new pathlength amplification configuration for shock tubes provides an opportunity to address a gap in the experimental validation data for the heavy-particle excitation of atomic air species at lower temperatures than previously studied, which simplifies the development of electronic excitation mechanisms. This new technology also enables simultaneous amplified measurements of multiple atomic species, probing their interactions during electronic excitation. A ring-amplified shock tube was used to simultaneously measure O(5S) at 777 nm and N(4P) at 868 nm with pathlengths of 10.44 and 11.58 meters, respectively, which will inform a new combined excited state model. Measurements of both states in shock-heated 1% oxygen, 5% nitrogen in argon at two different conditions, 7600 K and 7100 K and pressures of 0.75 atm and 0.90 atm, respectively, are presented here.

A diagnostic technique for simultaneous measurements of temperature, pressure, and velocity at a point is demonstrated in high-enthalpy air environments using spectrally resolved laser-induced fluorescence. The technique uses narrow-linewidth, continuous wave lasers rapidly modulated about the D2 transition of atomic potassium vapor, which is used as a flow tracer. Temperature and pressure are inferred from the measured lineshape, while gas velocity is inferred from the shift in linecenter due to the Doppler effect. The technique was validated at temperatures from 1200-1900 K, gas velocities from 1100-1500 m/s, and pressures below 0.11 atm, with 100 kHz measurement rates. Other compressible flow system quantities, including mass flux and Mach number, can then be calculated. These results represent a key step towards spatially resolved flow field mapping of temperature, pressure, and velocity in high-enthalpy gas environments.

We deploy a suite of laser-based sensors to instrument hypervelocity gas flows in the T5 Free-Piston Reflected Shock Tunnel. The six employed infrared lasers rely on Tunable Diode Laser Absorption Spectroscopy (TDLAS) to measure temperature and concentration of nitric oxide, carbon monoxide, carbon dioxide, and water in the Mach 5 freestream, and to detect electronically-excited atomic nitrogen and oxygen behind a Mach stem generated by a symmetric, opposing pair of wedge models. Hypervelocity computational fluid dynamics (CFD) modeling of the wedge models, completed in US3D, inform beam position selection for the atomic nitrogen- and oxygen-targeting TDLAS sensors relative to the opposing wedge models, and limited comparisons between experiments and these planning-phase simulations are conducted. This work encompasses two separate experiments in T5, both conducted at the same 16 MJ/kg nominal condition. The freestream sensors collect independent measurements of rotational and vibrational temperatures but find the test gas to be in thermal equilibrium, in general agreement with previous TDLAS-based measurements in the T5 freestream at this condition. The Mach-stem sensors detect electronically excited states of atomic nitrogen and oxygen at parts-per-billion concentrations. We detect the targeted atomic oxygen state during the test time, but atomic nitrogen is only measurable during the unsteady facility startup process. Measurements in both regions are made at 50 kHz. Because electronically excited atomic species are important experimental targets for developing hypervelocity CFD, these measurements demonstrate the capability of TDLAS data to contribute to improving hypervelocity models.

The results of atomic resonance absorption spectroscopy (ARAS) experiments used to measure the low-lying excited states of atomic nitrogen, N(2D) and N(2P), in shock tube experiments are presented here. The probed conditions range from 5100 K to 6400 K in 0.6 atm to 0.8 atm, 1 % nitrogen in argon mixtures, with test times on the order of 1 ms. N(2D) and N(2P) were measured using absorption spectroscopy at vacuum ultraviolet wavelengths of 149 nm and 174 nm, respectively. The vacuum ultraviolet light was produced using a nitrogen-helium microwave discharge lamp and spectrally filtered with a vacuum monochromator, both of which were directly mounted to the shock tube. Self-absorption in the discharge lamp was characterized and simulated using the measured relative strength of the multiplets at each state in order to infer number density. The measured number densities as a function of time of N(2D) and N(2P) were used to infer kinetic rates relevant to hypersonic flow conditions using a preliminary one dimensional, reduced order, single temperature collisional-radiative model. To the best of the authors’ knowledge, this is the first time that N(2D) and N(2P) have been measured in a shock tube.

We demonstrate large-range tuning of the optical transition of Tin-Vacancies (SnV) in diamond using electro-mechanical-induced strain, realizing &gt; 40 GHz tuning. We employ real-time feedback on the strain environment to stabilize the resonant frequency.

Distributed feedback lasers, which feature rapid wavelength tunability, are not presently available in the yellow and orange spectral regions, impeding spectroscopic studies of short-lived species that absorb light in this range. To meet this need, a rapidly tunable laser system was constructed, characterized, and demonstrated for measurements of the NH2 radical at 597.4 nm. The system consisted of three main parts: (1) a distributed feedback diode laser operating at 1194.8 nm, (2) a fiber-coupled optical amplifier, and (3) a periodically poled lithium niobate (PPLN) waveguide for second-harmonic generation. A phase-matching optical frequency bandwidth of 118 GHz and a second-harmonic generation efficiency of 109%/W were determined for the PPLN waveguide, and the intensity and wavelength stability of the system were measured. The rapid-tuning capabilities of the laser system were characterized to explore its potential for use in scanned-direct absorption and wavelength modulation spectroscopy experiments. The feasibility of scanned-direct absorption up to a scan rate of 900 kHz and wavelength modulation spectroscopy at modulation frequencies up to 800 kHz were demonstrated. Finally, the system was deployed in a series of shock tube experiments in which the concentration of NH2 radicals was measured during the decomposition of NH3 behind reflected shock waves.

The ability to sense and control nuclear spins near solid-state defects might enable a range of quantum technologies. Dynamically Decoupled Radio-Frequency (DDRF) control offers a high degree of design flexibility and long electron-spin coherence times. However, previous studies considered simplified models and little is known about optimal gate design and fundamental limits. Here, we develop a generalised DDRF framework that has important implications for spin sensing and control. Our analytical model, which we corroborate by experiments on a single NV center in diamond, reveals the mechanisms that govern the selectivity of gates and their effective Rabi frequencies, and enables flexible detuned gate designs. We apply these insights to numerically show a 60x sensitivity enhancement for detecting weakly coupled spins and study the optimisation of quantum gates in multi-qubit registers. These results advance the understanding for a broad class of gates and provide a toolbox for application-specific design, enabling improved quantum control and sensing.

We report on near-body measurements of temperature and nitric oxide (NO) concentration in the hypersonic flows around a cylindrical test article in the Caltech T5 reflected shock tunnel. Flow measurements were made at 50 kHz using tunable diode laser absorption spectroscopy, deploying six lasers to probe an array of quantum state-specific transitions. Laser beams were positioned both in the freestream and behind the bow shock at specific locations deemed pertinent to computational fluid dynamics comparison and kinetic model evaluation. The fractions of laser beam pathlengths behind the shock in different spatial regions were also discerned, thus providing a measurement of shock location. This study consists of six total experiments (“shots”) across two Mach [Formula: see text] conditions, characterized by total enthalpies of 8 and [Formula: see text] and freestream velocities of 3.5 and [Formula: see text], respectively. Freestream measurements generally concur with prior works in the T5, but with some non-trivial differences. Shock-layer measurements span from 2000 to 6000 K and feature noteworthy and expected variety among different zones within the post-shock region. Thermal equilibrium is generally held throughout the flowfield, but chemical nonequilibrium is commonly observed. NO is the primary spectroscopic target, but measurements of carbon monoxide, carbon dioxide, water, and atomic oxygen provide supplementary insights.