Femtosecond laser tagging measures velocity in hypersonic arc-jet plasma flow

Engineers who test heat shields on the ground face an awkward problem. The facilities that best reproduce re-entry heating are arc-jet tunnels, which push electrically heated, partly ionised air past a sample. That plasma is hot enough to destroy a probe and bright enough to blind a camera. A team at the University of Texas at Arlington's Aerodynamics Research Center has now published a laser-based velocity measurement made inside that environment. The paper appeared in Communications Engineering. The authors are graduate students Aaron Gieder, Nikkhil Chander and Alyssa Velky and professor Luca Maddalena, with co-authors from Texas A&M, Hy-SET LLC and Speckodyne Inc. Reference lists also name Laura and Arthur Dogariu and Daniel Palmquist among the authors.

According to the university's announcement, the technique reaches into the shock layer. This is the thin region between the bow shock and the vehicle surface, where the gas is hottest and where boundary-layer behaviour sets the heat load.

01Writing a tracer into the gas

The method is femtosecond laser electronic excitation tagging, or FLEET. Conventional velocimetry seeds a flow with particles and tracks them. That fails in hypersonic flow because particles lag the gas across shocks, can contaminate the tunnel and can't be injected cleanly into a plasma. FLEET avoids the problem by making the tracer from the gas itself.

As UT Arlington describes it, a train of extremely short laser pulses, each lasting a quadrillionth of a second, dissociates nitrogen molecules in the air. When the atoms recombine into molecules, they emit light, creating a thin line that moves with the airflow, and a high-speed camera tracks the line over time. A NASA technical report on FLEET explains the arithmetic. The tag is imaged by a time-delayed, fast-gated camera, and velocity comes from the displacement during the delay. A separate Chinese hypersonic-tunnel study notes that the fluorescence lasts on the order of microseconds while the camera gate is set to hundreds of nanoseconds. So the tag is a very short-lived marker, and the gate timing has to match it.

The laser is also non-intrusive. Maddalena said the method lets the team measure turbulence "really close to the surface" of the material in the shock layer, a region that a physical probe would disturb or destroy. The team says the laser doesn't perturb the flow. Dissociating a small fraction of nitrogen along a line is a very light touch compared with a probe in a plasma.

FLEET is not new in hypersonics. Researchers have used it in large facilities, for example to measure streamwise velocity at Mach 8 in Sandia's Hypersonic Wind Tunnel. The harder question was whether it would work in an arc-jet, with its extreme background light.

02Why arc-jets are hard

UT Arlington's arc-jet tunnel, known as Leste, came online in 2019. The university calls it the first of its kind at a university and one of only a handful of arc-jet wind tunnels in the United States. The group reported the first FLEET measurement in an arc-jet in 2021. The tunnel was a 1.6 MW arc-jet plasma facility, and the average plume gas velocity for the condition studied was 1.78 km/s.

The 2020 release on that first demonstration named the central difficulty: separating the faint afterglow tracer signal from bright background emission produced by the plasma. A later turbulence study from the same group used a conventional optical fix. It applied a 725 nm long-pass filter and a background-subtraction algorithm to isolate the FLEET signal from arc-jet radiation. That work was in a Mach 4 nitrogen nozzle flow, a far easier case than air. It also shows how selective the data can be. Of 4,722 images, only 563 were usable for velocity measurements.

Moving from the freestream to the shock layer in air is a bigger step, for two reasons:

• Signal quenching. Oxygen in hot air weakens the emission from the nitrogen molecules. That makes the tag dimmer and harder to image, and tagging air at realistic conditions is the harder case than pure nitrogen.
• Background. The shock layer is where the plasma glows most brightly, so the signal-to-background ratio is at its worst.

According to the Tech Xplore summary of the paper, the team obtained velocity measurements despite intense plasma emission and oxygen-related signal loss. A summary on Bioengineer.org says velocities in the jet core were consistent with the facility's operating conditions and with complementary diagnostics, and that spatially resolved profiles showed structure that point measurements would average away. I could not access the full paper, so I can't report uncertainties, spatial resolution, laser repetition rate or the exact test conditions. Those numbers decide how far other labs can reproduce the result.

03Why turbulence drives heat-shield thickness

The practical interest is in heat transfer. UT Arlington notes that when smooth airflow turns turbulent, heat load on a vehicle can rise by 10 times or more. Thermal protection systems must be sized for the worst credible heating. If ground tests overstate or understate turbulence, or if designers can't verify what the tunnel produced, the margin must cover the uncertainty. That margin costs mass, and mass is expensive on both spacecraft and hypersonic vehicles.

Until now, much of what happens at the wall in a plasma tunnel has been inferred. Simulations of reacting, ionised, turbulent flow carry large model uncertainties, and they need near-wall data to be checked. Maddalena's group has pursued this under Office of Naval Research funding. A 2022 UT Arlington release described a grant to study how turbulent hypersonic flow affects thermal-protection materials, particularly at leading edges and noses, using femtosecond TALIF, FLEET and coherent anti-Stokes Raman spectroscopy (CARS). The arc-jet group's own site lists a separate grant to add CARS to the arc-jet femtosecond-laser diagnostics. CARS measures temperature and species rather than velocity. Combining the diagnostics would give velocity, temperature and composition in the same flow.

04Ground tests versus flight

Maddalena makes an unusual point about test realism. In his words, the team is "testing in worse conditions" than actual flight, because the plasma flow generated in ground facilities is more turbulent than what a vehicle experiences. A tunnel that adds turbulence flatters nothing: it tends to over-predict heating compared with flight. The group's aim is to quantify that effect so that data from any tunnel can be corrected toward flight conditions. He says, "no matter which wind tunnel is used."

That is a goal rather than a result. The paper shows the measurement is possible. It doesn't show that the correction works. Turning near-wall velocity statistics into a validated flight-extrapolation method will need comparison with computational models, heat-flux data on actual samples and, ideally, flight data. Open questions that the public material doesn't answer include:

• how thin a layer next to the wall can be resolved;
• how the oxygen quenching correction is handled quantitatively;
• how repeatable single-shot measurements are across tunnel conditions.

The paper is a measurement-capability milestone. It makes a previously invisible quantity observable in one of the harshest test environments available, and that is the prerequisite for the heat-shield sizing work described above.